ME3301 Press Tool Design [PDF]

Zitiervorschau

Hundreds of Sheet Metal parts form the Car body

Manufacturing Process Manufacturing Process Non Metals (Injection moulding)

Metals

Joining (welding, riveting)

Shaping

Melting (Casting)

Material removal (machining)

Plastic deformation

others

Bulk forming Forging Rolling

Sheet forming Deep drawing

Extrusion, wire drawing

Stretch forming Shearing, bending etc.

Bulk forming and sheet forming 𝑺𝒖𝒓𝒇𝒂𝒄𝒆 𝒂𝒓𝒆𝒂 𝒗𝒐𝒍𝒖𝒎𝒆 𝒐𝒓 𝒕𝒉𝒊𝒄𝒌𝒏𝒆𝒔𝒔

• Low -> Bulk forming http://bharatforge.com/facilities/forging.html

• High-> Sheet metal forming https://www.thyssenkrupp-steel-europe.com/en/press/press-releases/press-release-6553.html

Work of deformation • Uniaxial tension – Plastic True Stress, 𝜎

𝜀

𝜀

𝑊=

True Strain, 𝜀

𝜎 . 𝑑𝜀 0

𝜀

𝜀 𝑛 . 𝑑𝜀

𝑊=𝐾 0

𝑊=

𝐾𝜀 𝑛+1 𝑛+1

=

𝐾𝜀 𝑛 𝑛+1

𝜀 = 𝜎𝑎𝑣𝑔 𝜀

– Assuming Holloman relation, 𝜎 = 𝐾𝜀 𝑛 𝜎𝑎𝑣𝑔

𝐾𝜀 𝑛 = 𝑛+1

Work of deformation •

Multiaxial – Plastic

Effective Stress, 𝜎

𝜀

𝜀

𝑑𝑊 =

𝐾𝜀 𝑛+1 𝑊= = 𝜎𝑎𝑣𝑔 𝜀 𝑛+1

𝜎 . 𝑑𝜀 0

𝜎𝑎𝑣𝑔 •

𝐾𝜀 𝑛 = 𝑛+1

Effective Strain, 𝜀

Load required for deformation

𝑝 = 𝜎𝑎𝑣𝑔 . A

(𝑝= average load A= instantaneous area)

‘W’ here refers to the ideal work required to deform a material to a given shape

𝑑𝑊 = 𝜎1 . 𝑑𝜀1 + 𝜎2 . 𝑑𝜀2 + 𝜎3 . 𝑑𝜀3

Deformation efficiency • (a) to (b) - Ideal deformation – Ideal work - 𝑑𝑊 =

𝜀 𝜎 0

. 𝑑𝜀

• Additional work due to – Friction (by frictional forces) – Redundant deformation (c )- (b)

• Total work=𝑊𝑡𝑜𝑡𝑎𝑙 = 𝑊𝑖𝑑𝑒𝑎𝑙 + 𝑊𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛 + 𝑊𝑟𝑒𝑑𝑢𝑛𝑑𝑎𝑛𝑡 • (a ) before deformation (b) Ideal frictionless deformation (c) Actual deformation

Efficiency=𝜂 =

𝑊𝑖𝑑𝑒𝑎𝑙 𝑊𝑡𝑜𝑡𝑎𝑙

Source : Kalpakjain and Schmid, Manufacturing processes for Engg Materials

Analysis of deformation processes • • • •

Slab analysis Slip line field theory Upper and lower bounds Finite element method

• Material factors – Yielding (yield criteria), friction, temperature & strain rate effect

Forging

• • •

One of the oldest metal working operations (5000 BC) Compressive forces Applications – – – – – –

•

Cold, warm and hot forging

Rolling • > 90% of all metals by metal working • >6 mm- plate and < 6mm- sheet • Various flat rolling and shape rolling operations

Automotive axles, crank shafts, wheels etc Gears Turbine disks Valve bodies Rails And a lot more

Source (Wikipedia)

Roll flattening and spreading Roll flattening Need for camber

Spreading Need for edger mill

Source : Kalpakjain and Schmid, Manufacturing processes for Engg Materials

Defects in rolling • Camber defects

Insufficient camber (a): edge wrinkling (b), warping (c), centerline cracking (d), and residual stresses (e).

Hosford, Metal forming :Mechanics and Metallurgy

Defects in rolling • Camber

Effects of over-cambering (a): wavy center (b), centerline splitting (c), edge cracking (d), and residual stresses (e).

Hosford, Metal forming :Mechanics and Metallurgy

EXTRUSION

Direct

Indirect Direct

http://www.allproducts.com/manufacture97/opoint/showroom1.html http://www.designworldonline.com/misumi-highlights-its-aluminum-extrusions-and-accessories-at-atx-west-2010/#_ http://www.alproextrusions.com/ http://lakelandplastics.com/about-us/guidelines/

Defects in extrusion • Surface cracking – High temp, friction and velocity – Reduce billet temp & speed

• Extrusion defect (piping) – Drawing impurities to centre – Modify flow pattern, control friction, remove surface scale

• Internal cracking – Secondary tensile stress – proper die geometry and extrusion variables Text in italics and blue font indicates solutions

http://vosbltd.com/images/Extrusion%20Defects/pdf2.PDF Source : Kalpakjain and Schmid, Manufacturing processes for Engg Materials

Wire and rod drawing Drawing Extrusion

• Push force • Friction b/w billet and container • Usually hot working

Source : Kalpakjain and Schmid, Manufacturing processes for Engg Materials

http://www.dreamstime.com/stock-image-old-bicycle-wheel-grunge-wall-image39791771

• Pull force • No friction b/w billet and container • Usually cold working

Sheet metal forming •

Sheet metal forming is a process of imparting a desired geometry to a work piece by plastically deforming it using a set of tools (punch, die and blank holder) • Several processes – shearing , blanking, bending, coining, deep drawing, stretching, roll forming, flanging…. etc.

Source: www.nd.edu/~manufact/pdfs/Ch16.pdf

Sheet Metal Bending Bending: In this operation, the material in the form of flat sheet or strip, is uniformly strained around a linear axis which lies in the neutral plane

and perpendicular to the lengthwise direction of the sheet or metal

V-bending

Edge bending

Sheet metal bending

Bending • Bend allowance 𝐿𝑏 – length of neutral axis

• Usually neutral axis – no change in length w

• 𝐿𝑏 = 𝑅 +

𝑡 2

𝛼 = 𝑅+𝑘𝑡 𝛼

– k=0.5 for larger bend radius (R/t >2) and k=0.3 for small radii (R/t 0.5, then its deep drawing

Stages in deep drawing

Metal flow during deep drawing

Deep drawing factors • Punch radius-4 to 10 times metal thickness; for lesser radius, restrike • Draw radius-4 times thickness, excessive radius lead to wrinkling – Shallow draw without blank holder- 6 to 8 times thickness

Draw radius nomograph

Deep drawing - % reduction D

• P=( D-d)*100/d • P- percent reduction • Theoretical max P- 50%; better to restrict to 40%

d h

Deep drawing –no of stages • h/d is another indicator of deep drawing • Max h/d per stage 0.75 • If h/d > 0.75, introduce another stage

Blank size determination

Aluminum Two-Piece Beverage Cans

Top of Aluminum can

Metal-Forming Process for Beverage Can The metal-forming processes involved in manufacturing a twopiece aluminum beverage can

Shearing

Shearing, blanking, trimming, piercing, fine blanking….. © wikipedia

Shearing - clearance • Clearance – important role

Excess clearance leads to tensile pulling of material into die

Source: Dieter, Mechanical Metallurgy, Kalpakjain and Schmid, Manufacturing Processes for Engineering Materials, 5th Ed

Shearing process- Force • Punch force depends upon – Material shear strength – Cross section area • Empirical relation • 𝐹 = 0.7 ∗ 𝑈𝑇𝑆 ∗ 𝑡 𝐿

– t=sheet thickness – L=shear length Source: Dieter, Mechanical Metallurgy, Kalpakjain and Schmid, Manufacturing Processes for Engineering Materials, 5th Ed

Cutting force calculation - Blanking • A mechanical press of maximum 1 tons is used to punch a square hole of 8 mm from an 1 mm thick sheet. Find the feasibility for – Material A- UTS=400 MPa – Material B- UTS=600 MPa

• Is it possible to reduce the cutting force during blanking ?

Reduction of blanking load • Stepped punch

• Shear on tools • Upto 30% reduction in cutting force possible in shear • Double shear better as it is balanced • Blanking- shear in die • Piercing- Shear in punch

Shearing

(a) Shaving a sheared edge. (b) Shearing and shaving, combined in one stroke.

Fine blanking

Shaving

Trimming

Tool design

http://calculusdesigns.com/press_tool_design.html

Die, fixtures, layout etc..

Sheet metal –die design

Image source: http://www.thyssenkrupp-system-engineering.com/en/automotive-industry/prototypes/prototype-tools-parts.html

Traditional die design

Die design

Die manufacturing

Tool tryout

FE based die design

http://www.deskeng.com/de/optimize-sheet-metal-forming/ http://www.autoform.com/en/products/autoform-diedesigner-plus/

Die set • Punch and Die mounted on punch holder and die shoe respectively • Advantage – alignment • At least 16 mm clearance to be given between punch and guide block for resharpening • Sufficient allowance in the die shoe for mounting

Types of dies- few eg • Punch attached to die shoe and die to punch holder Advantage : 1) Less bending and useful in thin sheets Disadvantage 1) High cost

Progressive die • Many stages are combined in a single die. • Each station has one operation done in the strip. • Station to Station distance constant • Disadvantage- less support

Compound die • Multiple operations in a single station • Slower than progressive die • Closer tolerance • Small press for large components

Combination die • Similar to compound die. • Difference-At least one noncutting operation is combined

Stamping press • Sheet metal working tool with a stationary bed and a powered ram driven towards the bed or away from the bed to apply force or required pressure for various metal forming operations

Components of a typical mechanical drive stamping press

Components of Presses •

A frame: that supports

the

bolster (2) that is a plate attached to the bed. The bolster has tapped holes, T-slots, or other means for

attaching the bottom die to the press. •

A drive mechanism (3) that moves the slide (4) at a right angle to the bolster

•

The Gibs

(5)

that guide the

reciprocating motion of the slide and ensure the parallelism of the slide to the bolster

Metal forming press • Press: Machine tool that apply force to change the shape of metal • Classification of presses Frame type – Mechanism • Hydraulic • Mechanical

– Function • Stamping, forging, press brake

– No. of slides • Single action • Double action • Triple action

• C frame • OBI etc

Different press frames

Open Back Inclined (OBI) C frame

Open Back

Widely used for blanking and piercing of small work pieces

Mechanical press •

A mechanical press drive consists of an electric motor, a flywheel, a clutch/brake, a drive shaft, a connection (also called

a

pitman

arm

or

connecting rod), and a slide (ram) guided by the gibs. •

Depending on the design, a press may have gear set(s) to reduce

the

speed

increase the torque

and

Drives in mechanical press

Drives in mechanical press

(a) Simple crank drive (b) slide motion

Crank drive • Crank link is attached to a drive shaft • Crank link rotates with the drive shaft and is attached to a connecting rod by a rotational joint • Connecting rod rocks back and forth during the motion of the crank • The ram operates in a slider joint and travels a one dimensional path in both directions

Eccentric drive • Motor drives an eccentric shaft, rotating in a connecting rod • The connecting rod moves a ram in a slider joint • As the motor rotates, the center of the drive remains stable but the overall center of the shaft changes. This causes the shaft to change position, providing motion

Knuckle drive • The knuckle joint press translates the energy of a motor through a powerful linkage design, and is capable of delivering a tremendous amount of force • The drive shaft crank rotates completely. The links are well grounded to support such pressure

Rack and pinion drive • The rack and pinion press delivers the motors energy from a gear directly connected to the drive shaft • The rack is actually a round gear of infinite radius

• A rotating gear (pinion), provides force through the rack. This gives the one dimensional,

translational

desired of press machines

motion

Screw drive • It uses rotational energy of a motor to turn a large screw. • Typically, a friction disk is used to translate the force from the drive shaft to the screw's head. The screw pushes a ram with great mechanical advantage. • Screw presses are similar to hydraulic presses in that they are relatively slow and require a longer contact with the work. • Some screw press machine tools in modern industry

can

produce

(62,000,000 lbs), of force.

31,000

tons,

Gear Reduction • In mechanical press, to increase energy per stroke, gear reductions are used.

(a) Direct drive (non-geared), (b) single gear reduction-single end drive, (c) single gear reduction-twin end drive, and (d) double gear reduction.

Single ,double and triple action press

Single action https://www.youtube.com/watch?v=QpHoKDkQ0FM

Double action https://www.youtube.com/watch?v=aoGvVC5QQbQ

Selection criteria for stamping press • • • • • •

Type of forming operation, and dies and tooling required Size and shape of work pieces Length of stroke of the slide, stroke per minute, speed and shut height (distance from the top of the bed to the bottom of the slide, with the stroke down) Number of slides (single action, double action and triple action) Maximum force required (press capacity, tonnage rating) Safety features

Schematic illustration of types of press frames for sheet- forming operations

Mechanical press Vs. Hydraulic press  The main difference between a hydraulic and mechanical press is the drive mechanism

Servo press • Main motor, flywheel, and clutch, replaced with a servomotor that focused energy only where needed and, in effect, made the ram a controllable axis • The stroke, slide motion, slide position, and speed are programmable to allow many different combinations that can work with a wide variety of dies, part types, and production speeds

Servo press

Injection molding • Injection molding is generally used to produce thermoplastic polymers

• The plastic material is received by the molder in granular form • It is placed in the hopper of an injection-molding machine, from which it is fed to a heated cylinder • Mold defines final shape as material cools and solidifies

Characteristics of injection moulding process • Intricate parts in large quantities • One molded part can replace what would otherwise be an assembly

of components • Secondary finishing operations are not necessary • Suited for lower stressed application area • Housings and covers are common uses rather than, for example, frames and connecting rods

• Production must be large enough so that the mold cost can be amortized over the quantity manufactured; >10,000 parts

Design for Injection moulding • Gate and ejector pin locations • Surface finish of the part depends upon the location of gate and ejector pin • Ejector pins usually located on the underside of a part if it has an both outside and an underside • Gates can be located in a number of locations

Various Gating Systems in Injection moulding

• In case of round and cylindrical parts center gating is preferred and for

large area parts near center-gating is recommended

Design guidelines

Maintain uniform wall thickness and avoid abrupt change in wall thickness and make it gradual change

Design guidelines

Minimum spacing for holes and sidewalls

Design guidelines

Minimum spacing for holes and sidewalls

Design guidelines • Through hole is preferred than a blind hole • Hole in the bottom of the part is preferable than the side • Blind hole should not be more than two diameters deep. If the diameter is 1.5 mm or less, then one diameter is recommended

Recommended depth limits for blind holes.

Design guidelines • To increase the depth of a blind hole steps are used thereby enabling employment of stronger core pin

If a blind hole must be deep, use a stepped diameter

Design guidelines • For through holes, cut-out sections in the part can shorten the length of a small diameter pin

Improved design on the right provides better rigidity of the mould core pin

Parting line rules • Parting line should not adversely affect the appearance or function of the part • It is preferred to keep the parting line at the edge of the part where

already a sharp edge exists. • If it is not possible to keep the parting line at the edge, a bead on the parting line facilitates removal of the mould flash

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This

or This