Lathe cutting tapers.

My lathe has suddenly started cutting tapers.

Using a three jaw chuck and no tailstock it is 0.05mm taper over 40mm on 16mm stock and on 40mm stock it is 0.02mm over 4cm.

It was fine a couple of days ago.

Any ideas?

Hi Chris

Dirt under the saddle?

Tool height is over or under your normal centre height. Or (But even so it should turn straight)

The tool is incorrectly set and is pushing the work away as it cuts. If that is happening it will turn a larger diameter towards the tailstock. Was the work you did very thin?

Edit Hmm missed the 16mm. That should be fairly stiff over 4cm? Was the tool in good condition.

But the fact that the thicker test was half the error suggests the centre height may be the culprit. In smaller diameters quite critical or you get a taper. Maybe there was a bit of deflection too on the smaller diameter cut.

Place a dial indicator against the side of the chuck with a heavy bar gripped in it as a leaver push it to and away from you by hand. Does the indicator move? you may get a few tenths of movement if it has plain bearings. any more and the spindle may need adjustment.

Regards
john

Edited By John McNamara on 19/02/2014 14:01:42

Edited By John McNamara on 19/02/2014 14:06:17

Edited By John McNamara on 19/02/2014 14:13:15

Hello Chris,

Have you had the chuck off and back on again in the last 2 days? Register cleaning is a possible source of the problem.

Have you swapped the jaws round for another job in the meantime? Again this might be down to cleaning with swarf in the scroll.

No other ideas yet

Brian

Brian,

The register of the machine won't have anything to do with it turning a taper.

Steve.

Blunt tool? Colin

I don't think it's the chuck, it hasn't been changed recently.

There is 0.05mm of play on the chuck when an 18" bar is in the chuck. When chuck is removed and bar inserted into spindle, play on spindle is 0.02mm

tried various tools and checked the height. 

Edited By Chris123 on 19/02/2014 16:32:15

Probably a silly suggestion, but you are not using the topslide for this by any chance?

Mine has been known to 'self adjust' by a very small amount at odd times particularly if some force has occurred on a previous operation.

Edited By V8Eng on 19/02/2014 16:38:38

Edited By V8Eng on 19/02/2014 16:39:50

No, it's on auto-feed.

No slack holding down bolts / nuts on the lathe mounts?

Tightened headstock bearings.

Checked bolts.

Still the same.

Steamer 1915-

Hello Steve, Yes of course you are right, chuck fitting doesn't come into it; must be going soft int ed.

Sorry to make a fool of myself really

Brian

Other ideas:

Check all gib strips and make sure the feed nuts of the various slides have not come slack, expecially the cross-slide.

Try a very sharp tool with a relatively fine cut. If this gives better results it suggests play somewhere may be the cause. If not, is it pssible the headstock has shifted?

Neil

Have tried sharp tools with 0.1mm cuts.

All gibs etc have been checked.

Its a Harrison lathe, I wouldn't have thought the headstock would move suddenly.

Scraped the ways.

Is there anything in the saddle / carriage that would affect this?

Edited By WALLACE on 19/02/2014 20:27:12

Edited By WALLACE on 19/02/2014 20:27:58

I've lifted it off its mounts and it's the same (or worse), the lathe weighs 700kg so I've always assumed it wouldn't twist?

the ground has been very wet here recently and hasn't dried out for a month or so.

Ive tried turning short and long pieces of metal.

I have done two test now that I believe indicate the saddle is lifting slightly near the chuck.

1) DTI on top of chuck, attached to saddle, approx 0.05mm difference over 7cm, highest figure at tailstock end of chuck.

2) DTI on ways, attached to saddle, approx 0.05mm difference over 7cm, highest figure at headstock end.

Is it a gap bed lathe? And if so has the gap been moved recently, or is it fitted properly? This has happened to me, and it was also the subject of an earlier thread on this forum with, if I recall correctly, a Harrison M500. All sorts of complex solutions involving everything up to complete rebuilds were suggested. It turned out that the gap piece hadn't been fitted properly.

Regards,

Andrew

Yes I saw that thread!

It's not a gap bed though.

Chris,

How old is your Harrison? could the bed be worn?

I once bought a Chipmaster at a price I could not resist. It had been used as a copy lathe for years. The bed was so worn, it had a noticeable dip in the middle and so the saddle movement was no longer parallel with the spindle axis. If you set the tool to the correct height next to the chuck, it would be out as it was moved towards the tail stock. The result was it always cut a taper, never parallel. The taper was more noticeable with smaller diameters and shorter workpieces. The tail stock would also fall into the dip and out on line, with short workpieces.

Mount a DTI in the toolpost and run it along the top or bottom of some ground bar held in the chuck.

Mike

It's fairly old, but metric.

I was boring last week over a longer distance and it was fine.

I've had it 8 months or so, it was relatively unused for 10 years before I had it. Before that it had spent it's life in a technical college. It's in very good condition for its age IMHO.

Will try try the bar in chuck when I get home.

Chris

My worn Chipmaster was better over the longer distances. The dip in the bed was near to the chuck.

A boring bar would of course move the saddle further away from the chuck onto a less worn part.

Make the DTI over as long as distance as possible

Mike