question regarding 0-1 drill rod?

blaser.306

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When working with 0-1 drill rod there was info posted regarding heat treating and stability / expantion rates. My question is if the material ( expands ) .0015" when heat treated per 1" of cross section , would a hole bored thru the center of a bar get smaller in diameter " whitch could either be seen as expantion or contraction". Or get larger following the outer dimention. I am wanting to make some sizing dies and am wanting to know how to adjust the way I need to cut and polish thes to allow for final lapping after hardening. Thanks in advance.
 
The hole is going to shrink and the amount will depend on how consistant your heat treat process is. The shrinkage may also not be even resulting hour glass shaped holes. In tool and die work it is normal practice to finish holes with expanding laps and lapping compound.
 
Metal expands in all directions when heat treated so the hole will get smaller. If there were no hole only the diameter and length would change.

"Billy G"
 
When a bolt gets stuck in a hole, you heat the fixture because the metal expands in all directions, the hole enlarges and the bolt stands a better chance of coming out. That said, I have a complete AISI book of materials covering heat treating. Let me know if you want me to look up expansion properties. If you're looking for coefficients of expansion so you can work on hot metal and predict the final size, for O-1, it is here: http://fordtoolsteels.com/wp-content/themes/twentytwelve/pdf/LSS_O1.pdf Note that the data sheet has no expansion characteristic information re: heat treating. That's because penultimate and ultimate differences in sizes is barely measurable for most steels. Hot working coefficients actually make a difference. Since I switched to KoolMist, it's much less of a problem.

EDIT: If you'd like to see the mathematical proof of how expansion enlarges a hole, let me know. It can be done by extending linear coefficients of expansion to volumetric coefficients of expansion and comparing the relative volumes of the part while cool and then while hot.

Ray
 
This is a very interesting mental exercise! My initial guess was that the hole gets bigger upon expansion, I can see the argument both ways but I am still with Ray C on this.
 
I don't believe there is a universal answer to that problem. Part of it stems from unknown stress factors in the material previous to machining. Machining changes the effect those stresses have on the shape and size of the part. Different cross sections change in different amounts and in different directions. The amount and direction that a hollow part move while hot isn't necessarily representative of those factors when it runs through the changes hardening and tempering bring about. Yes, the amount of expansion can be measured and calculated to a fairly accurate degree, but the shape of the part plays too heavily into it to try and hold close tolerances ahead of the ht procedure. That is why virtually every part that is made to be hardened at the finished sizes will have honing/lapping/grinding/hardturning allowances in the pre-hardened condition.

There are procedures to minimize the changes during the ht process, but none to totally eliminate it. And some things are beyond the control of the machinist if the work is sent to a commercial facility for hardening.
 
Heating a metal part to get to it expand to remove a stuck bolt is totally different than what happens to material when it gone thru a heat treatment process.

If you don't believe the holes will shrink then why do tap manufacturers make oversize taps for parts than will be heat treated.

oversizetaps.JPG

oversizetaps.JPG
 
When you heat the piece of metal, the hole gets bigger. Depending on the material, I would assume that some will change the size of the hole AFTER heat treatment.
 
Plus the symmetry or lack of it affects the direction of change in the grain size and structure, which the root cause of all this. Things that grow in one direction can shrink in another. That's why this whole thing is a bit of guesswork at first, anyway.
 
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