Cpk of profile tolerance?
Cpk of profile tolerance?
(OP)
I have a cast part that is described with only basic dimensions. There is a profile of a surface (all over) call out on this part.
Some at my work have a desire to require that some of the basic dimensions (size features) used to describe the true profile be critical characteristics. Traditionally, at my work, a critical characteristic requires capability to be shown.
I believe that what these individuals are actually after is some kind of way to track tooling degradation in certain areas of the die/mold and/or give the operator, making the part, a quick caliper check to ensure that the parts are good without having to have a scanner or CMM at their work station.
I have relayed the information that just because the size is within the profile tolerance does not mean that this area of the part is fully complying with the profile tolerance (location & orientation may still be out). They understand this but still want some measurement (less than the entire part) to be made and to have this data used to evaluate the part/process.
1) Has anyone ever required something like this?
2) Does this qualify as “NONMANDATORY (MFG DATA)” (section 1.4 - f)?
3) If not then what would/should this requirement be called? Additional Size Requirement? Supplementary Size Requirement? Tooling check? Etc.
4) How do you show capability of a profile call out? How do you show capability of a small portion of a profile call out?
Some at my work have a desire to require that some of the basic dimensions (size features) used to describe the true profile be critical characteristics. Traditionally, at my work, a critical characteristic requires capability to be shown.
I believe that what these individuals are actually after is some kind of way to track tooling degradation in certain areas of the die/mold and/or give the operator, making the part, a quick caliper check to ensure that the parts are good without having to have a scanner or CMM at their work station.
I have relayed the information that just because the size is within the profile tolerance does not mean that this area of the part is fully complying with the profile tolerance (location & orientation may still be out). They understand this but still want some measurement (less than the entire part) to be made and to have this data used to evaluate the part/process.
1) Has anyone ever required something like this?
2) Does this qualify as “NONMANDATORY (MFG DATA)” (section 1.4 - f)?
3) If not then what would/should this requirement be called? Additional Size Requirement? Supplementary Size Requirement? Tooling check? Etc.
4) How do you show capability of a profile call out? How do you show capability of a small portion of a profile call out?





RE: Cpk of profile tolerance?
If you are the customer/designer, you care that the part meets your tolerances, in this case, an all-over profile tolerance. Otherwise, you don't care what the fabricator does, short of sacrificing virgins. I see no reason why manufacturing should not take measurements to monitor their process. There may be a dimensional drift that tells manufacturing when to repair or replace tools, even though all the parts meet your specification. Since this is a manufacturing process, I see no reason for it to be on your specification/fabrication drawing.
Would a simple fixture allow them to monitor the process by measuring from datums?
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JHG
RE: Cpk of profile tolerance?
We could have a "simple" fixture to verify the parts but it would only provide attribute data (good part or bad part) so we could not calculate Cpk.
Our part has no datums. The all over surface profile call out is independent ("best fit" approach). When I said "just because the size is within the profile tolerance does not mean that this area of the part is fully complying with the profile tolerance (location & orientation may still be out)." I meant the location and orientation of that region of the part could be out with respect to the rest of the true profile.
RE: Cpk of profile tolerance?
2) I don't think so.
3) That's up to your company.
4) The first part of your question: If your process is in control and the distribution is normal then just run it on any Capability calculator. Non-normal distributions may need transforming unless there's historical data to show the distribution is going to normal. The second part of this question: DEtermine which part of the profile you need to analyze then just gather data from there.
John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
RE: Cpk of profile tolerance?
A BASIC dimension does not have a capability requirement. Capability only applies to a specification. In your case, it is the overall profile. Well, do what you say and say what you do. If you have an overall profile specification, then you measure it and report it properly. If you only want to measure partial specifications, then you have to establish multiple specifications so that you can measure it accordingly.
Best regards,
Alex
RE: Cpk of profile tolerance?
I am a hard-liner. I would say that there is a single requirement for your part. Single. If you want to study the statistical behavior of meeting that requirement, I would argue that you must determine the narrowest best fit all-over profile zone that would contain the surfaces of the part. That may or may not provide some useful data. It would be interesting to see the numbers, but I'd fear they'd be awfully muddy, depending on the complexity of the part. Ball bearing, sure. Complex casting, no.
For a complex part, you could map those numbers onto the part and have a sort of hot/cold color model for variation. I have absolutely no idea how to do this.
My opinion? Leave the geometric tolerance alone to define the part. Don't try to add some weird additional requirement scheme just for the purposes of gathering data. Don't pollute the tolerancing to achieve goals unrelated to part definition. Then, in a separate document, outline what you want to measure and study, the how, the why.
RE: Cpk of profile tolerance?
I toy-ed with the idea of proposing a "critical region" instead of applying a CC to a basic dimension. This would ask you to focus more measurement effort in this region (and we can argue about how to demonstrate capability later). However, this does not address the other aspect that my co-workers wanted, which was a "quick" way to check parts. But putting this type of info. on a separate document might help this aspect.
RE: Cpk of profile tolerance?
John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
RE: Cpk of profile tolerance?
I questioned if this was a good/bad idea and how one could (easily) show capability of a binary (good vs. bad) measurement.
RE: Cpk of profile tolerance?
2) There is no general requirement of capability for a TS 16949 (assuming Automotive) Controlled Characteristic. A specific company may have such a requirement but TS 16949 does not. It's a terrible standard because the definition of Controlled Characteristic is left up to each and every customer. We use 3 Controlled Characteristic symbols, only one requires SPC.
3) Manufacturing is free to put what ever they want on process documentation as long as they include the product definition requirements. QA can put extra measurements on their control plans if they see fit. Maintenance can put out preventative maintenance requirements to keep the process humming. None of these belong on the product drawing.
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