Lathe Levelling - how level?

May seem a daft question, but how much discrepancy in levelling the bed is too much?

I'm aware it's bed twist that really matters, rather than end-to-end or exact side-to-side levelling, but how much twist?

I've found that my grandad's WW1 clinometer (used for setting up the sights on Vickers .303 machine guns - he was the final inspector in one of the factories!) is accurate to 1 minute of angle , which I think is an inch in about 95 yards, and can be set +/- 15 degrees consistently- accurate enough, too accurate?

Thanks all,

Dave H.

Reply to
Dave H.
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There were articles about making a machine level in Model Engineers Workshop a few years ago, but if I remember right a gun clinometer was considered the thing to get.

John H

Reply to
John

Hello Dave, You want it as good as you can sensibly get. 1 minute is certainly not too accurate, it's almost exactly 1:3600 or an inch in 100 yds as you say, but more usefully a thou in 3.6".

A decent lathe should have twist of less than that, say your tool height is 4.5", then one minute of twist would move the tool postiion by a bit about 00125 or if you'd rather, put 0025 taper on the work. Assuming no other errors etc. obviously.

Richard

Reply to
Richard Shute

OK, thanks Richard - so it'd probably be best to level it as accurately as poss' with the clinometer, then once running take a few test cuts (assuming I can get the tailstock in line...) to check for any taper and adjust the feet to suit?

The clino's *calibrated* in m.o.a., but I can probably estimate down to about 10-15 seconds in a pinch, so that should get me within a thou" of taper (centre height's approx' 6-3/8" above the ways, slightly less than half the swing) if I follow the calculation correctly? I hope the 2 Imperial Tons of Old Iron should be pretty rigid to start with, particularly with an (almost) integral cast-iron box cabinet to sit on...

Dave H.

Reply to
Dave H.

My understanding is that test cuts for bed twist are done without the use of the tailstock. Once the degree of taper is acceptable, a test bar is fitted between centres and the tailstock adjusted so that the bar can be clocked with minimal deviation along the length of the bed.

Both these methods do need the saddle and bed to be in a good state of adjustment.

Bob

Reply to
Bob Minchin

Possibly a dumb question: is it fair to assume that lathe levelling is not so much of an issue when the bed is attached to a humungous and solid base? I have a Holbrook C10 (1500kg, or so) and have it mounted on machine feet. It's level'ish, but not to the accuracy discussed here.

Dave S

Reply to
NoSpam

Hello Dave, Broadly, Yes. It's not so much a matter of the lathe mass, but whether the base is itself very rigid with a comparatively smaller bed on top so that the machine is a rigid whole. If that is so there's usually not much to be done about any apparent bed twist. In effect, the base is part of the bed or bed is part of the base, either way, it makes the whole machine a very rigid structure. You can measure for twist, but probably not do much to adjust it, jacking the feet of the base will/should have little effect, 'though it may be measurable and unbolting the bed from the base is also unlikely to improve matters and might easily make them worse.

When you get to MUCH bigger machines, the case is often different again. Very often for a machine that has, say, a 10m bed, you would expect to use multiple jacking feed along the bed/base against a suitably serious concrete floor so that the concrete floor effectively becomes part of the machine.

I had not realised when I replied to the OP that it was a fairly serious machine (6.5" centre height), I was assuming something 'home workshop' sized and probably bench mounted where the appropriate levelling is pretty well mandatory. Likewise, an old machine mounted on legs rather than a monolithic base (eg a Southbend Heavy 10) will also need levelling as legs at either end can induce significant twist into a bed having no other support or 'external rigidity' added.

As Bob said, you are better off testing for taper without the tailstock and then setting the tailstock afterwards. There are plenty of write-ups on how to measure machine straightness and to partially account for wear, it depends on the construction of the machine and also what tools you have to hand. You can get accurate results using just a DTI and a bit of ground round bar that actually doesn't need to be especially straight if you look for the mid point of the DTI range and compare that at each end of carriage travel.

Richard

Reply to
Richard Shute

The Heavy 10's bed rests on a pivot pin under the tailstock, allowing it to be set to cut straight without shimming the feet. It pretty much straightens itself when the two opposing adjustment screws are loosened.

jsw

Reply to
Jim Wilkins

Humungous is definitely the word - I have a C13 (2 tons) and I reckon the base alone weighs as much as a Chipmaster! I still want to check the bed for twist though, Just In Case.

Are you also a member of the Yahoo Holbrook group? Lots of useful info there (particularly on the C's) and some very helpful folks with advice on rebuilding my C13! I'm working on hooking an ABB VFD/inverter to the 3-speed motor (not too far off the final design now), are you blessed with real

3-phase?

Cheers, Dave H.

Reply to
Dave H.

. The use of a precision level simply a method of measuring bed twist. It really doesn't matter whether the lathe is level or not provided the bed isn't twisted.

Most texts describe placing the level directly on the bed ways and moving it along the ways to measure the level at all points. More convenient and, slightly more accurate is to mount the level on the crossslide and simply observe the change in bubble position as the carriage is traversed end to end.

For aligning the tailstock I prefer to use a lever type dial gauge mounted by a 3 or 4 jaw chuck with the measurement tip bearing on the INSIDE of the tailstock bore.

Because the dial gauge is rotated by the chuck it uniquely defines the headstock rotation axis.

Because the tip bears directly on the tailstock bore it eliminates errors relating to the use of tailstock centres.

It also shows up directly the shifts resulting from clamping the headstock to the bed or the barrel to the body. It is also easy to check the change in alignment at different extensions of the tailstock barrel .

Jim

Reply to
pentagrid

Yes, I joined the Holbrook group as soon as I bought the lathe.

I struggled to get the C10 to run at top speed - initially from a Transwave static converter, then a rotary converter, then a 3 phase inverter (415 3ph in/out, with the input stage modified to act as a voltage doubler). I haven't had time to do much for the last year or so but I'm fairly certain that the headstock oil is too viscous (the spec is very thin indeed!) so when I get the right round tuit I'll replace the oil.

Dave

Reply to
NoSpam

Add another tool to the Xmas list then - a lever DTI! Although I have a decent (half-thou") plunger indicator or three, and I've seen a few turn-it-to-a-lever adaptors in the DIY tools mag's / web pages, may have the urge to make one!

Without adjusting / levelling, I ran the carriage from head to tail and back a few times: once I had the Clino' set I couldn't see much bubble movement if any at all, so I assume I'm free of twist, although it slopes about 30 m.o.a transversely and about a degree and a half head-down at the moment...

Re the "ground test bar" that's been mentioned, I have a number of print-head rails (about 18" long. 3/8" diameter), I suppose these would be adequate (they're uniform to 0.0001" on diameter and at least as straight as my surface plate's flat) if a little short?

Thanks again, Dave H.

Reply to
Dave H.

Fair enough, my fault. I haven't actually palyed with a Heavy 10 and assumed too much from its appearance.

Richard

Reply to
Richard Shute

Try Rollie's Dad's Method of Lathe Alignment (Google for it). Worked for me.

Iain

Reply to
Iain

Before I retired several years ago I heard an interesting tale of levelling a machine.

The guy installing a new machine at a major engineering company worked at various adjustments to get the machine level. He had got very near the desired result then went of lunch or maybe it was overnight. Anyway on his return it was not a level as when left. This puzzled him especially when the effect was repeated later. It turned out that the site was on a tidal estuary and the land moved as the tide came in and out!

Henry

Reply to
Dragon

The problem is chucking them so they run true enough, though Rollie's Dad's Method works if the runout is within the short range of an

0.0001" indicator.

I prefer a test bar (or cylinder square) with a center hole running in the dead center, since it represents working conditions more closely and you can use it to recenter the tailstock after turning a taper.

jsw

Reply to
Jim Wilkins

I owned mine for ten years before noticing the pivot and its adjustment screws under many layers of trade-school paint, applied yearly in lieu of real maintenance.

Rollie + other goodies:

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jsw

Reply to
Jim Wilkins

That is the method I alluded to, although I didn't know it by that name. I also found it eminently effective, and confirmed my levelling using a 10sec level on the carriage.

Richard

Reply to
Richard Shute

Used properly, although it may be convenient, a test bar does not have to be ground, straight and constant diameter.

The essential requirement is just for the bar surface to be truly circular at the near and far measurement locations. At these locations the headstock rotation axis is uniquely defined by the average {half (max reading minus min reading)} clock reading and is independent of both local bar diameter and eccentricity.

A few nicely finished turned locations on a mild steel or light alloy bar is all that is needed

Jim

Reply to
pentagrid

In article , snipped-for-privacy@yahoo.com writes

But surely Jim, though that may be a good test for concentricity of spindle rotation with spindle nose fitting, it would do nothing to test whether the spindle axis was parallel with the bed.

What the non-ground bar you describe *would* be useful for is to make a test cut; if the "near and far" positions are given a light cut, and come out at exactly the same diameter, that does show that spindle and bed are parallel. In fact, you could say that is the most revealing test of all, as that is mostly what you want from a lathe.

David

Reply to
David Littlewood

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