Heating Metal
October 13, 2004 2:31 PM   Subscribe

If you heat up a metal plate with a hole drilled in it, does the diameter of the hole get bigger or smaller as the metal expands?
posted by rhruska to Science & Nature (14 answers total)
smaller. metal expands into the hole.
posted by Hackworth at 2:35 PM on October 13, 2004

posted by whatnotever at 2:57 PM on October 13, 2004

An analogy that may be easier to think about: imagine blowing up a child's (doughnut-shaped) innertube. Imagine you can overinflate it as much as you like and it won't pop. A cross section of the innertube would look like two circles side by side, separated by a moderate distance. As you inflate, the circles' diameters increase, growing outward on the outside of the doughnut and inward on the "hole side." The hole shrinks.
posted by jfuller at 3:01 PM on October 13, 2004

The hole expands. The reason jfuller's analogy doesnt work is that metal is not a gas.

In the innertube, gas molecules can expand to fill the available space but in a heated metal, the atoms are vibrating but they are still linked to each other. In the metal disk, for the hole to "close up" the metal atoms would have to move *closer together* to constrict the hole. What they want to do, however, is get away from each other so the whole disk expands, including the size of the hole, to make more space for everyone (everyatom)
posted by vacapinta at 3:18 PM on October 13, 2004


When a material increases in volume because of thermal expansion, all its dimensions increase. In metals, the amount that any dimension will increase can be calculated from the particulat metal's coefficient of thermal expansion, which is a simple linear factor.

In the case of a metal plate with a hole in it, the plate's outer dimensions will increase a greater amount than the inner dimension of the hole increases, which will account for the overall incriese in volume of the body.

Things become much more complex, however, when different metals, with different coefficients of expansion, are used to make parts that are joined, and the resulting component is heated.
posted by normy at 3:20 PM on October 13, 2004

apologies for the crappy editing
posted by normy at 3:21 PM on October 13, 2004

Response by poster: Thanks all. Looks like bigger wins (from whatnotever's google link).
posted by rhruska at 4:06 PM on October 13, 2004

I never knew I wanted to know that, but I'm glad I do, now.
posted by Blue Stone at 4:43 PM on October 13, 2004

Anecdotally, bigger. If you want to assemble something, say a bearing race, into a brake rotor, and there's an interference fit (i.e. the race is .003" bigger in diameter than the hole it's fitting into), you heat the brake rotor and freeze the race. They fit together all nice, then the temperatures normalize, and they're locked together.
posted by notsnot at 10:56 PM on October 13, 2004

The technique notsnot describes is termed "shrink fitting" by mechanical engineers. It's often used with very large components, where other joining methods would be insufficiently strong or prohibitively expensive.
posted by normy at 11:52 PM on October 13, 2004

Have you never held a jar under hot water to get the top off? The rim of the metal lid is effectively around a hole (with a disc of metal stuck to it). Clearly the metal expands and the hole gets bigger as it gets hotter.
posted by biffa at 3:44 AM on October 14, 2004

I just read this one aloud to my partner and he said: "It's a classic thought experiment. Imagine that you cut a hole out of a sheet of metal, and then you placed the round disc of metal back in. Heat the whole thing up. It all expands, right? And what happened to the hole? It expanded." I get it now. See, this is why I'm marrying him.
posted by web-goddess at 5:48 AM on October 14, 2004

Metafilter: I never knew I wanted to know that.
posted by ssmith at 2:25 PM on October 14, 2004 [1 favorite]

Well, here's an interesting followup, these five days later.

The opposite has been tested: when shrinking a coin through exposure to high levels of electricity, the hole shrinks, even disappears. The pictures are impressive. With both a 5 yen piece and a Subway token, the hole puckers on up.
posted by waldo at 9:58 AM on October 19, 2004

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