Re: How do I get the sample for GRR study
You should determine the reason for performing the MSA prior to planning the sample collection. There are two basic reasons to perform an MSA.
Reason #1) to determine whether the gage can distinguish a good part from a bad part for acceptance measurements. You need to compare the measurement error to the tolerance using P/T Ratio. In this case, your sample selection method is the appropriate method.
Reason #2) to determine whether the gage can distinguish one part from another part for statistical measurements such as used for process control (SPC) or process improvement studies (DOE, t-tests, etc.). You need to compare the measurement error to the process variation using %GRR. In this situation, your sample selection criteria will most likely give you false results depending on your actual process variation. You should select sample using the same criteria that you would use to collect samples for a capability study because the standard deviation of your samples must match the standard deviation of the short-term process capability.
If you have a capable process, your method would result in an artificially high %GRR because you have provided samples with a standard deviation greater than the actual process variation.
"A fool can learn from his own experiences; the wise learn from the experience of others." - Democritus, 460-370 B.C.