Calipers (New Standard ISO-ASME) [PDF]

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NCSL International Workshop & Symposium | Measurements of Tomorrow August 27-30, 2018 | Portland, Oregon

The New American Standard for Digital, Dial, and Vernier Calipers James G. Salsbury Mitutoyo America Corporation 965 Corporate Boulevard Aurora, IL 60502 Phone: 630-723-3619 Email: [email protected] Abstract A new American National Standard for digital, dial, and vernier calipers, ASME B89.1.14, was approved by the ASME B89 dimensional metrology standards committee in 2017, and final publication of the standard is expected in 2018. The purpose of this paper is to introduce the new standard and discuss some of the key developments. In particular, the standard includes default specifications and calibration test methods that will have a major impact to calibration services. This paper will review the history of the development of the standard and discuss all the major sections of the standard, including specifications, rated operating conditions, test methods, conformance decision rules, and measurement uncertainty. The relationship between ASME B89.1.14 and the international standard for calipers, ISO 13385-1, will also be discussed. Learning Objectives   

Identify the new ASME B89.1.14 standard and understand the key concepts. Apply the new ASME B89.1.14 to calibration practices. Understand the differences and development of the American standard versus the international standard, ISO 13385-1.

1 Introduction At the time of this writing, there is no American National Standard for the most common dimensional measuring instrument – the digital, dial, or vernier caliper. See Figure 1. That situation should change by the time this paper is presented at the NCSL International Workshop and Symposium in summer 2018. The first American National Standard for calipers, the new ASME B89.1.14-2018 [1] has completed all levels of approval and publication is pending. In the United States, the American Society of Mechanical Engineers (ASME) has the responsibility for developing American National Standards in the dimensional metrology area. That work is managed by the ASME B89 committee. Back in the mid-1990’s, the ASME B89 committee initiated projects to develop standards for three different measuring instruments – micrometers, indicators, and calipers. The standards for micrometers [2] and indicators [3] were completed in 2001, and the micrometer standard was later revised in 2013 [4]. Unfortunately, the work for calipers was not completed at that time. In 2011 the International Standard for calipers, ISO 13385-1:2011 [5], was published and this provided for some resurgence of the B89.1.14 project team. In addition, the ISO working group 1

NCSL International Workshop & Symposium | Measurements of Tomorrow August 27-30, 2018 | Portland, Oregon

began a revision process for ISO 13385-1 a few years ago, and this created an opportunity for the ASME group to harmonize efforts with ISO. In addition to the pending publication of ASME B89.1.14, the ISO 13385-1 revision recently was approved at the draft international standard (DIS) level [6], and final publication of that standard is forecasted for 2019. The chair of the ASME B89.1.14 project team is Kevin Skinner from the United States Air Force. This author is the chair of the ASME Division 1 subcommittee as well as the task force leader for the ISO 13385-1 revision. The contents of this paper should not be seen as an interpretation of the ASME B89.1.14 standard but rather just a summary of the new standard as well as providing some insight into the differences between the ASME and ISO standards for calipers. The standard should be consulted for official text and additional details.

Figure 1. Example digital caliper showing (1) outside measuring faces, (2) crossed knife-edge inside measuring faces, (3) depth measuring faces, and (4) step measuring faces. 2 Scope The new American standard on calipers leverages the work done in ISO. ISO 13385-1 provides design and metrological characteristics of calipers that are important in the manufacture, specification, and purchase of calipers. Instead of repeating all that information, ASME B89.1.14 references ISO 13385-1 and then focuses on content important to the calibration business, including detailed calibration methods, default specification values, measurement uncertainty, and conformity decision rules. ASME B89.1.14 does not contradict ISO 13385-1 and is a standalone document intended for use by organizations involved with the calibration of calipers. With the publication of the new ISO/IEC 17025:2017 [7], and the increased focus on decision rules, the new ASME B89.1.14 should be quite helpful to accredited laboratories that calibrate calipers. 3 Measuring Methods As shown in Figure 1, the most common configuration of caliper has four different modes of measurement – outside, inside, depth, and step. There are, of course, many different configurations and sizes of calipers available on the market, and ASME B89.1.14 tries to address all of the most common situations. There are two primary metrological characteristics for calipers defined in ASME B89.1.14 (and ISO 13385-1). These characteristics are called the partial surface contact error, E, and the scale 2

NCSL International Workshop & Symposium | Measurements of Tomorrow August 27-30, 2018 | Portland, Oregon

shift error, S. For both of these, there is a defined tolerance, or maximum permissible error (MPE), which are noted simply as EMPE and SMPE. Manufacturers of calipers should supply the MPE values, but ASME B89.1.14 contains a table of default MPE values that apply if not otherwise stated. A portion of the tolerance table from ASME B89.1.14 is shown here in Table 1. Additional default MPE values for other resolutions and larger size calipers (up to 1000 mm or 40 in.) are available in the standard. In looking at Table 1, it is important to note that these default MPE values are a function of the measured length and not the entire measuring range. This means that all calipers, regardless of the overall measuring range, have the same MPE values for the same measured length. In addition, the MPE values increase with the measured length. The MPE values increase in a stepwise manner, and due to the resolution of the caliper, the steps are equal to integer multiples of the resolution. Due to this, it is important to calibrate the caliper using the same units of measure (inches or mm) as the applicable MPE value and not convert the measured values or the MPE values. In addition, there is no requirement in the standard to calibrate using both units of measure (on calipers that support both units). Table 1. Example of the default MPE values in ASME B89.1.14. Digital Resolution of Caliper Measured Length, L 0.0005 in. 0.01 mm mm

in.

EMPE, in.

SMPE, in.

EMPE, mm

SMPE, mm

0  L  50

0L2

± 0.0010

± 0.0010

± 0.02

± 0.03

50  L  150

2L6

± 0.0010

± 0.0020

± 0.03

± 0.05

150  L  200

6L8

± 0.0015

± 0.0020

± 0.03

± 0.05

200  L  300

8  L  12

± 0.0015

± 0.0025

± 0.04

± 0.06

4 Partial Surface Contact Error, E The partial surface contact error is a length measuring error of indication that applies to the outside measuring faces. On most calipers, the outside measuring faces are long, and the concept of “partial surface” is that only a small portion of the outside measuring faces should be used for any given test point. This is usually accomplished in practice by using a measurement standard, e.g. gage blocks, oriented in a manner to minimize the contact with the measuring faces. In addition, the partial surface contact error applies at any point on the outside measuring faces so it is important that the calibration include test points with the measurement standard at various distances from the beam of the caliper. See Figure 2. In accordance to the ASME B89.1.14 standard, the partial surface contact error needs to be tested across the measuring range of the caliper with approximately equally distributed test points. The standard defines a minimum number of test points, which ranges from three to five points depending on the measuring range of the caliper.

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NCSL International Workshop & Symposium | Measurements of Tomorrow August 27-30, 2018 | Portland, Oregon

Most calipers on the market have an adjustable zero point. In the determination of the partial surface contact error, as well as all the other errors to be discussed later, the caliper is to be properly zeroed after bringing the outside measuring faces together. This zero point, having a defined error of indication of zero, is therefore not a test point. There are some styles of calipers, e.g. some vernier calipers, where the zero point is not adjustable. In those cases, there may be a non-zero error when closing the outside measuring faces. In accordance to the ASME B89.1.14 standard, this error is to be evaluated as an error and not corrected for at any of the test points when assessing conformity to specification.

Figure 2. Example showing two test points for the partial surface contact error. 5 Scale Shift Error, S The scale shift error is a length measuring error of indication that applies to all measurements not using the outside measuring faces. The concept behind the name of this error is that the partial surface contact error tests the measuring scale of the caliper (which is used for all measurements) but there may be an additional error, a shift or offset, when using the other measuring faces. As the zero point is defined using the outside measuring faces, this additional shift applies to all the other modes of measurement available for the particular configuration caliper. As the purpose of this test is to assess any shift, ASME B89.1.14 only requires a single test point using each of the other available measuring faces, with the default lengths being less than 50 mm (2 in.). Example test configurations for inside, depth, and step measurements are shown in Figure 3. As shown in Figure 1, a common caliper design uses crossed knife-edge measuring faces for inside measurements. Due to the width of the knife-edges and the gap between them, there is an additional error that can occur when measuring small cylindrical holes. This error is considered another type of scale shift error and is tested by measuring a small ring gage with the default diameter being 5 mm (0.2 in.). In accordance to ASME B89.1.14, this test only applies to calipers with a measuring range up to 300 mm (12 in.). Larger calipers typically have much thicker knife-edge measuring faces and measuring small internal diameters is not recommended. The standard states that calipers with a measuring range over 300 (12 in.) are not rated for the measurement of internal diameters smaller than 20 mm (0.75 in.).

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NCSL International Workshop & Symposium | Measurements of Tomorrow August 27-30, 2018 | Portland, Oregon

Fig. 3. Examples of testing the scale shift error for inside, depth, and step measurements. 6 Line Contact Error As discussed earlier, the partial surface contact error, E, applies across the length of the outside measuring faces. In this manner, the influence of the parallelism of the large outside measuring jaws is included in the evaluation of the partial surface contact error. The ASME B89.1.14 standard recognizes that there may be times where a more direct assessment of this parallelism is of value. In addition, there may be localized flatness errors or wear on the outside measuring faces. The standard recommends bringing the outside measuring faces close together and observing the light gap between the jaws. If any discrepancy is observed, then the line contact error test is recommended. This test involves measuring a small pin gage at any position across the outside measuring faces. The test is typically done by placing the pin gage near the beam and then moving the pin towards the tip of the jaws while observing the caliper readings. The range of the results is then compared to the MPE value of the partial surface contact error. 7 Repeatability In accordance to ASME B89.1.14, the specified MPE values apply to all unique measurement indications made under reasonable use of the caliper. In this manner, it is not permitted to take multiple readings at one test point and then compare the average error to the MPE value. As such, all the test points are influenced by any lack of repeatability of the caliper. If an issue with repeatability is observed during calibration, then a specific assessment of repeatability may be of value. As defined in ASME B89.1.14, a repeatability test can be done using the outside measuring faces by measuring three repeat readings at the same test point. The range of the results is then compared to the MPE value of the partial surface contact error. 8 Rated Operating Conditions ASME B89.1.14 states that caliper specifications have a rated operating condition of 20°C (68°F). As such, any test values observed in calibration should be corrected to 20°C (68°F), at least in theory. In practice, the consequences of not being at 20°C (68°F) are typically included in the evaluation of the measurement uncertainty. Calipers are manually operated indicating measuring instruments, and therefore the measuring skills of the operator will influence the measurement results. ASME B89.1.14 states that all specifications apply when a reasonably skilled operator uses the caliper in a manner consistent with normal operation and in accordance with the recommendations of the manufacturer. The 5

NCSL International Workshop & Symposium | Measurements of Tomorrow August 27-30, 2018 | Portland, Oregon

intent of this concept is that a reasonably trained and skilled operator should be able to operate a caliper and expect the caliper to perform within specification. The caliper needs to be designed, manufactured, and maintained to perform as specified when used as intended. The reasonably skilled operator therefore becomes a rated operating condition when testing for conformity of the caliper to specification. Variation in results due to the operator is expected, and part of the purpose of the conformity assessment test is to determine those errors. In other words, the caliper needs to be tested in a manner similar to its use, and not by some “robot” or other means that would eliminate any influences by a reasonably skilled operator. 9 Measurement Uncertainty ASME B89.1.14 includes an appendix with general guidance for the estimation of measurement uncertainty. The example uncertainty analysis specifically includes uncertainty associated with the reference standards used in testing, as well as any uncertainty associated with temperature. Specifically excluded from the uncertainty are contributions from the errors of the caliper, such as the caliper repeatability or resolution. In addition, for the reasons discussed above, any variation from a reasonably skilled operator is also not a source of measurement uncertainty. For additional information on the important and emerging concept of “test uncertainty”, see [8] to [11]. 10 Decision Rules ASME B89.1.14 defines a default decision rule that applies when making statements of conformity to specification. The decision rule is simple 4:1 acceptance in accordance to ASME B89.7.3.1 [12], also known as simple acceptance with a measurement capability index, Cm, equal to four [13], or the 4:1 test uncertainty ratio (TUR) rule [14]. The decision rule in ASME B89.1.14 follows the recommendations of ASME B89.7.1 [11], which states that ASME B89 standards should explicitly state a simple 4:1 acceptance decision rule unless there is specific justification for an alternative decision rule. In dimensional metrology, the concepts of decision rules have been used for some time (see [12]). The recent update to ISO/IEC 17025:2017 [7] includes the use of the term and concepts of explicit decision rules stating that “unless inherent in the requested specification or standard, the decision rule selected shall be communicated to, and agreed with, the customer.” Having the decision rule stated in ASME B89.1.14 should enable easier adoption of this new requirement for accredited calibration laboratories and provide additional protection for customers of calibration services. 11 ASME B89.1.14 versus ISO 13385-1 The current ISO standard for calipers, ISO 13385-1, was published in 2011. That document outlines the design and metrological characteristics of calipers, and those concepts have been adopted in ASME B89.1.14. The current ISO standard, however, contains no default specifications, has limited description of test methods, and offers no information about contributors to measurement uncertainty. As such, the beneficial use of ISO 13385-1:2011 for calibration laboratories is limited. 6

NCSL International Workshop & Symposium | Measurements of Tomorrow August 27-30, 2018 | Portland, Oregon

The current revision of ISO 13385-1 [6] is incorporating many of the same concepts as ASME B89.1.14. There is good harmonization in the standards development between ASME and ISO; for example, the default metric specification values and decision rules are identical in the two standards. Due to the influence of different experts worldwide, it is expected that there will be some differences between the two published standards; however, there is a general direction within the working groups towards international harmonization. The more recent caliper standardization efforts in Japan [15] and Germany [16] indicate that desires to harmonize internationally are not unique to the United States. 12 Example Reporting An example of how the test results in accordance to ASME B89.1.14 might be reported for the calibration of a typical 0-12 inch digital caliper is shown in Figure 4. Description: Caliper, 0 – 12 inches Serial Number: 123456 Digital Resolution: 0.0005 inches Nominal Length 1 2 4 Outside 6 8 12 Inside 1 Knife-Edge 0.2 Step 0.5 Depth 0.5 All values in inches.

Tolerance ± 0.0010 ± 0.0010 ± 0.0010 ± 0.0010 ± 0.0015 ± 0.0015 ± 0.0010 ± 0.0010 ± 0.0010 ± 0.0010

Test Value 0.0000 + 0.0005 0.0000 – 0.0005 + 0.0005 + 0.0005 – 0.0005 – 0.0010 0.0005 – 0.0010

Pass/Fail Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass

This caliper was tested in accordance with ASME B89.1.14-2018. Statements of conformity are based on the test values shown above, the default accuracy specifications in ASME B89.1.14-2018, and when accounting for the measurement uncertainty using a simple 4:1 acceptance decision rule as defined in ASME B89.7.3.1-2001. The measurement uncertainty = ± 0.0001 inches (k=2), which achieves better than a 4:1 test uncertainty ratio.

Figure 4. Example calibration reporting following the test methods in ASME B89.1.14.

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NCSL International Workshop & Symposium | Measurements of Tomorrow August 27-30, 2018 | Portland, Oregon

13 Conclusion The first American National Standard for calipers, ASME B89.1.14-2018, is expected to be available by the summer of 2018. The default specification values, calibration test methods, measurement uncertainty, and decision rules in the new standard will have an impact in the calibration business. The purpose of this paper was to bring awareness of the new standard, discuss the history and development, and to summarize some of the major technical points. References 1. 2. 3. 4. 5.

ASME B89.1.14-2018 Calipers (publication pending). ASME B89.1.13-2001, Micrometers. ASME B89.1.10M-2001, Dial Indicators (for linear measurements). ASME B89.1.13-2013, Micrometers. ISO 13385-1:2011, Geometrical product specifications (GPS) — Dimensional measuring equipment – Part 1: Callipers; Design and metrological characteristics. 6. ISO/DIS 13385-1:2017, Geometrical product specifications (GPS) — Dimensional measuring equipment – Part 1: Callipers; Design and metrological characteristics. 7. ISO/IEC 17025:2017, General requirements for the competence of testing and calibration laboratories. 8. ISO 14253-5:2015, Geometrical product specifications (GPS) — Inspection by measurement of workpieces and measuring equipment — Part 5: Uncertainty in verification testing of indicating measuring instruments. 9. J. G. Salsbury and E. P. Morse. Measurement uncertainty in the performance verification of indicating measuring instruments. Precision Engineering, 36 (2012):218-222. 10. ISO/DIS 14978:2017, Geometrical product specifications (GPS) — General concepts and requirements for GPS measuring equipment. 11. ASME B89.7.1-2016, Guidelines for Addressing Measurement Uncertainty in the Development and Application of ASME B89 Standards. 12. ASME B89.7.3.1-2001, Guidelines for Decision Rules: Considering Measurement Uncertainty in Determining Conformance to Specifications. 13. ISO/TR 14253-6:2015, Geometrical product specifications (GPS) — Inspection by measurement of workpieces and measuring equipment — Part 6: Generalized decision rules for the acceptance and rejection of instruments and workpieces. 14. ANSI/NCSL Z540.3-2006 – Requirements for the calibration of measuring and test equipment. 15. JIS B 7507:2016, Vernier, dial, and digital callipers (English edition). 16. DIN 862:2015-03, Geometrical product specifications (GPS) — Callipers – Maximum permissible errors (English edition).

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