Eng. Mgt. 385 Statistical Process Control Stephen A. Raper Chapter 9 – Statistical Analysis of Process Capability and For Process Improvement

Process Capability Processes have variability – SPC seeks to identify that variability, and proceed to reduce and in some ways “control” the variability Once the above is accomplished, comparison is then made to the “design specifications” Recall, the process drives SPC whereas specifications are by intent or design How then do the two compare? What is the capability of the process?

Capability Study A capability study seeks to determine how the process (with respect to a given variable) is performing. With respect to process centering and dispersion. Natural tolerances limits are compared to specification limits. Natural process centering is compared to specification centering. Specific actions can then be taken.

Possible Courses of Action No Action: Natural tolerances well within specs, may use control chart for attributes, if needed. Action to Adjust Centering: Natural tolerance range and spec range are about the same, but may need to adjust process centering to produce almost all product within specs. Action to Reduce Variability: Not meeting specs, seek to find sources of variation, target process centering, problem solving needed. Actions to Change Specifications: Maybe specs are too tight and they need adjustment. (may be resistance to this move) Resignation to Losses: Scrap and rework will occur, inspection of product may be necessary.

Steps In The Analysis of Process Capability Establish Control over the process (SPC) Analyze Process Data by a variety of TQM tools. Analyze Sources of Variation: From simple means to highly complex means. (figure out what is going on)

Process Capability and Performance Indexes Previous looks compared 6  with the specification spread, the smaller 6  is in relation to the spread, the better. Various Indices have been developed and can be used to gage process capability C p = – Most closely related to process capability potential; any value greater than 1 has the potential for meeting specs if held in control centered between the upper and lower specs; A minimum target value of 1.33 has been suggested.

Process Capability and Performance Indexes C pL = C pU = C pk = min(C pL,C pU ) C pL and C pU relate the difference between the current estimate of process mean and the respective specification limits, L and U, to half the natural process spread. Minimum target value of 1.

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