Question:
My question is regarding a threading process. There is 100% inspection for go/no go check and about 5% rejection/rework. The batch size is 5,000 nos and is completed in 3 days of production. Two such batches are produced in a month.
What type of control chart should be used to monitor the process? How should the process capability be calculated in this case?
Answer:
The type of control chart first depends on what type of data you are measuring. If you are doing go/no go then you are limited to a “P” chart or a “C” chart. A “P” chart looks at % good (or bad). A “C” chart looks at the number of defects found.
If you are measuring thickness or strength, (something that can be measured), then you can use a X-bar/R chart or an X-bar/S chart depending on many samples are taken.
That is the simple answer; part of this depends on how you are taking samples and how often. If samples are taken at the start and the finish, then I would probably recommend the “P” chart.
If you can measure throughout the manufacturing process, and you look at the type of defects, then I recommend a “C” chart.
Ideally, if you can get measurement data, you are better off with the X-bar/R or the X-bar/S charts. These tend to be better predictors and it is easier to calculate capability.
With the capability for the go/no go data, you can get % defective, (or % good) and multiply that by 1,000,000 to get your capability estimate in defects per million.
Jim Bossert
SVP Process Design Manger, Process Optimization
Bank of America
ASQ Fellow, CQE, CQA, CMQ/OE, CSSBB, CSSMBB
Fort Worth, TX
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