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  Statistical Techniques and 6 Sigma
  SPC? How in the world?

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Author Topic:   SPC? How in the world?
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Posts: 19
Registered: May 99

posted 11 May 1999 01:47 PM     Click Here to See the Profile for Joe_1   Click Here to Email Joe_1     Edit/Delete Message   Reply w/Quote
O.k., first of all, we are a small business that is currently seeking QS9000 certification. We are a machine shop that offers CNC and conventional machining for all types of applications. Getting to the point, we do a lot of jobs that are "one" timers and we may never do them again. This is mostly true in our conventional machining area. On the other hand, the CNC does run production "type" jobs. We may machine 200-300 parts here and there but not on a repeatative basis. I mean the same CNC machine may run 4-5 different parts a week! I am just about frustrated with the whole SPC thing! How in the world do you apply SPC to this type of a business? I can see where the SPC methods would apply to a machine shop that made the same parts over and over, but that's simply not the case here. What can I do to ensure that we will meet the standard? How can I apply statistical methods here? Please help!

Joe W. Guy,
QS9000 Administrator

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Don Winton
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Posts: 498
From:Tullahoma, TN

posted 11 May 1999 11:40 PM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote

How in the world do you apply SPC to this type of a business?

In a nutshell, don't! Seriously, any SPC program must be value added. If the program does not add value to your operation, do not use it. If a job is a one shot wonder, do not use it. My personal opinion: Short run SPC is for decoration purposes and, IMHO, serves with minimal benefit. From your description above, we have eliminated the need for SPC on part your processes, let's look at the other part.

We may machine 200-300 parts here and there...

Most generally regard SPC as measuring and plotting 5 of every 20 widgets and plotting the data on a chart. This is a very limited view and is more correctly SQC. Now, if you are determined to use SPC on the machines, use them for the processes. A CNC machine that runs '4-5 different parts a week' has other opportunities for statistical techniques.

You could use Pareto to observe faults in the system (which dimensions are off-quality by category, monthly downtime by reason, etc.). You could use process capability to determine if the machine is capable of producing to specification. You can use process capability to determine if the rate of production is to cost. You could use Xbar-R for spindle speeds, feed rates, etc. And on and on. Or, you could simply use none.

Examine the processes to determine where SPC will add value. Collect data to determine which areas would benefit from the application of statistical techniques. If you can, get a copy of "Handbook of Statistical Methods in Manufacturing" by Richard Barrett Clements published by Prentice Hall. It gives many opportunities for statistical techniques that most would not realize. But remember, any technique selected must add value to the operation.

In regards to QS 9000, I believe the standard requires you identify need in your control plans. We have just done that. There is no need. In the control plan state:

Statistical Techniques Needed: None

Perhaps those more knowledgeable in the QS field can elaborate on this. I am using the 29JUL98 version of ISO 16949 as a reference.



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