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Critical Characteristics, SPC Improvement Initiative |
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2007 |
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Any feature on a Gentex product specification drawing deemed critical by the Design Engineer was identified with a symbol indicating a statistical process control requirement. The specific requirement was for every such feature to be measured on a minimum of thirty pieces and exhibit a Cpk of at least 1.67 on an initial capability study. |
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Between the years 2004 to 2007: the average number of points to be measured on each print increased 130%; the number of critical characteristics (CC) increased 800%; and the number of critical characteristic data points increased by more than 1,100%. |
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The increase in data points could be attributed to two factors: an increase in the complexity of parts being designed and, more significantly, an increase in the number of profile tolerances designated as critical. |
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A profile tolerance denoted as a CC would require a 30-piece capability study for each of the data points taken. For example, if a profile dimension required 18 data points per part to be measured, a capability study would require a total of 540 data points to be collected for a single print requirement. |
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At a supplier-estimated cost of $1.50 for each data point, Gentex was spending an additional $4,000 on every drawing layout and subsequent PPAP in the year 2007 vs. 2004. |
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2008 |
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For 2008, I introduced two new critical characteristic symbols: CC-A and CC-V |
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A feature designated by the CC-A symbol requires the supplier to verify the feature�s compliance to the print by measuring three initial parts and to use an attribute gauge on an ongoing basis. |
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A feature designated by the CC-V symbol requires the supplier to verify the feature�s compliance to the print by measuring three initial parts, use a variable gauge on an ongoing basis, and meet an initial capability of at least 1.33 Cpk. |
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The original CC symbol is still in use, with the same meaning. However, the new tools allow the Design Engineer more specificity in how a critical feature is treated. This flexibility also saves thousands of dollars in measurement time, reporting, and print updates. |
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