Question the Status Quo

Even measurement pros can be lulled into complacency by automated tools

Let me start with a confession: I’m an imposter in the metrology community. My background is in the design engineering community—well-meaning folks who don’t quite grasp the complexities of the manufacturing process and the measurement systems that support it.

Measurement people know the effect of arbitrary decisions in the design process. We designers put 0.005-in. tolerances on a part because it’s as easy as a mouse click in CAD. Any engineer should understand that most materials expand when heated and contract when cooled. Poll design engineers and ask what temperature their CAD models were when they applied the dimensions. Many will stare at you with a blank look.

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Comments

Clear and insightful

Thanks for a well written piece elucidating design and metric issues. Another helpful tip for auditors trying to assess appropriateness of criteria for design outputs and verification.

Temperature compensation in composite assemblies

If you are making just one or two, temperature soak them to 68F/20C.

If you are making lots of them, the assembly variation in the final dimension will probably be more significant than the thermal variation...

Nice article

First of all, let me say it's a nice and interesting article, especially liking the first few paragraphs :-)

As for your question, I am no experts, but going KISS leads me to think this:

1) There is two tempratures to compensate for.

- One is of the measuring defice itself and one is for the product. A metal ruler for example will have a different scale size at 100C then it has on 20C, so the actual value needs to be calculated back to the 20C. This should go for every measurement tool

- Secondly is the workpiece itself. If like you suggested, it's a homogenious temprature and material, or a serial connected chain of materials with known tempratures, the calculation back from your measured size, known materials and tempratures, it can still be easily calculated, as the behavior is still linear.

- When like you suggest, it is a combination of materials in a complex shape, one would think it becomes more complex to measure. However, if the design is know, the calculation is still possible, because each individuel element will behave linear, but because of the combination, the deformation will not be in a linear direction, but freeform, which will required 3D compensation in the measurement data.

Only in case the design of a mix of complex shape is not known, you cannot calculate and compensate... which is logical, as you won't know the coefficent for the material in the first place.

My Conclusion => Temprature compensation for the measuring tool makes sence, as to ensure that the same absolute size is found with a measurement. The distance from paint A to point B in a space.

Where this point A and B is on the actual product, will change, due to the temprature. That compensation is mathmatical and can only be done once one knows the design of the part. If the design of the part is known however, one should be able to compensate for any and all changed, regardless of the mix of materials used.

If you disagree with any of the above, I'd love to hear it and why.