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Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ

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Expansion of metal

I polish xray mirrors on a cast iron wheel of 1.2 meters and my question is by how much does this metal expand in increasing temperature in 1 degree increments.
The metal is 20mm thick and the polishing temperture is between 18 to 22 degrees.
I have found that the wheel changes from as much as 20 micron convex to 20 micron concave if the temp suddenly changes from say 18 to 25 degrees.

Gordon Shiel
optical engineering - UK
2005

You are discovering that temperature control is vital for precision machining. I don't know what material your mirrors are made of, but almost all materials exhibit expansion that is proportional to temperature change. The number you are looking for is called "the thermal coefficient of linear expansion"

The coefficient for aluminum is 24 x 10^-6, which means that for every degree C, the length will expand by .000024 x its length at the original temperature. The coefficient for iron and steel is about half of that.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2005

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