question regarding 0-1 drill rod?

Issue #1: Hot working.

Take a think piece of wire 1" long, make a perfect circle, measure the diameter.
Take the same piece of wire, heat it up and while still hot, make a perfect circle. This one is bigger in diameter.

No need to argue this. It's been solved a million times in basic text books in mechanical engineering. And by the way, when I press pulleys onto shafts, the first thing I do is put the shaft in the freezer and the pulley in the toaster oven and sweat them together.

Issue #2: Heat Treating.

I was wrong. I looked at the reference books and they show no difference in size pre and post heat treating. I do recall reading there is a small change ranging from micro-meters to nano-meters (x10[SUP]-6[/SUP] and x10[SUP]-9[/SUP]) that for all practical purposes is zero. The effect is about the same dimensional changes as from bluing or black oxide. There may be other effects such as scale build-up and sag. Scale build-up is controlled by using gaseous or vacuum chambers. Sag is prevented by using pyrometers and not color charts to reach the critical temperature.

Ray


tigtorch said:
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.
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There's really no guess work. I very well remember this problem in my beginning physics classes. Irregular structure or not, atomic forces need a place to push. Those forces find nothing to push against inside an empty hole thus, each atom therefore pushes against the ones next to it. At the very border of the inner circle, you can image it being a wire of negligible thickness or infinitely thin. As in the case of the wire example below, it stretches it's length and increases in diameter. This problem is very, very solved -by people many, many years ago.



Ray

Tony Wells said:
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 canshrink in another. That's why this whole thing is a bit of guesswork at first, anyway.
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By the way guys, I'm not just posting my theory on what happens to a hole when heated-up... My first instinct would be to think the same thing but, no kidding, this is a classic textbook physics problem that most engineers and all physics students had to solve or was exposed to at one time. This is not theory, it's a physical fact.

Ray
 
Thanks for all the replies and views! I am going to give a couple of test runs of parts and do some carefull measurements pre and post treatment. I was just concerned with having to try and lap out more than needed once the material was file hard, I would rather stay just under size than spend a week lapping! I do also have some A-2 that will be worked with somewhere along the line , but that I will send to a local heat treater for finan work, as they have the knowledge and proper equipment to handle the task.
 
Ray C said:
By the way guys, I'm not just posting my theory on what happens to a hole when heated-up... My first instinct would be to think the same thing but, no kidding, this is a classic textbook physics problem that most engineers and all physics students had to solve or was exposed to at one time. This is not theory, it's a physical fact.

Ray
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I don't think anybody was saying the hole doesn't get larger when heated. It's what happens when it cools back down that you get some distortion. If you want to read it in print get a hold of the book "The Tool Steel Troubleshooter" from Bethleehem Steel Corp. and read pages73-74.
 
You are correct Ray, when heated it will get larger but. when cooling it gets smaller. That is why you have to ream or hone after heat treat. Nobody said after heating, they said after heat treat.

"Billy G"
 
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Nice thread, I'll give my practical input.

Where I work we routinely make rings of different sorts out of S-7, D-2, A-2 as components in dies.
Lets say the ring was 6" in diameter with a 4" hole in it...
After heat treat both the ID and OD will be cylindrically ground to the final dimensions, and we will typically leave between .015 and .030 of grind stock on both the ID and OD. Now, when I actually do the grinding, I can tell you that there is more material to take off the outside than the inside. Which is to say that both the ID and OD have increased in diameter. I suspect that something different happens with a smaller hole in a large piece, but I can't testify exactly what. As to the oversized taps used in tool steel? It could be as much to counteract the deformation of the threads which occurs during heat treat, as it is the hole shrinking.
 
Ray C said:
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
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Ray,
I'm glad you posted this. I'm not much of a machinist but I'm a long time mechanic. For forty years I've been getting frozen nuts, bushings and sleeves off shafts by expanding them with heat so if a hole contracts when heated then I've been doing this all wrong.
Ed P
 
I really gotta dig into this issue as, I believe all the folks who say the hole shrinks a little after treating but, I cannot find a shred of evidence about why it does. I have noticed a couple things in my meager experiments so far. One thing is that scale builds up due to oxidizing under high heat and mixing with air. The scale is very crusty and is easily removed. Also, I learned the hard way that pack carburizing can really stick like hard diamonds! Very difficult to get it off. That was my mistake for not having the foil sealed-up very well. That mistake will never happen again in my shop! I also noticed that threaded holes need to be touched up but this is easily explained. Simply, the thin narrow threads are getting cooked too long for their narrow cross sections. This is unavoidable. With heat treating, you're only supposed to cook for the amount of time based on the thinnest cross section of the material. In my case, I had a plate 1.2" thick with threaded holes and I cooked for the amount of time based on 1.2". This is the correct course of action as if I cooked based on threads (which are only say, 50 thou thick) the rest of the plate would not get treated to any desirable effect.

I did treat some 2-3" disks with 1" holes (no threads) and I carefully measured them before and after. There was no difference in any dimension that I could measure with a mic reading ten-thous. This however is based on one type of material (1045) so it's by far an exhaustive test that draws a conclusion under a very limited sample...

More learning on my part is needed...


Ray



Bill Gruby said:
You are correct Ray, when heated it will get larger but. when cooling it gets smaller. That is why you have to ream or hone after heat treat. Nobody said after heating, they said after heat treat.

"Billy G"
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epanzella said:
Ray,
I'm glad you posted this. I'm not much of a machinist but I'm a long time mechanic. For forty years I've been getting frozen nuts, bushings and sleeves off shafts by expanding them with heat so if a hole contracts when heated then I've been doing this all wrong.
Ed P
Click to expand...


You also are correct. The hole does not contract while heating. It gets bigger. While it is cooling it goes back to where it was or slightly less in diameter. When I replace bearings in model airplane engines I heat the case and the bearing will usually drop out. Replacing them I heat the case and freeze the bearing. Nice tight fit when case cools.

"Billy G"
 
Bill Gruby said:
You also are correct. The hole does not contract while heating. It gets bigger. While it is cooling it goes back to where it was or slightly less in diameter. When I replace bearings in model airplane engines I heat the case and the bearing will usually drop out. Replacing them I heat the case and freeze the bearing. Nice tight fit when case cools.

"Billy G"
Click to expand...
Thanks for the clarification. I've done the freezer trick my self and It's pretty slick.
 
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