Mandrel to machine large copper tube

A friend asked if I could machine the ends of a soft copper tube around 70mm diameter to be flat and square, and to an exact length. Not easy to hold in the lathe, so I made up some expanding mandrels from engineering plastic.

I made a silver steel shaft with an M12 threads on one end and drilled a centre. The other end mounts in a 12mm ER40 collet and there is a bracing tube so tightening the nut forces the the tapered plugs into the split mandrels and they open and press very tightly against the inside of the copper tube. The mandrels when uncompressed are a tight fit in the tube.

Using this technique allows full access to both ends and the whole outside of the tube for machining and polishing.

The raw tube ready to machine
Split mandrel with a tapered bore and a tapered plug on a threaded shaft
Mandrel in place on the shaft
Mandrels in place in the tube, there is a spacer tube inside to keep the mandrels apart
Finished product

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