Lathe turning out of round parts

[Former MemberParticipant @formermember32069]

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[Former MemberParticipant @formermember32069]

[HopperParticipant @hopper]

Posted by Barrie Lever 1 on 13/08/2018 00:02:08:…

…a stepped shaft made in free machining aluminium, 5mm dia going to 6mm and then 8mm, length of the 5 and 6 mm dia sections is 35mm each ….

That's a long thin job that is going to be subject to quite a bit of flexing. I would suspect this aspect, or some defective material that has a hard spot etc through it, rather than a lathe malfunction. You are right, chuck etc should not cause out of round turning. It should "find its own centre" as you say.

Are you using a tailstock revolving centre? Is it running true, or running a tad out of whack? Maybe try a fixed centre?

Are you using HSS toolbit? Carbide tooling might tend to rub and cause problems on a long thin ally job like this.

Have you tried some test pieces of other, larger material to see if they do the same? If the problem is a lathe defect, it should show up on a piece of half-inch steel bar or even larger turned in similar manner with tailstock centre support. If they turn OK, then the problem would seem to be either the material, or the long thin nature of the aluminium job you are attempting. You may have to make some kind of travelling steady to support that job. Or turn it in shorter sections, sliding the stock out of the chuck/collet as you complete each shorter section.

[Former MemberParticipant @formermember32069]

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[JasonBModerator @jasonb]

Try it with a non revolving ctr to eliminate any problem with that.

Does the error measure bigger towards the head or the tailstock? If it is one of these then I would expect it to be worse at one end than the other.

As Hopper says what tooling were you using, if taking fine 0.005mm cuts there will almost be some element of rubbing.unless razor sharp

Edited By JasonB on 13/08/2018 07:31:47

[JasonBModerator @jasonb]

Another thought is that the bearings have been stood vertically for what 20+ years? it is always said they should be stored flat and only for a set length of time so you may have a problem there

[Former MemberParticipant @formermember32069]

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[HopperParticipant @hopper]

Yes, if its doing it on larger pieces without a tailstock centre, in the chuck and in collets, it's looking more like bearings, or poor fit of spindle to bearings or bearings to housing etc. Might be worth checking the headstock hold-down bolts to, while you are in there.

+1 on long term storage not good for bearings. I see it in the gearboxes of BMW motorbikes that have been stored for years. Keep riding them they last for many decades. Park them up and they go to hell in a few years.

Edited By Hopper on 13/08/2018 11:22:26

[larry phelan 1Participant @larryphelan1]

My thanks to JasonB for that information,that,s something I did not know [along with many other things ].

Seems there is a lot more to bearings than I thought !

Should machines be run from time to time,even dry runs,to help avoid some of these problems ?.

IS it possible that the material was out-of-true to begin with ? this is not unusual with some of the stuff we get here.

[Howard LewisParticipant @howardlewis46836]

If the bearings are ball or roller, and the lathe has been stored with bthe spindle vertical, and even worse, subjected to vibration, it is possible that the outer raceways have become "Brinelled".

The likely result of this would be that the out of round becomes regular, a bit like a very small serration, mimicing the depressions in the raceway of the bearing.

Replacing the bearings (and seals) should rectify the problem.

If the bearings are plain, (parallel or tapered) then one wonders if there is some wear (unlikely in a little used machine) or a lack of adjustment (slack?) allowing the spindle and the workpiece to try to climb around the cutting tool.

H T H

Howard

[Former MemberParticipant @formermember32069]

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