What is Cc? Capability Study on Key Critical Dimensions

S

stevoc

Hi

Can anyone help please - I've been preparing some specification sheets for manufacture of parts for a medical device. On this are listed the requirements for in-process and outgoing controls and includes a capability study on key critical dimensions (Cpk>1.33). The client has reviewed these and asked that we also add a requirement that Cc<0.25.

Problem is that neither I or anyone else here knows what Cc is. I can find no mention in the Six Sigma Handbook and google hasn't returned anything.

many thanks

Steve

Bev D

Heretical Statistician
Staff member
Super Moderator
have you aked your client? perhaps it's something special to them?

D

Darius

Thanks Atul, so CC >> like Taguchi's loss function; to the target, the less variation of the mean against the target the better.

S

stevoc

Thanks for that - I'd managed to find that reference as well. I also found this definition:

"Cc is a capability index comparing the process average with the target and specification limits to assess the centring of the process. A value of 0.25 means the average is in the middle quarter of the spec. A value of 0.50 means the average is in the middle half of the spec. Unlike Cp and Cpk, smaller values of Cc are better. Ideally a Cc of 0.25 should be obtained. This in combination with a Cp of 2 results in Six Sigma Quality.
Cc can only be calculated in the event of two specification limits and target. Cc does not consider the process variation. Variation is evaluated using Cp and Cpk."

Also, the formula shown on that web page is incorrect - the denominator for the second half should be USL-target, not USL only.

There's also another formula:

Cc = 1.0 - (Cpk/Cp)

Having looked at our IPC data, I'm slightly alarmed that whereas before we achieved our quality targets (Cpk >= 1.33) by some way (generally Cpk is > 3.0) we now fail on the Cc requirement (Cc < 0.25)

A

Atul Khandekar

so CC >> like Taguchi's loss function; to the target, the less variation of the mean against the target the better.
I was wondering if this is meant to be used for processes with asymmetric tolerances: Target not at the midpoint of specs ??

D

Darius

Just playing with the equations...
Ideally a Cc of 0.25 should be obtained.
, agree that the manual say so, but it's "Ideally Cc < 0.25" (sort of sigma shift). As I say before to the target, the closer to Zero the best.
I was wondering if this is meant to be used for processes with asymmetric tolerances: Target not at the midpoint of specs ??
It should work, but insteed of the mean, it should use Median. The question is, what happen when the Target is the spec limit? (the equation goes to infinitum = 1/0)

bobdoering

Trusted Information Resource
One caveat - customers place capability requirements on print without knowing the underlying process. Cpk - and sometimes Cp - are not always applicable. If not, how you calculate this may be affected. Beware of the rubber stamp!

S

stevoc

We've been developing a series of medical devices for a pharma client. Until recently they've been unable to supply any quality related information and have said it's for us to determine - so we've been applying Cpk >= 1.33 to all critical to qaulity dimensions (in line with similar devices). We've actually been achieving 6 sigma in most instances (Cpk > 1.5, Cp > 2.0).

Now they've brought in quality people to fill the hole in their organisation, who have specified the Cc < 0.25 requirement. They can't give us any justification for it's use and it seems to be based on what they used in their previous employment.

The end result is that over 50% of our controlling dimensions now fail this requirement and removing/modifying from the specification is not proving easy!

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