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Chrome plated pin gage is oversize

Q. I purchased Chrome plated Pin gages that seem to be expanding more in the heat than normal pins. I have class ZZ Pins that are .3135. Yesterday on the shop floor, they Measured(with micrometer) .31375 in the heat of the shop. After sitting in the Metrology lab (72 °F) it still reads .3137. I purchased a few of these Pins. I measured another pin from the same package at .3135 right where it should measure. Can someone explain why this could be?

iliana gutierrez
- Kenosha, Wisconsin
July 6, 2018

July 2018

A. Hi Iliana. I think that, for whatever reason, you have a defective or improperly marked pin which is actually .3137 dia. at 72 °F

The coefficient of expansion of chromium is lower than that of tool steel, which is about 11.5 x 10^-6 per degree K. If you measured .3137 at 72 ° and .31375 at 96 °F, that would sound just about right. 0.3137 x (1 + 11.5 x 10^-6 x 24 °F x 5/9) = 0.31375

Regards,

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

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