Conflicting dimensions - Cable and wire industry and application of SPC

I work in the cable and wire industry and application of SPC seems to be somewhat hard. The automotive specs almost allways include requirements on minimum and average wall thickness plus minimum and maximum diameter of the insulation. The specs are not written by engineers taking into regard the fantastic possibility of SPC and Cpk's above 1.33 for all dimensions. According to my experience I have not yet seen a spec where these four dimensional requirements have not been conflicting with eachother, i.e when diameter can be held at Cpk > 1,33 either min or av wall thickness will be < 1.33. To satisfy the customer and mostly the assesor we have choosen diameter to be a characteristic to study. I would like to know if anyone else have any good solutions on this problem? Please don't tell me to discuss it with the responsible engineer writing the specs cause they just won't listen.

Isaberg

[Don Winton]

<FONT COLOR="BLUE"><BLOCKQUOTE>To satisfy the customer and mostly the assessor we have chosen diameter to be a characteristic to study.</BLOCKQUOTE></FONT></P>

As long as the customer is satisfied, you may be OK.

<FONT COLOR="BLUE"><BLOCKQUOTE>I would like to know if anyone else have any good solutions on this problem?</BLOCKQUOTE></FONT></P>

I do not personally, but I may suggest something. Typically, you would choose, from among the available variables, key input variable(s). For example, if you identify wire diameter as key and wall thickness as secondary to the key variable, identify diameter as Cpk >1.33 while wall thickness as Cpk >1.00 (or whatever). The inverse could also be true.

Typically, when conflicting dimensions exist, it is better to grade the requirements against criticality.

Regards,
Don

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Just the ramblings of an Old Wizard Warrior.