I am currently designing a mechanical tensile testing setup to operate at different temperatures (-20 to 100 C) in a wet environment. I plan on machining two sets, one from stainless and one from PMMA. Of course, this setup will have a clamp setup similar to the below picture. enter image description here

When I tolerance the drawing prior to machining, I would probably want to tolerance both of the threaded shafts and all four of the smooth bolts for a sliding fit. How can I (or should I even try to) assess the need to consider the various temperatures when tolerancing these parts?

  • $\begingroup$ If all the parts are made of the same material, then they will change by the same relative amount per degree change in temperature... It's when you have different materials with a close fit.... $\endgroup$
    – Solar Mike
    Nov 8 '18 at 16:18
  • $\begingroup$ Any engineering material specification should include its thermal expansion coefficient. If your parts are different materials or at different temperatures, work back to the dimensions and tolerances they should have while they are being manufactured (presumably, at room temperature). You may also need to consider if the device can be assembled at room temperature, with those tolerances $\endgroup$
    – alephzero
    Nov 8 '18 at 17:21
  • $\begingroup$ Perhaps you can modify your design to not be effected by the change so much? Looks like its pretty much just a clamp to hold your specimen as you try and pull the specimen apart? If so, how tight do your tolerances need to be for the clamp to work? I think you can put the appropriate amount of clearance in the thread engagement of the screw or even use a more loose fitting acme thread. Same with the guide rods, if their purpose is to only prevent the jaws from spinning, sufficient clearance can be put in the rod/bores so they move freely in each temperature case. $\endgroup$
    – Corey
    Nov 9 '18 at 13:14
  • $\begingroup$ Of course, any shakiness is highly undesireable, so I worry that if I tolerance the screw/threaded hole that moves the grippers back and forth, the grippers will either be loose or too tight depending on temperature. This is my real concern. $\endgroup$
    – User2341
    Nov 9 '18 at 22:28

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