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why CPK <1.0 regarding two pieces of control charts of torque wrench?


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  Post Number #1  
Old 30th November 2010, 11:19 AM
Terry

 
 
Total Posts: 28
why CPK <1.0 regarding two pieces of control charts of torque wrench?

Hello everybody,

please refer to the attached file regarding two pieces of control charts of torque wrench, could you please help me to analyse the reason why CPK <1.0? what measures shoud I be taken? I hope you can give your advices to me, you know i am new guys for SPC knowledges, now i am studying them.

Thanks in advance!

Terry
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File Type: xls 中文cpk.xls (300.5 KB, 1298 views)

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  Post Number #2  
Old 30th November 2010, 11:36 AM
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bobdoering

 
 
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Re: why CPK <1.0 regarding two pieces of control charts of torque wrench?

It appears they state CPK <1.0 because they are using the entire specification with their torque variation. The reasons can be many - most would should be listed in the preparation of a total variance equation. But they might include variation from the test method, the gage and the torquing of the wrench used for assembly. There could even be such detail as thread and mating surface finish. The evidence will be tough to sort out where and how much each participant is providing to the total variance. Part of the problem is it is destructive testing - it is impossible to duplicate to verify gage R&R without some type of destructive gage R&R approach.
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  Post Number #3  
Old 30th November 2010, 12:21 PM
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Sturmkind

 
 
Total Posts: 268
Re: why CPK <1.0 regarding two pieces of control charts of torque wrench?

Terry;

I agree with Bob that all variation components need to be quantified. The standard GR&R just doesn't work well with Torque since the thread forms are distorted at final full torque so there is rarely correlation between re-start torque or break-away.

There may be a good correlation between torque angle & rundown if your gun transducers capture that information. If the test is a manual torque a large contributor can be operator-to-operator and how perpendicular to the fastened joint they are holding the wrench.

I did confirm that your spreadsheet is correct that Cpk is between 0.85/0.86.
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