Advise on R Chart with Ppk of 17.85 for Ford

mlee97

Involved In Discussions
#1
Hi guys! I have a particular problem with a machining capability study in which I am butting heads with a Ford SSBB SQA.

We performed a Surface finish run on a capability run. 300+ pieces sampled to get 125 pieces (25 sets of 5). Ford Requires a PPK of 1.67 on these and control.

When we crunched the data in Minitab, we got a PPK of 17.85 but we had a control point out on the R chart of the required 6-pack.

I complained because the control limits are so tight because of the very limited amount of variation in the machining process. The span of the control limits is less than the unit of measurement (RZ) and the sigma is less than the resolution found in the MSA. He didn't buy it. He said I had to remeasure, but I balk because I know that I am simply in the noise zone of the measuring process.

How do you guys argue this? Any advice?
 
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A

adamsjm

#2
Re: Need advise on R chart with PpK of 17.85 for Ford

If you will post your data and required spec limits or your analysis of the data, it will be easier to post a valid response.
 
A

adamsjm

#3
Re: Need advise on R chart with PpK of 17.85 for Ford

You need to "Look at the data visually" not just read the numbers from the analysis program. Will you share your R&R calculations?
 

bobdoering

Stop X-bar/R Madness!!
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#4
Re: Need advise on R chart with PpK of 17.85 for Ford

I complained because the control limits are so tight because of the very limited amount of variation in the machining process. The span of the control limits is less than the unit of measurement (RZ) and the sigma is less than the resolution found in the MSA. He didn't buy it. He said I had to remeasure, but I balk because I know that I am simply in the noise zone of the measuring process.
Good observation, but it would help us if your provided the raw data.
 

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#6
Re: Need advise on R chart with PpK of 17.85 for Ford

OK, those are the mini-tab printouts. Would be useful still to see the raw data. Then we can see the fineness of the measurement capability. Could you post the raw data as an excel file?
 

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#8
Re: Need advise on R chart with PpK of 17.85 for Ford

Thanks. I don't have minitab. I'll work up some charts in Excel at home tonight (off the job). Off hand - looking at the measurements there seems to be plenty of variation (as compared to the precision) in the data. What subgroup size are you using and why that size?
 

mlee97

Involved In Discussions
#9
Re: Need advise on R chart with PpK of 17.85 for Ford

Subgroup size of five is a Ford requirement. Max Tolerance was 10 Rz. (Lower Bound - 1 sided)
 

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#10
Re: Need advise on R chart with PpK of 17.85 for Ford

Thanks. I still have an old copy of Ford's "Continuing Process Control" book so will apply its methodology, subgroup =5. I'll also try x-individuals to see if that says anything different.
 
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