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OLD MAN 06-24-2009 09:43 PM

Hardened By Heat
I bought some 3/4" circles that had been cut out by a torch. They are center spotted and they used a circle cutter to cut them. They are pretty close. I want to mill them but I need to sand or grind some of the edge off to get the hardened metal out of the way. How much has to come off for the mill to not have any trouble? Plain steel from a sheet. They are about 8" across.:):)

digger doug 06-25-2009 09:38 AM

Mild steel really doesn't get hard, but the scale formed
in cutting them with a torch is very hard.

Grinding off that scale is hard to do, can you drop them in
a cement mixer ?

At work we roto-blast, which is an extreme sandlbast
(airless) using steel shot. Unless you've got a line
on one of these, let's see what is avail to you.

Regular sandblasting works, do you have a little
cabinet unit in the shop ? You really only need to
concentrate on that cut edge.

I have machined these before, as an apprentice, and
we just went at them with carbide (negative rake)
in the lathe, scale flying everywhere.

Ironman 06-25-2009 11:01 AM

If I am correct, mild steel will caseharden in a oxy acetylene flame. The mild stell will absorb carbon from a neutral or fuel rich flame and this skin will be tough on drill bits, but should not affect a good machine tool.

USMCPOP 06-25-2009 11:33 AM

Can you slowly spin them, then hit the edge with an angle grinder?

digger doug 06-25-2009 12:15 PM


Originally Posted by USMCPOP (Post 290483)
Can you slowly spin them, then hit the edge with an angle grinder?

It's gonna make allot of dust, I think O.M. would not want allot
of dust.

A cement mixer can be filled with water during the "run"
and there shouldn't be any dust.

Walker 06-26-2009 11:35 AM

Just knock the surface off with a sanding disk. My experience has been mostly with a die grinder and I have found that once the scale is ground off it is easy to work.

OLD MAN 06-26-2009 05:21 PM

1 Attachment(s)
I took a photo of one of the circles, I have several.:)

Charlie C 06-26-2009 07:30 PM

Jerry, I would grind the disk untill I could not see any flame marks from the cutting torch.

digr 06-26-2009 08:08 PM

I would face off a piece of 2" pipe and tack it to the center of the plate and center it with a four jaw chuck and turn the od on the lathe. I have done it that way and it works good.

Barn Owl 06-26-2009 08:21 PM

If you knock the edge of the circle swinging the hammer toward the center of the disk the dross should pop right off. If it was flame cut right there should be very little. If the travel speed was too slow or the heat was too high it is going to make removal harder. If there is not too much (couldn't tell from the photo) grinding is the best way but if there is a lot that is really stuck then a well placed blow with a hammer and it will come off in chunks.
We don't allow the students to cut their plates with the band saw after it has been flame cut because we have found that it reduces the life of the saw blade exponentially. Milling bits might be a bit tougher than a band saw blade. If you have a big belt sander, that will take most of the hardened surface away and save your bits when on the lathe. I've got a big 4 hp sander and it really eats through rough edges.

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