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What are the general rules / guidelines when doing a machine capability study?


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  Post Number #1  
Old 1st September 2005, 03:25 PM
maxschnau

 
 
Total Posts: 5
Let Me Help You What are the general rules / guidelines when doing a machine capability study?

WHat are the general rules / guidelines when doing say a 30 pc measurement capability study on a dimension on a part. Say one part is out of spec, can you get another part?, can you refixture the part?, etc, and if you have the anser to these rules, what source do they come from?

THANKS!!!

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  Post Number #2  
Old 1st September 2005, 03:34 PM
Jim Wynne's Avatar
Jim Wynne

 
 
Total Posts: 14,149
Quote:
In Reply to Parent Post by maxschnau

WHat are the general rules / guidelines when doing say a 30 pc measurement capability study on a dimension on a part. Say one part is out of spec, can you get another part?, can you refixture the part?, etc, and if you have the anser to these rules, what source do they come from?

THANKS!!!
Please explain what you mean by "capability study." What information are you hoping to get from the study?
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  Post Number #3  
Old 1st September 2005, 03:44 PM
maxschnau

 
 
Total Posts: 5
I mean capability study - Initial Capability Studay as is required by Automotive. I would be looking to achieve a 1.33 or above CPK.
  Post Number #4  
Old 1st September 2005, 03:51 PM
Jim Wynne's Avatar
Jim Wynne

 
 
Total Posts: 14,149
Quote:
In Reply to Parent Post by maxschnau

I mean capability study - Initial Capability Studay as is required by Automotive. I would be looking to achieve a 1.33 or above CPK.
You can't do that (by default) with just 30 pieces. Here are the default requirements as stated in the AIAG PPAP manual (3rd Edition):
Quote:
For those characteristics that can be studied using X-bar and R charts, a short term study should be based on a minimum of 25 subgroups containing at least 100 readings from consecutive parts of the significant production run...
Furthermore, you can't calculate Cpk without first verifying that the process is in a state of statistical control, and I don't know how you can do that with a 30-piece sample.
  Post Number #5  
Old 1st September 2005, 03:54 PM
maxschnau

 
 
Total Posts: 5
OK, Let's say it's a 300 Piece Study. What are the rules of remeasuring or refixturing or replacing a part that is considered a flyer by the data.........
  Post Number #6  
Old 1st September 2005, 03:55 PM
ralphsulser's Avatar
ralphsulser

 
 
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Plus, when you find something in your sample size out of spec. you can't throw it out and ignore it, or change the gauge either
  Post Number #7  
Old 1st September 2005, 04:10 PM
maxschnau

 
 
Total Posts: 5
OK, Let's say it's a 300 Piece Study. What are the rules of remeasuring or refixturing or replacing a part that is considered a flyer by the data.........
  Post Number #8  
Old 1st September 2005, 04:19 PM
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Miner

 
 
Total Posts: 3,788
Quote:
In Reply to Parent Post by maxschnau

OK, Let's say it's a 300 Piece Study. What are the rules of remeasuring or refixturing or replacing a part that is considered a flyer by the data.........
You cannot ignore a part that is flyer unless there is a legitimate reason. By your questions about refixturing, it appears that you suspect that the part was incorrectly loaded into a measurement fixture, or there was debris between the part and fixture that threw the reading out. In a case such as these, you could legitimately clean the fixture, remeasure the same part and substitute the results.

However, in another example, this would not be true. Let's say that during the manufacture of product, a change occurred (material lot, etc.), the process shifted, then was adjusted back. In this example, the "flyer" is a legitimate part of the data that is telling you that the process is not always in control. You cannot simply throw this data out. You have to bring the process into control and re-study.
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