Page 49 - MetalForming-Aug-2018-issue

The Science of Forming
Property Distribution—Three Suppliers
0.60 0.50 0.40 0.30 0.20 0.10 0.00
160 180
200
220 240 Strength (MPa)
260 280
Lower Specification Limit Upper Specification Limit
Mill 1: Capability 170-270 MPa, average 220 MPa Mill 2: Capability 175-255 MPa, average 215 MPa Mill 3: Capability 200-260 MPa, average 230 MPa
range. With this reduction in incoming variability, the stamping plant may bet- ter control the parts it produces.
A mathematical term, standard deviation, describes this variability. This is represented by the lowercase Greek letter sigma, or 􏰀. Knowing the standard deviation lets you estimate the likelihood that any one test result will fall within tolerance, and its prox- imity to the average. Two-thirds of all test results will fall within one standard deviation of the average (Fig. 3), mean- ing that one-third of all test results will fall outside of this range. However, more than 95 percent of all results will fall within two standard deviations of the average value (average ±2􏰀 ). A range of ±6􏰀 (six standard deviations) encompasses 99.73 percent of all results. This corresponds to a Process Capability Index, or Cpk, of 1.0. A Cpk
Fig. 2
of 1.33 is associated with readings within ±4􏰀 of average (63 out-of-com-
pliance results for every one-million tests), while a Cpk of 1.67 corresponds
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