Cpk's for five different dimensions on castings

[abhattacharya]

Cpk for casting dimensions

We are manufacturing ductile iron castings and supplying to the automotive sector in India. We are in the process of implementing QS9000 systems.
In one of the castings, the customer has given us more than five critical dimensions for which we have to give the Cpk values with every lot. We find that getting CpK values > 1 for each critical dimension simultaneously is quite difficult for founding operations (We are using green sand process for manufacturing castings).

Are we to reject the castings even because one out of the five dimensions has CpK lower than one? What's the way out?

 

[Marc]

I'm bumping this up on the listing. I'd love to hear from someone who has an ideas on this situation.

 

[proudliberal]

This is something that should have been negotiated prior to production. If your "critical" dimensions are really critical to function, you will probably be required by your customer to implement some system to prevent non-conforming product from reaching his dock.

Additionally, I would get an agreement from him to eliminate the Cpk requirement on the dimension based on your action plan. This is assuming that your process cannot meet the requirement. If you can demonstrate that the variables in your process (temperature, cooling time, etc)are controllable and controlled (via Cpk), he might be more amiable to a alternate plan.

Remember, the intent of Cpk is to assure the shipment of conforming product only. Focus your and his attention on that.

 

[Marc]

So - you're saying that what is possible with consideration to 'state-of-the-art', materials, tolerances (combined), should be known and considered prior to acceptance of contract. I have seen this in the injection molding industry. The materials were such that there was no way to meet the requirement tolerances. This was not 'discovered' until run-off, of course. Now we need an engineering change.....