Not a wood question..

Re: Not a wood question..

Larry Rupprecht

The expansion of the inner circumference is in keeping with the idea that heating a substance usually causes an expansion. If you consider the material at the very edge of the inner circumference and ask it to expand, the only space available is to grow its diameter. If it were to expand towards the center of the hole, it would run into its neighboring material which is also trying to expand towards the center. Other than becoming denser, the only option for all of this inner diameter material to expand is outward. End result is a larger hole.

This is one of the first basic science experiments for elementary schools. A ball and ring are displayed wherein the ball is too large to pass through the ring. If the ring is heated, the ball passes through. Material science at its simplest!

A second lesson was available for those of us who just had to touch the ring!

Re: Not a wood question..

Mike Schwing

Well it wasn't a lesson in any of my basic education! I've had several people explain this to me this afternoon, your description is probably the easiest to understand. It makes sense but just doesn't seem right on the surface. Thanks!

That wasn't covered in my elementary school either

John in NM

High school physics class is where I saw that demonstration. Maybe a good junior high science class would cover it.

The easy way to think of it is that the inner circumference has to get longer, just like the outer one does. If it got smaller, the metal would be in compression, not expansion.

Re: Not a wood question..

Don Stephan

Not intuitive to me either.

Re: Not a wood question..

Brad Chenoweth

Another example of this is when we heat bearings in hot oil to make them larger in order to press fit them on a shaft.

John's explanation describes what is going on best

How about these.....

Barry Irby

Imagine the circle is a square....and the square is heated. You would expect the top and bottom would expand both lengthwise and widthwise. The same for the left and right sides. Since the sides are far longer than they are wide or thick, you would expect the square to get larger both inner and outer.

Imagine the the circle is solid, no hole, and its heated. You would imagine it would expand uniformly. So if you drilled out the center while it was hot, the hole would become smaller when it cooled.

Or, Diameter of a circle is D and the circumference is Pi D. So when heated the rim expands 3.14 times as much as the Diameter.

excellent explanation

Re: Not a wood question..

Mike Schwing

I knew there were a lot of smart people in here. Thanks!

Re: Not a wood question..

TomD

Epoxy it in. Heat is not your friend with Al, though I don't know how much you are using.

Re: Not a wood question..

Mike Schwing

Hi Tom. I think I prefer this method, discovered accidentally, over epoxy because it is reversible. It is a super strong, tight fit and the amount of heat used was way less than the the welded joint just a few inches away.