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Process Capability

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  1. Process Capability Operations Management Dr. Ron Tibben-Lembke

  2. Process Capability • First question: is our process capable of producing with as low a level of defects as our customers require? • The reality is not all parts are the same. Some are bigger, some are smaller. • We describe that variation with the concepts of average , and standard deviation, .

  3. Process Capability • Voice of the Process • Upper, Lower Natural Limits: • UNL / LNL = mean +/- 3  • Voice of the Customer • USL -- Upper Specification Limit • LSL -- Lower Specification Limit • completely unrelated to natural variation. • Design tolerance: specified size +/- 0.005 inches • No connection between tolerance and 

  4. Process Capability Process Capability -- When the process is under control, is it likely that all produced parts will fall within the design specifications? If the answer is yes, the process is “capable.” If no, even when we are in control, we will be making unacceptable parts. This is a problem.

  5. LTL LTL UTL UTL LTL UTL Process Capability Capable Not Capable LTL UTL

  6. Process Capability • Specs: 1.5 +/- 0.005 • Mean: 1.490 Std. Dev. = 0.002 • Are we in trouble?

  7. Process Capability • Summarize process potential to meet acceptable tolerances (USL-LSL) • Tells how well parts produced fit into specs • Cp = 1.0 means process will generate virtually all of its output within specification limits

  8. Process Capability • Specs: 1.5 +/- 0.005 • USL = 1.505 • LSL = 1.495 • USL – LSL = 0.01 • Mean: 1.490 Std. Dev. = 0.002 • UNL = 1.496 • LNL = 1.482 • UNL – LNL = 0.012 • Cp = 0.01/0.012 = 0.833 • We’re in trouble- too much variability

  9. Capability Index • Capability Index (Cpk) will tell the position of the control limits relative to the two-sided design specifications. • Cpk>= 1.0, process is capable • Cpk< 1.0, process is not capable

  10. Process Capability • Specs: 1.5 +/- 0.005 • Mean: 1.490 Std. Dev. = 0.002 • LCL = 1.49 - 3*0.002 = 1.484 • UCL = 1.49 + 0.006 = 1.496 Process Specs 1.484 1.495 1.496 1.505

  11. Process Capability • Tells how well parts produced fit into specs • For our example: • Cpk= Min[ -0.833, 2.5] We’re really in trouble.

  12. Process Capability • Tells how well parts produced fit into specs • Cpk= Min[ -0.833, 2.5] We’re in big trouble.

  13. Final Thought IBM Canada Ltd. ordered some parts from a new supplier in Japan. The acceptable quality level allowed for 1.5%defects. The Japanese firm sent the order with a few parts packaged separately, & the following letter ... © 1995 Corel Corp.

  14. Final Thought Dear IBM: We don’t know why you want 1.5% defective parts, but for your convenience we have packaged them separately. Sincerely, © 1995 Corel Corp.