Chris:
1) Despite many good comments to this post, there is no such thing as “very small part variation.” Rather, there is only measurement tools unable to measure the variation present. It would be like trying to use a yardstick to measure microns. Here, you indicate that you may have up to .002 range and discrimination level of .0001, so you have a possibility of 20 levels, so should be OK. Do you actually observe this range or is that just the possible level?
2) Your customer insisting on using the actual parts just goes to prove that they don’t understand what an MSA/GRR is meant to demonstrate. You are evaluating the TOOL not the parts. Does it matter if I measure a die, a cube of similar size, or two lines drawn a similar distance apart? Only from the standpoint of how the operators use the tool. This is a common issue with MSA – I often find that the operators want to use “good” parts, thus the GRR turns out poorly. If needed, intentionally fabricate the parts to represent the range of allowable variation. Be careful with this, though. By creating these parts with larger variation, your customer may then come back and accuse you of producing “crap.” You need to educate them.
3) Gary makes a very good point on also looking at the capability. In fact, this is a scenario that I often use in evaluating the skill set of folks – you have a GR&R with a study variation of 38%, the process which is being measured with this gage is currently exhibiting a Cpk of 1.67. What actions do you take? (since I will be using this in the future I won’t disclose the appropriate answer, but if it isn’t obvious to you, you can send me an e-mail at [email protected] and I’ll send you the answer).
.
