Teach me about case hardening

[zahc]

I need to make a broach to make hex and/or square-ish 1/2" holes in aluminum.  It will probably be 6" or 8" long.

I would use tool steel, but it's like $30 just for a piece of O-1 that big, and I can't find it in hex at all.

I'm thinking about trying in mild steel which is like $5 instead. Given it's aluminum I'm cutting, it might work for a few holes. Is there any point in trying to harden 1045 steel at all? If not, what about case hardening?

I don't have a furnace so I will have to make one somehow.

[Northwoods]

I can ask our metallurgist at work, but I don’t think mild steels case harden.  You can harden 1045, but not as much as higher carbon steels.  I forget the temps but you’d heat it to the appropriate color and quench in oil.  Then temper at the heat and dwell appropriate for the hardness you want, somewhere around 400F iirc for a hour for 58-60 Rc, which is typical for a knife.  Depending on the aluminum alloy that might work ok.  But I’m definitely not a subject matter expert.

[bedlamite]

You can case harden low carbon steel, all that means is you are adding carbon to the surface, Use a torch or oven to get it red hot, and bury it in carbon powder, get it red hot again, then quench. Depending on the steel and how hard you want it, two or three times in the carbon powder might be necessary.

Honestly, for cutting tools, just spend the money on tool steel.

[K Frame]

Kasenit.

This stuff: https://rosemill.com/heat-treating-compounds/?gclid=Cj0KCQiAjKqABhDLARIsABbJrGlEmrDOqfWJTblBSNnUfrWaT4IoiPS08pZQV1-LmrnrvxkXI5UaK7IaAvivEALw_wcB

I bought a jar of Kasenit from Brownell's (they now call it surface hardening compound or something similar).

Does a FANTASTIC job.

I've hardened a bunch of stuff and it works great, including common nails, which I believe are mild steel. Went from cutting them with a file as if they were butter to barely scratching the surface on them without multiple passes.

[K Frame]

OK, looks like Kasenit is no longer made, and the Cherry Red stuff I linked, at least from that site, is only sold in 10 pound quantities. You can get lesser quantities online.

How much do you think you'll need?

I've barely touched my can of Kasenit. I'd be more than happy to mail you a couple of ounces.

[Jim147]

Are you going to use a press or a hammer to make the holes?

I might have some shorter 1/2" around here that could be attached to a handle of brazed to some mild steel.

[230RN]

I was going to offer Kasenit as a solution for the "carbon powder," then noticed Mike Irwin said it was no longer made.

I had good results with it, hardening regular screws by torch-heating them and dipping in Kasinite several times and quenching the last repeat.  I was only interested in making them (the surface) as hard as possible without tempering. I forgot what the project was, but I remember I was happy with the results.

[French G.]

I would look for reclaimed steel. Piece of spring, something that was hard. Old chisel is a go to for me. Not the right way but I have hardened tools just running it to cherry red with my tig torch then oil quenching. I did one short pin punch like that and about 6000 chain links broken later it still hasn’t deformed or broken.

[cordex]

Could you not buy a hex wrench of appropriate size/length and cut off the short leg?
https://www.mcmaster.com/7122A54/

If you are forming a broach you could then round off and taper one end and then grind your cutting edges.

Probably be better option than case-hardened mild steel.

[zahc]

I assume that Allen wrenches are too hard. I need to be able to turn it in a lathe.

[Nick1911]

I assume that Allen wrenches are too hard. I need to be able to turn it in a lathe.

Yea, I think they are made out of S2.  Unfun to cut in the hardened state.  You could anneal it, cut, then re-harden.  I've done that before with an old jackhammer chisel, which was also S2.

I've done a bit of case hardening, using Kasenit.  Apparently I'm lucky to have bought some back when it was made.

Personally, for a tool in aluminum that sounds like it's not going to be used heavily, I'd attempt it with 1045 and a hardening cycle.

Selected quotes from The Book:

SAE 1030, 1033, 1034, 1035, 1036, 1038, 1039, 1040, 1041, 1042, 1043, 1045, 1046, 1049, 1050, 1052: These steels, of the medium-carbon type, are selected for uses where higher mechanical properties are needed and are frequently further hardened and strengthened by heat treatment or by cold work. These grades are ordinarily produced as killed steels.

In general, any of the grades over 0.30 carbon may be selectively hardened by induction or flame methods. 

0.45–0.50 per cent - Heat-treated parts requiring fairly high hardness and strength with moderate toughness.

The superscript b indicates a water quench.  Hardness is listed in Brinell, here's how you can walk that to rockwell C, which is probably a system you're more familiar with: https://www.engineersedge.com/hardness_conversion.htm

[zxcvbob]

What Nick said.  1040 and 1045 contain enough carbon to harden them with a simple oil quench, or a water quench then temper.  Another good choice might be a piece of rebar; I think it's generally rolled from old railroad rails so it should have some carbon; I can't remember how much.

[Hawkmoon]

https://www.brownells.com/gunsmith-tools-supplies/metal-prep-coloring/color-case-hardening/surface-hardening-compound-prod27119.aspx

Temporarily out of stock, but available for back order so they should be getting it in again. You could call and ask when.

[tokugawa]

It is difficult to give tool advice without knowing the job.

What sort of aluminum? How thick?  How precise a hole do you need? How many?

Soft aluminum, 1/2" hex hole,  I would try  try drilling it to the width of the flats and broaching the points  through with a 1/2" allen wrench socket mounted in a arbor press. Sharpen it square on the cutting end on a grinder and lube it.

More than this, talk to the folks at practical machinist forum.

 

[Nick1911]

It is difficult to give tool advice without knowing the job.

What sort of aluminum? How thick?  How precise a hole do you need? How many?

Soft aluminum, 1/2" hex hole,  I would try  try drilling it to the width of the flats and broaching the points  through with a 1/2" allen wrench socket mounted in a arbor press. Sharpen it square on the cutting end on a grinder and lube it.

That's a decent idea.  Also, if it's a one off, you might drill out the bulk and file out the detail.  Slow work, but effective.

More than this, talk to the folks at practical machinist forum.

Ehh... Personally, I've found the culture at PM tends to not be kind to us home shop guys.

[tokugawa]

That's a decent idea.  Also, if it's a one off, you might drill out the bulk and file out the detail.  Slow work, but effective.

Ehh... Personally, I've found the culture at PM tends to not be kind to us home shop guys.

 Maybe try home Shop Machinist?   I started off rebuilding an old USA milling machine on PM, so that might have cut me some slack over there.
 There are definitely a few jerks, though.

 zahc,
 I just read your post again- is the broached hole 6" long, or were you referring to the tool?
 
 If the hole is deeper, with a carbide tool, a 1/2" allen wrench could be cut off and broach teeth cut, on the lathe, (I think). Then tapered on a grinder, or belt sander.

[zahc]

The hole is in 1/2 or 5/8 thick aluminum.

The hole doesn't have to be exactly on-size but it needs to be consistent, like +/- 0.001". The problem with making the square holes on my mini-mill is I'm more like +/-0.002 on each side, that's 0.004" overall so it ends up being between 0.496 and 0.504 between the flats, which isn't consistent enough unless I make the shaft to match as a matched assembly.

If I switch to broaching I would probably change to hex instead of square because it's easier to broach and leaves more material behind so it should be stronger.

Allen wrenches are too hard to turn on the lathe. I might try grinding a ghetto pseudo-broach on the bench grinder though.

[brimic]

https://youtu.be/yHuwBwmV1OE

Not really going to help with case hardening, but might give you other ideas...

[Hawkmoon]

The hole is in 1/2 or 5/8 thick aluminum.

The hole doesn't have to be exactly on-size but it needs to be consistent, like +/- 0.001". The problem with making the square holes on my mini-mill is I'm more like +/-0.002 on each side, that's 0.004" overall so it ends up being between 0.496 and 0.504 between the flats, which isn't consistent enough unless I make the shaft to match as a matched assembly.

If I switch to broaching I would probably change to hex instead of square because it's easier to broach and leaves more material behind so it should be stronger.

Allen wrenches are too hard to turn on the lathe. I might try grinding a ghetto pseudo-broach on the bench grinder though.

Precision grinding is "in."

There's a company in Pennsylvania that makes [very] high end 1911s -- Cabot Guns. They are a spin-off of a company that does precision grinding, and sells precision grinding tools and equipment. I toured their factory several years ago. Their 1911s aren't "machined" -- every part is made by computer controlled grinders. And they don't start with forged frames -- every frame and every slide starts out as a billet. And they just grind away until all that's left is a frame or a slide.

[brimic]

Precision grinding is "in."

There's a company in Pennsylvania that makes [very] high end 1911s -- Cabot Guns. They are a spin-off of a company that does precision grinding, and sells precision grinding tools and equipment. I toured their factory several years ago. Their 1911s aren't "machined" -- every part is made by computer controlled grinders. And they don't start with forged frames -- every frame and every slide starts out as a billet. And they just grind away until all that's left is a frame or a slide.

that's a great way to get the oiled glass on glass feel, but oh boy, expensive.

[tokugawa]

The hole is in 1/2 or 5/8 thick aluminum.

The hole doesn't have to be exactly on-size but it needs to be consistent, like +/- 0.001". The problem with making the square holes on my mini-mill is I'm more like +/-0.002 on each side, that's 0.004" overall so it ends up being between 0.496 and 0.504 between the flats, which isn't consistent enough unless I make the shaft to match as a matched assembly.

If I switch to broaching I would probably change to hex instead of square because it's easier to broach and leaves more material behind so it should be stronger.

Allen wrenches are too hard to turn on the lathe. I might try grinding a ghetto pseudo-broach on the bench grinder though.

  How many of these do you need to make?  What does it do? Is the shaft a sliding fit, or a press fit? Is the location of the hole critical, as well as the size? How were you squaring the corners on the mill-or are they just cut deeper with a small end mill?
 I ask because I am fascinated with making things-  you know, sometimes a complete rethink of how to do something happens from odd questions-

[zahc]

It's a bicycle crankarm. I could press-fit one side, but the other side needs to be able to be disassembled, so I'm using a pinch bolt. It works fine if all the pinchbolt has to do is lock it on. But if it has to take up any slack at all, then they eventually start to creak or wobble. So it needs to be a real nice sliding fit with minimal wobble even before tightening the pinchbolt.

Right now it's a 15mm shaft with a 1/2" square "spline". So the corners aren't sharp. You can make the socket with a 1/4 endmill or file.

I think I might try having some pieces of 1095 laser-cut with the hole profile. Then I can sandwich the aluminum between them in the vise and file it out with a chainsaw file. The 1095 should be hard enough for the file to slide over, hopefully. I might be able to make a similar thing for the shaft, like sort of a miter-box for filing the flats.

[tokugawa]

It's a bicycle crankarm. I could press-fit one side, but the other side needs to be able to be disassembled, so I'm using a pinch bolt. It works fine if all the pinchbolt has to do is lock it on. But if it has to take up any slack at all, then they eventually start to creak or wobble. So it needs to be a real nice sliding fit with minimal wobble even before tightening the pinchbolt.

Right now it's a 15mm shaft with a 1/2" square "spline". So the corners aren't sharp. You can make the socket with a 1/4 endmill or file.

I think I might try having some pieces of 1095 laser-cut with the hole profile. Then I can sandwich the aluminum between them in the vise and file it out with a chainsaw file. The 1095 should be hard enough for the file to slide over, hopefully. I might be able to make a similar thing for the shaft, like sort of a miter-box for filing the flats.

 I don't know anything about bicycles- so just kicking around ideas-  I assume weight is critical? What do other manufacturers do? Is this a standard method? It does not need to slide to operate, , that is just to allow assembly/disassembly?

 The first thing that came to mind was using a taper , key and nut to pull out any play.   Like a prop on a shaft.
 
  It is likely any steel template hard enough to guide a file will also dull it quickly.

[Jim147]

Chainsaw files have safe sides so no need to worry about dulling it.

[zahc]

There's three basic designs on bike cranks. Tapered cotter pins (obsolete), pinchbolt, and square taper. I don't think I'm going to come up with anything better than the pinchbolt arrangement. Square taper would probably be even harder to make.

I made a broach from some $5 piece of 1/2" key stock today, but I think it's too soft. If I'm lucky it will work a couple of tines, and I don't have the stuff to try to case harden it