ASME B89.7.3.1:2001 pdf download

ASME B89.7.3.1:2001 pdf download CUIDELINES FOR DECISION RULES: CONSIDERING MEASUREMENT UNCERTAINTY IN DETERMINING CONFORRIIANtE TO SPECIFICATIONS
mean measurement result: results of repeated measure- ments are arithmetically averaged to yield a mean measurement result. The mean result is used to deter- mine acceptance or rejection. data rejection with cause: repeated measurements may indicate that one or more measurement results signifi- cantly deviate from the rest of the results of measure- ment. If the measurement procedure has a documented policy for addressing measurement rejection then this policy takes precedence. Otherwise, measurement re- sults may only be rejected if a physical cause can be established. Examples of physical causes for measure- ment rejection include: improper instrument settings, loose or improperly fixtured components, known tran- sient events such as vibrations caused by doors slamming.
3 REQUIREMENTS FOR DECISION RULES
3.1 Zone Identification A decision rule must have a well-documented method of determining the location of the acceptance, rejection, and any transition zones.
3.2 Decision Outcome Each zone of a decision rule must correspond to a documented decision that will be implemented should the result of measurement lie in that zone. While this is automatic for the acceptance and rejection zones by definition, any transition zones must have their corresponding decision outcome documented.
3.3 Repeated Measurements A decision rule must state the procedure for addressing repeated measurements of the same characteris- tic on the same workpiece or instrument. See Appendix B for further discussion of this issue.
3.4 Data Rejection A decision rule must state the procedure for allowing data rejection with cause, that is, rejection of “outliers.” See Appendix C for further discussion of outliers.
4 ACCEPTANCE AND REJECTION ZONES IN DECISION RULES
4.1 Simple Acceptance and Rejection Using an N: Decision Rule This is the most common form of acceptance and rejection used in industry and is the descendant of MIL- STD 45662A. Simple acceptance means that product conformance is verified2’ if the measurement result lies in the specification zone and rejection is verified otherwise (see Fig. 2), provided that the magnitude of the measurement uncertainty interval is no larger than the fraction 1/N of the specification zone. In recent years, as tolerances have become increasingly tighter, the well-known ten-to-one ratio has transitioned to a more commonly used ratio of four-to-one (see MIL- STD 45662A) or even three-to-one (see International Standard 10012-1). A 4:l decision rule means the uncertainty interval associated with the measurement result should be no larger than one-fourth of the allow- able product variation, which requires the expanded uncertainty, U, to be no larger than one-eighth of the specification zone. Once the uncertainty requirement is satisfied, then the product is accepted if the measurement result lies within the specification zone and rejected otherwise. Note that instrumentation is sometimes speci- fied by a maximum permissible error (MPE), which places a limit on the magnitude of the error regardless of sign. Hence the specification zone has a width of twice the MPE, i.e., -tMPE, and a four-to-one ratio requires the expanded uncertainty to be one-fourth the MPE value; see Appendix D for further details. While the simple acceptance and rejection approach is straight- forward, difficulties develop for measurement results close to the specification limits. Even using the mean of repeated measurements, if the mean result is near the specification limit there may be a significant chance that a product characteristic with simple acceptance verified is actually out-of-specification and vice versa. To address this issue, an alternative decision rule based on “guard banding” can increase confidence in accept- ance decisions.
4.2 Stringent Acceptance and Relaxed Rejection Using a 2% Guard Band Stringent acceptance increases confidence in product quality by reducing the probability of accepting an out-of-specification product through the use of guard banding. The acceptance zone is created by reducing the specification zone by the guard band amount(s) as deemed necessary for economic or other reasons, thus ensuring product compliance at a specified level of confidence, or conversely, at an acceptable level of risk. In a binary decision rule, stringent acceptance is accompanied by relaxed rejection. Relaxed rejection allows the rejection of products even when the measure- ment result lies within the specification zone by the guard band amount. The size of the guard band is expressed as a percentage of the expanded uncertainty. It is typically the customer who requests stringent acceptance of the supplier and enforces this through the contract. Some of the factors that should be consid- ered when establishing the size of the guard band are given in Appendix E.