Hi!

I need some good definitions for "process capacity" and "overadjustment".

Thank you in advance.

FG

Felix

I have not heard the term 'process capacity', do you mean 'process capability' ?

'Overadjustment' could be defined as adjusting a process to correct an 'out of specification' reading such that the process is then 'out of specification' at the other end of the specification range. i.e. if I am turning down a diameter on a shaft, and I monitor my process my measuring the shaft od I might measure 9.5mm against a spec of 10mm +/- 0.1mm and therefore adjust my machine offsets such that the next piece produced is 10.5mm - again out of spec.

This can occur more subtely where a decision is taken to adjust a process based on a process reading that is actually indicative of the capability of the process, and doesnt actually require the process to be adjusted. Correct use of run charts should eliminate such problems.

M Greenaway,

Thanks for your reply. You are right, I mean 'process capability'.
¿Any definition?...

Felix

Process Capability: Definition and more:
http://www.symphonytech.com/articles/processcapability.htm

Jim

Yes, this statement is so wooly I am sure it will be poorly implemented. I think it should be a statistical measure of process capability - this is what I believe is key to the 'process approach' loosely defined in ISO9001, i.e. not drawing pretty pictures of process models, but understanding process performance, variation and causes of variation, and utilising statistical methods for this.

Just my opinion though......

When in 8..2.3, ISO 9001 talks about demonstrating "the ability of the processes to achieve planned results" is it talking, do you think, of process capability as commonly understood?

Living in the shadow of the B3, this has always seemed to mean CpK and such. However, not all processes are suitable for this type of study. TS, goes as far as calling them “process studies” (8.2.3.1) which supports CpKs. On an ISO system, as you say Jim we need to think beyond process capability studies. The key, IMNSHO is to determine if your processes meet plans. Also, don’t just think production…consider all your processes that can affect customer satisfaction.

db

What sorts of processes would not be suitable for such a study ??

MG,
Processes that require only visual inspection as a means of acceptance.

Sam

Attribute data can equally be analysed statistically.

You would determine process capability by discovering how often your process produces visual defects.

Unless I'm not understanding the process (in which case I'm certain you guys will help me out) Process Capability Studies do not do well with onesy twosey processes. I work with companies that build single complex products. Although, there can be some sub-process tracking (such as welding), much of the product is so unique that statistical application, much less CpK is difficult at best. Organizations might also have trouble with non-clause 7 processes, such as hiring, training, sales etc. Even 8.2.3.1 limits its application to manufacturing processes.

I think felixgh's original question was probably based on manufacturing, and process capability would definitely apply. That is unless the process is generating single units.

db

Even companies that percieve that they only manufacture small batches, or even on or two offs do actually operate processes that are common to the batches/products.

It can be done.

That is why I stated sub-processes. If I have a welding operation, the operation is probably close enough on each job to gather statistical data. That is where a lot of companies got into trouble with Qs by saying 4.20 didn't apply, when in fact it did.

In many cases, however, the data gained might not be worth the effort, especially if you only weld three or four times a year. The occurences are spaced too far apart for statistical value.

Sure thing db.