Can Inspection Frequency be based on Cp or Cpk?

C

csterling

#11
What would you recommend for attribute data? In my case the Inspector not only sample for SPC but also for attributes. Does the 3 to 5 samples apply?
Does the lot or bath size being produced matters?

What would be a good justification for using 3 to 5 samples (in-process inspection) for variable and attribute data idependently of the frequency and lot size?

Thanks in advanced.
 
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bobdoering

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#12
I am in the metalworking industry (Milling, Turning, Drilling, Grinding) and i decided to do this, based on common sense:

CPK < 1.33 => Use 100% In-process inspection AND reduce process variability (CPK and PPK) !
CPK > 1.33 AND PPK < 1.33 => Increase In-process inspection frequency (Decreasing time from subgroup to subgroup) in orden to early detect subgroup process variation (PPK) (shifts, drifts or trends) and make adjustments (in order to increase PPK) and have a better long term process performance. Continue reducing short term process variation in order to have a better CPK.
CPK >> 1.33 AND PPK >> 1.33 => I am allowed to decrease In-process inspection frequency (increasing time from subgroup to subgroup).

Optimal In-process sampling frequency if i have:
CPK >> 1.33 AND PPK >= 1.33
In this case if i increase in-process inspection frequency (decreasing time from subgroup to subgroup) i am only increasing costs, not quality.
First, it is important to not that if you are calculating Cpk in precision machining for lengths and diameters, you are most likely not properly controlling your process. Second, attempting to calculate Cpk based on a sample from a properly controlled metalworking process is nearly impossible due to severe effects of sampling error. One needs to control the process correctly, and use the rule of thumb 5 to 7 checks per adjustment driven by the charting methodology, and the problem is solved - for precision machining.

One important clue: If you are precision machining, and you have a normal distribution, your controls are wrong and you are overadjusting.

For information on the correct way to control precision machining see:
Statistical process control for precision machining
 
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