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  Statistical Techniques and 6 Sigma
  Debalancing machine - is SPC needed?

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Author Topic:   Debalancing machine - is SPC needed?
VojkoKrizman
Lurker (<10 Posts)

Posts: 2
From:Nova Gorica, SLOVENIA
Registered: Nov 2000

posted 17 November 2000 05:11 AM     Click Here to See the Profile for VojkoKrizman   Click Here to Email VojkoKrizman     Edit/Delete Message   Reply w/Quote
Debalancing machine is consisted from two parts:1 measurement instrument and debalancing part. ALL parts (rotors for DC motors) are measured (100% inspection). If balance is more than prespecified target (eg. > 40 gmm) then parts go through balancing operation that lowers the debalance below target (measured again). Parts, that go to the next operation, than comes from two distributions (supposed nonnormal):1. good parts from initial measurement and 2. balanced parts. How to measure the Ppk (I understand that Cpk can not be computed). Is it necessary to compute Ppk, since it seems all parts are good but parts from the first distribution can be near the target (but below!)

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Vojko Krizman
SLOVENIA

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Laura M
Forum Contributor

Posts: 299
From:Rochester, NY US
Registered: Aug 1999

posted 17 November 2000 04:51 PM     Click Here to See the Profile for Laura M   Click Here to Email Laura M     Edit/Delete Message   Reply w/Quote
My initial thoughts are you have 2 separate calculations to do, because you essentially have 2 different populations. You would want to know how capable the rework process is.

Also wanted to mention that my Great grandparents were born in Slovenia!

Laura

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VojkoKrizman
Lurker (<10 Posts)

Posts: 2
From:Nova Gorica, SLOVENIA
Registered: Nov 2000

posted 20 November 2000 04:32 AM     Click Here to See the Profile for VojkoKrizman   Click Here to Email VojkoKrizman     Edit/Delete Message   Reply w/Quote
Debalancing operation cannot be omitted.The result of this operation are parts that come from two distributions and we have 100% inspection. My main question is: If we have 100% inspection and all parts are good, do we have to compute Ppk, especially if all parts are good (but from distribution it comes out we have some fraction of nonconform parts!?)

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Laura M
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Posts: 299
From:Rochester, NY US
Registered: Aug 1999

posted 22 November 2000 09:51 AM     Click Here to See the Profile for Laura M   Click Here to Email Laura M     Edit/Delete Message   Reply w/Quote
100% inspection is often the alternative if you do not meet process capability requirements. If you have a bimodal distribution due to 2 processes rolled in to one data set, then it is possible that each process is separatly capable, but when you combine them the standard deviation increases and the capabiiity index would indicate parts out of spec. Without knowing more about the prcocess, that's probably as much as I should say so I don't mislead you.

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Rick Goodson
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Posts: 102
From:Wuakesha, Wisconsin, USA
Registered: Aug 2000

posted 22 November 2000 10:58 AM     Click Here to See the Profile for Rick Goodson   Click Here to Email Rick Goodson     Edit/Delete Message   Reply w/Quote
I agree with the advise Laura has given but wanted to add a thought. If the two separate processes are capable, why combine them and then and force yourself into a 100% inspection operation that may not be necessary? If you run both populations across the same secondary operation you could consider a 'short run' SPC technique such as a 'Nominal' or a 'Target' chart. Once again, as Laura stated, I would need to know more about the process before we could definitively say short run is the best option. If you want to investigate short run further try the International Quality Institute, http://www.i-q-i.com/index.htm or Dr Don Wheeler's website http://www.spcpress.com/

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Mark M
unregistered
posted 10 January 2001 09:49 AM           Edit/Delete Message   Reply w/Quote
Adding to the Balancing Process Characterization Issue ...

How are you going about calculating the process capability for a polar oriented process (imbalance amount and angle) when the usual capability calculation assume a linear measurement? Of course, the polar coordinates can be converted, but you still have two dimensions instead of one.

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