Influence of High Cutting Speeds [PDF]

Zitiervorschau

Report No. ERC/NSM - S-96-19

INFLUENCE OF HIGH CUTTING SPEEDS ON THE QUALITY OF BLANKED PARTS by

Martin Grünbaum, Visiting Scholar University of Stuttgart, Germany

and Jochen Breitling, Staff Engineer, ERC/NSM Taylan Altan, Professor and Director, ERC/NSM

NSF Engineering Research Center for Net Shape Manufacturing The Ohio State University 1971 Neil Avenue Columbus, Ohio 43210

May, 1996 Advanced Copy For Limited Distribution Only

(This report is an advance copy subject to modification and is distributed only to members of the ERC for Net Shape Manufacturing. Approval must be requested from the ERC prior to distribution to other organizations or individuals.)

FOREWORD This document has been prepared for the Engineering Research Center for Net Shape Manufacturing (ERC/NSM). The Center was established on May 1, 1986 and is funded by the National Science Foundation and the member companies. The focus of the Center is net shape manufacturing with emphasis on costeffective manufacturing of discrete parts. The research concentrates on manufacturing from engineering materials to finish or near finish dimensions via processes that use dies and molds. In addition, to conduct industrially relevant engineering research, the Center has the objectives to a) establish close cooperation between industry and the university, b) train students and c) transfer the research results to interested companies.

This report summarizes experimental and simulation work, investigating the effects of very high cutting speeds. The goal of the first part of the project was to monitor

and

analyze

the

velocity

profile

obtained

with

a

Lourdes

electromagnetic impact press. For that purpose the press was equipped with a velocity and proximity sensor in order to monitor the velocity-stroke curve. In addition, the influence of material properties, cutting velocity and punch-die clearance on the quality of the part edge was investigated.

Information about the ERC for Net Shape Manufacturing can be obtained from the office of the Director, Taylan Altan, located at the Baker Systems Engineering Building, 1971 Neil Avenue, Columbus, Ohio 43210-1271, phone 614/292-5063.

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INFLUENCE OF HIGH CUTTING SPEEDS ON THE QUALITY OF BLANKED PARTS Martin Grünbaum, Visiting Scholar Report No. ERC/NSM - S-96-19 EXECUTIVE SUMMARY This report summarizes the influences of different parameters on the part edge quality of blanked parts. Experiments have been conducted using different materials, punch-die clearances and cutting speeds. In order to determine the reachable cutting speeds and to calculate the energy required for blanking, velocity-stroke curves were obtained. In addition, blanking simulations with DEFORM 2D have been performed. The results of these simulations have then been compared with the results obtained by the experiments.

The evaluation of the part edges shows that higher cutting speeds can improve the part edge quality, resulting in smaller burr height and rollover, and a larger shear zone. Furthermore, it could be observed that the part quality improvement when blanking with high cutting speeds (up to 12 ft/sec) is much more distinct for steel than for copper or aluminum. According to theory, this improvement was expected because copper and aluminum have much higher heat conduction coefficients. Therefore, the heat dissipates faster and the desired stress relief effect does not take place to the same degree as for steel.

ERC/Net shape Manufacturing 339 Baker Systems / 1971 Neil Avenue Columbus, OH 43210 ph: 614-292-9267 fax: 614-292-7219

ii

TABLE OF CONTENTS

FOREWORD.................................................................................................................. i EXECUTIVE SUMMARY............................................................................................. ii TABLE OF CONTENTS.............................................................................................. iii LIST OF TABLES.......................................................................................................... v LIST OF FIGURES....................................................................................................... vi

PAGE

CHAPTERS

1. INTRODUCTION .....................................................................................................................................1 2. THE BLANKING PROCESS ..................................................................................................................2 2.1 DEFINITION OF SHEARING AND BLANKING .................................................................................................2 2.2 INFLUENCES ON THE BLANKING PROCESS ...................................................................................................2 2.3 PHASES OF THE BLANKING PROCESS ..........................................................................................................3 2.4 STRESS CONDITIONS IN SHEARING ..............................................................................................................6 2.5 FORMATION OF THE PART EDGE ..................................................................................................................8 2.6 HIGH SPEED BLANKING .............................................................................................................................10 3. EQUIPMENT USED...............................................................................................................................13 3.1 THE LOURDES PRESS 100-OH ..................................................................................................................13 3.2 EXPERIMENTAL SETUP .............................................................................................................................17 3.2.1 Punches and Dies ............................................................................................................................17 3.2.2 Stock materials ................................................................................................................................19 3.2.3 Sensors.............................................................................................................................................20 3.2.3.1 Analog Proximity Sensor ..........................................................................................................................20 3.2.3.2 Linear velocity transducer .........................................................................................................................20

3.2.4 Data Acquisition and Signal Analysis .............................................................................................21 3.3 TECHNIQUES FOR EVALUATING THE PART EDGE .......................................................................................22 3.3.1 Measuring the penetration depth, the shear zone and the rollover .................................................22 3.3.2 Technique for burr height measuring ..............................................................................................23 3.3.2.1 Definition of the burr height......................................................................................................................23 3.3.2.2 Requirements and design of the device .....................................................................................................24

iii

4. EXPERIMENTAL PROCEDURE ........................................................................................................27 4.1 INVESTIGATION OF PRESS CHARACTERISTICS ............................................................................................28 4.2 VELOCITY INVESTIGATIONS ......................................................................................................................29 4.3 PART QUALITY INVESTIGATIONS ...............................................................................................................30 5. EXPERIMENTAL RESULTS ...............................................................................................................31 5.1 PRESS CHARACTERISTICS AND PRELIMINARY RESULTS .............................................................................31 5.2 VELOCITY INVESTIGATIONS ......................................................................................................................33 5.3 PART QUALITY INVESTIGATIONS ...............................................................................................................41 5.3.1 Experiments with low carbon steel ..................................................................................................42 5.3.2 Experiments with high strength steel...............................................................................................50 5.3.3 Experiments with aluminum ............................................................................................................57 5.3.4 Experiments with copper .................................................................................................................64 5.3.5 Material comparison .......................................................................................................................71 6. FEM SIMULATIONS WITH DEFORM 2D........................................................................................78 6.1 THE FINITE ELEMENT CODE DEFORM 2D ..............................................................................................78 6.1.1 Pre-processor ..................................................................................................................................78 6.1.2 Simulation engine ............................................................................................................................78 6.1.3 Post-processor .................................................................................................................................79 6.2 SIMULATIONS OF THE HIGH SPEED BLANKING PROCESS ............................................................................79 6.2.1 Simulation settings...........................................................................................................................79 6.2.2 Simulation results ............................................................................................................................81 6.2.3 Comparison of the experimental and simulated results...................................................................82 7. SUMMARY AND CONCLUSIONS......................................................................................................84 8. LIST OF REFERENCES .......................................................................................................................87

APPENDIX A APPENDIX B APPENDIX C APPENDIX D

iii

LIST OF TABLES PAGE

CHAPTERS

TABLE 1: INFLUENCES ON THE FORMATION OF THE CUTTING EDGE /10/, /4/, /8/, /11/, /12/................................10 TABLE 2: HEAT CONDUCTION COEFFICIENTS FOR DIFFERENT MATERIALS /19/ ..................................................11 TABLE 3: LOURDES 100-OH PRESS SPECIFICATIONS (ACCORDING TO THE MANUFACTURER) .............................14 TABLE 4: CLEARANCES IN PERCENT FOR DIFFERENT PUNCH DIAMETERS AND MATERIAL THICKNESSES (DIE BUTTON DIAMETER: 0.500")...................................................................................................................19

TABLE 5: CLEARANCES IN PERCENT FOR DIFFERENT PUNCH DIAMETERS AND MATERIAL THICKNESSES (DIE BUTTON DIAMETER: 0.514")...................................................................................................................19

TABLE 6: CUTTING SPEEDS AT DIFFERENT POWER LEVELS OF THE PRESS FOR DIFFERENT MATERIALS .................32 TABLE 7: CUTTING FORCE FOR DIFFERENT MATERIALS .....................................................................................38 TABLE 8: ENERGY LEVEL AT DIFFERENT CUTTING SPEEDS.................................................................................40 TABLE 9: CUTTING ENERGY FOR THE STOCK MATERIALS ...................................................................................41 TABLE 10: CROSS SECTION AND SIDE VIEW FOR LOW CARBON STEEL. CLEARANCE: 3/4%...................................47 TABLE 11: CROSS-SECTION AND SIDE VIEW FOR LOW CARBON STEEL. CLEARANCE: 15%. ..................................48 TABLE 12: CROSS-SECTION AND SIDE VIEW FOR LOW CARBON STEEL. CLEARANCE: 21/24%. .............................49 TABLE 13: CROSS-SECTION AND SIDE VIEW FOR HIGH STRENGTH STEEL. CLEARANCE: 3%.................................54 TABLE 14: CROSS-SECTION AND SIDE VIEW FOR HIGH STRENGTH STEEL. CLEARANCE: 14%...............................55 TABLE 15: CROSS-SECTION AND SIDE VIEW FOR HIGH STRENGTH STEEL. CLEARANCE: 20/24%..........................56 TABLE 16: CROSS-SECTION AND SIDE VIEW FOR ALUMINUM. CLEARANCE: 5%. .................................................61 TABLE 17: CROSS-SECTION AND SIDE VIEW FOR ALUMINUM. CLEARANCE: 15%. ...............................................62 TABLE 18: CROSS-SECTION AND SIDE VIEW FOR ALUMINUM. CLEARANCE: 21%. ...............................................63 TABLE 19: CROSS-SECTION AND SIDE VIEW FOR COPPER. CLEARANCE: 6%.......................................................68 TABLE 20: CROSS-SECTION AND SIDE VIEW FOR COPPER. CLEARANCE: 13%.....................................................69 TABLE 21: CROSS-SECTION AND SIDE VIEW FOR COPPER. CLEARANCE: 19%.....................................................70 TABLE 22: INPUT DATA OF THE LOW CARBON STEEL SHEET AND THE PUNCH......................................................80 TABLE 23: INPUT DATA OF THE DIE BUTTON AND THE BLANK HOLDER ...............................................................80 TABLE 24: COMPARISON OF THE EXPERIMENTAL AND SIMULATED RESULTS (LOW CARBON STEEL) ......................83

v

LIST OF FIGURES PAGE

CHAPTERS

FIGURE 1: SCHEMATIC ILLUSTRATION OF THE SHEARING PROCESS.......................................................................3 FIGURE 2: PHASES OF THE BLANKING PROCESS /6/ ............................................................................................6 FIGURE 3: SHEARING FORCES ............................................................................................................................7 FIGURE 4: STRESS CONDITIONS IN THE SHEARING ZONE ......................................................................................8 FIGURE 5: DIFFERENT ZONES OF THE PART EDGE ...............................................................................................9 FIGURE 6: THE LOURDES PRESS 100-OH ........................................................................................................13 FIGURE 7: POSITION OF MATERIAL STRIP IN THE PRESS .....................................................................................14 FIGURE 8: SCHEMATIC PICTURE OF THE LOURDES PRESS 100 - OH..................................................................15 FIGURE 9: PUNCH VELOCITY VERSUS THE POWER LEVELS OF THE PRESS DEPENDING ON THE STROKE LENGTH. NO CUTTING CONDITION. .............................................................................................................................17

FIGURE 10: STRIPPER, DIE BUTTON AND PUNCH ...............................................................................................18 FIGURE 11: SENSOR WIRING (SCHEMATICALLY).................................................................................................22 FIGURE 12: MICROSCOPE PICTURE SHOWING THE CROSS SECTION OF A SLUG /4/...............................................23 FIGURE 13: BURR HEIGHT MEASUREMENT DEVICE ............................................................................................24 FIGURE 14: METHOD OF WORKING OF THE BURR HEIGHT MEASURING DEVICE...................................................25 FIGURE 15: VELOCITY/DISPLACEMENT-TIME CURVE FOR NO CUTTING CONDITION AT POWER LEVEL 5 OF THE PRESS ....................................................................................................................................................28

FIGURE 16: VELOCITY-STROKE CURVE FOR NO CUTTING CONDITION. POWER LEVEL 5. ......................................29 FIGURE 17:MAXIMUM CUTTING SPEED FOR DIFFERENT PUNCH-DIE CLEARANCES AND STOCK MATERIALS ...........31 FIGURE 18: UNIFORMITY OF PART EDGE (LOW CARBON STEEL, 14% CLEARANCE, 4 FT/SEC)...............................33 FIGURE 19: VELOCITY/DISPLACEMENT-TIME CURVE. MATERIAL: LOW CARBON STEEL. POWER LEVEL 3, STROKE LENGTH 0.5"..........................................................................................................................................34

FIGURE 20: VELOCITY/DISPLACEMENT-TIME CURVE. MATERIAL: LOW CARBON STEEL. POWER LEVEL 9, STROKE LENGTH 1.5"..........................................................................................................................................35

FIGURE 21: PUNCH VELOCITY VERSUS DISPLACEMENT. MATERIAL: LOW CARBON STEEL. POWER LEVEL 3, STROKE LENGTH 0.5"..........................................................................................................................................36

FIGURE 22: PUNCH VELOCITY VERSUS DISPLACEMENT. MATERIAL: LOW CARBON STEEL. POWER LEVEL 9, STROKE LENGTH 1.5"..........................................................................................................................................37

FIGURE 23: BURR HEIGHT VERSUS PUNCH-DIE CLEARANCE (LOW CARBON STEEL)..............................................42 FIGURE 24: % SHEAR VERSUS PUNCH-DIE CLEARANCE (LOW CARBON STEEL).....................................................43 FIGURE 25: % ROLLOVER VERSUS PUNCH-DIE CLEARANCE (LOW CARBON STEEL)...............................................45 FIGURE 26: % PENETRATION VERSUS PUNCH-DIE CLEARANCE (LOW CARBON STEEL)..........................................46 FIGURE 27: BURR HEIGHT VERSUS PUNCH-DIE CLEARANCE (HIGH STRENGTH STEEL) .........................................50

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FIGURE 28: % SHEAR VERSUS PUNCH-DIE CLEARANCE (HIGH STRENGTH STEEL) ................................................51 FIGURE 29: % ROLLOVER VERSUS PUNCH-DIE CLEARANCE (HIGH STRENGTH STEEL) ..........................................52 FIGURE 30: % PENETRATION VERSUS PUNCH-DIE CLEARANCE (HIGH STRENGTH STEEL) .....................................53 FIGURE 31: BURR HEIGHT VERSUS PUNCH-DIE CLEARANCE (ALUMINUM) ..........................................................57 FIGURE 32: % SHEAR VERSUS PUNCH-DIE CLEARANCE (ALUMINUM)..................................................................58 FIGURE 33: % ROLLOVER VERSUS PUNCH-DIE CLEARANCE (ALUMINUM)............................................................59 FIGURE 34: % PENETRATION VERSUS PUNCH-DIE CLEARANCE (ALUMINUM).......................................................60 FIGURE 35: % SHEAR VERSUS PUNCH-DIE CLEARANCE (COPPER) ......................................................................65 FIGURE 36: % ROLLOVER VERSUS PUNCH-DIE CLEARANCE (COPPER) ................................................................66 FIGURE 37: % PENETRATION VERSUS PUNCH-DIE CLEARANCE (COPPER) ...........................................................67 FIGURE 38: BURR HEIGHT VERSUS PUNCH-DIE CLEARANCE, 0.5 FT/SEC.............................................................71 FIGURE 39: BURR HEIGHT VERSUS PUNCH-DIE CLEARANCE, 12 FT/SEC..............................................................71 FIGURE 40: % SHEAR VERSUS PUNCH-DIE CLEARANCE, 0.5 FT/SEC ....................................................................73 FIGURE 41: % SHEAR VERSUS PUNCH-DIE CLEARANCE, 12 FT/SEC .....................................................................73 FIGURE 42: % ROLLOVER VERSUS PUNCH-DIE CLEARANCE, 0.5 FT/SEC ..............................................................75 FIGURE 43: % ROLLOVER VERSUS PUNCH-DIE CLEARANCE, 12 FT/SEC ...............................................................75 FIGURE 44: % PENETRATION VERSUS PUNCH-DIE CLEARANCE, 0.5 FT/SEC .........................................................76 FIGURE 45: % PENETRATION VERSUS PUNCH-DIE CLEARANCE, 12 FT/SEC ..........................................................76

vi

1. Introduction Since raw-material as well as energy will become scarce and hence more expensive in the future, the costs in the production industry will increase. Despite the pressure of rising costs, it is very important to stay competitive in the market. Therefore it is essential to think about economic production and high productivity during the early part of the design phase.

Netshape or near netshape manufacturing is becoming more important to decrease the costs of production. The final part has to be produced with fewer manufacturing processes and natural resources. Many sheet metal parts are produced by a blanking operation or include blanking within their manufacturing process. In particular, when blanking with high punch velocities it is possible to manufacture near netshape parts by improving the part edge quality. The burr height, the rollover and the penetration depth are decreased and the shear zone is increased when blanking at high speeds /1/. The overall goal is to improve the blanking process in such a way that the produced part does not need to be reworked. Therefore, the process parameters have to be optimized carefully.

In order to determine the influence of different parameters on the part edge quality, investigations have been made by varying process parameters, like the punch-die clearance and the cutting speed for different materials. The punch velocity and displacement was continously monitored by means of a velocity and proximity sensor. Based on these two values it was possible to calculate the required energy for blanking a part.

1

2. The blanking process The following sections provide general information about the blanking process, the different phases of the cutting operation and the various parameters that influence the process. In addition, the formation of the resulting part edge will be discussed.

2.1 Definition of Shearing and Blanking Shearing is defined as the cutting of a workpiece between two die components. The material is stressed between two cutting edges to the point of fracture or beyond its ultimate strength. During this process, the material is subjected to tensile as well as compressive stresses /2/.

Various metal forming operations are based on the shearing process. Blanking is cutting of parts out of sheet material to a predetermined contour. The contour is defined by the punch and the die (Figure 1). The ejected slug is the part and the remaining skeleton is considered scrap, in contrast with punching where the sheared slug is discarded and the rest is the part /2/.

2.2 Influences on the blanking process The cutting process is influenced by many parameters. The primary variables affecting the cutting process are listed below /3/, /4/: •

punch - die clearance

•

punch velocity

•

stock material (thickness, mechanical properties, chemical composition, microstructure and grain size)

•

cutting tools (materials, cutting edge, tool wear)

•

lubrication

2

•

alignment of the tools and

•

strain rate.

pu nch m otion

p u nch strip p er w orkp iece

d ie

Figure 1: Schematic illustration of the shearing process Figure 1 shows the shearing process in principle. In industrial applications a stripper or blankholder is used to strip or remove the material from the punches. The simpliest type of stripper is the fixed one, in which the material is guided in a gap between the die and stripper plate (fixed channel stripper) /5/.

2.3 Phases of the Blanking Process The different phases of the cutting process are shown schematically in Figure 2. Schüssler explains the process in terms of the following steps /6/, /7/:

Phase1:

The punch moves downwards in the direction of the sheet with a certain velocity. There is no contact between punch and die.

3

Phase 2:

The punch reaches the stock. Due to elastic and plastic deformation, the contact area increases until enough force is applied for material deformation.

Phase 3:

The applied forces deform the stock elastically. The amount of the bending moment and the elastic bending of the sheet depends on the clearance between punch and die. The bending force consists of tension and compression.

Phase 4:

As the cutting elements penetrate further into the material, the stresses within the deformation zone reach the shearing strength of the material. The material starts yielding into the die. In this phase, the draw in zone/rounded edge of the final part is created.

Phase 5:

The material is sheared along the cutting edge. During this phase the material which flows into the cutting gap is strain hardened within the deformation area. The maximum cutting force is reached in this step.

Phase 6:

As soon as the shear stress reaches the tensile strength of the material, material rupture starts. Due to high radial and tangential stresses rupture starts behind the cutting edge on the surface area of the die and spreads out to the surface area of the punch. This phase is completed when the maximum rupture force of the material is reached.

Phase 7:

The blanked part is separated from the stock. If the incipient cracks which start at the cutting edge of punch and die are not aligned and 4

do not meet, the material is still not completely separated. In this case the complete material separation occurs after the cutting elements move further together.

Phase 8:

The two parts are now separated and the shape of the shear area is fully developed. Due to the elastic springback effects, the diameter of the slug increases and the diameter of the skeleton decreases. The result is a pressure on the surface of the cutting elements.

Phase 9:

The punch moves down to the bottom dead center and ejects the blanked part. During this phase, the surface pressure is still effective.

Phase 10:

At the bottom dead center, the direction of motion is inverted. Due to the friction between the stock and the surface of the punch, the surface pressure is intensified. A stripper or blank holder has to strip the blank from the punch.

5

Figure 2: Phases of the Blanking Process /6/

2.4 Stress conditions in shearing The cutting forces do not act linearly at the cutting edge. Instead, the vertical force FV and horizontal force FH act in a small area near the cutting edge (Figure 3). The distribution of those compressive forces is nonuniform. The distance l between the forces causes a moment which either bends or tilts the workpiece. This moment has to be compensated by a counterbending moment which results in bending stresses and horizontal normal stresses on the workpiece and tool /4/, /8/. In addition to the above forces, frictional forces also act on the tooling. The horizontal forces result in the frictional forces µFH and µF'H and the shearing forces in µFV and µF'V .

6

Figure 3: Shearing forces The stress condition in the shearing zone during crack formation is triaxial. According to Tresca, the flow criterion is given by (Figure 4) /9/:

τ max =

σ1 −σ3 2

=

σf

(1)

2

σ1

principal tensile stress

σ3

principal compressive stress

σf

flow stress

τ max

maximum shear stress

During the process, the shear yield stress increases because of the strain hardening effect. As shown in Figure 4, the principal stress circle enlarges until the shearing strength is reached /8/.

7

As described in chapter 2.3, the stress condition changes throughout the deformation process. The cracks propagate in the direction of the maximum shear stresses /9/.

shear stress shearing strength

shear yield stress

τ max

fractu re

com p ression

shearing σ 3

norm al stress

σ

1

tension

Figure 4: Stress conditions in the shearing zone

2.5 Formation of the part edge The part edge is characterized by distinct regions as shown below (Figure 5) /4/. The smooth and shiny area created by shearing the material is called shear zone. The rough surface of the rupture zone is caused by material fracture/3/. The penetration depth of the cracks depends on the material and the clearance between punch and die. If the cracks do not run towards each other, secondary shear formation may occur /8/. Stresses between the tool and the workpiece, and between the two sheared surfaces, generate friction that stretches the metal into a thin, ragged protrusion called a burr (see also 2.4). The side opposite the burr

8

develops a rounded edge called rollover, as material is drawn away from the surface /1/. The rollover is caused by plastic deformation, which is mainly affected by material ductility, tool wear and clearance.

The quality of the part edge (ratio of the different zones) is mainly influenced by the punch-die clearance, material properties, material thickness, cutting speed and tool wear (see also chapter 3.2.1). The burr height, for instance, increases with increasing clearance and increasing ductility of the material. Previous investigations have shown that the cutting speed can have a remarkable influence on the formation of the part edge (see also chapter 2.6) /6/. The influences on the formation of the different zones of the part edge are summarized in Table 1.

rollover shear zone secondary shear rupture zone burr

depth of the crack penetration

Figure 5: Different zones of the part edge

9

zone

mainly influenced by:

burr

ductility of the material, clearance, tool wear, cutting speed

rollover

material properties, clearance, tool wear,

penetration depth

tool wear, material properties, clearance

shear zone

material properties, tool wear, cutting speed, clearance

secondary shear

ductility of the material, clearance, sheet thickness

Table 1: Influences on the formation of the cutting edge /4/, /8/, /10/, /11/, /12/

2.6 High speed blanking Blanking at high speeds could mean that the process speed/stroke rate is high or that the punch speed is high, or both. In this study, high speed blanking is refered to as blanking with high punch velocities. The result are high strain rates within the material. The strain rate within a deforming material describes the variation of strain over time. The strain rate (dimension: [ s −1 ]) affects the temperature of the workpiece as well as of the tool. Huml found out that as much as 95% of all the work performed when forming and cutting materials is converted into heat /13/. This means that an increase in cutting velocity results in a temperature increase in the forming zone and the tool surface. Since the heat is generated faster than it can dissipate into the material (depending on the heat conduction coefficient, Table 2), the result is a very high temperature concentration in a narrow shearing zone.

10

This effect produces three main benefits: •

High temperatures of up to 1800°F in the shearing zone create a stress release effect within the material /14/, /15/. The higher the speed, the more the effect of stress release is apparent. The strain hardening effect caused by the material deformation may even be neutralized. Because of the stress release the material can withstand more shearing until the shearing strength is exceeded and the material fractures.

•

Due to the small deformation area, the springback of the parts is negligible compared to blanking at lower shearing speeds. This phenomena results in less return stroke load and therefore less tool wear /16/.

•

Temperature related internal stresses and fractures are dramatically reduced /17/.

These effects result in an improvement of the part edge quality by means of decreasing the burr height and the rollover and increasing the shear zone. In addition, less distortion is created than in blanks produced at low speeds /18/.

The heat transfer is characterized by the heat conduction within a material. A characteristic value for the heat conduction is the heat conduction coefficient. The following table provides a comparison for different materials:

Material

Heat conduction coefficient [W/Km]

Steel (0.2 % C)

50

Steel (0.6 % C)

46

Copper

350...370

Aluminum (99.5%)

221

Brass

80...120

Table 2: Heat conduction coefficients for different materials /19/

11

Table 2 shows that the heat conduction coefficient of copper and aluminum is much higher than the one for steel. Therefore, the heat which is created by high cutting velocities is also dissipating faster when cutting copper or aluminum. This results in lower quality improvement for copper and aluminum when blanking with high punch speeds.

Contrary to high speed blanking, the stress relief effects are negligible while blanking at lower speeds. The edge quality of the blanked part is mainly affected by the material properties and the tool geometry. Due to the stress profile and the movement of the tools, the material strain hardens in the area close to the cutting tool surface. Since this area can withstand higher stresses than the material next to it, the rupture takes place in the direction of the maximum shear stress within the unhardened material. The resulting stress profile created by the force couple has two separate shear zones which grow towards each other from opposite sides of the workpiece. In the zone between the two shear planes, the stress builds up to the material’s tensile strength, causing it to rupture. This results in an S-shaped edge on the blanked part /1/.

All these effects, seen at low speeds, result in blanked parts with a larger deformation zone, more part deformation, a higher burr and a rough rupture zone compared to high speed blanking /20/.

Ideal process conditions in high speed blanking are only achieved when the strain hardening effect is reduced as much as possible by the stress relief effects. This means that the optimum shearing speed has to be high enough that stress relief occurs, but not too high in order to allow enough time for stress relieving, which is a time dependent process.

12

3. Equipment used In the following chapters the press, tooling, instrumentation and data acquisition system are described. Furthermore, a technique for measuring the burr height and the materials used for the experiments will be introduced.

3.1 The Lourdes Press 100-OH

Figure 6: The Lourdes Press 100-OH The experiments were conducted with a Lourdes Electro Activated Die Set with the following specifications:

13

Force (0.030” above bottom)

10 tons

Overall dimensions

10”x10”x19”

Maximum work area

4.5”x10”

Stroke length

0.5" to 1.5”

Open height

5.0”

Shut height

3.5”

Approx. weight

100 lbs

Approx. weight of untooled top plate

35 lbs

Table 3: Lourdes 100-OH press specifications (according to the manufacturer) The Lourdes High Speed Press 100-OH uses high tool speed, rather than force or pressure to perform work. It accelerates the tooling to speeds up to approximately 12 ft/sec /17/.

Figure 7: Position of material strip in the press The Lourdes Electromagnetic Press uses tractive Solenoids, comprised of a coil structure, a ferro-magnetic flux path, and an Armature, to accelerate the motion plate. The magnetic accelerator mounts directly to a precision die set (as shown in Figure 8) /21/.

14

The microprocessor control precisely energizes the accelerator causing the punch to be rapidly accelerated towards the die. The control regulates the tool speed and disconnects the driving forces just before the tool impacts the material. The kinetic energy or momentum of the moving tool holder is converted to work as the tooling impacts the material. Finally, any unused energy is absorbed by the urethane stops and the tool holder plate is returned, aided by spring force, to the initial position /17/. AIR IN

COOLING FAN

ARMATURE SPACERS

ARMATURE

RETURN RATE ADJUSTING

GUARD (TRANSPARENT FOR CLARITY ONLY)

RETURN SPRING

COIL ENCLOSURE

POWER CABLE

AIR OUT

TO CONTROL

AIR OUT

ARMATURE SUPPORT

URETHANE RETURN STOP BALL BEARING BUSHING

STROKE ADJUSTING

GUIDEPIN BALL RETAINER ADJUSTOR URETHANE STOP

Figure 8: Schematic picture of the Lourdes Press 100 - OH In order to minimize wear on the die set and tooling and to get the best blanking results, three different press adjustments have to be made:

15

1) Return Rate Adjustment The return rate adjustment is made by two nuts, located on the top of each Return Rate Spring Rod (Figure 8). This spring force only serves to return the punch plate to the top of its stroke after the Die Set has been fired. It is not intended for the use of material stripping. This return force has to be adjusted depending on the upper die weight. In order to leave the maximum amount of energy available for the actual application, the return rate has to be set to a minimum. 2) Stroke Adjustment This adjustment is located between the motion plate and top plate, and plays an important part in developing the maximum force of the unit. The stroke length has to be adjusted depending on the selected power level. 3) Armature Spacer Adjustment The armature spacers (washers) located on top of the armature have to be adjusted only if the tooling shut height differs from the press shut height. The armature should sit flush with the top of the spring loaded T-bar that enters the coil from the bottom side when the tooling is closed.

As soon as the adjustments described above have been made, the cutting speed which is nesessary for each specific application has to be determined. For the Lourdes 100 - OH press there are 9 power levels available. The cutting speed varies between 2.5 (power level 2) and 12 ft/sec (power level 9), depending on the blanked material.

The following graph shows the dependence of the cutting speed on the stroke length for each power level. Shown are the settings for 0.5" (minimum stroke length) and 1.5" (maximum stroke length). However, it is possible to set the stroke length at any number in between these two extremes.

16

velocity [ft/sec]

15 12 9 6 3

stroke length 1.5" stroke length 0.5"

0 1

2

3

4

5

6

7

8

9

power level

Figure 9: Punch velocity versus the power levels of the press depending on the stroke length. No cutting condition. Figure 9 shows that up to power level 4 it is more efficient to set the stroke length to 0.5" in order to reach the maximum punch velocity, whereas for power level 5 and higher, the maximum punch velocity can only be reached by setting the stroke length to 1.5 ".

3.2 Experimental Setup 3.2.1 Punches and Dies Usually the punch-die clearance is defined as a relative clearance per side in percent of the material thickness (equation (2)) /17/.

c=

dd − d p 2t

⋅ 100%

(2)

c

radial clearance [%]

dd diameter of the die dp diameter of the punch t

17

material thickness

The radial clearance is important, since it will affect the part edge quality, distortion of the blanks, tool wear and production costs in general /18/. According to previous investigations, increasing the punch-die clearance has the following effects /3/, /6/, /10/, /18/, /22/:

•

more rollover

•

higher burr

•

less shear and more rupture.

In general, small clearances (< 8%) create high strains on punch and die /6/. Therefore, proper alignment of the punch and the die is necessary in order to minimize tool wear. On the other hand, the part quality increases with decreasing the punch-die clearance. To reduce strain, clearances higher than 8% are preferred when using high strength steels /6/.

The tooling of the Lourdes Press consists of a punch, a die button and a polymer stripper, which are shown in the following figure. All punches and die buttons which were used for the experiments are made of M-2 high speed steel and are mounted to the retainers by means of a ball lock (for fast punch and die change). The polymer stripper is mounted directly to the punch and is used for stripping the skeleton off the punch after blanking /23/.

Figure 10: Stripper, die button and punch

18

In order to obtain different punch-die clearances a whole set of round punches and die buttons was available. The following two tables show the resulting clearances depending on the material thickness.

punch

material thickness [in]

diameter [in] 0.488 0.491 0.494 0.496 0.497 0.498 0.499

0.016” 38 28 19 13 9 6 3

0.033” 18 14 9 6. 4.5 3 1.5

0.041" 14.5 11 7 5 3.5 2.5 1.2

0.054” 11 8.5 5.5 3.5 3 2 1

Table 4: Clearances in percent for different punch diameters and material thicknesses (die button diameter: 0.500") punch

material thickness [in]

diameter [in] 0.488 0.491 0.494 0.496 0.497 0.498 0.499

0.033” / / 30 27 26 24 23

0.041" / 28 24 22 21 20 18

0.054” 24 21 19 17 16 15 14

Table 5: Clearances in percent for different punch diameters and material thicknesses (die button diameter: 0.514")

3.2.2 Stock materials Four different stock materials were used for conducting the experiments:

•

Low carbon steel (0.0033" thickness),

•

high strength steel (0.054" thickness),

•

copper 110, annealed temper (0.016" thickness), 19

•

aluminum 2008 (0.041" thickness) and

•

brass (0.031" thickness). Brass was only used for preliminary experiments.

More detailed information about these materials and their properties can be found in the appendix A. 3.2.3 Sensors In order to monitor the velocity profil over the stroke, the press was equipped with a velocity transducer and a proximity sensor. 3.2.3.1 Analog Proximity Sensor For monitoring the relative position of the punch an inductive proximity sensor was used. The sensor (resolution: 0.0002", linearity: ± 4%, range: 1") was mounted to the base plate and was detecting the motion plate (Figure 6). An inductive proximity sensor consists of a coil and a ferrite core arrangement. The oscillator creates a high frequency field, radiating from the coil in front of the sensor, centered around the axis of the coil. The ferrite core bundles and directs the electro-magnetic field to the front.

When a metal object (target) enters the high-frequency field, eddy currents are induced in the surface of the target. This results in a decrease of energy in the oscillator circuit and, consequently, a smaller amplitude of oscillation /24/. The signal was further processed through an oscillator/demodulator, a signal conditioner and an operational amplifier. For monitoring the relative position of the slide, the probes were connected to a data acquisition board, which will be described in chapter 3.2.4. 3.2.3.2 Linear velocity transducer In addition to the proximity sensor, a linear velocity transducer was used in order to monitor the velocity of the punch at different stages during the cutting process. The velocity transducer consists of high coercive force permanent 20

magnet cores which induce sizable DC voltage while moving concentrically within shielded coils.

As shown in Figure 6, the shielded coil is mounted to the stationary chassis and the permanent magnet core via a brass rod to the motion plate of the press. The induced output voltage of the coil is directly proportional to the magnet's relative velocity and field strength. For reducing the noise of the signal, an aluminum foil shielding around the probes was used to improve the results. However, for getting the most accurate results, one should operate in a magnetically shielded enclosure /25/. The signal was also converted from high to low impedance by means of an operational amplifier in order to stabilize the signal and avoid compatibility problems with the data acquisition board. 3.2.4 Data Acquisition and Signal Analysis The data acquisition PC was equipped with a National Instruments AT-MIO16F-5 data acquisition board in conjunction with the National Instruments LabVIEW software used for real time monitoring of the sensor signals up to 200 kHz. A Labview program (the program code is shown in the appendix B) was written for our specific application.

The program has the following features:

•

It displays the velocity as well as the penetration of the punch in respect to the time. One can read the correlating punch velocity for a given punch position.

•

It has an option to save the acquired data for further investigations/analysis of the cutting speed.

Figure 11 shows the scheme of the discussed sensor wiring.

21

linear velocity transducer

voltage divider operational amplifier

proximity sensor

oscillator/ demodulator

signal conditioner

DAQ

PC

Board

Figure 11: Sensor wiring (schematically)

3.3 Techniques for evaluating the part edge According to Lange there are basically four values, that should be considered when evaluating the part edge of a blanked part (Figure 12) /4/:

•

the burr height hb ,

•

the percentage of shear with respect to the material thickness (called % shear),

•

the percentage of rollover with respect to the material thickness (called % rollover),

•

the penetration depth tc .

Depending on the final use of the part, an ideal part edge has a minimum of rollover and burr and at least 75 % shear. 3.3.1 Measuring the penetration depth, the shear zone and the rollover The values for the penetration depth and the % rollover have been obtained from the cross section of the slugs by using a microscope. To achieve the cross sections, the slug first has to be sheared close to the centerline, then be mounted in polymer, and finally be ground and polished in order to get a smooth surface.

For investigating the amount and the constancy of shear and rupture, the slug has been microscoped from the side. Since the % shear value is not as constant as the penetration depth or the % rollover for a certain clearance and cutting speed, each slug has been measured at 4 different locations around the circumference. 22

Pictures for different materials showing cross sections as well as side-views can be found in chapter 5.3. with:

se

s

s

material thickness

se

edge draw-in/rollover

ss

shear zone

sr

rupture zone

sr

hb

burr height

hb

tc

penetration depth

ss

cross section

% rollover =

tc

% shear =

se s

ss s

Figure 12: Microscope picture showing the cross section of a slug /4/

3.3.2 Technique for burr height measuring A device for fast and reliable burr height measurements had to be developed. Requirements for such a measuring device, a short overview about existing methods and the final design are discussed in this chapter. 3.3.2.1 Definition of the burr height Cross sections of the cutting edge show that the burrs may be either sharp-edged or rounded. Due to different material properties the part may remain flat or becomes domed during the blanking process. For that reason, the burr height is defined as the difference between the highest point of the burr and the surface of the part immediately adjacent to the burr.

23

3.3.2.2 Requirements and design of the device In order to measure the burr height in a precise and repeatable way, a new device had to be developed. Since the burr is very small and soft, it is very important that the burr is not damaged by the device while measuring. High accuracy of the measurement tool guarantees repeatable results.

A literature review showed that there have been basically 3 different principles of measuring the burr height:

•

A device based on the principle of a caliper /26/,

•

measuring the surface profile of the skeleton /22/,

•

optical solutions /27/.

The advantages and disadvantages of these principles have already been discussed in a previous report /28/. A technique based on the "caliper-method" was chosen.

1 micrometer head 2 reference tip

1 2

Figure 13: Burr height measurement device

24

Figure 13 shows a picture of the designed burr height measurement device, which works as follows (Figure 14):

The head has to be moved down until the slug is clamped by the reference pin. The pin clamps the part 0.1 mm (ca. 0.0039 in) beside the burr. After that, the user turns the micrometer dial until the tip of the micrometer touches the burr. The height of the burr can be read from the micrometer dial. The metal tip of the micrometer and the plane table (with the part) are electrically isolated from each other. A device which senses conductivity is connected across the micrometer head and the table. As soon as the tip of the micrometer touches the burr, the instrument indicates that the electrical circuit is closed. The main advantage of this principle is a very small load on the burr during the measurement.

0.0039 in

tip of micrometer head reference pin

slug flat surface

Figure 14: Method of working of the burr height measuring device

25

Calibration of the device: The device is calibrated by putting the reference pin directly on the plane table. The micrometer is then lowered until it touches the table. The calibration value, which has to be deducted from the measured burr height, is the value read on the scale of the micrometer.

This device reduces the load on the burr to a minimum and has a resolution of approximately 0.0003 inches. In contrast to other solutions, the device can also be used to measure the skeleton.

26

4. Experimental Procedure Different experiments have been conducted in order to achieve the following objectives: 1. Investigating the performance characteristics of the press. 2. Investigations of characteristics of the punch velocity during one stroke (velocity investigations). 3. Determining the influence of high punch velocities in conjunction with different punch-die clearances on the part edge quality depending on different materials (part quality investigations). Before performing any experiments with the press, preliminary work had to be completed:

•

Four different stock materials had to be chosen (chapter 3.2.2).

•

The tooling had to be selected. Chapter 3.2.1 contains more detailed information on the punches and dies. With the chosen punches and dies it is possible to conduct experiments in a clearance range from 4% up to 24%.

•

Punch-die alignment. The retainers had to be adjusted precisely.

In order to obtain repeatable results, the following points had to be taken into account:

•

Once the retainers were adjusted well, the adjustment was kept for all the experiments. Only the punches and die buttons had to be changed for the experiments with different punch-die clearances.

•

During each measurement both channels (displacement and velocity) were monitored continously through the stroke with a maximum sampling rate of 10,000 samples per second. This sampling rate was necessary so as not to lose small signal peaks.

•

All the experiments were conducted with sharp punches and die buttons.

27

4.1 Investigation of press characteristics First of all it is important to know in which velocity range the press is performing. Therefore experiments using different power levels of the press have been conducted under no cutting condition. The displacement as well as the velocity of the punch have been monitored by means of a proximity sensor (chapter 3.2.3.1) and a velocity transducer (chapter 3.2.3.2). Thus, a displacementtime- and velocity-time-curve could be obtained (Figure 15). In addition, a velocity-stroke-curve could be obtained by combining the information of these two curves (Figure 16).

12

1 0.75

6

0.5 BDC

3

0.25 stop blocks

0

0 0

0.005

0.01

0.015

0.02

0.025

-3

displacement [in]

punch velocity [ft\sec]

9

-0.25 velocity displacement

-6 -9

-0.5 -0.75

time [sec]

Figure 15: Velocity/displacement-time curve for no cutting condition at power level 5 of the press

These first experiments under no cutting condition gave a rough idea, which punch velocity could theoretically be reached at the 9 power levels of the press. In addition, several influences on the performance of the press (like the

28

adjustment of the return springs or the stroke length) have been examined. For determining the minimum and maximum cutting velocity the different materials were blanked. For each material five measurements were made, then the average value was taken.

4.2 Velocity investigations The main goal of these investigations was to obtain characteristic displacementtime, velocity-time and the resulting velocity-stroke curves, as shown in Figure 16. The two curves in respect to the time (Figure 15) are important for roughly calculating the cutting force (chapter 5.2) later on. They are also used to show when blanking starts, when blanking is completed and when the polymer stop blocks are reached.

10

punch velocity [ft/sec]

8

6

stop blocks

4

2

0 -1

-0.5

0

displacement [in]

Figure 16: Velocity-stroke curve for no cutting condition. Power level 5.

29

0.5

In order to check the accuracy of the velocity transducer a monitored displacement-time-curve was derived at particular points and the results were compared. The measured and calculated velocity were corresponding with a variation of ± 5%.

4.3 Part quality investigations The following four characteristic values for evaluating the part edge quality were measured and are shown in 4 different graphs for each material (chapter 5.3):

•

burr height (measured with the burr height measurement device, chapter 3.3.2),

•

% shear (expressed as a percentage of the material thickness, measured by using the side view, that can be seen under a microscope),

•

% rollover (expressed as a percentage of the material thickness, measured at the cross-sections of the parts which were mounted in epoxy),

•

% penetration (expressed as a percentage of the material thickness, measured at the cross-sections of the parts which were mounted in epoxy),

The values mentioned above are shown on the y-axis whereas the x-axis shows the punch-die clearance. Separate curves are shown for different cutting velocities.

Before conducting the experiments to get the actual graphs that are shown in chapter 5.3, preliminary experiments (using 3 different cutting speeds, 2 different clearances and 6 different materials) have been conducted in order to find out which materials, cutting speeds and clearances are the most promising to investigate further.

30

5. Experimental Results 5.1 Press characteristics and preliminary results Figure 17 shows the highest cutting speed that could be reached for the different stock materials. It can be seen that there is almost no influence of the clearance on the cutting speed. The shown values represent the average of five measurements. For each of the four materials the highest cutting speed is approximately 12 ft/sec.

12.5

cutting speed [ft/sec]

12

11.5

11 low carbon steel Al 2008

10.5

110 Copper high strength steel

10 0

5

10

15

20

25

clearance[%]

Figure 17:Maximum cutting speed for different punch-die clearances and stock materials The following table shows the average value of the cutting speed that could be reached at different power levels for each material. The measurements were made with ideal stroke length.

31

materials low carbon

high strength

aluminum

copper

steel

steel

power level 2

4.5 ft/sec

6 ft/sec (PL 3)

4.5 ft/sec

3 ft/sec

power level 5

9 ft/sec

8.5 ft/sec

9 ft/sec

9 ft/sec

power level 9

12 ft/sec

12 ft/sec

12 ft/sec

12 ft/sec

Table 6: Cutting speeds at different power levels of the press for different materials

The lowest cutting speed that could be reached with this press is about 4 ft/sec. This is already regarded as high speed. In order to obtain curves at low cutting speeds, additional experiments were conducted manually. This means that the tooled plate of the press was moved down by means of an extension arm, resulting in approximately 0.5 ft/sec.

As mentioned in chapter 4, the punch-die alignment was a very important issue before conducting the experiments. The retainers for the punch and for the diebutton had to be adjusted in order to minimize wear on these tools and to achieve a uniform part edge around the circumference. The retainer adjustment was then checked by punching low carbon steel (14 % clearance, punch velocity: 4 ft/sec) and measuring the burr height and percentage of shear at four different locations around the circumference of the slug. The results of these experiments are shown in Figure 18. The burr height varied within a range of 3/10,000 of an inch, the percentage of shear within a range of 5 %. Both values are regarded as acceptable.

32

50

0.002

40

0.0015

30 2

0.001 1

0.0005

slug

20 feeding direction

3

1

10

burr height % shear

4

0

2

3

% shear

burr height (in)

0.0025

0 4

locations around the part

Figure 18: Uniformity of part edge (low carbon steel, 14% clearance, 4 ft/sec)

5.2 Velocity investigations Since the velocity-time, displacement-time and velocity-stroke curves look similar for the different materials, only the curves for low carbon steel are shown and discussed in this chapter. The according graphs for the other stock materials can be found in appendix C.

33

12

1 start blanking stripper

blanking completed

0.75

6

0.5

3

0.25

0

0 0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

-3

displacement [in]

punch velocity [ft\sec]

9

-0.25

-6

velocity displacement

-9

-0.5 -0.75

time [sec]

Figure 19: Velocity/displacement-time curve. Material: low carbon steel. Power level 3, stroke length 0.5"

Figure 19 and Figure 20 show the velocity and displacement of the punch versus time for low carbon steel. The following points could be observed:

•

The polymer stripper touches the material.

•

The punch hits the sheet and blanking starts. This also causes vibrations due to which the curve is oscillating afterwards.

•

Blanking is completed, the punch "enters" the die button.

•

The polymer stop blocks are reached. They are compressed and absorb the remaining energy.

The bottom dead center (BDC) is reached. The moving direction of the punch is inverted which results in zero velocity.

34

12

1 blanking completed

6

0.75 0.5

stripper BDC start blanking

3

0.25

0

0 0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

-3

displacement [in]

punch velocity [ft\sec]

9

-0.25 stop blocks

-6 -9

velocity displacement

-0.5 -0.75

time [sec]

Figure 20: Velocity/displacement-time curve. Material: low carbon steel. Power level 9, stroke length 1.5"

Until blanking starts the punch velocity is constantly increasing. Compressing the urethane stripper does not result in a velocity decrease. However , if the maximum velocity is only 4 ft/sec, the blanking force is high enough to result in a velocity decrease. As soon as the material fractures the velocity curve starts oscillating due to the energy release of all structural components of the press (also known as "snap-through" effect).

The following graphs show the punch velocity over the stroke. These curves were used for calculating the forces that are shown in Table 7. Whereas Figure 21 shows a velocity decrease, Figure 22 shows an almost constant velocity during blanking.

35

6

punch velocity [ft/sec]

5

4

vm

vst 3

2

vd 1

0 -0.05

-0.025

0

0.025

displacement [in]

Figure 21: Punch velocity versus displacement. Material: low carbon steel. Power level 3, stroke length 0.5"

In Figure 21 and Figure 22 the following abbreviations are used: vSt = punch velocity when the stripper touches the material. vm = punch touches the material (blanking starts). vd = blanking completed. vs = polymer stop blocks are reached.

36

0.05

14

punch velocity [ft\sec]

12 10

vst

vm

vd

8

vs

6 4 2 0 -0.05

0

0.05

0.1

0.15

0.2

displacement [in]

Figure 22: Punch velocity versus displacement. Material: low carbon steel. Power level 9, stroke length 1.5"

The cutting force when blanking with low strain rates and cutting velocities can be calculated by using the shear resistance of the blanked material and the tool geometries /4/, /29/. A table including the shear resistance of the different materials is shown in the appendix A.

Fc = π ⋅ d ⋅ t ⋅ σ s

(3)

with: Fc = blanking force

σs = shear resistance d = disk diameter t = sheet thickness

37

On the other hand, it is possible to calculate approximately the blanking force by using the change of speed during the cutting operation. The force is calculated by using the following equation:

F = m⋅a = m⋅

dv dt

(4)

with: F = cutting force a = acceleration m = weight of the moving mass

If the time range of the deceleration is very short, the equation can be simplified to:

F = m⋅

v − v2 ∆v = m⋅ 1 ∆t t1 − t2

(5)

The following table shows cutting forces calculated with the two different equations described above for the four stock materials:

material

thickness

cutting force, calculated

cutting force, calculated

[ in ]

with equation (5) at

according to equation (3)

4 ft/sec low carbon steel

0.032

10.7 kN

334 kN/in

8.1 kN

253 kN/in

high strength steel

0.054

30.1 kN

557 kN/in

17.5 kN

324 kN/in

Al 2008

0.041

5.1 kN

124 kN/in

5.0 kN

122 kN/in

110 Copper

0.016

1.8

111 kN/in

3.2 kN

290 kN/in

Table 7: Cutting force for different materials

38

In order to compare the forces of the different material thickness' which were blanked, the actual forces have been divided by the material thickness.

A comparison of the numbers shown in Table 7 shows that there is no reliable accordance between the forces calculated in different ways. Therefore, it is not advisable to calculate the force derived from a measured velocity. If one wants to know the cutting force, the experimental setup should be equipped with a force measurement device.

In addition to the force, the required energy for blanking was investigated. There are two different ways to approach this problem. Since the cutting force calculated with equation (3) and the entry depth of the punch before fracturing begins are known, it is possible to calculate the energy as follows:

Ec = Fc ⋅ l f

(6)

with: Fc=cutting force lf=entry depth until fracture

The entry depth before fracturing begins is approximately the experimentally observed length of the shear zone (see also % shear curves). On the other hand, the cutting velocity and the energy used for blanking are related through the following equation:

E=

1 2 mv 2

(7)

with: E = energy [J] m = weight of the moving mass [kg] v = velocity of the moving mass [m/s]

39

Since the two distinct points, start of blanking and blanking completed, are known, the cutting energy can be calculated as the energy difference between these two points:

E c = E1 − E 2 =

1 m( v 12 − v 22 ) 2

(8)

The weight of the tooled plate of the press is 40 lbs. This leads to the following calculation for the two curves shown: Ec represents the energy used for blanking with a speed of 4 ft/sec (using equation (8)):

ft  ft 2 ≈ 6.3 J Ec  4  = 150lb  sec  sec 2

In contrast to this equation, it is not possible to calculate the energy needed for blanking with 12 ft/sec. The velocity is staying almost constant during the blanking operation, Figure 22. This means that the energy required for cutting is very small compared to the available energy of the moving mass (compare with Table 8).

Energy available for cutting at 4 ft/sec in [ J ]:

14

Energy available for cutting at 12 ft/sec in [ J ]:

121

Energy used for cutting (at 4 ft/sec) in [ J ]: Table 8: Energy level at different cutting speeds

40

6

material

thickness

entry

cutting

cutting

cutting

[ in ]

depth until

velocity

energy

energy

fracture

[ ft/sec ]

according to

according to

(6) in [ J ]

(8) in [ J ]

[in] low carbon steel

0.032

0.0128

4

2.64

6.3

high strength steel

0.054

0.0162

5

7.21

19.5

Al 2008

0.041

0.0144

4

1.82

8.2

110 Copper

0.016

0.0112

2.5

0.92

1

Table 9: Cutting energy for the stock materials

Table 9 shows approximate energy numbers for cutting the four different stock materials. It shows that the thinner and softer materials require less energy for cutting. But also in this case it could be observed that the energy derived from the cutting velocity does not correlate very well with the calculated numbers. This shows that the fastest and most reliable way to get information about blanking force and energy is by implementing a load sensor in the experimental setup. This will be done in the next phase of the project.

5.3 Part quality investigations It should be mentioned that the discussion of the results is divided based on the blanked materials. The part edge quality is evaluated for the different parameter settings and the four stock materials (see also appendix A). After drawing conclusions regarding the process conditions for each material, the results of the different materials are compared.

41

5.3.1 Experiments with low carbon steel

The results of the experiments conducted with low carbon steel are shown in the next four graphs. In addition, seven cross-sections and side views are shown.

6

burr height [0.001 in]

5 4 3 2 12 ft/sec 9 ft/sec 4 ft/sec 0.5 ft/sec

1 0 0

5

10 15 punch-die clearance [%]

20

25

Figure 23: Burr height versus punch-die clearance (low carbon steel) Figure 23 shows the burr height versus the punch-die clearance for different cutting speeds. The following characteristics were observed:

•

The burr of the slugs blanked with 0.5 ft/sec is constantly increasing with an increasing clearance. Table 10, pictures 1, 7 and 11 confirm that.

•

For velocities above 4 ft/sec the burr height is almost constant at 0.0013" up to 14 % clearance. Increasing the clearance further results in a linear burr height growth.

•

When blanking with low cutting speeds of 0.5 ft/sec and a small clearance the burr is about 0.0035", which is about three times as much as when blanking with high speed.

42

This graph shows that in the commonly used clearance range between 5 and 10% the burr height is less than half as large when blanking at cutting speeds above 4 ft/sec compared to low speed blanking.

100 12 ft/sec 9 ft/sec 4 ft/sec 0.5 ft/sec

% shear

80 60 40 20 0 0

5

10

15

20

25

punch-die clearance [%]

Figure 24: % shear versus punch-die clearance (low carbon steel) Figure 24 shows the influence of the cutting speed on the formation of the shear zone. The following characteristics were observed:

•

Increasing the cutting speed above 4 ft/sec has no influence on the amount of shear.

•

The punch-die clearance has a major influence on the percentage of shear. For all cutting speeds there is a large difference in the percentage of shear between 4% clearance and 14 % (compare Table 10, Table 11: pictures 2 and 8 or pictures 6 and 10). Increasing the clearance further has only a minor influence on the amount of shear.

43

•

The influence of high velocities on the percentage of shear is not as distinct as the clearance influence. However, the percentage of shear is about 5-20 % higher for parts blanked with high speeds compared to parts blanked with low speeds (see also Table 11, pictures 8 and 10).

Although there is an increase in the percentage of shear when using high speeds, it is not enough to compensate the decrease of % shear which is caused by an increasing clearance. This means that for getting a part edge of 75% shear, one still has to go with a relatively small clearance of about 5 to 8%.

The next graph (Figure 25) shows the % rollover in respect to the punch-die clearance for cutting speeds of 0.5 ft/sec (low speed) and 12 ft/sec (high speed). The following characteristics can be seen:

•

Up to 15% clearance, the % rollover of the slugs is constantly increasing for all cutting velocities (see Table 10, Table 11: picture 1 and 7 and pictures 5 and 9). For a further clearance increase the rollover stays constant.

•

At a clearance of 15%, the % rollover is about 3 times as big as at 4% clearance.

•

Using high cutting speeds results in a decrease of % rollover (compare Table 10: pictures 1, 3 and 5). For 4 % clearance, the % rollover is more than twice as big at low speeds than at high speeds.

44

30 25

% rollover

20 15 10 0.5 ft/sec 12 ft/sec

5 0 0

5

10 15 punch-die clearance [%]

20

25

Figure 25: % rollover versus punch-die clearance (low carbon steel)

It is obvious that the selected clearance has the main influences on the plastic deformation of the part which results in the rollover zone. However, for a given clearance the % rollover can be always reduced by blanking with high speeds.

Figure 26 shows the % penetration in respect to the punch-die clearance for high and low cutting speeds:

•

For both velocities the curves show a constant increase of % penetration with an increasing punch-die clearance (Table 10, Table 11, Table 12: pictures 5, 9 and 13).

•

High cutting speeds result in less penetration depth, whatever clearance is selected (Table 10: pictures 1, 3, 5). At small clearances, around 4% to 50% reduction could be seen. 45

Since the edge of the slugs should be as straight as possible for most applications, it is important that the penetration depth is as small as possible. With a low cutting speed it is not possible to produce parts with a % penetration smaller than 8%, even if the clearance is very small. By using high cutting speeds, however, it is possible to reach a % penetration as low as 4% (Table 10, picture 5).

20

% penetration

15

10

5

0.5 ft/sec 12 ft/sec

0 0

5

10 15 punch-die clearance [%]

20

Figure 26: % penetration versus punch-die clearance (low carbon steel)

46

25

Cross-section

side view Clearance: 4 %. Characteristic part edge for 0.5 ft/sec punch velocity.

1

2 Clearance: 3%. Characteristic part edge for 4 ft/sec and 9 ft/sec punch velocities.

4

3

Clearance: 3 %. Characteristic part edge for 12 ft/sec punch velocity

5

6

Table 10: Cross section and side view for low carbon steel. Clearance: 3/4%.

47

Cross-section

side view Clearance: 15 % Characteristic part edge for 0.5 ft/sec punch velocity.

8

7

clearance: 14 % Characteristic part edge for 4 ft/sec, 9 ft/sec and 12 ft/sec punch velocites.

10

9

Table 11: Cross-section and side view for low carbon steel. Clearance: 15%.

48

Cross-section

side view Clearance: 21 % Characteristic part edge for 0.5 ft/sec punch velocity

11

12 clearance: 24 % Characteristic part edge for 4 ft/sec, 9 ft/sec and 12 ft/sec punch velocities.

14

13

Table 12: Cross-section and side view for low carbon steel. Clearance: 21/24%.

49

5.3.2 Experiments with high strength steel

In this chapter the results of the experiments conducted with high strength steel are discussed. In particular, the influences of clearance and cutting speed on the formation of the different zones of the part edge are discussed.

7

12 ft/sec 8.5 ft/sec 6 ft/sec 0.5 ft/sec

burr height [0.001 in]

6 5 4 3 2 1 0 0

5

10 15 punch-die clearance [%]

20

25

Figure 27: Burr height versus punch-die clearance (high strength steel) Figure 27 shows the burr height versus the punch-die clearance for different cutting speeds. The following characteristics were observed:

•

The major changes in the burr height are taking place between 3% and 6% clearance. The smallest burr is obtained at 6% clearance. A further increase of the clearance has only a minor effect on the formation of the burr.

•

The cutting speed has an influence on the formation of the burr. The largest burr height decrease is seen between 0.5 and 6 ft/sec cutting speed. Increasing the cutting speed further than 8.5 ft/sec does not give any improvement concerning the burr.

50

The previous graph shows that at the commonly used clearance of 6% the burr height is reduced by 70% when blanking at high speeds.

100 12 ft/sec 8.5 ft/sec 6 ft/sec 0.5 ft/sec

% shear

80 60 40 20 0 0

5

10

15

20

25

punch-die clearance [%]

Figure 28: % shear versus punch-die clearance (high strength steel) The effect of the punch-die clearance and the cutting speed on the % shear is shown in Figure 28:

•

Increasing the cutting speed above 6 ft/sec has no remarkable effect on the amount of shear (see also Table 14: pictures 8 and 10).

•

There is a large influence of the punch-die clearance on the % shear when using clearances smaller than 14%. Larger clearances have only a minor influence on the amount of shear (compare Table 13: picture 4 and Table 14: picture 10).

•

An increase of the percentage of shear when using higher cutting speeds can only be noted at clearances smaller than 14%.

51

50

% rollover

40

30

20 0.5 ft/sec

10

12 ft/sec

0 0

5

10 15 punch-die clearance [%]

20

25

Figure 29: % rollover versus punch-die clearance (high strength steel) Figure 29 shows the % rollover in dependance of the punch-die clearance and the cutting speed. The following characteristics were observed:

•

In general, the % rollover of the slugs is linearly increasing with increasing clearance for all cutting speeds (see also Table 13: pictures 1, 7 and 11).

•

For small clearances the % rollover can be reduced by 50% when blanking with high speeds (compare Table 13: pictures 1 and 3).

•

For getting less than 10% rollover one has to blank with high speeds and small clearances.

The next graph shows the % penetration in respect to the punch-die clearance for cutting speeds of 0.5 ft/sec and 12 ft/sec. The following characteristics can be seen:

52

•

Independent of the cutting speed, the % penetration is constantly increasing with an increasing punch-die clearance (compare Table 13: pictures 1, 7 and 11).

•

High cutting speeds result in less penetration. To get a straight part edge with less than 10% penetration one has to go with high speeds and small clearances (see also Table 13: pictures 1 and 3).

30

% penetration

25 20 15 10 0.5 ft/sec

5

12 ft/sec

0 0

5

10 15 punch-die clearance [%]

20

Figure 30: % penetration versus punch-die clearance (high strength steel)

53

25

Cross-section

side view Clearance: 4 %. Characteristic part edge for 0.5 ft/sec punch velocity.

1

2 Clearance: 3%. Characteristic part edge for 6 ft/sec, 8.5 ft/sec and 12 ft/sec punch velocities.

4

3

Clearance: 3 % Characteristic part edge for 12 ft/sec punch velocity. Ca. 100% shear!

5

6

Table 13: Cross-section and side view for high strength steel. Clearance: 3%.

54

Cross-section

side view Clearance: 14 % Characteristic part edge for 0.5 ft/sec and 6 ft/sec punch velocity.

8

7

clearance: 14 % Characteristic part edge for 8.5 ft/sec and 12 ft/sec punch velocities.

10

9

Table 14: Cross-section and side view for high strength steel. Clearance: 14%.

55

Cross-section

side view Clearance: 20 % Characteristic part edge for 0.5 ft/sec punch velocity

11

12 clearance: 24 % Characteristic part edge for 6 ft/sec, 8.5 ft/sec and 12 ft/sec punch velocities.

14

13

Table 15: Cross-section and side view for high strength steel. Clearance: 20/24%.

56

5.3.3 Experiments with aluminum

The results of the experiments conducted with aluminum are shown in the next four graphs. As in the previous chapters, cross-sections and side views are discussed as well.

Figure 31 shows the burr height versus the punch-die clearance for different cutting velocities. The following characteristics were observed:

•

The major influence on the formation of the burr is the punch-die clearance. High speed has no effect on the formation of the burr.

•

For all cutting speeds there is an optimum clearance of about 7%, where the burr is smallest.

•

For clearances smaller or bigger than 7% the burr is constantly increasing regardless of the cutting speed.

burr height [0.001 in]

1.5

1

12 ft/sec 9 ft/sec 4 ft/sec 0.5 ft/sec

0.5

0 0

5

10 15 punch-die-clearance [%]

Figure 31: Burr height versus punch-die clearance (aluminum)

57

20

25

The next figure shows the percentage of shear versus the punch-die clearance. The following characteristics can be seen:

100 12 ft/sec 9 ft/sec 4 ft/sec 0.5 ft/sec

% shear

80

60

40

20

0 0

5

10 15 punch-die clearance [%]

20

25

Figure 32: % shear versus punch-die clearance (aluminum)

•

The bigger the punch-die clearance gets, the less % shear occurs. The percentage of shear doubles by decreasing the clearance from 21% to 5% (see also Table 16, Table 17, Table 18: pictures 2, 6 and 10).

•

The cutting speed has almost no influence on the percentage of shear (compare Table 10: pictures 2 and 4).

58

40

% rollover

30

20

10

0.5 ft/sec 12 ft/sec

0 0

5

10 15 punch-die clearance [%]

20

25

Figure 33: % rollover versus punch-die clearance (aluminum) Figure 33 shows the % rollover versus the punch-die clearance for high and low speed. It could be observed that:

•

Up to clearances of 15% neither the clearance nor the cutting speed is influencing the % rollover (see also Table 16, Table 17: pictures 1, 3, 5 and 7).

•

Only at clearances around 20% can the plastic deformation be decreased by choosing high cutting speed.

The next figure shows the influence of the punch-die clearance and the cutting speed on the % penetration. The following characteristics can be seen:

•

Since the material fractures earlier when blanking with large clearances also more penetration is observed.

•

Like with the rollover, high cutting speeds show the biggest improvement in combination with large clearances (Table 18: pictures 9 and 11).

59

20

% penetration

15

10

5

0.5 ft/sec 12 ft/sec

0 0

5

10 15 punch-die clearance [%]

Figure 34: % penetration versus punch-die clearance (aluminum)

60

20

25

Cross-section

side view Clearance: 5 %. Characteristic part edge for 0.5 ft/sec punch velocity.

1

2 Clearance: 5%. Characteristic part edge for 4 ft/sec, 9 ft/sec and 12 ft/sec punch velocities.

4

3 Table 16: Cross-section and side view for aluminum. Clearance: 5%.

61

Cross-section

side view Clearance: 15 % Characteristic part edge for 0.5 ft/sec punch velocity.

6

57

clearance: 15 % Characteristic part edge for 4 ft/sec, 9 ft/sec and 12 ft/sec punch velocities.

8

7 Table 17: Cross-section and side view for aluminum. Clearance: 15%.

62

Cross-section

side view Clearance: 21 % Characteristic part edge for 0.5 ft/sec punch velocity

9

10 clearance: 21 % Characteristic part edge for 4 ft/sec, 9 ft/sec and 12 ft/sec punch velocities.

12

11

Table 18: Cross-section and side view for aluminum. Clearance: 21%.

63

5.3.4 Experiments with copper

The softest material used in the experiments was copper. It also has the highest heat conduction coefficient in comparison to the other investigated materials. The results of the experiments conducted with copper are shown in the following graphs, side views and cross-sections.

There is no graph showing the burr height versus the punch-die clearance, because the burrs of the blanked parts were too small to be measured with the measurement device described in chapter 3.3.2.2. Since the resolution of that device is 0.0003", it can be noted that the burr of the copper slugs (no matter which clearance or cutting velocity) was always less than 0.0003".

In comparison to the other materials, the copper slugs were domed. This made the mounting in epoxy as well as all other measurements more complicated, and is also the reason for the inclined shear zone (see part cross sections).

Figure 35 shows the percentage of shear versus the punch-die clearance. The following characteristics were observed:

•

Increasing the punch-die clearance results in a decrease of % shear. Between 6 and 19 % clearance this decrease is as much as 25% (see also Table 19 and Table 21: pictures 4 and 12).

•

Increasing the cutting speed up to 12 ft/sec results in an increase of %shear of approximately 15% (Table 19: pictures 2 and 4).

64

100

% shear

80

60

40 12 ft/sec 9 ft/sec 4 ft/sec 0.5 ft/sec

20

0 0

5

10

15

20

25

punch-die clearance [%]

Figure 35: % shear versus punch-die clearance (copper)

The influence of the cutting speed and the punch-die clearance on the % rollover is shown in the next figure. In particular, it can be observed that

•

the clearance has the main effect on the % rollover. Like with most other materials, the larger the clearance the larger the % rollover (compare Table 19 and Table 21: pictures 3 and 11).

•

Also, with this material it was observed that the high cutting speeds only show improvement when combined with large clearances.

65

30

% rollover

20

10 0.5 ft/sec 12 ft/sec

0 0

5

10 15 punch-die clearance [%]

20

25

Figure 36: % rollover versus punch-die clearance (copper) Figure 37 shows the % penetration versus the punch-die clearance depending on the cutting speed. The following characteristics can be observed:

•

There is no obvious influence of high cutting speeds on the formation of the % penetration.

•

The penetration of the material fracture is only influenced by the clearance up to 13%.

66

20

% penetration

15

10

5

0.5 ft/sec 12 ft/sec

0 0

5

10 15 punch-die clearance [%]

Figure 37: % penetration versus punch-die clearance (copper)

67

20

25

Cross-section

side view Clearance: 6 %. Characteristic part edge for 0.5 ft/sec punch velocity.

1

2 Clearance: 6%. Characteristic part edge for 4 ft/sec, 9 ft/sec and 12 ft/sec punch velocities.

4

3 Table 19: Cross-section and side view for copper. Clearance: 6%.

68

Cross-section

side view Clearance: 13 % Characteristic part edge for 0.5 ft/sec punch velocity.

6

57

clearance: 13 % Characteristic part edge for 12 ft/sec punch velocity.

8

7 Table 20: Cross-section and side view for copper. Clearance: 13%.

69

Cross-section

side view Clearance: 19 % Characteristic part edge for 0.5 ft/sec punch velocity

9

10 clearance: 19 % Characteristic part edge for 4 ft/sec, 9 ft/sec and 12 ft/sec punch velocities.

12

11 Table 21: Cross-section and side view for copper. Clearance: 19%.

70

5.3.5 Material comparison

After discussing the influence of different parameters on the formation of the part edge for every material, the different materials will be compared between each other in this chapter. 4 low carbon steel high strength steel Al 2008 Copper 110

burr height [0.001 in]

3

2

1

0 0

5

10 15 punch-die clearance [%]

20

25

20

25

Figure 38: Burr height versus punch-die clearance, 0.5 ft/sec 4 low carbon steel high strength steel Al 2008 Copper 110

burr height [0.001 in]

3

2

1

0 0

5

10 15 punch-die clearance [%]

Figure 39: Burr height versus punch-die clearance, 12 ft/sec

71

Figure 38 and Figure 39 show the burr height versus the punch-die clearance for low and high cutting speeds. The following conclusions could be drawn:

•

At low cutting speeds, low carbon steel shows by far the highest burr.

•

For copper, aluminum, and high strength steel the punch-die clearance has the main influence on the formation of the burr. Only for low carbon steel is the burr height decreased when blanking with high speeds.

•

The two softest materials used in the experiments, copper and aluminum, show the smallest burr, regardless of speed.

Another important value for the quality of the blanked part is the percentage of shear, Figure 40 and Figure 41. The following characteristics were observed:

•

For all materials the major influence on the percentage of shear is the clearance.

•

The only material which shows a shear zone increase due to high cutting velocities is copper. When blanking with small clearances and high speeds a shear zone of up to 95% could be reached.

•

Regardless of speed, low carbon steel can be made to fracture after 80% of shearing only when blanking with small clearances.

72

100 low carbon steel high strength steel Al 2008 Copper 110

% shear

80

60

40

20

0 0

5

10 15 punch-die clearance [%]

20

25

Figure 40: % shear versus punch-die clearance, 0.5 ft/sec

100 low carbon steel high strength steel Al 2008 Copper 110

% shear

80

60

40

20

0 0

5

10 15 punch-die clearance [%]

Figure 41: % shear versus punch-die clearance, 12 ft/sec

73

20

25

The next two figures show the percentage of rollover versus the punch-die clearance for low and high cutting speeds, Figure 42 and Figure 43. The following characteristics can be seen:

•

In the commonly used clearance range of between 5 and 15%, there is an obvious decrease of the % rollover by increasing the cutting velocity only for high strength steel. The other materials show no velocity influence.

•

When blanking low carbon steel with small clearances (< 5%) and high cutting velocities, the plastic deformation could be decreased.

•

The influence of high cutting speeds is more distinct when blanking with more than 15% clearance.

Figure 44 and Figure 45 show the percentage of penetration versus the punch-die clearance. It could be observed that:

•

High strength steel shows the highest percentage of penetration.

•

As expected, for all materials the % penetration is increasing with an increasing punch-die clearance. That means that the clearance has a major influence on the % penetration.

•

For aluminum and high strength steel, increasing the cutting speed results in a decrease of % penetration for all clearances.

•

For low carbon steel and copper, a higher cutting speed results in less % penetration for clearances smaller than 6%.

74

50

% rollover

40

30

20 low carbon steel high strength steel Al 2008 Copper 110

10

0 0

5

10 15 punch-die clearance [%]

20

25

20

25

Figure 42: % rollover versus punch-die clearance, 0.5 ft/sec

50 low carbon steel high strength steel Al 2008 Copper 110

% rollover

40

30

20

10

0 0

5

10 15 punch-die clearance [%]

Figure 43: % rollover versus punch-die clearance, 12 ft/sec

75

25

% penetration

20

15

10 low carbon steel high strength steel Al 2008 Copper 110

5

0 0

5

10 15 punch-die clearance [%]

20

25

Figure 44: % penetration versus punch-die clearance, 0.5 ft/sec

25

% penetration

20

15

10 low carbon steel high strength steel Al 2008 Copper 110

5

0 0

5

10 15 punch-die clearance [%]

Figure 45: % penetration versus punch-die clearance, 12 ft/sec

76

20

25

Overall, it can be noted that a positive influence of high cutting speeds on the quality of the part edge is more obvious (especially concerning the burr height and the percentage of rollover) for low carbon and high strength steels. Benefits for aluminum and copper could be only observed for a few parameter combinations. This agrees with the theory described in chapter 2.6: most of the positive effects of high cutting speeds are temperature related. Copper and aluminum have much higher heat conduction coefficients than steel. That means that the heat produced by the increased cutting velocity is dissipating relatively quick compared to steel. Thus, the benefits of high cutting speeds are not as large for copper and aluminum as for steel. However, the punch-die clearance has the major influence on the formation of the part edge, independent of which material is blanked and which cutting speed is used.

77

6. FEM simulations with DEFORM 2D 6.1 The Finite Element Code DEFORM 2D For simulating the blanking process a modified version of the FEM-code DEFORM 2D (Version 4.1.5) was used. DEFORM includes three parts:

•

The pre-processor,

•

the simulation engine and

•

the post-processor.

These parts will be described in the following /30/. 6.1.1 Pre-processor

The pre-processor consists of:

•

An input module for introducing the model geometry and the process conditions,

•

an automatic mesh generation program, which creates a mesh taking various process parameters into consideration (die and workpiece geometry, strain, strain-rate and temperature),

•

an interpolation module for interpolating the deformation history of the old distorted mesh into the newly generated mesh.

These three tasks, called automatic remeshing, make it possible to perform a continous simulation without any intervention by the user, even if several remeshing steps are required. The automatic remeshing capability reduces the total calculation time of the FE analysis. All the data generated in the preprocessor is saved in a data base.

6.1.2 Simulation engine

This program provides different choices in the analysis mode:

78

•

Isothermal, non-isothermal or heat transfer,

•

rigid, plastic, elastic, elasto-plastic, porous object type.

As mentioned before, the simulation results are stored in a binary format and are accessed by the post-processor. 6.1.3 Post-processor

The

post-processor

displays

the

simulation

results

in

graphical

or

alphanumerical form. The graphic presentation includes the mesh, contour plots (line or continous tone in colors) of strain, strain rate, temperature, velocity vectors and load-stroke curves. Two other important capabilities are 'point tracking' (provides deformation histories of selected points in the workpiece throughout the deformation) and 'flownet' (allows the user to observe the deformation of the circles or rectangles defined on the underdeformed workpiece).

6.2 Simulations of the high speed blanking process 6.2.1 Simulation settings

Four simulations have been performed for low carbon steel. The geometry of the tooling was designed and meshed on CAEDS and then imported into DEFORM 2D via an universal file. The simulations were performed using standard units. The input parameters of the different components of the simulation are shown in the following tables:

79

Component Sheet

Punch

rigid-plastic

rigid

Number of elements

~4500

~200

Material properties

AISI 1015

none

68 F

68 F

5.082 E-4

3.74 E-4

0.03106

0.03106

0.25

0.45

Object type

Temperature at beginning Thermal conductivity [Btu/sec/in/F] 3

Heat capacity [Btu/in /F] Emissivity

Table 22: Input data of the low carbon steel sheet and the punch

Component Die button

Blank Holder

Object type

rigid

rigid

Number of elements

~230

~40

Material properties

none

none

Temperature at beginning

68 F

68 F

Thermal conductivity [Btu/sec/in/F]

3.74 E-4

3.74 E-4

Heat capacity [Btu/in3/F]

0.03106

0.03106

0.45

0.45

Emissivity

Table 23: Input data of the die button and the blank holder The simulations were performed with the following settings:

•

Material: low carbon steel. Properties from AISI 1015 were taken, because they are very close to the properties of the low carbon steel that was actually used. The sheet thickness was 0.033".

80

•

Clearances: 5% and 18% (punch diameters: 0.488 in and 0.497 in; die button: 0.500 in),

•

punch velocities: 0.5 and 12 ft/sec,

•

non isothermal status,

•

axisymmetric problem: though only one half of the geometry was simulated.

The inter object relationship was defined with a constant friction coefficient of

µ=0.05 for all contacts between tool and sheet. This value is typically used for cold forming. The interface heat transfer coefficient was 0.3397 E-2 Btu/sec/in2/F.

Furthermore, the following restrictions apply:

•

One has to be careful when interpreting the results of the simulations: high speed blanking deals with high strains and high strain rates ( ε ≈ 500% and

ε& ≈ 10000 / s ). Since there are only data for low strains (~70%) and low strain rates (90/s) available, DEFORM extrapolates the values for higher strains and strain rates. This is a very critical point concerning the simulation results, because the accuracy of the simulation is influenced by the material flowstress curve. •

The non isothermal simulation mode was used. In this mode the program was not able (softwarewise) to fracture the material. Therefore conclusions can only be drawn about the plastic deformation at the beginning of blanking resulting in rollover. This shows that we are still in the beginning stage of simulating the high speed blanking process.

6.2.2 Simulation results

The pictures shown in the appendix D show results of the simulations for different stages of the process. Each picture contains of three figures: 81

•

A load versus time curve,

•

the workpiece, the die button and the punch,

•

a magnification of the area where the actual shearing takes place.

Simulations with a punch-die clearance of 18%: Figure D-1 through D-5 show the simulated shearing process for low speed (0.5 ft/sec). Two intermediate steps are shown in addition to step1 (starting position, figure D-1) and step 102 (shearing completed, figure D-5). Like in the experiments, only the slug (left part of the sheared sheet in the pictures) was of interest. As mentioned earlier, in this stage of the simulation software, only the formation of the rollover could be observed. For 18% clearance and low as well as high speed it is around 20 %. That means the simulations do not show an influence of the high speeds on the formation of the rollover.

Simulations with a punch-die clearance of 5%: The same characteristics as with 18% clearance were also seen with a punch-die clearance of 5%: Increasing the cutting speed does not result in a decrease of the rollover.

According to the simulations, the punch-die clearance is the main influence on the formation of the rollover.

6.2.3 Comparison of the experimental and simulated results

Experimental and simulated results exist only for low carbon steel at clearances of 5 and 18% and for low and high cutting speeds. Table 24 shows the percentage of rollover for the different cases.

82

5% clearance

18% clearance

low speed

high speed

low speed

high speed

experiment

12%

7%

26%

22%

simulation

12%

13%

21%

21%

Table 24: Comparison of the experimental and simulated results (low carbon steel) Although the simulation results do not show the influence of high cutting speed, the experimental and simulation results show a good correlation. Only for small clearance and high speeds do the results not correlate.

It will be important in the future to be able to simulate the fracture as well and accrue information about the burr height , % shear and penetration depth. These values are necessary to predict the quality of the part edge by means of simulations.

A more in depth comparison between simulations and experiments will be possible as soon as the FEM program is able to handle high strains and strain rates for the fracture simulation. This study will provide a large database for fine-tuning the FEM simulation.

83

7. Summary and conclusions This report discusses the influences of several parameters on the part edge quality of blanked parts. Different experiments as well as blanking simulations have been conducted in order to investigate the characteristics of the velocitystroke curve and to determine the influence of high punch velocities in conjunction with different punch-die clearances on the part edge quality. Four different materials: low carbon steel, high strength steel, aluminum and copper have been blanked with punch-die clearances between 4% and 24%. All the experiments were conducted with a Lourdes impact press which reaches cutting speeds of up to12 ft/sec.

The displacement-time, velocity-time, and velocity-stroke curves were monitored by means of a velocity and proximity sensor. This setup provided the velocity when blanking starts and the velocity decrease during blanking. It was shown that a velocity decrease due to blanking is only measurable for low cutting speeds of around 4 ft/sec. The kinetic energy at high speeds is much higher than the energy required for blanking. That results in an almost constant punch speed while blanking. Blanking force and energy were determined based on the velocity changes measured at low cutting speeds. The numbers matched only in part with the numbers calculated from the shear strength of the different materials. If one wants to know the exact cutting force the experimental setup should be equipped with a force measurement device.

When evaluating the part edge quality of the blanked parts, all the different zones were taken into consideration (burr height, shear, rupture/penetration depth, and rollover). The results of the part quality investigations show that the positive influence of high cutting speeds on the quality of the part edge is more obvious for low carbon and high strength steels. Benefits for aluminum and 84

copper could only be observed for a few parameter combinations. This agrees with the theory that most of the positive effects of high cutting speeds are temperature related. Since copper and aluminum have much higher heat conduction coefficients than steel, the heat generated by the increased cutting velocity dissipates relatively fast. The temperatures in the shear band are not as high as when blanking steel materials. Therefore, the benefits of high cutting speeds are not as large for copper and aluminum as for steel. However, the punch-die clearance has the major influence on the formation of the part edge, regardless which material is blanked or which cutting speed is used.

Blanking simulations were performed for low carbon steel at different cutting velocities and punch-die clearances. At the current stage of the development of the simulation program, only the plastic deformation and shearing of the material could be simulated. The experimental and simulation results show a good correlation concerning the investigated zones of the part edge. However, simulating small clearances when blanking with high velocities will need additional investigations. For predicting the part quality by means of simulations it will be necessary to simulate the whole process. Thus, a more in depth comparison between simulations and experiments will be possible as soon as the FEM program is able to handle high strains and strain rates for the fracture simulation.

In the future it will be of particular interest to obtain more knowledge about the effects of the high cutting velocities on tool wear. In conjunction, it will be interesting to investigate the influence of lubricants and the running mode of the press (either single or continous operation) on the tool wear. Also, it would be desirable to obtain additional information about the high speed effects on the part edge. Therefore, investigations concerning the microstructure of the part edge as well as temperature measurements during blanking could be helpful. As 85

shown in previous investigations, further part edge quality improvements can be achieved by increasing the cutting speed to 40 ft/sec /1/.

86

8. List of references /1/

Svahn, O.

Superfast blanking prevents defects Pressworking Industry Quaterly, Volume 8, No.4, 1993

/2/

Smith, D.A.

Die Design Handbook, Society of Manufacturing Engineers, Dearborn, Michigan, 1990

/3/

Lascoe, O.D.,

Handbook of Fabrication Processes ASM International, 1988

/4/

Lange, K.

Blanking and Piercing Handbook of Metal Forming The McGraw-Hill Book Company, 1985

/5/

Smith, D. A.

Fundamentals of Pressworking Society of Manufacturing Engineers Dearborn, Michigan, 1994

/6/

Schüssler, M.

Hochgeschwindigkeitsscherschneiden im geschlossenem Schnitt zur Verbesserung der Teilequalität Dissertation, University of Darmstadt, 1990

/7/

Breitling, J.

Investigations of different loading conditions

Wallace, D.

in a high speed mechanical press

87

Journal of Materials Processing Technology, Volume PRO 059/1-2, pp. 18-23

/8/

Dannenmann

Umdruck zur Vorlesung Schneiden University of Stuttgart, Germany, 1992

/9/

Lange, K.

Umformtechnik Band3: Blechbearbeitung Berlin, Heidelberg, New York, 1990

/10/ Neumann, C.-P.

Die Schneidbarkeit von Elektroblech und ihre Prüfung unter besonderer Berücksichtigung von Blechwerkstoff und Schneidspalt Dissertation, University of Hannover, Germany, 1979

/11/ Schenk, H. Prölss, E.

/12/ Kühne, H.-J.

Schneidspaltoptimierung fuer Elektrobleche Fertigung 4/77, Germany, 1977

Der Schneidvorgang selbst und die Stempelgeometrie als Ursache für Maßungenauigkeiten und Spannungen beim Scherschneiden von Elektroblechen Magazin Trennen Rohre Profile, 1991

/13/ Huml, P.

Der Einfluß der hohen Geschwindigkeit auf das Schneiden von Metallen Institute of metal forming, Stockholm Annals of the CIRP, Volume 23/1/1974

88

/14/ Davies, R. Austin, E.R.

Developments in High Speed Metalforming The Machine Publishing Company Ltd BN1 4 NH, Brighton, Sussex, 1970

/15/ Turkovich, B.F.

On a class of Thermo-Mechanical Process during rapid Plastic Deformation (with special reference to metal cutting) Annals of the CIRP, Volume 21/1/1972

/16/ Tobias, S.A.

Hochgeschwindigkeitsumformen Das Petro-Forge-Umformsysytem Fertigung 1/1971

/17/ N.N.

Brute Force vs. High Tool Speed Bulletin, Lourdes Systems, Inc.

/18/ Jana, S. Ong, N.S.

Effect of punch clearance in the High-Speed Blanking of thick metals using an accelerator designed for a mechanical press Journal of Mechanical Working Technology

/19/ Beitz, W. Küttner, K.-H.

Dubbel, Taschenbuch für den Maschinenbau 17. Auflage, Springer Verlag, Heidelberg, 1990

/20/ Hippenstiel, H.-R. Elektrodynamische Hochgeschwindigkeitspresse Röttger, R.

Werkstatt und Betrieb, S. 683-687, Nr. 110, Germany 1977

89

/21/ N.N.

Operating Instructions for Lourdes Electro Activated die sets overhead units Lourdes Systems Inc.

/22/ Cammann, J. H.

Untersuchungen zur Verschleißminderung an Scherschneidwerkzeugen der Blechbearbeitung durch Einsatz geeigneter Werkstoffe und Beschichtungen, Dissertation University of Darmstadt, Germany, 1986

/23/ N.N.

Pivot Basic Series, catalog 1000, Pivot Punch Corporation, New York 1993

/24/ N.N.

Sensors catalog Turck Inc., 1992

/25/ N.N.

Linear velocity transducer, Series 100 Transtek Inc., Bulletin S012-0028

/26/ Borchert, P.

Einflüsse der Werkzeuggeometrie und der Maschine beim Schneiden von kaltgewalztem Elektroblech Dissertation, University of Hannover, Germany, 1976

/27/ Seidenberg, H.

Presseneinwirkungen auf Werkzeugverschleiß und Grathöhe beim Schneiden von Feinblech im geschlossenen Schnitt Dissertation, University Hannover, Germany, 1965

90

/28/ Pfeiffer, B.

Investigations of the performance of in-die sensors for high speed blanking ERC Report, Columbus, 1996

/29/ N.N.

Aida Press Handbook Third Edition, Aida Engineering, Ltd., 1992

/30/ Wolff, Christian

Metal flow simulations for flashless-forging of a cross grooved inner-race ERC Report NSM-B-95-25, Columbus, 1995

91

APPENDIX A Material

low carbon

high strength

Al 2011

110 Copper

steel

steel

Thickness

0.033”

0.054”

0.041"

0.016"

Grade/heat treat.

EDDQ

50-XF

T3

annealed temper

5.5

99.90

0.4 0.4 0

0.04

Chemical composition (wt %)

C Mn P S Si Cu Ni Cr Mo V Sn Al Ti Cb N B Ca Bi Pb O Sb

0.003 0.1 0.006 0.007 0.01 0.01 0.02 0.02 0.01 0.002 0.003 0.049 0.056 0.001 0.004 0 0.0004 0 0 0 0.0052

0.07 0.39 0.006 0.004 0.063 0.02 0.01 0.02 0.01 0.004 0.005 0.058 0.01 0.005 0.0001 0.004 0 0 0 0.001

Yield [ksi]

20.55

n/a

43

10-53

UTS [ksi]

40.98

n/a

55

32-66

Shear resistance σs

~25

~32

~12

~20

Tensile Properties

[kgf/mm2] Table A-25: Material properties of the stock materials used

A1 - 1

APPENDIX B

Figure B - 1: Labview program code

B-1

APPENDIX C

1 start blanking

blanking completed

punch velocity [ft\sec]

9 6

0.75 0.5

3

0.25

stripper

0

0 0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

-3

displacement [in]

12

-0.25

-6

velocity displacement

-9

-0.5 -0.75

time [sec]

Figure C - 1: Velocity/displacement-time curve. Material: high strength steel. Power level 3, stroke length 0.5"

6

5 punch velocity [ft/sec]

vst

vm

4

3

vd

2

1

0 -0.05

-0.025

0

0.025

0.05

0.075

displacement [in]

Figure C - 2: Punch velocity versus displacement. Material: high strength steel. Power level 3, stroke length 0.5"

C - 11

12

1 0.75 BDC

start blanking

6

0.5

blanking completed

3

displacement [in]

punch velocity [ft/sec]

9

0.25

0

0 0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

-3

-0.25 stop blocks

-6

velocity displacement

-9

-0.5 -0.75

time [sec]

Figure C - 3: Velocity/displacement-time curve. Material: high strength steel. Power level 9, stroke length 1.5"

14

punch velocity [ft/sec]

12 10

vs

vd

vst vm

8 6 4 2 0 -0.05

0

0.05

0.1

0.15

0.2

displacement [in]

Figure C - 4: Punch velocity versus displacement. Material: high strength steel. Power level 9, stroke length 1.5"

C - 22

12

1 blanking completed

start blanking

6

0.75 0.5

stripper

3

0.25

0

0 0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

-3

displacement [in]

punch velocity [ft/sec]

9

-0.25

-6

velocity displacement

-9

-0.5 -0.75

time [sec]

Figure C - 5: Velocity/displacement-time curve. Material: aluminum 2008. Power level 2, stroke length 0.5"

4

punch velocity [ft/sec]

vst

vm

3

vd 2

1

0 -0.05

-0.025

0

0.025

0.05

0.075

displacement [in]

Figure C - 6: Punch velocity versus displacement. Material: aluminum 2008. Power level 2, stroke length 0.5"

C - 33

12

1 0.75 stripper

6

BDC

0.5 displacement [in]

punch velocity [ft/sec]

9

start blanking

3

0.25 blanking completed

0

0 0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

-3

-0.25 stop blocks

-6

velocity displacement

-9

-0.5 -0.75

time [sec]

Figure C - 7: Velocity/displacement-time curve. Material: aluminum 2008. Power level 9, stroke length 1.5"

12

punch velocity [ft/sec]

10

vst

vm

vd

8

vs 6

4

2

0 -0.05

0

0.05

0.1

0.15

0.2

displacement [in]

Figure C - 8: Punch velocity versus displacement. Material: aluminum 2008. Power level 9, stroke length 1.5"

C - 44

12

1 start blanking

9

blanking completed

0.75

6

0.5

stripper

3

0.25

0

0 0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

-3

displacement [in]

punch velocity [ft/sec]

BDC

-0.25 velocity displacement

-6 -9

-0.5 -0.75

time [sec]

Figure C - 9: Velocity/displacement-time curve. Material: copper 110. Power level 2, stroke length 0.5"

punch velocity [ft/sec]

4

3

2

vst

vm

vd

1

0 -0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

displacement [in]

Figure C - 10: Punch velocity versus displacement. Material: copper 110. Power level 2, stroke length 0.5"

C - 55

12

1 0.75 start blanking

6

BDC

0.5

blanking completed

3

0.25

0

0 0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

-3

displacement [in]

punch velocity [ft/sec]

9

-0.25 stop blocks

-6

velocity displacement

-9

-0.5 -0.75

time [sec]

Figure C - 11: Velocity/displacement-time curve. Material: copper 110. Power level 9, stroke length 1.5"

12

punch velocity [ft/sec]

vst

vm

vs

vd

9

6

3

0 -0.5

0

0.5

1

1.5

displacement [in]

Figure C - 12: Punch velocity versus displacement. Material: copper 110. Power level 9, stroke length 1.5"

C - 66

APPENDIX D

Figure D - 1: Simulation model, Step 1 of 102, 18% clearance, 0.5 ft/sec

D - 11

Figure D - 2: Simulation model, Step 40 of 102, 18% clearance, 0.5 ft/sec

D - 22

Figure D - 3: Simulation model, Step 80 of 102, 18% clearance, 0.5 ft/sec

D - 33

Figure D - 4: Simulation model, Step 102 of 102, 18% clearance, 0.5 ft/sec

D-4 4

Figure D - 5: Simulation model, Step 105 of 105, 5% clearance, 0.5 ft/sec

D-5 5

Figure D - 6: Simulation model, Step 110 of 183, 5% clearance, 12 ft/sec

D-6 6

Figure D - 7: Simulation model, Step 56 of 56, 18% clearance, 12 ft/sec

D-7 1