Unwanted Taper

I've always used the method as advised in the Myford handbook where you actually turn a piece of metal & shim the tail stock feet to get parallel. Loads of threads about this subject on here & on the net. Last time I saw a machinists level was 2 years ago & yes it was gathering dust in a cupboard.

Tony

That should be plenty good enough for most bench lathes.

If you have a machine with 2 pedestals like a Bantam, the first thing to do sould be to level the bed and fix the feet to the floor, with shims of required to achieve level. I rebuilt a Bantam and as an exercise I put a dial gauge on the saddle and against a part in the spindle, then I moved it about on my workshop slab - the dial gauge swung all over the place just from the variations in floor level and the lathe stood normally on it, and my floor isn't bad at all. It was quite the eye-opener.

Thanks for all the comments

when I remeasured my work this morning the taper was back 😢 no idea how but it clearly is

So I thought I would try what several have suggested and turn a new piece of brass rod without tail support and try shimming the tail end to get it parallel. So I did that…

my first measurement was 935 at the headstock and 961 at the far end. I added shims tightened it back up and took several more cuts and measurements.

now I have quite a lot of shim (about 030) and my level (love it or hate it) indicates a significant amount of twist that wasn’t there before. Headstock:

tailstock:

shims :

my latest measurement is 908 at the headstock end and 920 at the tail end, so quite a lot better but nothing like close enough. I am starting to think this is nonsense, because I seem to be adding twist to try to reduce taper. In addition the finish is bad at the far end of the bar, which is what one would expect I believe, and probably enough to account for the taper.

basically it doesn’t seem to be working. I’m worried too I could damage the lathe.

arghhhh!

Steve

Steve you might well damage your lathe If you keep that up. You don't say what lathe is actually is. It could be that your bed is level but the headstock is not pointing straight down it.

Steve, you may be a beginner, but you certainly are not an idiot. There is always a learning curve for anything and by making mistakes and solving the problems you will soon become proficient and be able to help others.

It's possible bed wear is dropping the tool height towards the tail stock so you effectively take less of a cut.

Are those marks on the bed enough to catch your finger nail?

Have you tried gripping an alignment bar (A solid piece of Silver Steel, say 1" diameter, would suffice for a first attempt ) and running a clock, set at centre height, along the bed check Headstock alignment ?

This assumes that the chuck is accurately seated onto the Mandrel and its backplate.

Are the chuck jaws worn /bell mouthed so that the work is held at an angle, not firmly?.

Howard

On the bed there are a few nicks along the edges, and some wide circular grinding marks that look like they are from the original manufacture. It looks in pretty good nick overall.

The chuck is relatively speaking new and looks to have a good grip on the piece, and it seems well seated in that if the bolts are removed it is still difficult to get the backplate off the chuck.

I do think the you and an earlier poster may be on to something with the headstock alignment though. Everything else on the lathe was out of alignment , no reason why the headstock should be accurately aligned. It’s actually the only part I haven’t messed with so far. The spindle has a 1MT fitting. Is it worth me investing in a 1MT alignment bar? They don’t look to be too expensive, but are usually quite short. I’ve used of all my other stock messing around recently doing test cuts.

Steve

The chuck is relatively speaking new and looks to have a good grip on the piece, and it seems well seated in that if the bolts are removed it is still difficult to get the backplate off the chuck.

I do think the you and an earlier poster may be on to something with the headstock alignment though. Everything else on the lathe was out of alignment , no reason why the headstock should be accurately aligned. It’s actually the only part I haven’t messed with so far. The spindle has a 1MT fitting. Is it worth me investing in a 1MT alignment bar? They don’t look to be too expensive, but are usually quite short. I’ve used of all my other stock messing around recently doing test cuts.

Steve

With a 1 MT Headstock, you can't really make your own Alignment bar, since anything slim enough to pass through will be too slender to be rigid, even if you do have a 4 jaw independent chuck.

So it looks like buying an alignment bar, even if it won't see much use. The cost will be offset by the savings on scrap that you don't make once everything is lined up.

When clocking along the bar, make sure that the Saddle gibs are sensibly, but not over, tight.

If the Saddle is flopping about the readings will be useless.

Hopefully, aligning the headstock in the horizontal m,plane will only entail slackening the fastenings and a little tapping.

But you may as well go the whole hog and remove to ensure that all the mating faces are clean and free of burrs.

If you have any doubts about the fixings, check them.

A friend had a ML4 that produced tapers. Someone had removed the Headstock and cross threaded one of the studs. As soon as that nut was tightened it went out of line. We made up an alignment fixture, clamped in the opposite stud, to drill out the hole gradually from 1/4 BSF until it could be tapped square as 3/8 BSF. We had already made up a 3/8 BSF / 1/4 BSF bush, and Loctited into p[lace. the Headstock could then be aligned and tightened, and stay in alignment.

You may need to go down the same road.

Once the Headstock alignment is certain, the Tailstockm is next on the list, firstly horizontally, again with no slop in the gobs, then, hopefully the Tailstock and Headstock centres will be on the same level

You just keeping eliminating errors, one at a time, until the machine is as good as you can get it

Hopefully; you will get there!

Howard

Hi Steve, re headstock alignment; all along I've been thinking that your Zyto would have the headstock cast integral with the bed. After looking at some photos, I now think that it might be a separate casting bolted onto the bed. Is that the case? If so then I think misalignment there could be at the root of your problem. Are there any alignment dowels in this joint. Before lashing out on a test -bar I'd try a quick and dirty check with a length of reasonably accurate bar held in the chuck and tested by traversing your dial gauge against it, ( but I'm a cheapskate). Check at several points of spindle rotation to even out run-out.

Even a length of bright mid steel would do for a start. Ground silver steel even better. Doesn't have to be too large a diameter.

If your problem does lie there then a test bar might be worth considering.

I thought the centres had been aligned? Which should mean up and down as well as side to side.

Bright mild steel is not always round. Silver steel or ground mild steel is round and straight.

If you use the top slide instead of the carriage is the effect the same?

Thanks for taking the time to explain all that Dave.

Thanks for taking the time to explain all that Dave.

My little rabone level is obviously an engineers level of sorts, it is adjustable, with a ground bed and far more sensitive that a regular spirit level. Whilst it may not of give me absolutely precise measurements, what it did do was give me a “direction of travel“ and magnitude for the amount of twist that there might be. When I started shimming up 1 foot of the bed, it moved substantially, and was useful in that it told me the direction of twist and that it was quite considerable, and if I continued shimming, it would probably damage the lathe.

I do need to work on my micrometre technique, positioning it in exactly the right place and square, particularly on a round surface, does seem tricky. I’m not yet convinced I get the same answer every time.

Your Rabone might be part of the problem. Thing is, hitting the target is downright difficult if the level isn't sensitive enough for this job. It may be necessary to remove or add so much shim to move an insensitive bubble that the bed twist is always over or under-corrected. For this reason have a look at Rollie's Dad's Method which only needs a rod and DTI. Although the instructions look complicated at first, the Rollie's is actually quite simple. Don't fixate on the Rabone just because you have one!

An indication of sensitivity is how fast the bubble moves. Engineering levels are poor for putting up shelves because tilting the shelf half a millimetre causes the bubble to shoot across the scale and disappear. Conversely, when measuring a fine level correctly, an engineering bubble moves very slowly across the scale maybe requiring a minute or two to settle before the measurement is taken. Another difference, Machine Levels are usually graduated, whilst ordinary levels aren't.

It's possible to estimate the Rabone's sensitivity by laying it on a stiff beam, say 1 metre of 2 by 4" or similar, and packing one end of the beam so the Rabone bubble is centred. Flipping the Rabone around should put the bubble back in exactly the same position. Assuming it passes that test, use a strip of aluminium cut from a drinks can to lift one end of the beam. (About 0.15mm). If the Rabone really is a Machine Level, the bubble should move to indicate the tiny change of angle caused by inserting the strip. Actually, a Machine Level should be sensitive enough detect even smaller changes.

Given it's unclear the Rabone is fit for purpose, don't rush to twist the bed based on it - cast-iron breaks easily if overstressed in tension.

Stick with it - realigning a lathe from scratch may be time consuming but it's highly educational! As a beginner wanting to get on with metalwork rather than fixing machines, I bought a new Chinese Lathe to avoid this sort of shenanigans. The problem with second-hand is condition is uncertain, varying from 'as-new' to 'completely knackered' via 'maladjusted' and the other circles of lathe hell. The good news is 'maladjusted' can be fixed, but depending on what's wrong it may take a fair amount of effort, especially if skills and tools are limited. For example, in the factory the headstock of an inexpensive early lathe would have been aligned in a few minutes by a trained man using a jig. Newbie in a shed 70 years later has to work from first principles, which is much harder.

Dave

Dave

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Please permit me to say, Dave, that the little Rabone does appear to have adequate sensitivity ... even though it does not have the additional graduations.

Steve has demonstrated to us that the bubble moves substantially with a 4 thou' tilt, so one thou should be easily detectable. ... It think it may be more a matter of technique.

MichaelG.

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Q.E.D.

MichaelG.

Not at all nonsense, it's a tried & tested method, the shims are removing twist, also if the finish is bad at the far end I guess the tool isn't cutting properly.

Tony

Just to (probably not) settle the question of whether my little Rabone level is any use or not, I remembered I had picked up some other gizmos in my eBay travels - some gauge blocks and a sine bar.

here’s the level on the plate…

5 thou….

1 thou….

so, well, inconclusive. 5 thou is very clear, the level isn’t quite straight in the 1 thou pic. I will give it a good clean on the base.

Hi Tony

Of course, I am aware that taper does not come and go. It’s either there or it isn’t. When I thought it had been rectified the night before, I must’ve measured it incorrectly. My fault/inexperience.

I am certain that the method is a good one, I’ve watched quite a few YouTube videos showing people going through that process in large workshops, to install or adjust large lathes.

but I had added about 0.040 of shim, and tightened it down, which according to my level (and the recent experiment done to see how accurate it is) caused the bed to go from zero twist, to about 0.015 twist. This resulted in a reduction of the taper by about 0.008.

so it seemed to me that the cause of the taper is not twist in the bed, and I better stop introducing twist before I damage the bed by over twisting it.

I have ordered a test bar, apparently coming today, which will allow me to see if the headstock is aligned. I’ve got no reason to suppose it is well aligned, everything else on the lathe was out of alignment when I got it.

Steve

Edited By Steve355 on 04/10/2021 12:34:42

Edited By Steve355 on 04/10/2021 12:50:23