Lathe run out

My lathe is turning tapers where I do not want tapers

34" of overhang in the 3 jaw chuck. At the far end it has 0.01" of runout on the DTI, not bad for a rusty old bar of 1" cold rolled.

However the far end is 0.25mm forward and 1.11mm below where the tailstock wants it. This may or may not agree with the test spec. that came with the machine, they forget to define their units, could be Siamese.

If I face down a bar leaving a tiny nub in the middle and then dent it with a hard center, I find the tailstock is fractionally low and forward. Unfortunately making everything worse not better.

Could it be in need of levelling? I never bothered. Seems unlikely but could the uneven garage floor be twisting it?

The "head stock" appears to be held on by 4 chunky screws. I cannot see any taper pins locking in the misalignment.

Should I send out for shim and start rotating until it turns cylinders, or, would loosening the four bolts be a huge mistake?

I never offset a tailstock in my life for taper turning so I am quite hapy to shim it for the vertical alignment.

Decisions, decisions... What would you do? Has anyone dared loosen those bolts? How did it go?

Not sure where you’re coming from.? When is it turning tapers chuck mounted or tailstock supported

34" of 1" bar is basically a noodle. It could be anywhere. Even if it were held by the tailstock centre it would still not turn parallel because the centre would whip. This is a time that you need to use a travelling steady.

Edited By Pete Rimmer on 10/05/2021 18:47:57

There are plenty of threads on here about lathe alignment (often called lathe levelling). DO NOT undo the four headstock bolts that hold the headstock to the bed - Here Be Dragons.

The long bar is useless as either an indicator or as a means of alignment. Need basically to take any twist from the bed caused by uneven floor before doing anything else, this is done without use of the tailstock.

Once untwisted the tailstock can be checked for alignment .

John

“not bad for a rusty old bar of 1" cold rolled.” So did you run the dti all along the bar at the front and on the top? Bet it won’t be held true in the Chuck and it will probably be bent accounting for the difference and this bar even if short  will not be held true due to the rust and  rolling tolerance, it’s not an accurate material 

is the runout you quote on the rotating bar?
As said above you need tailstock support and a travelling steady

have you done the standard bobbin turning test to check for any bed twist?

have you done the between centres test to check the tailstock position?

moving the headstock will not be easy due I think it being sat on the v ways on the bed

tread slowly!

Edited By Zan on 10/05/2021 19:01:31

I was trying to turn 5" of 1" hex steel down to 17mm to fit some bearings and stuff. They didn't go on very far, it got fatter approaching the chuck. Like me, fatter. The tailstock was quite extended due to a DRO scale, not much chance of it forcing a 17mm steel bar into alignment

I could try it on 3 feet, a tripod, that only involves sticking in a bit of broom handle under the light end so it can rock.

The headstock appears to sit on a flat plate which is part of the bed. Two bolts from above, two from below.

Do you have a trusted dead centre?
I am assuming, as it looks like a fairly new lathe that bed wear can be ignored.

Chuck a short piece of scrapbinium ( something like 10mm diameter or so with 1 diameter stick out) and turn a point on it. Move the tailstock up.

compare points. They should line up exactly in both planes. Trapping a 6” ruler between them is an often mentioned way to amplify any offset, but I’ve never done that. If they match it’s fairly easy to see with a good light and some magnification (I use an optivisor).

IAssuming that the points do match you know the tailstock lines are up.
The headstock might still not be aligned with the ways, or the ways may be twisted.

I had a Harrison L5 not on a cast stand, so it stood on the headstock plinth and a 2 legged tailstock support.

IIRC it needed about 50 thou of shim to turn parallel. That was stood on a smooth level concrete floor, and L5s are not exactly “lightweight” - probably a chunk stiffer than your modern lathe.

look up the 2 collar test - and do it with a sufficiently large diameter that the bar is stiff enough to not deflect significantly.

I don’t know how stiff the cabinet stand is, but you can shim under it or under where the bed bolts to it.

I would not touch the headstock bolts until a great deal more measuring and checking has been done. Then *if* you

do need to at least you’ll know why.

Dave

If I were you I would:

Check the bed for straightness with a sensitive level. Adjust to suit (if it's possible). If you have no level or it's not adjustable (maybe it's on three feet) just carry on down this list.

Chuck up a stout piece of ally in the chuck (perhaps 2" dia and 6" long) take skim passes and check for parallel. If the bed is not twisted and the part dosn't turn parallel then you have no choice but to loosen the headstock and adjust.

Once you have it turning parallel time to adjust the tailstock. Measure the tailstock ram with a micrometer. Turn the piece in the chuck to the same diameter. Bring the tailstock up to the part you have turned.

Put a mag base on the saddle with a tenth-reading dial gauge. Put the dial gauge against the side of the ram and move the saddle so it crosses over to the turned bar. It should read the same. If it doesn't adjust using the off-setting adjuster until it does.

Now do the same but running the dial gauge along the top of the parts. If the tailstock is low shim between the body and base to suit. Check the ram is level with the dial gauge or very slightly rising (never falling).

If you do all that you'll be about as good as you can get.

I think Pete has it. I want to turn a cylinder so the obvious test is to turn a cylinder.

However, that means it must be nutted down tight which rathe precludes adjustment.

Perhaps turn the truncated cone it is set for.

Put a DTI on the tool post, which will show perfectly straight.

Adjust it until the DTI shows the correct error.

But first, I am moving into uncharted territory, I am not a shim person...

These are the bolts holding the headstock down at the front

I have cleaned the plaster and paint away from where the shim will go. Don't want to do a Hubble.

The shim will be 0.26mm thick, 180mm long, have two 12mm clearance holes for those bolts, centred 16mm back from the front edge.

How wide should it be? I see problems for both too much and too little. Is there a best practice?

STOP! STOP! STOP!

You are putting the cart way before the horse and getting several separate issues mixed up. Moving your headstock is an absolute last resort and should be avoided if at all possible. It is rarely needed.

Your tailstock is adjustable and is usually adjusted to match the headstock, not vice versa.

First, you need to realise there are two taper turning causes, with two tests and two solutions:

1. Bed Alignment.  Stick a piece of 1" diameter bar in the chuck with about 4 to 6" sticking out. NO tailstock centre in place for this test. Take a fine finishing cut along the length of it and measure the job for taper. It should be within a thou or less (0.025mm). If it's not, the adjustment is made by shimming ONE of the mounting feet where the lathe attaches to the bench, at the tailstock end. This "twists" the bed to get it aligned to your headstock spindle axis.

2. Tailstock Alignment. Stick a short piece of bar in the chuck and turn a 60 degree point on it as close to the chuck as possible. Then put a known good centre in the tailstock and slide it up so the two points almost meet. Pinch a thin steel ruler between the two points, with the quill extended about the amount you normally use it at. The steel ruler should stand up vertical and also should lay square to the main lathe axis when viewed from above. If it does not, adjustment is done by adjustment bolts or screws in the tailstock base that move it from side to side. If the tailstock centre is lower than the headstock centre, you will have to put shim between the base and main body of the tailstock to bring it up to headstock spindle level. Further fine adustment is made by turning a piece of 1" diameter bar 6 to 12" long between centres and measuring the resulting taper after a fine cut. Adjust tailstock offset until less than a thou of taper.

The best and simplest written/pictorial description of how how set up your lathe's bed alignment and tailstock alignment I have seen is in the front of the Myford ML7 Owners Manual. PDF copies are available free all over the net. It applies to all lathes, not just Myfords. You should read it carefully before adjusting anything. It gives two ways of doing the bed alignment, using either an expensive precision level, or by the simple turning test outlined in 1. above, which is all I ever use in the home workshop. Then they describe the tailstock alignment by turning test as in 2. above.

The test you have described with a very long piece of unmachined bar does not really give an accurate reading of anything. Ditto mounting a DTI on the toolpost at this stage. Do the above tests 1 and 2 before adjusting anything and you will at least know where you are starting from . At first blush it sounds like your main problem is a tailstock that needs adjusting. You may possibly not have to do any more than that.

Edited By Hopper on 11/05/2021 04:37:28

I strongly suggest you listen to Hopper!!

Tony

Do the tests Hopper suggests and make sure the tool is sharp so you get zero deflection of work and tool

You say the tailstock is forward of the "dent" you make with the ctr this can simply be cured by adjusting it horizontally and this will likely get rid of the taper you describe turning as the tailstock end will now be thicker.

From the same test you say the tailstock is low. So pack up the tailstock, can't see why you are packing the head up as that will make it worse.

Edited By JasonB on 11/05/2021 07:45:17

Re reading my post it isn’t clear that the 50 thou of shim was on the floor, under one of the tailstock end feet.

I wouldn’t touch the headstock until proven that it needs it. Then again it’s your machine. I recently took the spindle on my surface grinder to pieces to reset the end play, even tho the “internet” said don’t do it, that’s never a problem and your poor surface is caused by “xyz”

However I did all the tests and measurements to be certain that was the issue and went ahead and fixed it.

Now grinds beautifully.

Do the legwork first and eliminate the obvious things.

Dave

BTW, how much taper are we talking about on this 5" long job? What are the diameters at each end as turned?

This listening to good advice trick really works, I must do it more often.

I removed all twist from the bed by replacing the oak block with a steel roller.

I turned 6" of steel down without the tailstock and measured the ends 25.14 and 24.975

It tapers 0.165mm, 0.0065 inches.

Pretty crappy BUT it's going the other way.

Somewhere between what I had and what I have now lies perfection.

I think I feel a quest coming on

Robin, your 'roller' is actually a tube, I would suggest taking out all the bits & pieces & let the stand sit directly on the concrete floor, then just shim/pack the corner[s] to remove rocking & also bring level if you want. You can then proceed to add appropriate shims between the lathe bed tail stock end [front or back] & the stand so the lathe cuts parallel. I strongly suggest you read the Myford manual so you understand what is going on. P.S. Just replacing the wood block with a roller tube may or may not eliminate lathe bed twist.

Tony

Edited By Tony Pratt 1 on 11/05/2021 15:13:01

Robin, at the risk at sounding a bit Irish (no offense intended to the Irish of course) your lathe does not have to be level to be level. By which I mean, the lathe bed has to be level to itself but not necessarily perfectly horizontally level.

That is why people suggest using a sensitive and accurate engineering level across the bed - it needs to be reading a constant anywhere along the bed, but the bed doesn't have to be level to any horizontal line.

Consider a lathe on a ship, That lathe bed is very rarely if ever level to the horizontal, but providing the bed has been set level to itself all along the bed then it will turn true.

Just saying!

Chris

If not already done the cabinet stand should preferably be bolted to the floor. These stands are not particularly stiff compared to the lathe bed and trying to shim the tailstock end of the bed to the cabinet is just as likely to move the cabinet as untwist the bed.

As Tony says above, get the cabinet as level as needed (doesn't have to be perfect and you may want to slope the tray to the suds drain point) shim under the cabinet feet to stop it rocking then bolt it to the floor. You now have a solid immoveable foundation from which to untwist the lathe bed. Loosen the tailstock end bolts between cabinet and bed, slide in shims as required, tighten bolts and test. Repeat as necessary turning and checking the test bar between tries. If the right hand end of the test piece is bigger than the left hand end put a shim under the front tailstock foot and of course 'vice versa' shouldn't take more than a couple of goes to get it bang on.

John