ASME B89.7.3.1 pdf free download

ASME B89.7.3.1-2001 pdf free download.GUIDEL INES FOR DECISION RULES: CONSIDERING MEASUREMENT UNCERTAINTY IN DETERMINING CONFORMANCE TO SPECIFICATIONS.
4 ACCEPTANCE AND REJECTION ZONES IN DECISION RULES
4.1 Simple Acceptance and Rejection Using an N:1 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 verified’ if the measurement result lies in the specifcation zone and rejection is verfied 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-lo-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 4:1 decision rule means the uncertainty interval associated with the measurement result should be no larger than one-fouth 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 widh of twice the MPE, i.e, +MPE, 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 specifcation 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 vrified is actually out-of-specifcation and vice versa. To address this issue, an alternative decision rule based on“guard banding” can increase confidence in accepl- ance decisions.
4.2 Stringent Acceptance and Relaxed Rejection Using a Z% 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. Figure 3 illustrates examples of stringent acceptance/ relaxed rejection. The guard band applied within the specification zone, gIn, usually is determined by establishing an“acceptable risk” of accepting out of-specifi- cation products. One-sided stringent acceptance is used to guard band only one of the specification limits. For example, workpiece “form error”is always positive by definition, hence the lower limit (zero) does not require a guard band. Measurement results that lie in the acceptance zone are considered to verify the product to its specification. 4.3 Stringent Rejection and Relaxed Acceptance Using a Z% Guard Band Stringent rejection increases confidence that a rejected product is actually out-of- specification. Adding the guard band amount(s) to the specification zone creates the rejection zone. It is typically the supplier who requests stringent rejection of the customer who may be seeking a refund for a product that is claimed to be out-of-specification. In a binary decision rule stringent rejection is accompanied by relaxed acceptance. Relaxed acceptance allows acceptance of products with measure- ment results that lie outside the specification zone by the guard band amount. Relaxed acceptance is often used when a state-of-the-art measurement system still has such large uncertainty that a significant number of good products would be rejected under simple or stringent acceptance rules. Figure 4 is an example of a binary decision rule using relaxed acceptance.ASME B89.7.3.1 pdf download.