Fixture / jig clamp for 20mm round stock in lathe

I'm looking for some ideas on how to repeatedly and hold some 20mm OD PVC round bar in a lathe chuck. I'm turning batches of 20 pieces - they start as a 25mm length of 20mm OD PVC rod and end up with:

a) an 11mm thru' bore b) a 17mm x 5mm counterbore. c) a 17mm OD step on the outside at one end.

Hopefully the cross-section below will make sense of my ramblings:

__________ _____| ____] |___________]

11mm thru 17mm internal counterbore bore ___________ |_____ ]____ |__________]

17mm 20mm OD step

Is there a neat way I can fabricate some sort of jig into which I can quickly clamp the 25 L x 20 OD PVC pieces so they will be in the same location? Some sort of simple collet??

I need to achieve

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According to Robbo :

I'm not sure how the bores are meeting -- or whether they are expected to meet.

Yep. start out with a piece of aluminum or brass round stock large enough so you can offset a 20mm diameter bore to produce the desired offset when the piece is chucked in a 3-jaw chuck. (You'll need a 4-jaw chuck to bore the hole of course). Make it long enough to hold the length (exclusive of the length to be machined on one end), and solid for perhaps 50mm after that to give the 3-jaw chuck a good grip. While you have it in the 4-jaw chuck, turn the outside to be concentric with the pocket for the full 25mm length, with perhaps a 4mm wall thickness. (Hmm ... make it shallower -- the pocket should be no deeper than the length of the workpiece full-diameter body plus the longer of the two turned down projections (assuming that they are not of equal length).

Once you have the pocket machined, go to a vise, and use a hacksaw to cut three cuts through the center of the pocket at 60 degree angles so you have six evenly-spaced slots.

Deburr the saw slots on the ID so you don't scratch your workpiece.

Mount the solid length in a standard 3-jaw chuck (assuming that the accuracy of a 3-jaw is good enough. Slip the workpiece into the pocket, and put a worm-gear hose clamp around the OD as far from the chuck as possible. Tighten that with a screwdriver, and it will grip the workpiece.

If you want, you can make two of these -- the second will be shallower, and then with a hole drilled for the first projection, so the orientation from the first to the second will be consistent.

That -- plus a hacksaw, or a bandsaw -- should be all that you need.

Good Luck, DoN.

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DoN. Nichols

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