Cutting on return traverse?

Good afternoon,

I've searched the forum and the internet, but to no avail. Maybe I'm not using the right search words.

Irrespective of the material, when cylindrical turning, the tool cuts slightly when returning the carriage for the next cut. Power feed or manual. Even with a relatively small 0.12mm depth of cut. 30mm stock protrusion, 12mm dia. Tried various spindle speeds and feed rates without success.

Cross slide and compound gibs are adjusted and headstock bearings feel OK. Lathe is a Warco WM250.

This is caused by there being flex/play in the system and is pretty much normal in most amateur set-ups....you can eventually get rid of it by doing repeated cuts on the same setting until the tool effectively runs out of material to cut. When you return the carriage after the slow feed cut you usually return at faster speed so a thread like outline is imparted to the workpiece.

This 'problem' is much less evident in heavier lathes with nice big castings and plenty of mass...indeed, although rarely mentioned it's one of the main reasons to have a 'pro' lathe. In our club workshop we have a line of S&B 'L' lathes which although small(ish) are made with a massive cast base which weighs in at a quarter ton....not surprisingly they turn 'dead' with very little spring or flex in normal cuts.

Hello Bo'sun,

It is a common observation, caused by a slight slewing of the carriage on return, due to either wear in an older machine or even resetting of the carriage using the normal clearances on a new machine.

Nothing to worry about. If you are trying to bore to a fixed size for, say a bearing pocket, then take the finishing cuts on the inward travel and wind off the feed to avoid re-cutting again on the return. The alternative is to do the final sizing cuts on the return, it is a little more tricky to manage but perfectly feasible.

Regards

Brian

It happens, for the reasons given, even on industrial lathes, with roughing cuts. When roughing I don't sweat over it, although I may take a second pass before final measurement. For finishing I simply dial in what I want to remove and retract the tool at the end of the cut. Heaven forbid, but if I boo-boo'd and need to do another finishing pass I simply move the carriage back and reset the tool (plus another cut) using the cross slide dial.

Andrew

Another reason is the tool flexes slightly one way on the forward cut, and springs further back on reverse letting the tool present a different edge to the job.

Fun can be had by sticking a DTI on cutting tools and protruding work to see how far they move when pressed firmly with a thumb. More obvious on small machines than big ones but metal bends! That nice cutter is really a just a spring with a cutting edge. The more rigid the better, so avoid overhangs and snug everything up. I should use steadies more than I do...

Dave

This is by no means unusual.

As an apprentice, I was berated by the Fitting instructor (barging in on the Turning instructor's realm ) for leaving a tool withdrawal mark when I withdrew the tool at the end of the roughing cut, with the lathe stopped.

I pointed out to him that at the end of the finishing cut, the tool would be removed from the work before withdrawing

had the work still been rotating, he would have complained a helical mark all around the work rather than the short linear mark!

Why does it happen?

Probably because of spring in the work and in the tooling, caused by the cutting forces. Sometimes although no change of any setting has been made, a second traverse over the work will result in a shallow cut. This is why such things are described as a "spring cut " Often used when boring to fine limits, (i have known as many as six passes before the tool stops removing metal )

Howard

If your lathe cuts when moving away from the chuck if you are on a finishing pass before returning the tool apply and maintain light pressure on the toolpost to keep the tool off the work when moving the tool away from the turned part.
This is usually enough to prevent the tool marking the part and you still have the last pass slide setting.

Emgee

Edited By Emgee on 07/02/2021 14:07:40

At my last place of work we had an old Dean Smith and Grace lathe, a big one but with a lot of wear in the carriage. It was a magnificent machine despite that.

It was not unusual to take a further 1/4 mm cut or more when retracting the tool and you could utilise that in roughing out a bore, you could actually see the saddle slew slightly. It was still quite possible to machine a bearing pocket to size as long as you did as Andrew has described

Brian

+1 to what Andrew says.

Work to the dials and set the finishing cut "knowing" it will turn out right. Brain farts excluded (not gonna admit what my lifetime brain fart count is!).

If your lathe won't do that then time to go right through it and fettle things so it does. Not wasted time. So much less stressful and quicker to get jobs done when the machine does what its told to within sensible tolerances. If you can't get it to behave unload it and get something that will. Life is too short to measure every cut.

Like Howard I accept a dragged line on a stopped workpiece when its time to measure before a final cut.

One nasty gotcha on finishing cuts is that lathe tool spring may be less than on heavier cuts so the finishing cut comes out oversize. One of those times when you need to know your lathe and, to a degree, how good your tool grinding is.

Clive

The Smart & Brown model A does it every time a small ammount. We get about 0.001" on the diameter, or the familiar helical groove if the saddle is withdrawn quickly. Don't worry about it, just back off the tool if the little extra cut is going to cause problems.

The leading cutting edge of the tool, having done the hard work, may be less sharp than the trailing edge which is cutting on the carriage return. Colin

Or stop at the chuck end of the cut, measure, and then decide whether to withdraw the tool or reverse feed direction and finish the cut that way. With a sharp tool, you'll soon know how much spring cut you'll get from a typical (pre-) finishing cut. If necessary - say with an unfamiliar material or tool - try it a time or two on the way in to determine the value.

Thanks everyone for the comments and suggestions. I had no idea the issue was so common. Yes, the WM250 lathe may not be as robust as some, but it did take me a bit by surprise. I can see a few trials coming up to see which method works best.

Am I mistaken or does Sparey refer to this in his book, when talking about boring ??

Just wondering ! I get it all the time, it,s no big deal so I dont worry about it.

I've used lathes from many respected British makers in the remote past, as well as my current WM250V, and I can't readily recall one that didn't do this in some conditions.

I thought Emgee’s post sums it up quite succinctly (when understanding what he is accepting). The tool is deflected away from the workpiece by the cutting pressure. He is demonstrating how little force is needed, to move the cutter just a tad off the cut surface for a clean return of the carriage.

IF the cut was made absolutely perpendicular to the direction of travel, so no resultant force, towards the centre of the work, existed (so no equal and opposing force from the workpiece), with a lathe with no ‘looseness in the carriage to bed, there might not be any line at all. We are assuming there is no sideways deflection of the chuck, as well, when under load.

Utopia rarely exists in a hobby workshop.🙂

Even if all the slack is taken up (etc) so the carriage etc cannot swivel, this can still happen. The cutting itself puts stress on the lathe parts, pushing them away from the workpiece. As soon as the stress is off, - ie you get to the end of the cut - the metal springs back to where it was before you started. Return the cut, and there is much less stress pushing the tool away, as most of the metal has been removed, so the distortion is less and so the tool is pushed away less. But it is still pushed away a bit. And it takes a tiny extra shaving off.

There is another effect causing this to happen. On the first pass, you are cutting a thread, in effect. Not a proper V thread, but a spiral of some sort, depending on the exact shape of the tool and the rate of feed. On the return pass, you follow the opposite hand of spiral, and so the tool can remove metal from the high ridges (even though they may be only 0.0005" deep) and so you notice extra swarf.

So - nothing to worry about, as really, in the real world, impossible to prevent.

Cheers, Tim

I wonder if this would happen when boring in reverse with a left hand tool? I can't do the tests until the lockdown ends.

Old mart - yes, I can see no reason why the effects would be different. They might vary in detail, as any slack, and and pressures, would be in somewhat different directions, but the effects would still be there.

Cheers, Tim