Temperature of metal

[dblcrona]

I am trying to settle a dispute and can find nothing in the machinery's handbook or online to support or deny either side of this issue. In high tolerance machining, temperature changes in your material can make a difference. When a piece of metal is heated it expands. How does this affect the size of holes bored in the piece. Will the holes enlarge with the part or will the material expand into the holes making them smaller? If you can provide proof of your answer I would appreciate it.

Thanks,
Dblcrona

 

[Compositepro]

This is a common question in first year physics class. A material expands uniformly in all directions, therefore any hole in the material also expands.

Take any two points in a block of material. Draw an imaginary line along any path between these two points. If the material along this path expands the line get longer. The points have moved farther apart. It doesn't matter if there is a hole in the material directly between these points.

 

[dblcrona]

That's what I thought. Thanks a lot!

 

[KENAT]

We did a demonstration of this at school, probably the equivalent of juniour high.

Can't remember exactly but basically:

We had a metal ring with a ball that didn't quite fit thorugh it.

We heated the ring, tried again and the ball fitted.

It's the same reason that in some interference fits you'll slightly heat the female item with the hole while cooling the male component.

KENAT, probably the least qualified checker you'll ever meet...

 

[rerig]

However, that is why a very close tolerance hole will be drilled to under-size then reamed to final dimension.

 

[unclesyd]

Measurements on all close tolerance holes or any part you will normally see a call out for a temperature where the measurements are to be taken. You don't have to have all that much precision for temperature to affect a measurements. Measurements in the tenths will be affected by small temperature changes.

Temperatures around 70F are considered nominal.

 

[IRstuff]

MIL-STD-883 used 25ºC as "normal" which translates into 77ºF, so you need to know what the consensus agreement within your specific industry or community calls for.

TTFN

FAQ731-376

 

[40818]

Thermal stresses/strains created between aluminium and carbon joint interfaces are an example of the problems with differential thermal expansion. In some instances they are dominant loading for aircraft.