Hougen mag drill with Broken Spindle

[metric_taper]

I have some silver steel in oil and water hardening that I stock.
I've run into air hardening accidentally a few times with mystery metal I got from surplus. I was machining away, and the material got hot and self hardened before I got finished. It didn't get that hot. That ended up scrapping the part, one time I broke a tap off. I have used my heat treat furnace to anneal tool steels so I could repair some part of them, then heat treat them again (and I've removed broken taps that way). For oil hardening, the 400F was the desired temp for a bearing surface.

I did make a draw bar for the band saw (blade tensioning), as the original metric bolt they used had thread failure (just soft no grade bolt). So I had a piece of oil hardening rod in the right diameter, ran the die on it, then heat treated it, so the threads did not get buggered up again. I did not temper, it snapped during band tightening, so I remade it and tempered it. It was one of those cheap imports 6x4 bandsaws. I didn't think tempering would be needed.

I believe that air hardening would be for cutting tools, you may make it work for this application. But you have lots of machining to do. The oil hardening does expand on hardening, memory is about .002/inch of material. Should not be a problem if you do the final grinding with tool post grinder on your lathe, as you're leaving material to grind to the bearing fit.

I just have a can of either engine oil or vegetable oil, they bot seem to work. But I'm no expert in making a long shaft that is straight when done.