Question about a basic additive variance/tolerance example

stm08007

Starting to get Involved
#1
I'm preparing for the CQE, and I'm reviewing the CQE primer.. this seems fairly basic to me but I do not understand the last sentence about the affect on internal scrap.

I understand that the total expected variance of a final assembly is much less than simply adding the individual variances of the subcomponents. It then says to consider this affect on the final product and the potential for unnecessary internal scrap. The effect on final product is a process that is closer to the nominal value.. but where's the scrap comment coming from? Is it saying that setting the spec limits to the lesser value (based on the theoretical 4 sigma of the assembly) is unnecessarily tight and simply adding them is still appropriate? That doesn't seem to make sense but I'm not seeing what else it could mean
 

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John Predmore

Quite Involved in Discussions
#2
The unnecessary internal scrap is the parts you throw away thinking they are unusable, but the tolerance you calculated (incorrectly) is not correct.
The key word is assignment of tolerances in the title and first sentence. There are two elements to the problem, the variation, which is a natural result of the manufacturing process, and the tolerance, which is a decision made by the engineer.

If you calculate (incorrectly) total variation of the assembly is 30+/-0.014, you may decide to put tighter tolerances on one or more parts to reduce variation in components selected for the assembly. If the tolerance on a component is tighter than good engineering practice would specify, there is a good chance of scrapping what would have been usable component parts.

The correctly calculated variation is +/-0.0082. That is 41% smaller result might meet the engineer's requirements for the assembly, and tighter tolerance on any of the components may be unnecessary, avoiding unnecessary scrap.
 
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stm08007

Starting to get Involved
#3
The unnecessary internal scrap is the parts you throw away thinking they are unusable, but the tolerance you calculated (incorrectly) is not correct.
The key word is assignment of tolerances in the title and first sentence. There are two elements to the problem, the variation, which is a natural result of the manufacturing process, and the tolerance, which is a decision made by the engineer.

If you calculate (incorrectly) total variation of the assembly is 30+/-0.014, you may decide to put tighter tolerances on one or more parts to reduce variation in components selected for the assembly. If the tolerance on a component is tighter than good engineering practice would specify, there is a good chance of scrapping what would have been usable component parts.

The correctly calculated variation is +/-0.0082. That 41% smaller result might meet the engineer's requirements for the assembly, and tighter tolerance on any of the components may be unnecessary, avoiding unnecessary scrap.
Ah gotcha, that makes sense--- I was thinking it was talking about unnecessary scrap of final assemblies (rather than components)... If you incorrectly calculated the variance of the assembly (as the example did), it may result in an even wider tolerance (which wouldnt result in unnecessary scrap, quite the opposite). What you're saying makes sense, thanks!
 
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