[How-To] Tool Making Questions regarding heat treating

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ArmyDoc

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General question: How does heat treating affect tool making?
Background / more detailed Question:
If I want to make a set of 1-2-3 blocks, how does the heat treat affect the part? From what I've read, I should make the part a thousands or so over sized, and then grind it to final dimensions with a surface grinder. Only, I don't have a surface grinder (ok, I don't have anything right now, and this is more of an intellectual exercise... but still.) How much will the heat treat distort the part? Can I decrease that by annealing the part before machining it? (I don't know if I'm using the right term... What I mean is heat the stock up and allow it to cool slowly to take out the stresses in it, so that when you heat treat it later it won't warp.)
Thanks
 
You can expect the metal to move under heat treatment by 0.000,5" to 0.001" more like surface irregularities than bending, twisting.
Done properly, the metal changes from something you can machine without much trouble, to something that can only be machined with abrasives--but you have control on how hard the part gets and how much you temper it back to normal. It is all temperatures and time.

Annealing only serves to enable the machining, the metal will still distort under heat treatment.
Carbon atoms in a ferrous matrix will ALWAYS have residual stresses in the solid state. It is simply part of the alloy.
 
As in most things, it depends. What tool steel are you using? Some distort more than others. How are you going to heat treat it? If the atmosphere/process is not controlled, you might get heavy scale and/or distortion during quenching and that is something to consider. How square/true will the part be machined prior to heat treatment? The closer it is roughed, the less you'll need to leave for grinding. Etc. Etc.

For anything I might make, my general rule of thumb is to leave 0.002-0.005" per side.

Ted
 
.002" allowance would not be enough if there is any decarburization at all during heat treating, .005 PLUS would be more like it. I made a pair of blocks when I was an apprentice, and the came out fine, and are stable and kept size over the over 50 years that have passed; I think they were made of 0-1 steel, but the shop had dedicated heat treating facilities to avoid decarb, (articles buried in peach pit charcoal in a stainless box) and precise temp. control and a similarly temp. controlled draw bath; most articles that I heat treated there were dies made of sophisticated tool steels and were soaked at heat for at least one hour before quenching. What I'm getting at, is that for a good job of heat treat, a torch and coffee can of drain oil is not going to do it. The household oven may serve for tempering if the domestic situation allows it.
 
So, once hardened, is a surface grinder the only option for bringing it into final dimensions? Or is hand polishing on a surface plate an option?
 
ArmyDoc said:
So, once hardened, is a surface grinder the only option for bringing it into final dimensions? Or is hand polishing on a surface plate an option?
Click to expand...
Hand polishing would likely not be a good option, accuracy wise, and that is what you want from blocks; I suspect that you could find a new friend to help with the grinding.
 
Actually, Doc, for the things that require a surface grinder (like 123 blocks) you just buy the thing. Heat treating in a hobby shop is really useful when making tools that are made for a specific purpose - cutters, drills, etc - and it is really good to learn how to do it. Everything warps with heat so if the part you're making rotates then you will have a small bit of warpage but things like custom lathe cutters that are static work just fine. I've been making small hardened parts for my personal use for a very long time and don't own a surface grinder; haven't had my tools not work yet. I'm one of those torch and toaster oven backyard guys and don't fit in with the big boys but that's okay, my tools still work.
 
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