Allowing for heat expansion

[DavidR8]

Is there a rule of thumb that applies to how much to allow for heat expansion?
For example, aluminum seems to really expand if it gets warm but I don't know how much expansion I should account for when trying to hit a dimension.
Any hints?

 

[dpb]

I don’t have the numbers offhand, but expansion from temperature change of various materials is readily available online, and in other resources.
That said, unless you have either put a lot of heat into the material (too hot to touch), or are trying to hit tolerances inside a thousandth, I don’t think you need to worry about it, much. I can routinely turn 416, or aluminum to within a couple tenths, and it's still there, later.

 

[mikey]

I don't personally follow any rules beyond cool it to ambient before measuring when accuracy counts. This applies especially when fits are tight, like bearing bores and bushing fits. All materials expand so I treat them all the same - cool to ambient.

 

[Janderso]

I don’t know the answer but. I agree, aluminum seems to expand more than steel when warm.
Cut a aluminum pilot bushing for my rotary table yesterda. I wanted a snug fit, went in to have lunch and put it in the refrigerator for a bit. When I tried the fit again it slipped in nice and snug. Had I left it to warm to ambient, I’d never get it off.

 

[macardoso]

Thermal Expansion - Linear Expansion Coefficients

Linear temperature expansion coefficients for common materials like aluminum, copper, glass, iron and many more.

www.engineeringtoolbox.com

These coefficients are easy to use to calculate how much a plain round bar will increase in length with a change in temperature, however any complex geometry will cause this expansion to be greater in some directions and places. This can certainly make it difficult to compensate. Best to let it cool before your final cut.

 

[SLK001]

+1 on letting your piece cool to ambient PRIOR to your final measurement and cut. Get to within a couple of thou, then walk away for an hour or two.

 

[Flyinfool]

There are to many variables to use a rule of thumb. The exact temp being a major one of the variables and there is not an easy way to know or control that exactly. As mentioned above if the dimension is that critical, let it cool to room temp and then take the final measurement and then the final cut.

 

[RJSakowski]

A one inch piece of aluminum will grow by about .0017" if the temperature is raised from 20º to 100ºC. If raised from 20º to 36ºC (warm to the touch), that same piece will have grown by .0003". If you are doing close fits, it would be advisable to allow the piece to cool down prior to making final measurements.

 

[silence dogood]

In high school machine shop we had to turn a cylinder to a certain dimension. After taking sandpaper to it, it measured right on. Gleefully took it the instructor. He said, nope, too hot. Much to my surprise, when he did measure, it was some three or four thousands short.

 

[SLK001]

I sometimes cut tapers in 1018 steel. Hogging off the extra material really heats things up. To speed up the process, I remove the part and drop it into a bucket of cold water. Five minutes later, the part is ready to make the final cut.