Spindle Play in Sieg SC4 Lathe

Recently my (only about 20 month old) Sieg SC4 began to behave oddly - alternating between cutting deeply and not at all over a cycle of many revolutions. This pretty much independent of speed but most obvious on light cuts (which alternated between a good and hopelessly scored cut along the length making a good finish impossible) and parting off where no cut at all alternated with a very deep cut even occasionally digging in). After some 'beginners difficulties' as a total novice with a new lathe I was parting off or producing a good finish quite satisfactorily so this is a problem which has only recently developed. Sharpening HSS tools or replacing carbide tips with new has not helped, nether has speed variation or careful checking of tool height.

I clocked the spindle and found that with moderate (finger) pressure - certainly no more radial force than I would expect to be exerted by a tool - the spindle was deflecting up to 0.02mm from one extreme to the other.

I don't know whether this amount of play is just expectable 'spring' or indicative of the early onset of bearing wear and the cause of my problems - so the opinion of old hands would be welcome.

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0.02mm play, or around 8 thou in old money, is definately not acceptable. On a deep cut the tool pressure will most likely push the spindle back and give more consistent results. On a light cut the tool pressure will do very little. With that much play, I suspect parting off is nigh on impossible.

I would check the adjustment of the bearings and nip them up, not too tight, just so the play is removed and the spindle gets no more than gently warm after a long period of running.

I would just like to point out that 0.02mm is not .008”, Old money or new, it is around eight tenths of a thou.

There seems to be confusion over measurements; 0.02mm is not 8 thou, more like 0.8 thou. On my (industrial) lathe putting a cheater bar up the spindle and pushing/pulling as hard as I can resulted in no movement when pushing down, and 0.01mm when pulling up. So 0.02mm of movement isn't great but probably won't be noticed. If the movement is 0.2mm then something is seriously wrong.

The forces generated when turning can be surprisingly high. The formal way of showing them is with the Merchant diagram. When turning the workpiece is trying to pivot up and over the tool point. Let's look at the torque available. My lathe has a 2200W motor. Suppose we're turning 25mm diameter bar at 500rpm. Spindle speed of 500rpm is equivalent to 52.4 radians per second. So the maximum torque is about 42Nm. At a radius of 12.5mm that produces a maximum force of 3360N, or ~336kg.

Andrew

Are you cutting brass now? That will grab.

Too much stick out on thin bar stock.

Is the chuck still holding within 3thou runout? If not it may be chips stuck in the chuck scroll.

Check the saddle rear gib has not come loose. Also try locking the saddle and use the top slide to take the sliding cut.

Thanks for responses so far:

br: An approach to Arc might certainly be useful - but trying to get a better idea of what is amiss first (i.e. little point in approaching them if it's my technique at fault).

Dave Halford: No problem at all with brass, which leads me to think that it's the greater forces involved in cutting steel which are at play. The problem was last experienced working on an 18mm steel bar projecting around 50mm from the chuck - so I don't think that it's deflection of the work (though correct me if I am wrong). I'll check the saddle.

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Dave Halford: You were very close there - tugged hard at top slide and enough movement to produce an audible clunk. Locked saddle and still the same. Cross slide gibs appear to need adjustment as this is where the play is. I'll have a go at them later and report back. Thanks.

Was Gonna say check all carriage, x slide, top slide, tool post gibs and fastenings before blaming the spindle.....

Questions: Do you lock the unused axis when cutting? Does this occur with cutting on both axes? Have you added a QCTP? What support, for the workpiece, are you using? What material are you cutting?

I expect it will be operator error. It usually is. Less than 0.01mm deflection is unlikely to cause problems if all usual precautions are taken, while machining.

Mount the indicator on the housing of the spindle to make sure you are measuring a spindle problem, not the play and deflection of the other parts.

If the spindle has tapered or contact angle bearings, these can be adjusted to reduce play. The manual should mention the bearing type.

That is too much movement for a spindle with roller or ball bearings under moderate finger pressure. The better way to test it is to clock spindle movement at the headstock while yanking as hard as you can on the end of a 12" long piece of stout bar or pipe held in the chuck. That way you find out how much slack you really have in the bearings. It will be more than light finger pressure at the spindle itself has shown.

The good news is, your bearings are easily adjustable for preload. Usually they run with a very light preload and zero slack, so you should get no measureable movement at the spindle. Adjustment is usually a simple threaded collar on the far left hand end of the spindle. See your manual or the ArcEurotrade website for details.

As it is a new (ish) lathe, it is pretty normal to tighten things like bearings and gib strips after a bit of use and things have bedded in. If the bearings continue to need frequent adjustment in the long run, replacement with better quality roller bearings from Timken or SKF or Naachi etc would be one way to fix it. Never use cheap Chinese bearings. But the Seig owners on here have not reported bearing problems in the past so I am pretty sure you will just need some adjustment, not replacement.

Edited By Hopper on 10/03/2022 04:50:18

Also do any tests on a warmed up lathe not one that has been siting in a near zero deg shed unused since the previous weekend.

^^^^ Good point about zero degrees. Such thoughts never occur to us here in the tropics. 44C in the shed without insulation. 33C with it.

Run your lathe at high speed for half an hour before setting bearing end float/preload.

Edited By Hopper on 10/03/2022 08:50:25

Could I make tentative suggestion that the problem might be caused by backlash on the cross slide.

David

I agree with suggestions the problem may be elsewhere, but even 0.2mm play is excessive for the SC4. You should be able to remove this completely by adjusting the spindle bearing preload. As mentioned above, warm the lathe up first.

Avoid excessive preload by monitoring bearing temperature.

Lots of good advice/guidance on other threads, mostly from Ketan @ Arc.

Neil

Well, thanks once again to all the helpful responses - my apologies for taking so long to reply, but I've been quite badly laid up with the dread Covid.
It's been a case of a little of everything, but pretty much all suggestions have contributed to largely getting rid of my problems: cross-feed gib adjustment, removing a small amount of saddle play, and a slight increase in headstock preload to reduce the small amount of play there.
No idea really on how much to increase preload but warmed machine up first, inserted long bar in chuck and tightened until no appreciable clock movement except due to the quivering of my arm. All the while turning mandrel to ensure not introducing any binding. Then ran machine at full speed for half an hour with no bearing heating beyond a slight warmth - so hopefully got that right!

Slightly worried over gib adjustment which required tightening to an extent that movement is quite stiff before play was eliminated - I can live with that, except slight worry that this might cause accelerated wear on the cross feed drive mechanism.

Sounds like you got the preload about right. Zero drag with zero shake is always a safe bet. A small amount of extra preload is good, but too much can shorten bearing life.

Cross slide should not need to be binding to eliminate shake. It might be worth pulling it apart and looking for burrs on the gib strip and dovetails. Dress them down carefully with a small flat slipstone or file. It is best t then set the gib strip adjustment without the feedscrew in place, so the slide moves smoothely with no binding or shake, then reinstall the feedscrew and adjust the end float of the screw spindle/handle/dial mechanism for minimum backlash.