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Posted by ChrisCase on Monday, October 27, 2008 at 16:18:37 :
In Reply to: Re: Oven no good, heat the 'nut' side only. posted by John R Dunn on Monday, October 27, 2008 at 10:52:46 :
That's what I thought until my first day in the machine shop. Think of it as un-rolling a piece of pipe- the length would be so much longer than it's thickness that if rolled back up, the diameter gets bigger than the ID shrinks.
For example, lets assume a bushing about 1/2" id., wall of 1/16". That's 1 1/2 circumference. When heated red hot, it expands 3%. So the circimference gets .045 longer, causing a .015 larger id. The thickness only gets about .002 thicker. I think this example would work exactly for a 1/2" nut. I suppose that in the middle of a solid block things don't work as exactly proportionately, but it does still work. Or always has for me.
You can usually heat something up about 1,000 degrees. You can only chill something oh, 200? So five times greater losenessosity can be had from heat.
So far as heating rusted parts, I also believe that heat breaks the rust down chemically. Rust has eight times the volume of the steel it used to be. So just like roasting limestone to cook the hydrogen/oxygen back out of it, heating the FeHOx will cook the hydrates out, making a smaller amount of junk in the joint. Just my theory, I don't have that much chemistry to back it up. But I do know that after heating rusted nuts and bolts to remove them, the joint is obviously looser, and strips threads easily. And that's why heat works better than penetrants. On my truck, YMMV.
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