Page 49 - MetalForming-Aug-2018-issue

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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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