Is there a way to cut down a disc diameter from the face?

Jim is right, be careful trying to make large diameter cuts like that- things can come loose and do all kind of damage to you and the machine.
Use slow speeds and eye protection- make sure the cutting tool has enough clearance not to get jammed up in the workpiece
-M
 
gard said:
I think if it took an hour to remove 1/2 inch from the OD of 3/8 thick aluminum you are doing something wrong with your lathe, cutting tip geometry or perhaps RPM would be my guess. I would think it should only take a couple of minutes and would be faster than trepanning.
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I'm with Gard. You should be able to hog the metal off on a plate that narrow. How are you holding the plate in the chuck? With a correctly set, sharp RH tool, you should be able to start with a .020 DOC. Then keep adding .005 each time until the machine complains and back off to the last good cut. I do steel plates and small shafts in only minutes.
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gard said:
I think if it took an hour to remove 1/2 inch from the OD of 3/8 thick aluminum you are doing something wrong with your lathe, cutting tip geometry or perhaps RPM would be my guess. I would think it should only take a couple of minutes and would be faster than trepanning.
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It's very likely that I am doing something wrong in my side cuts, but it just seems crazy to make that many chips instead of a nice ring. I will try the trepan this week.
 
Jim F said:
For your application, this may be better.

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My part has a center hole, and I have a nice fixture made up for turning. I am not confident to try the pressure hold on my jet lathe. It is not a "solid machine" in any sense. Thanks for the suggestion (@benmychree too).
 
First of all, you do not want a ring. Metal turning should look like "C" or "6" when coming off the tool. Long curly turning can turn into "bird's nest" and can foul up the work or at sometimes become dangerous. Trepanning is an art. You should watch a lot of videos on it before trying. A lot of times it is not cut through, but a few thousands are left and the piece is hammered out.
 
Hole saw in tail stock

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Unless you need the rings for something not sure I would bother with either a hole saw or trepanning. You need a good hole saw arbor and hole saw. The one I have, a Morse, is not true, and the holes that are cut are eccentric. This causes a lot of vibration. Trepanning is ok, but you need to grind your own tools. Since it is only 3/8" thick it won't be hard to do.

Easiest of all is to just turn down the diameter. If that's difficult then something is not set up properly. Perhaps the tool is not centered or is ground improperly. Reduction of diameter is what lathes are made for. It should be easy, especially on aluminum.

Show us some pictures of your setup, perhaps we can improve it and make your life easier.
 
I'm thinking the tool geometry or sharpness may be at fault- aluminum should cut like butter
We recommend everyone with a small lathe get a grinder and learn to grind their own HSS bits
 
Charles scozzari said:
Why?
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It keeps the piece from coming loose and flying around. Depends on size and weight and eventual use. Also depends on if the piece you are keeping is the center or the outside and the way you are holding it. A 3" center. piece flying through the air is not funny nor is a 1/4" ring wraping around the chuck or tool holder. And then there is the possibility that the piece will be damaged beyond use.

Been thinking about the size difference. You want 3" out of 4". That leaves 1/2 " ring. Not really. You need to leave a little for face cleanup. Say .030". Then you have the trepanning groove. If you are really lucky, 3/16", more like 1/4". That leaves you with a ring of about 1/4" width. Not really enough left to do anything with. Big difference if you are cutting this out of a 6" diameter or 6" square material. Then you have enough to make other items.

If you have a lathe or mill then you are in material reduction. All you do is make chips. Garbage cans full of them. Collect them and recycle.
The only way to reduce chips is by casting to a closer dimension, or EDM, laser cutting, plasma cutting or water jet cutting the part out.
 
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