If one has a piece of aluminum plate that one would like to make round using a rotary table on a mill, how does one clamp it down so that one can use an uninterrupted cut?
I can put a small (.200-ish) hole in the center, but that doesn't seem to be enough to hold a clamp decently. I can clamp it from the outside, but I'd have to move the clamps as I milled around the piece. I'm assuming that this wouldn't allow for an uninterrupted cut which I wanted for the best finish. Is this newbie missing anything? Suggestions?
Simple. Imagine the rotary table is to the left of the headstock so you're milling on the right edge of the piece. You arrange a clamp from the bed on the left side of the piece, bearing down on the centre of the piece, but this must act through a ball bearing so it can rotate as the rotary table rotates. If you have a lathe don't even bother though because on a lathe it's easy...
With the same principle, on a lathe you can grip a plate between two bungs using tailstock pressure and a revolving centre without needing a chuck or clamp, as long as you take light cuts. Making things round is a lathe job not a milling job.
I like Dave's bearing clamp idea, I doubt that I'd have thought of that. When I want a round cut out of a square, I bolt the corners to the table T-nuts with a a sheet of scrap underneath the workpiece. When you near the full depth, there will be a foil-thin amount holding the disc in place (that's a good time to stop). The disc will separate easily as it's gently pried up. The other method I use is to utilize a 4-jaw chuck on the rotary table. The same applies to the stop depth. When you get bored/tired of cranking the RT, motorize it.
Thanks. I've been thinking the same thing. I have a variable speed (something small like 1/6HP?) motor and controller that I thought would be a perfect compliment for my RT.. or the spindle on my mill...
Easiest way if you have a lathe and start out with a suare work piece is to drill four holes in the corner, screw the stock to a piece of 3/4" thick MDF (medium density fibreboard aka Medite). Mount in 4-jaw chuck or on a faceplate and cut out your circle. If the radius is large enough, the work thin enough and you have a not-too-high parting tool, you can use it otherwise you need a trepaning tool. A trepaning tool looks like a parting/groving tool except that it's curved so the sides don't rub.
Alternately, you can clamp the work piece to the MDF with a large washer and an a draw bar through the headstock. This is the best if the stock is rather thick. Rough it out with a band saw then machine it round.
If you really want to do it on the mill, here's how: My RT has a hole in the middle to take a MT-2 center. This hole extends right through at the smaller diameter of the taper. I made a special nut that is longish and hex at one end and steped down to a round that fits into the hole in the RT. This nut is threaded for 3/8-16 but could be 1/4 or even 3/16. The work piece has a hole drilled in what will be the center to take the bolt and a fairly large washer. The RT needs to be aligned with the mill. You can see my procedure for this in
The RT is set up on two parallels so a thin wrench can get under it to hold the nut while the bolt is tightened. The workpiece is set on a piece of MDF so I can cut right through the work piece without hurting the RT. Offset by the desired radius and mill away. Contact me off list if you need a picture.
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