Lathe unwanted taper

So I recently noticed (when I was turning a piece of long alu ) that the lathe is turning tapers. Had not noticed this before as most of the work I did was short and did not require any precise dimensions.

Over a length of 200mm I got a 0.2mm taper (wide end at the headstock side) - could not believe my eyes! Reasoning that its a new lathe so this should not be wear related I turned my attention to it's leveling.

Can I level it without a precision engineer level? I don't have one and don't feel like spending for something you seldom use...might try to build one myself!

Chris

Is it supported with the tailstock?
If so does the tailstock need adjusting?
You don't sat what diameter ally you are turning? could be deflection.

Did you use the tailstock for support?

Has the tailstock beed adjusted front/back to make sure it is lined up correctly? having that rope tied round it may not have helped

Edited By JasonB on 09/11/2017 07:45:33

If its not down to the tailstock being wrong, I'd be looking at the headstock not being inline.

I had that trouble on a couple of new lathes.

Good luck sorting it out

Jim

Nah, should not have done much as the weight was being taken by the strap near the headstock, the rope at the tailstock was just keeping balance - that said, tailstock might need aligning anyway.

I am using a 20mm dia round alu bar protruding by 200mm, taking very,very light cuts, unsupported at the tailstock end. Still getting 0.2mm taper.

I've just re-read the the first post, do you mean the big end is at the chuck??

Jim

John gives good advice.

Regarding level, the lathe could be at 45 degrees as long as the bed is 'level' from one end to the other!

If the bed were true and flat and level, the lathe should turn without taper - but conversely, if the lathe cuts without a taper, the bed should be true (even if not quite level in the absolute sense) This means that shimming of the tailstock end, to remove any taper cutting should level the bed.

Obviously, levelling the lathe initially is a good ploy, but things like the floor or bench/stand top can also introduce extra levelling requirements. I expect that shimming the tailstock end (if the the root cause of the problem is pinned down to a slight twist of the bed) would be more than adequate for our small lathes.

200 mm of 20 mm bar - That would deflect by that amount if you applied 1/2 a Kg on the end of the bar, so it is likely that it would whip/ bend whilst you were turning it. You say light cuts, but the tool still loads the end of the bar. An accurately aligned tailstock and centre was the way to go.
BobH

I would try your test again with a piece of 25mm diameter steel protruding 150mm out of the chuck. Just to make sure the problem is not deflection of the long thin piece of bendy aluminium before you start attacking your new machine tool

I would also check the tightness of the headstock bearings because if they are loose it can cause taper turning. With a newish lathe they might need adjusting after initial bedding in.

And check for burrs on the ways and slack in the carriage and cross slide ways.

Then slacken off all the foot mounting bolts where the lathe bolts to the bench. Try sliding a thin feeler gauge under each foot. If there is any gap at any foot, a piece of shim needs to be slid in there to take up the gap. Otherwise, you are pulling the bed out of shape when you cinch down the mounting bolts.

After all that, if still turning a taper, you may need to first "level" the bed using a precision level to make sure there is no twist along it.

Then, if it still turns a taper, you can go two ways. Either twist the bed by adding shims under one or other of the tailstock end mounting feet until the lathe turns true. This method is an ok bodge for small taper errors. Probably not for 0.2mm though. The more properer way would be to set the alignment of the headstock to the leveled bed so that it turns parallel.

Edited By Hopper on 09/11/2017 08:28:58

Hi Chris you should check the alignment of the headstock. If you have a test bar which fits into the headstock taper that is best but if not use something like a 20 mm diameter silver steel held in a 4 jaw chuck and clocked up to run true both ends. Then with the clock attached to the saddle and set at centre height traverse it left and right seeing if it is true, repeat it with the clock on the top of the bar and see if there is a rise and fall. There may be adjustment you can make to align it.

David

+1

Russell

Use a travelling steady!

Assuming Chris used the tailstock to support his rod, Jason may be on to something. I found the balance point on my WM280 to be much closer to the headstock. To position the strap at the balance point I had to remove the chuck guard (one bolt and it slides off a spindle). That rope in Chris' picture would pull the tailstock, possibly enough to shift it's alignment.

If Chris didn't use the tailstock to support the end of the rod, then deflection might well be causing the taper. Rods bend, even under their own weight! It's instructive to experiment with various diameters and lengths in the chuck. With a DTI on the far end of the work, pushing the rod's end with a finger will cause the DTI to register a deflection. You may be surprised at how much spring there is in a hefty lump of steel!

Dave

Thanks for all the replies, I spent most of this morning in the workshop adjusting bench feet and doing some test cuts on the lathe, also knocked out a spirit level out of some scrap I had around and a test tube - surprisingly it turned out much more accurate than a normal level, but takes time to read a the bubble is slow moving.

Anyway, so I leveled the bench as best I could, then loosened all 6 lathe mounting bolts and did some test cuts, taper reduced to 0.1mm, next I tightened two diagonally opposite mounts, one at the tailstock near side, and one at the headstock far side - took a test cut again, now taper is down to 0.02mm...much better but now tailstock is off. Did all remaining attachment bolts, but did not tighten them up, just snugged them. Checked the bed for level, the tailstock end is almost spot on, the headstock end is slightly off, good enough for now.

I'll call it a day for now as I'm on afternoon shift - will try again tomorrow with a piece of steel taking some of the advice posted and see how it goes.

Edited By ChrisB on 09/11/2017 10:41:08

I don't know what the experts think but the picture shown seems to imply that having the lathe a little closer to the ground might do a world of good.

Michael W

Were you not aware that Chris is 8' 4" tall?

I wonder if you are anywhere near Mid Wales, Chris? If so, I can lend you an M&W level which I have used for exactly this job recently. I am half way between Hereford and Shrewsbury, and about on a level with Ludlow.

Regards, Tim - 01547 529946

Tim,

Not unless Malta (or Wales) has moved!

ndiy says: Not unless Malta (or Wales) has moved!

A good case for ever closer union, I say !

Tim