this post was submitted on 26 Aug 2023
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As someone who knows almost nothing about the topic, wouldn't some (most?) of these parts be big enough that a small change in temperature or air pressure alone would cause these parts to expand/shrink enough to go over the tolerance limit?
Yes, and different materials will have different rates of thermal expansion. That's probably why the pixel 7 camera glass was cracking for no apparent reason when winter hit. Imagine coming out in the morning and finding all the glass in your car shattered because it got cold overnight. Or even worse you take it out of a heated garage on a cold day and the glass shatters while you're driving.
Thermal expansion of steel is .0000072" per degree F. All you would need is a 100 degree F temp change to blow that tolerance out of the water. And 100 degrees is not that much when it comes to cars. A freezing cold day versus a boiling hot day in the summer is a temperature swing more than 100 degrees. A ICE engine runs at roughly 250 degrees F so that right there would easily expand parts way more than the tolerance he is calling for. On an EV, I don't think anything gets that hot, but the motors still get warm.
Is that per inch or per foot?
That's inches per degree F.
But one unch of material doesn't dilate as much as one foot of the same material. I guess that's what they mean when asking "per inch per foot?"
We compensate for thermal expansion. The standard temperature things are measured at is 68F/20C. So if it's 72 degrees we'll compensate it back to 68 in software for the material we're measuring. We use scale bars of known length and similar material type to verify scale. (I run laser trackers and laser radars.)
For measurement equipment that's stationary, like CMMs, you just control the environment.