Dealing with Excessive Within-Part Variation when doing MSA and Cpk studies

Golfman25

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#1
So I am reading thru the AIAG MSA 4th ed. and section D (pg. 167) addresses excessive within-part variation. Unfortunately, their explanation is practically worthless. They give no specifics on exactly what to do and how to handle. So, I ask here -- How do you deal with excessive within-part variation when doing msa and cpk studies? Thanks in advance.
 
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Bev D

Heretical Statistician
Staff member
Super Moderator
#2
well first: within part variation counts and it matters.

for MSA the general approach is to 'mark' or designate specifically where in the part the measurements are made so the measurement error isn't confounded with the within part variation.

for Cpk - I simply don't do that. well, OK, I do capability studies I jsut don't calculate that infernal number. I DO include within part variation in the study. of course, this typically results in a non-Normal distribution so you may have difficulty with that Cpk index. There are two ways to handle this depending on what your Custoemr is REALLY looking for from the number.
  • If they want to estimate yields or defect rates then simply calculate the part level defect rate and back calculate the Cpk thru the Z table.
  • If they are looking for process spread vs spec limits, then take the range of the process spread and the range of the spec limits and calcualte Cpk that way.
 

bobdoering

Stop X-bar/R Madness!!
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#4
The key to dealing with within part variation for a gage R&R is to understand what question you are trying to answer.

If you are trying to answer "Is this gage capable of measuring this characteristic with sufficient statistical resolution?", then you omit the within part variation by measuring in one location as BevD indicated. It is not the gage's fault you have within part variation, and if the gage passes the gage R&R, it will be accurate enough to detect the variation when used correctly.

If you are trying to answer the question "If I ignore the fact that I have within part variation and allow the operator to measure the part any way they wish, is the measurement technique adequate?" then you will indeed will include the within part variation. It will be a measure of both gage variation and measurement error variation combined - because measuring a characteristic using a technique that ignores within part variation is measurement error. It will be adequate, however, if the within part variation is statistically insignificant. I can think of better ways to determine that rather than burying it in a gage R&R study, however.
 
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