If a characteristic of a part is being inspected but only the minimum and maximum measured value is being recorded is there a way to calculate CP & CPK against the spec limits?

If a characteristic of a part is being inspected but only the minimum and maximum measured value is being recorded is there a way to calculate CP & CPK against the spec limits?


Welcome to The Cove Forums! :bigwave: :bigwave:

Can you tell us a little more about your issue? What are you inspecting? With what? What process? If you are willing to provide us with some more specifics, that would help our Fellow Covers respond in an appropriate and timely manner. We enjoy puzzles, but not too many..;)

Stijloor.

It depends. Are the Min/Max from the same part or from a subgroup?

If from a subgroup, and you are sure that the process is in control, you can calculate Cp from the Ranges. This is exactly what you would do anyway, but you could normally confirm state of control.

Cpk is a little trickier, because you have to make an additional assumption on normality and average the Min/Max.

I would consider these as estimates only, but they would give you a "rough" idea of capability until you can collect proper data that would allow you to verify the assumptions.

Some time ago I recorded a pump max. pressure as a variable because it was important not to be to high, both max or min can be different variables and calculate Capability index individually if is important to keep track of the control in your process.

Here is some additional detail. The parts in question are raw castings. The inspector inspects a wall thickness using ultrasonic (UTT). He will inspect the entire surface to make sure the thickness falls between the BP limits, he then notes the minimum and maximum measured value. The data I am looking at is historical so there is no way to go back and collect the data in a different manner.

** The min/max value recorded is from a single casting. I have data from 20 to 30 pieces**

I would performed process capability analysis on min & max seperately and treat them as seperate features.