GR&R represents 99% of the measurement error caused by either operator or equipment. It is usually expressed as a percentage of the total variation (TV). The GR&R percentage =GR&R/TV,
which is the portion of the total variation consumed by the GR&R measurement error, can be derived from the following sources:
1. The specification limits have historically been used as the total variation, since it is assumed that the test of the product or part will cull out any parts outside the specifications.
2. The total variation is comprised of" KSS of the GR&R and the part variation (PV). The part variation, which is also the population variation used for six sigma calculations, can be derived from the GR&R data by multiplying the range of part averages as measured by the operators by the constant Kg. K3 is calculated from d2 in Table 5.6, depending on the number of parts examined in the GR&R measurements, as follows:
3. If the process variation is known through process capability studies and is based on six sigma, then σp can be derived independently from the GR&R study and used for PV and TV calculations, with PV= 5.15
The GR&R% of total variation can be used to determine if the measurement system is acceptable for its intended applications. General guidelines for the value of GR&R% are:
• If GR&R% < 10%, then the measurement system is acceptable
• If 10% < GR&R% < 30%, then the system may be acceptable, based on whether the part characteristic classification is not critical or from customer input
• If GR&R%0 > 30%, then the system is not acceptable. It is then desirable to seek resolution through the use of quality tools, better operator training, or the purchase of new inspection equipment.