Basic sheet metal work

[spike7638]

Dykem the part, use a compass to scribe a nice circle. Bandsaw outside the line. Finish on a disc sander bringin it up to the scribed line.
Frequent dunks in water to avoid overheating. Burned dykem is hell to clean off.

If I bandsaw/sand the aluminum/stainless/aluminum sandwich, the dykem will all be on the aluminum, so it won't be a big deal if it burns. This sounds like a good solution, as the OD isn't a critical measurement -- I just want it to not look ragged and ugly.

Visit harbor freight and get s hole saw kit.

Make hole in plywood first (3 inch) next clam plywood over work an position on drill press and remove pilot bit as plywood is guide.

Drill press as slow as it can go and short light pecks.

If a mill with back gear then slow constant heavy pressure

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I've got hole saws, but nothing with a 3.25" ID, alas. The "slow light pecks" is useful to learn -- thanks. I'll probably use that somewhere soon.

Circle on a mill, do the sandwich thing and use a hole saw. If you don't want the center pilot hole in the part, choke up on the pilot drill so it engages the sacrificial top, but doesn't hit the part. On a lathe, build the stack on an arbor, could be a bolt, and turn the outside down to size. Trammel cutters on a mill can work with a sharp pointed tool. Go slow, there's a lot swinging around and they look kind of scary. Boring head with a custom ground cutter would work similarly. This is better for hole making than circle making since holding the middle is harder than holding the outside.

Right --- the "holding the middle" thing was what worried me (not to mention using a trammel cutter at all, which always scares me a bit). I'm not yet at the point where I can custom grind a cutter, so that boring head idea won't go. The "stack on the lathe with an arbor" idea is a nice one...but I'll probably do the bandsaw-and-sander approach, as it's not essential that it be perfect, and I've already got to drill a much larger hole in the center of the piece, making the whole "arbor" thing tougher.

It amazes me how many ways you experienced folks come up with to answer my questions, when I can't think of even one. I'm much obliged.