StatisticalProcessControl.pptVIP

Process Capability: The Control Chart Method for Variables Data Construct the control chart and remove all special causes. NOTE: special causes are “special” only in that they come and go, not because their impact is either “good” or “bad”. Estimate the standard deviation. The approach used depends on whether a R or S chart is used to monitor process variability. ^ _ ^ _ ? = R / d2 ? = S / c4 Several capability indices are provided on the following slide. Process Capability Indices: Variables Data ^ ^ CP = (engineering tolerance)/6? = (USL – LSL) / 6? This index is generally used to evaluate machine capability. tolerance to the engineering requirements. Assuming that the process is (approximately) normally distributed and that the process average is centered between the specifications, an index value of “1” is considered to represent a “minimally capable” process. HOWEVER … allowing for a drift, a minimum value of 1.33 is ordinarily sought … bigger is better. A true “Six Sigma” process that allows for a 1.5 ? shift will have Cp = 2. Process Capability Indices: Variables Data ^ ^ CR = 100*6? / (Engineering Tolerance) = 100* 6? /(USL –LSL) This is called the “capability ration”. Effectively this is the reciprocal of Cp so that a value of less than 75% is generally needed and a Six Sigma process (with a 1.5? shift) will lead to a CR of 50%. Process Capability Indices: Variables Data ^ ^ CM = (engineering tolerance)/8? = (USL – LSL) / 8? This index is generally used to evaluate machine capability. Note … this is only MACHINE capability and NOT the capability of the full process. Given that there will be additional sources of variation (tooling, fixtures, materials, etc.) CM uses an 8? spread, rather than 6?. For a machine to be used on a Six Sigma process, a 10? spread would be used. Process Capability Indices: Variab