Turning Between Centres: The Setup You Can Take Out and Put Back

Face each end square, then drill a centre hole with a centre drill — the short stiff two-diameter drill that cuts a small pilot and a 60 degree countersink in one go. The 60 degree cone is what the lathe centre sits in, and it must be cut, not punched. A centre-punched dimple is neither round nor at 60 degrees, and the work will run out and wear the mark oval as it turns. Do not run the centre drill in too far. The small pilot tip is fragile, and if you bury it to the shoulder the countersink bottoms out and the centre bears on the wrong surface. Cut until the cone is a little larger than the centre's point and stop.

Put a live (rotating) centre in the tailstock where you can. A dead centre does not turn, so the work rubs on it and needs grease; a live centre carries bearings and turns with the work, which is why it tolerates speed and load without seizing or scoring. Before trusting the setup, bring the tailstock centre up to the headstock centre and look at the two points end on. They should meet tip to tip. If the tailstock is offset, every shaft you turn will come out TAPERED — and it will be a beautiful, consistent, entirely wrong taper. That offset is adjustable, and on many lathes it is deliberately offset at some point in the past to cut a taper and never put back.

The centres locate the work but cannot turn it — they are just cones in holes. A LATHE DOG clamps onto the work and its tail engages a slot in the faceplate, so the spindle drives the work while the centres decide where it sits. Tighten the dog on a part of the shaft that will not be finished, or protect the surface with a shim, because the screw will mark it. Check the tail has clearance in its slot — if it binds against the end of the slot it pushes the work off its centres and you lose the very thing this setup is for. Support the tailstock end with a little grease if the centre is dead, and re-check it after the work warms up: a shaft heated by cutting GROWS, tightens itself between the centres, and will squeal and score if nobody eases the tailstock.

Turn a diameter, measure it, remove the work completely, then remount it between the same centres and put an indicator on the turned diameter. It should run true to within a few micrometres — far better than any three-jaw will manage. Then turn a second diameter at the other end and check the two are concentric. That is the whole argument for this setup in one experiment. It is why a shaft that has to be hardened between roughing and finishing is turned between centres: the centre holes survive the heat treatment, so the finished part is concentric with the roughed one even though the two operations were weeks apart.