Unwanted Taper

[old martParticipant @oldmart]

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.

[Dave HalfordParticipant @davehalford22513]

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?

[Howard LewisParticipant @howardlewis46836]

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

[Steve355Participant @steve355]

Posted by Dave Halford on 03/10/2021 15:46:11:

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?

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.

[Steve355Participant @steve355]

Posted by Howard Lewis on 03/10/2021 16:59:00:

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

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

[Steve355Participant @steve355]

Posted by Howard Lewis on 03/10/2021 16:59:00:

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

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

[Howard LewisParticipant @howardlewis46836]

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

[Clive Brown 1Participant @clivebrown1]

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.

[Dave HalfordParticipant @davehalford22513]

Posted by Clive Brown 1 on 03/10/2021 18:27:38:

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?

[Steve355Participant @steve355]

Posted by SillyOldDuffer on 03/10/2021 10:16:07:

To be explicit, an ordinary bubble level won't cut the mustard. They're good for putting up shelves and most other purposes but aren't sensitive enough to detect a twisted lathe bed.

You need an Engineers aka Machine Level. They are made to detect tiny deviations from level, as required to set a lathe accurate to a thou or two over several inches. Typically, in a new installation the lathe stand is set up with an ordinary level first, and then the lathe is tested. If not cutting straight a Machine level is used to check for bed twist, which can be corrected by shimming a foot: some machines have screw-adjustable feet.

This example is made by Dasqua and sold by RDG.

In theory new lathes are confirmed to cut straight on a true level surface in the factory, so plonking them down on any decently stiff flat surface should be good enough. In practice, don't expect too much of inexpensive hobby or elderly lathes. Note flat rather than level. It's not necessary for lathes to be level provided the bed is straight: they work perfectly well in storm tossed ships. Levelling is just one way of eliminating twist, and a bent stand is one way of accidentally twisting a machine.

In my book Machine Levels aren't good value for money. Pricey and much too sensitive for ordinary work. Fussy and the bubble takes an age to settle. Good for anyone installing lots of machines, dubious for putting one into a shed! After being used once to set-up the owners lathe, I suspect most Machine Levels end up in a cupboard. As an alternative, Rollie's Dad's Method takes longer and is more complicated to do than detwisting with a good level, but it requires no special equipment.

A word of warning: high-precision measuring is so difficult it's quite easy to lead oneself up the garden path into a deep maze of confusion. A wobbly DTI moving on a slide will cause trouble. When measuring into the 0.02mm / 0.001" region don't rush to adjust anything based on amateur measurements: they're likely to be flawed because taking them properly is a skill you don't have!!! Therefore, keep measuring simple and use the right tools: detect taper with a micrometer, not a DTI. Check everything – it may take several hours practice to get a micrometer to repeatedly read the diameter of a precision rod consistently. Positioning the micrometer correctly is important, and so it applying the same pressure to the screw every time. The ratchet helps, but trained craftsmen rarely use it; they develop a 'feel'. Some are better than others, so find out how good you are by practising. In the meantime, don't jump to conclusions!

Dave

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.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Steve355 on 04/10/2021 08:47:31:

Posted by SillyOldDuffer on 03/10/2021 10:16:07:.

…

As an alternative, Rollie's Dad's Method …

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.

…

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

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 04/10/2021 10:40:29:

[…]

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. […]

Dave

.

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.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Steve355 on 02/10/2021 22:06:57:

[…]

here is a 0.004 feeler gauge under one end. I would imagine that makes it sensitive enough to be useful?

.

Q.E.D.

MichaelG.

[Tony Pratt 1Participant @tonypratt1]

Posted by Steve355 on 03/10/2021 14:38:44:

Thanks for all the comments

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

Your comment is worrying, a lathe is a machine tool not something magical which 'comes & goes' the taper was always there or not at all.

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.

Steve

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

[Steve355Participant @steve355]

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.

[Steve355Participant @steve355]

Posted by Tony Pratt 1 on 04/10/2021 12:05:14:

Posted by Steve355 on 03/10/2021 14:38:44:

Thanks for all the comments

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

Your comment is worrying, a lathe is a machine tool not something magical which 'comes & goes' the taper was always there or not at all.

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.

Steve

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

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

[blowlampParticipant @blowlamp]

It should be straightforward to get this right if the headstock is separate from the bed.

Use your level across the bed at headstock & tailstock end and shim to read the same – this will verify that the bed is straight and untwisted. It's the starting point for everything else.

A test cut without tailstock support will show the taper that's causing problems for you, so it's necessary to loosen the headstock mounting bolts and carefully reposition it until its axis runs parallel with the bed. Very light cuts with a sharp tool will confirm the adjustment.

If the headstock & bed are in one piece then you have a harder problem to solve.

Martin.

[not done it yetParticipant @notdoneityet]

Let’s be clear. Adding shims is to remove twist in the bed, not to add it.

There should be no twist. The bed should be flat. Unfortunately, using a spirit level is (in my opinion) a waste of effort with a lathe which is only secured at two points along the bed. Further, if the lathe is worn, using any spirit level is never going to improve anything. Lathes with several holding down points (or feet) do need levelling with a spirit level.

Do take notice of what Clive Brown has posted.

The normal simple checks and adjustment, once it is assured that the headstock is in alignment with the bed, is to turn a suitable stiff bar held (only in the chuck at about 2 1/2 to 3 times stick-out from the chuck) to check for twist in the bed (any change in cutter height should not affect the result – unless the bed is very severely worn). The bed is then levelled (twist removed) by fitting shims under one foot at the tailstock end. We are assuming a rigid base to which the lathe is attached – little point in shimming the bed further away instead of shimming the lathe, if you understand what I mean.

Only when the lathe is cutting without taper, is setting the tailstock carried out. Ideally a long bar with a waist and ‘bobbins’ at each end suspended between centres, taking light cuts from only the bobbins until the tailstock is adjusted to provide the same diameter on both bobbins. Think here, you should never be cutting at excessive stick-out without tailstock support. That is a recipe for disaster – either taper by deflection or a p. poor finish.

My lathe is secured tightly to the base at the headstock end but only loosely held at the tailstock end by a single bolt (it has a Thakery washer fitted which is not compressed tightly). Adjustment is by adding shims to one side of the bed at the tailstock end if it does not cut parallel. My example does not have any shims.

Edited By not done it yet on 04/10/2021 13:23:05

[SillyOldDufferModerator @sillyoldduffer]

Posted by Michael Gilligan on 04/10/2021 11:39:44:

Posted by Steve355 on 02/10/2021 22:06:57:

[…]

here is a 0.004 feeler gauge under one end. I would imagine that makes it sensitive enough to be useful?

 

.

Q.E.D.

MichaelG.

The case for the defence. I think I show my Machine Level to be considerably more sensitive than Steve's Rabone.

The apparatus is a 670x30x20mm bright mild steel bar laid on my milling table, with a Level in the middle, shimmed at one end so the bubble is nearly level. It's a bridge:

The beam isn't quite level: nearly 3 graduations right, 2 left, a slope of 0.02mm per metre, about 0.001°. It takes the bubble just over 10 seconds to settle in this position after hands-off.

Unlike the Rabone which was tested with a 4 thou feeler, a 3 thou feeler under the right hand side of my level moves the bubble well outside the graduations – the resulting slope is bigger than the level can measure:

Inserting a 0.15mm shim at extreme right (370mm away from the level, about 0.023° ), also moves the bubble off-scale. The graduations can't be trusted.

My thinnest feeler is 1.5thou (0.059mm). Inserting it 370mm from the level moves the bubble off-scale, but it can be guestimated at about 4 divisions. To really test the level by shifting the bubble a graduation or two, I need a shim about 0.02mm thick at the far end. Can't think of anything 0.02mm thick to use: cigarette paper maybe but I expect it's too soft to take the weight without deforming.

Happy to have the methodology criticised, for example that 20x30mm steel bar must be bending slightly! But I hope m'learned friend and the jury accept the Rabone isn't anything like as sensitive as my genuine Machine Level. Whether the Rabone is good enough to level a lathe is another question: I think not, but I'm often wrong…

For most other purposes the Rabone is a far more practical instrument than my Machine Level. It's a pain in the proverbial in action: far too sensitive, nothing but fiddle, fuss and bother. Not one of my best buys! But it does do accurate levelling when that's really needed, way better than a Wixey or an ordinary Spirit Level. Ordinary Spirit levels get far more use in my workshop though, simply because most practical levelling is done better with a sensibly insensitive instrument! As tiny amounts of twist cause a taper, lathe beds need a sensitive level. Or consider Rollie's Dad as a alternative!

Dave

Pesky smileys!

Edited By SillyOldDuffer on 04/10/2021 14:18:04

[Martin ConnellyParticipant @martinconnelly55370]

I have not been paying a great deal of attention to this thread having been away for the some of the weekend but there are a few things to note. Your original photo shows a piece held in a chuck and supported by a tailstock centre. If the tailstock is not correctly set up it will cause taper because even with a short workpiece that is not too slender it can bend it off the spindle centre of rotation. So any taper found using a setup like that could be from the tailstock. This means that you have to have a workpiece in the chuck that is not supported by a tailstock centre to test for taper. This has been mentioned already by Clive. You need something that is not slender but is sticking out less than 3 times the diameter and you should take fine cuts with a very sharp tool on his test piece to check for taper. This must be done by moving the carriage along the bed and not by moving the compound parallel to the bed. This test piece turned unsupported like this is what should indicate either the bed is twisted or the headstock is misaligned. Then once you have fixed this type of taper you should align the tailstock to ensure it is not deflecting the workpiece off the spindle's centre of rotation when the centre is put into the end of the workpiece. A workpiece that is relatively slender (more than 3 x diameter in stick out) that is not supported by the tailstock is likely to deflect away from the tool giving a taper that has a larger diameter at the unsupported end than near the chuck. The problem with this taper is it will vary with the material, the length of stick out, the sharpness of the tool, the speed of rotation and with some materials the temperature it is at (think plastics here) and I suspect other things like how homogenous the material is and if there is any coolant or lubricant being applied.

It is hard to tell from what you are saying you have done if you are doing the initial tests as described without the tailstock centre in use.

Anyway the point is that this is not necessarily a quick fix job as there are so many things that can contribute to taper so don't get too dis-heartened if it takes some time to sort our what is happening.

Martin C

Forgot that depth of cut and feed rate can affect taper of unsupported parts as well.

Edited By Martin Connelly on 04/10/2021 15:09:52

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 04/10/2021 14:16:10:

[…]

The case for the defence. I think I show my Machine Level to be considerably more sensitive than Steve's Rabone.

[…]

.

No problem with that, Dave … and thanks for the demonstration photos

At the risk of sounding like a Weaselly Politician or Lawyer : I did carefully write that “the little Rabone does appear to have adequate sensitivity ” … which is not to say that it is ideal for the job.

If Steve can use it to check that his lathe bed is free from any more than 0.001” of twist in 4” across the ways; then I would maintain that it has served the purpose quite adequately. … It will obviously need more ‘imagination’ than doing the same check with a more sensitive bubble and graduations; but on a modest small lathe, of some considerable age, any better than one thou in four inches is probably a pipe-dream.

MichaelG.

[Steve355Participant @steve355]

I agree Michael, I think it’s been a really useful level. It allows me to rule out bed twist as a cause, when it is disproportionate to the symptom I’m seeing. I may have 1 thou of twist but that would not cause an 8 thou taper on a 3 inch piece (I surmise).

Anyway, I think I might have fixed it …. The 1MT alignment bar came, I just spent an hour tapping then tightening, then loosening, tapping again and re-tightening more carefully. It was indeed about .008 out of alignment, now I have it down to about 0.0005.

The first few (very fine) test cuts saw the tool cutting only further from the chuck…. The cutting line faded and disappeared as I got closer to the chuck. I could literally see the taper disappear from the test piece in front of my eyes.

I will do more cuts and measurements after dinner, but I think (with everyone’s help) I might have cracked it.

[Michael GilliganParticipant @michaelgilligan61133]

Excellent news, Steve

Keep up the good work !!

MichaelG.

[Pete RimmerParticipant @peterimmer30576]

Well done Steve it;s nice to see people thinking about and tackling a problem themselves without having to have their hand held all the way.

[Dave HalfordParticipant @davehalford22513]

Posted by Pete Rimmer on 04/10/2021 20:08:34:

Well done Steve it;s nice to see people thinking about and tackling a problem themselves without having to have their hand held all the way.

Too true.

[Author]

Posts