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  FMEA and Control Plans
  Capability study

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Author Topic:   Capability study
ivo
Lurker (<10 Posts)

Posts: 2
From:
Registered: Feb 2000

posted 16 February 2000 09:06 AM     Click Here to See the Profile for ivo   Click Here to Email ivo     Edit/Delete Message   Reply w/Quote
For my traineeship, I have to improve a production process. I have done an FMEA and now I want to do a capability study. But at school I learned that the least amount of products you should produce (to test) is 50.
But the products that I have to produce are about 13 meters long and 100 kg. Is there a way to reduce the number of products? Please let me know..........

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Mike525
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Posts: 49
From:
Registered: Apr 99

posted 16 February 2000 02:55 PM     Click Here to See the Profile for Mike525   Click Here to Email Mike525     Edit/Delete Message   Reply w/Quote
ivo:

I'm not a statisticion, but I believe the fewest number of parts one can use for a capability study, and still be statistically correct, is 31 - has something to do with the distribution under a normal curve based on the Standard Normal (z) Table and Sample (t) Table. Two good books to reference are "Introduction to Statistical Quality Control" 2nd ed. by Douglas Montgomery, and "Understanding Statistical Process Control" 2nd ed., by Donald Wheeler and David Chambers. Hope this helps.

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Qualiman
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Posts: 35
From:Cuautitlan , Mexico
Registered: Feb 2000

posted 18 February 2000 03:19 AM     Click Here to See the Profile for Qualiman   Click Here to Email Qualiman     Edit/Delete Message   Reply w/Quote
ivo:

As a normal rule you cannot make a capability study unless you have already demonstrated the stability of your process, and you need at least 30 data if you use average lectures and more than 100 for individual moving range charts.
However,(I declare : I am not an expert in this field) you can run some short time period (or preliminary)studies to have an idea of your process capability using let us say.. 7 lectures, but in this case you must close your gap and instead of using 3 sigma you have to use 4, I think to "compensate" of using few points.

What other folks think ?

Qualiman

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Batman
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Posts: 111
From:Kane, PA 16735
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posted 27 February 2000 07:11 PM     Click Here to See the Profile for Batman   Click Here to Email Batman     Edit/Delete Message   Reply w/Quote
One should always plan the attack before going into the field. If one is charged with improving a process, one would plan the attack, which would include the "after" sampling, along with the comparison to the "before" stuff.

That said, in this instance, based on the description of the product there are few parts made per day, them how about just adding the new data onto the old in a run chart of some sort. I assume that there was historical data that suggested the need for improvement, so take some "after" parts (seperate enough from the "before" parts - maybe a "purge" of the process) and add the sample data to a run chart of the prior data - Average and moving range, for example. If the large size is an indication of being able to run only a few per hour or day, then I think only a few could be necessary. The improvement (or lack thereof) should be obvious. IF there is an improvement, continue to run more parts, enough to see expected variation over some time, then calculate capability.

I think the first step is to determine if the change was a real improvement.

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