AIAG SPC Manual (ver.1) Interpretations related to Gage R&R (GR&R) and MSA

P

pinpin - 2009

#1
Dear Covers,

I am very poor at GRR.

I have some questions that I need help, I use first version of AIAG SPC Manual:

1) Page 126 says "If the process variation is known....", how do you know or how can we tell?

2) What is this "process variation"?

3) Page 119 says to assess a measurement system prior to engaging in spc, if it is, we will have to wait until the production run is completed in order to select parts that represent the range of the process. Is it correct? Can we do GRR and SPC at the same time? especially when we run a new part?

4) I read somewhere in MSA and read from a post here, saying to select measuring system that is not more than 10% of the tolerance (part or process?), how to tell this % before we do GRR? and the system has not been used on this new part yet?

Please teach me...thank you!

:thanx:
 
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Miner

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Admin
#2
1) This would be from a capability study
2) This would be the variation of the process over short term. That is before long term sources of variation are allowed to creep in. These could be from material batch changes, shift changes etc.
3) You can in order to control the process. The manual is referring to the ideal, not the practical. The reason is that the gage may fail the %GRR and be found inadequate for SPC. However, you still have to control your process.
4) This refers to the resolution of the gage versus the tolerance. The resolution of the gage is the smallest possible measurement that the gage is capable of making.
 
P

pinpin - 2009

#3
1)
4) This refers to the resolution of the gage versus the tolerance. The resolution of the gage is the smallest possible measurement that the gage is capable of making.
Thank you so much Miner!

4) Sorry and just to reconfirm, many people told me is the % GRR and not resolution of the gage. I don't have the MSA manual at hand, will check on it.
 

Miner

Forum Moderator
Staff member
Admin
#4
The only way that you can determine 10% before performing an MSA is the method described above.

You are probably referring to the P/T Ratio <10% = Good; 10+% to 30% = Marginal; and, >30% = unacceptable rule. You must perform an MSA to determine this. No other method.

Note: there is a difference between %GRR (a.k.a. %TV) which has nothing to do with tolerance and P/T Ratio (a.k.a. %Tol) which is based on tolerance. There are many thread discussions on this. Search this forum for posts from Miner with keywords like P/T Ratio or %GRR and you will find detailed explanations of the difference.
 
P

pinpin - 2009

#5
1) This would be from a capability study
2) This would be the variation of the process over short term. That is before long term sources of variation are allowed to creep in. These could be from material batch changes, shift changes etc.
3) You can in order to control the process. The manual is referring to the ideal, not the practical. The reason is that the gage may fail the %GRR and be found inadequate for SPC. However, you still have to control your process.
4) This refers to the resolution of the gage versus the tolerance. The resolution of the gage is the smallest possible measurement that the gage is capable of making.
Dear Miner,

1) The process variation here means only the manufacturing process's 1 standard deviation or 6 standard deviation?

2) MSA page 21's Production gage variation curve means GRR or TV that stated in GRR Report?

:thanks::thanx:
 

Miner

Forum Moderator
Staff member
Admin
#6
1) The AIAG MSA 3rd edition is based on 6 standard deviations of the parts selected for measurement. These parts should be selected to represent the process variation.
2) The gage variation curve graphically displays more information , but is an extension of the P/T Ratio. It shows the probability of accepting a part as you approach the specification limit. The x-axis plots the true value of the part (and the tolerance limits) while the y-axis plots the probability of accepting that part. This illustrates the impact of a poor P/T Ratio.
 
P

pinpin - 2009

#7
1) The AIAG MSA 3rd edition is based on 6 standard deviations of the parts selected for measurement. These parts should be selected to represent the process variation.
2) The gage variation curve graphically displays more information , but is an extension of the P/T Ratio. It shows the probability of accepting a part as you approach the specification limit. The x-axis plots the true value of the part (and the tolerance limits) while the y-axis plots the probability of accepting that part. This illustrates the impact of a poor P/T Ratio.
What is P/T Ratio?:thanks::thanx:
 
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