Process Capability Operations Management
Dr. Ron Tibben-Lembke
Process Capability Operations Management
Dr. Ron Tibben-Lembke
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, .
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
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.
Specs: 1.5 +/- 0.005
Mean: 1.490 Std. Dev. = 0.002
Are we in trouble?
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
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
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