Re: SPC Problem

[PSIGROUP@aol.com]

Manoj Kumar Sharma wrote:

>Problem: Method engineer has proposed to introduce modification 
>to change specification limit by 0.5 sigma on either side. What 
>considerations must have gone into, before proposing such a 
<change? What would be effect of this change on process capability. 
<As Quality Engineer what steps would you take to ensure that all 
<the production lots are accepted with these modified limits.
<Any solution? Please help

Response:
1.  What are the customer requirements driving the change - has the voice of 
the customer been heard?

2.  What is the current performance to standards - has the voice of the 
process been heard?  Cp - Cpk - dpm -sigma level?

3.  Which way are the specs being moved?  In or out?  What are the compelling 
customer driven reasons for doing so?  Moving them in towards one another 
will decrease the Cpk value and sigma level (appear to make worse) for a 
given process performance - widening the specs will increase (appear to "make 
better") the Cpk and sigma (and thus one of the many illegitimate reasons for 
moving the specs outward!).

4.  To make sure that all parts are within spec, I'd relentlessly pursue the 
reduction of variation around the target value AND the reduction of process 
complexity (number of steps).  

5.  What will be the likely quality and monetary impact to the organization 
and to the customer if the standards are changed?

6.  As for the process?  It could care what you do to specification . . .  
Unless changed (or acted upon), the process will always do what it is doing.
Charles C. Hannabarger
Process Solutions International
"Global Perspectives - Local Solutions"
8659 Cordial Road
El Cajon, CA
619-443-3165
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