Theoretical question about travelling steadies

Something that's just ocurred to me whilst watching one of Abom79's recent videos, is the use of a travelling steady on stock that may not be perfectly round - imagine, for this question, it is markdly oval in cross-section. If the steady is bearing on the uncut portion of the work ahead of the tool, will it not force the cut to produce a slightly smaller oval?

And if you tried to true up the stock by taking an initial cut without the steady, it would flex away from the tool in the middle, leading to a piece that's thin at both ends and much, much thicker in the middle (as Monty Python would say, like a brontosaurus).

So should the steady bear on the stock that has just been cut for best results, the first few inches benefitting from the support of the tailstock before the steady is set up?

Rob

Yes the steady has to bear on a truly cylindrical diameter

Tony

If the work is thin it might be possible to generate a true surface close to the chuck at one end then cut back towards the chuck with the steady on the newly cut surface ? Noel.

If the material is out of round it should be held in a 4-jaw independent chuck, not your best 3-jaw one, but otherwise the best route is as Noel suggests

It may be possible to centre-drill the end, support that on a tailstock centre then turn that initial steady-bearing surface.

I did something similar only today, though with a fixed steady: centre-supported the bar to turn the outer surface. used the steady to support that, then removed the tailstock support to make way for the internal operations.

Travelling steadies are often called a follow steady, mostly in the US I think. Reason being it usually follows the tool so it bears on a smooth, cylindrical machined surface. Usually you will have a centre in the end to hold the end steady and you machine the first bit to diameter and set the travelling steady to bear on the machined bit before proceeding to the flexible centre area of the job. If you can't use a tailstock centre for some reason, you can machine the first end part with a fixed steady then fit the follow steady to fit that diameter.

Bit tricky on a Myford because the travelling steady fits on the leading edge of the carriage so you have to extend the topslide way out there to get ahead of it. A less than ideal design, but cheap to manufacture. So a workaround can be to take a light skim cut over your long job and set the leading steady to bear on that. Or you can set up a following steady block in the rear toolpost, just a piece of square brass bar with V notch in the end etc. It moves with the cross slide so can be a bit of a fiddle to set up but can be done.

If using bright mild steel etc that is reasonably round and set reasonably true to start with, you can sometimes get away with using the leading steady but results vary depending on the amount of runout etc. But obviously not possible with something like rough cast iron bar etc.

This is why capstan and turret lathes used Roller boxes with rollers leading or trailing the tool, depending on the workpiece.

With non round material, you have to turn a short length on which the rollers can run.

Howard

I don't think the tool will cut a smaller oval. if anything it will cut the inverse. So a wild leap might imply that over several passes the original ovality is reduced.

Andrew

The two roller boxes for my repetition lathe both have rollers that follow the tool. Presumably the first op is to turn a small spigot before extending the work to full length. For the two capstan roller boxes I have one has rollers that follow the tool and one has rollers that lead the tool. As Howard says this is more likely to do with the shape of the workpiece and the required cut than ovality of the stock.

Andrew

If you want an accurate thread on a long workpiece a travelling steady makes good sense. It is easy for the centre or end section to move away from the cutting tool producing shallower depths of cut. You are usually starting off with a round part before cutting the thread so that is not an issue.

Martin C

And it is helpful to use the travelling steady in the leading position like that when screwcutting because a following steady would have to bear on the freshly cut thread, which usually has a burr on the top of the thread sticking out.

Thanks for all the replies, chaps - seems like, as with most things, there's no hard and fast rule but you vary the approach to suit the job in hand.

Rob