Aluminum arc thermal expansion

[Craighell]

This may be easy for some of you, but I have a question regarding a peice made from aluminum and what happens as a result of thermal expansion. The part is machined from stock and is approx 6" X 2.5" x40" It is also cut in an arc. It is the radius of a 40" circle.

If specified to be machined to tolerances at 68 deg. F what will happen as the temp rises and falls? I know a circle will grow with the same coefficient of linear expansion as the material it is punched through, but what happens to a semi-circle? Can it be expected to hold shape? Does it wander? I know if I heat and cool a piston NOT placed inside a bore it will no longer be round when I am done, so I wonder does a "Banana" of aluminum expand uniformly or not?

I'm curious if I can expect my CMM to measure correct inner and outer radius with thermal compensation activated or does that go out the window since the part is not a complete circle?

 

[redpicker]

Unless there are external forces acting on the part (or internal residual stresses), the expansion will be uniform provided the temperature is uniform and there are no phase transformations involved.

I am not sure why you think a piston that is heated and cooled NOT placed inside a bore will not be round when you are done. Provided the heating and cooling is uniform, so will any expansion/contraction.

rp

 

[Craighell]

Thanks for the answer. That helps.

The piston thing? I learned that at a much younger age. Heated a piston in the same oven as my freshly painted cylinder for my dirt bike. I wanted to see how much it actually grew when heated to 550 deg F. I found out. It also wasn't round when cooled back to room temp. I had to by a fresh piston with paper route money.

 

[Compositepro]

Aluminum alloy heat treatment will be altered at temperatures above 350F. Grain size in the metal will grow.

 

[TVP]

I agree with redpicker, the semi-circular/banana shaped piece should expand when heated and contract when cooled, provided that the temperature change is uniform. There may be some amount of non-uniform distortion if there are residual stresses that have not been relieved during the manufacturing of the piece.