Shrink fitting. What is it?

[Szech]

I know Alpha does this with their heatsinks to get the copper embedded, but what does the process involve? I was thinking the copper could be super cooled, put in a narrow fitting space, then when it warmed up and expanded, it would be a tight fit.

Is this how it's done???

I have access to liquid Nitrogen at work, and I want to put a silver plate in the bottom of a heatsink (a la Noise Control Silverado). Is this the best way to go about doing it?

[ZX-Tex]

Shrink fitting is when one metal is heated and another is cooled to change their size for assembly.

For example, if you wanted to have a very tight fit for a ring on a pipe, you would heat the ring to a modest temperature, say 250 deg F. This increases the diameter of the ring. The pipe would be cooled, either in the freezer or with liquid nitrogen. This decreases the diameter of the pipe.

The two would slide together if assembled quickly. However, once they return to room temperature, the ring shrinks, the pipe expands, and they are locked together due to what is called an interference fit.

Keep in mind that this only works well with very precisely machined components. For something in the 1" to 6" range the diameters need to be precise within 0.001" or so. For example I have used this technique to fit bearings onto shafts. Bearings are machined to 0.0002" in some cases. The shaft tolerance would be about the same.

OK long winded but I am an engineer after all, no simple answers ;-) Hope this helps. If you want more info, go see a good machinist, or pick up a copy of 'The Machinery's Handbook'. It is THE Bible for such matters and many others.