Thread used on British Browning .303 jacket.

[Brit plumber]

Hi Patrick, Said cone is currently a tube, I need to make a mandrel and force the tube over to make the cone. The end result should look like this...I hope.

[Patrick Chadwick]

The mandrel should be made somewhat longer than the workpiece, with center-drilled recesses at both ends, so that the workpiece can be turned and/or threaded while it is mounted on the mandrel, which itself is either mounted between centers, and driven by that almost-forgotten device, a lathe dog, or with the one end of the mandrel in the chuck and the tailstock end running on a center.

Of course, it would be easier to cut the thread before creating the taper! The trouble with tubes is that holding them in a 3-jaw chuck is a recipe for disaster for anything but very light facing cuts. A 3-jaw squeezes the tube into a 3-lobed cross-section and/or presses grooves into the tube, so the usual method is to use a collet, and the difficulty is typically to find a collet that has an adequate through-bore to take the tube body, leaving just the portion to be threaded outside the chuck.

If you have the patience, but no suitable collet attachment, then one solution is to turn up a special "one-off" collet. Fix a piece of suitable brass bar in the chuck, face it off, bore out to the outside diameter of the tube as closely as you can, so that the tube just slides in if it is greased. File a mark on the faced-off end of what is now a brass bush to mark the orientation of the tube with respect to the chuck jaws. Marking with a pen is no good - been there, done that - the mark is bound to rub off in handling!

Remove the bush and cut 3 equally spaced longitudinal slits at one end of the bush, about 3/4 of the length. Repeat at the other end, with a 60 degree shift, so that the slits from the other end lie between the first set.

You now have a hand-made "concertina" collet that can be reinserted in the chuck (observe the jaw orientation mark!) and squeezed up ever so slightly to grip the tube perfectly, without causing any distortion. You can now turn the thread on the exposed portion.

And if you think this sounds like theory, it most definitely is not. I have used such a device to remove the bell-mouthing from the last millimetres of a black-powder barrel. Very, very slow, but it worked!