How level should a lathe be?

Hi all,

I'm building a shiny new bench to put my Southbend 9", and hide some of the other crap in the garage.

I am intending to mount the lathe on some levelling screws, as per Chris Heapy's site - large diameter, fine pitch bolts, and some large washers. Probably going to use M10 fine, easiest thing to get hold of.

My question is: how level do I need to get it? I don't have a (working) engineers level, and am reluctant to buy one at =A320 off ebay, if I'm only going to use it once. I have my grandfathers old wooden 'boat' style level.

Should I 'eye' it up on grandad's woodwork level, or bite the bullet & buy the proper thing?

Cheers,

Ed

Reply to
zedbert
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The lathe doesn't have to be level at all, it has to be straight. A precision level is the easy way to check if the bed is twisted or not.. I used one tonight and it took five minutes to verify that a particular lathe bed was straight within less than half a thou in ten inches over it's entire length. If you don't want to get a precision level, then try searching the interweb for "rollie's dad's method". That is a re-telling of the classical method for testing a lathe for straightness.

HTH

Mark Rand RTFM

Reply to
Mark Rand

I'd check your granddad's level by trying it on the same surface, both ways round, and see if the bubble lies the same distance off centre. (Perhaps stick a mm scale alongside bubble for clarity.) You can go further if necessary by checking the sensitivity against a known slope.

An expensive level should have a more sensitive bubble (bigger radius arc), but may not be set more accurately, initially. I've got an ultra-sensitive one using prisms to see both ends of the bubble at the same time. Almost impossible to use outside of the theodolite from which it was taken!

Reply to
Malcolm Stewart

No, the bed can be twisted and it can still cut parallel.

Yes - but it isn't very important that the bed is untwisted unless you are using it for milling work, or perhaps super-accurate pitch threading - and even then a twist in the bed isn't likely to be that important.

RDM won't test for straightness - but it will help test for parallelness of cut. RDM is a method for calculating relative measurements to the centreline of rotation of a lathe along different points of the bed, and these results can be used to adjust the head to obtain parallelity of cut, so that the centreline of rotation is equidistant from different points on the bed - but it can't per se tell whether the bed is twisted, you need a level as well (or instead) for that.

Imagine a lathe with a very twisted bed, so that it undergoes a quarter turn over it's length - as long as the centreline of rotation and the axis of the twist are in the same place it won't affect the accuracy of a facing or turning cut at all.

It would affect eg the pitch of a cut thread - it would have 1/4 extra/less turn over the length of the bed - but in practice twistedness doesn't matter much, as such an extreme twist is most unlikely.

Reply to
Peter Fairbrother

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In the sizes of lathe that we use the direction of gravity is irrelelevant. It's convenient for it to be setup roughly level - a carpenters level is more than good enough.

A bed with serious twist can only cut parallel if the axis of the twist is coaxial with the spindle axis. This is wildly improbable.

The twist axis of the bed is normally pretty close the the c of g of the bed cross section and in your lathe that will be at least 5"displaced from the spindle axis. Because of this a very small degree of twist will result in significant taper turn error.

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In practice, because of the wide separation between twist axis and spindle axis, small amounts of bed twist are important.

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Maybe I look at things differently - I have no real interest in conceptual straightness .My goal is parallelism of cut so I'm quite happy with RDM.

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********** Jim *************
Reply to
pentagrid

'Level' isn't important - lathes are used on ships at sea - but 'twist' is. If you send me your address off list I'll lend you a level.

Best regards --

Chris Edwards (in deepest Dorset) "....there *must* be an easier way!"

Reply to
Chris Edwards

Like others say, no need to set the lathe level for turning. But it can occasionally help if you also mill on your lathe; the engineer's level can be used to set the work level & thus square to the bed faster than a dial indicator, especially for non-flat pieces.

hth Guy

Reply to
Guy Griffin

In practice the path made by a tool along a mildly twisted bed will be a straight line for most everyday purposes, even though it is in fact a section of a screw - align the centre of rotation with the line and it should cut parallel.

You are right of course. I should think more before posting after midnight.

[..]

I agree - and RDM is probably the best way to do that.

Reply to
Peter Fairbrother

Mark, don't bother trying to level your Hardinge when you are ready to use it. They don't need it ;)

If you didn't know the beds just sit (Not bolted) on three conical rings, and the only leveling you need do it to allow the coolant to drain properly.

Wayne..

Reply to
Wayne Weedon

Mostly what I have been told, eg that a lathe with a twisted bed can still cut parallel if the head is adjusted correctly.

Which parts did you disagree with?

Reply to
Peter Fairbrother

Actually there's three bloody great studs holding it down to the stand.

But they don't need the "wring down until it's straight" procedure though.

Mark Rand RTFM

Reply to
Mark Rand

Yes there are studs and nuts and washers, but normally there are compression springs between the washers and the underside of the cabinet.

I've rebuilt quite a few ;) I've also seen them unsettled by IDIOTS slinging Hardinges from the bed! They can normally lift right off the seats before the nuts start taking the strain.

Wayne...

Reply to
Wayne Weedon

All ;-)

Reply to
Charles Lamont

What height have you selected ?

The reason is that my father visited my workshop the other day to look over the Cromwell lathe, and he said "you can see its a precision lathe because of the height". I had never considered this before.

The lathe centre is 44inches, the only other lathe I have on a genuine factory stand is a pre-war Zyto (40.5 inches). I suppose the ideal height must depend on how tall you are (or short in my case) as well as what size of work you use it for. I've got a Winfield at 45" that came on a Brown&Sharpe cast stand and that seems a nice height for small work.

So is it true that lathes for precision work are always taller, in which case a watchmakers lathe would be nose-level !

Steve

Reply to
Steve

This is quite true. And watchmakers lathes were used sitting on a chair, having the arms rest on the table. I've seen a photo where the operater sat on the tailstock's end and nearly embraced the lathe (I would embrace a Schaublin anyway).

Nick

Reply to
Nick Mueller

This is based on your experience? Or is it your "considered" opinion?

Tom

Reply to
Tom

Since you ask, all.

Reply to
Tom

Ed

I don't know what you are going to make your bench of, I hope it is metal as wood changes with the weather/humidity etc. and thus may be fine when you set it up and all over the place 2 weeks later. Didn't see any mention of this.

Steve

I'm building a shiny new bench to put my Southbend 9", and hide some of the other crap in the garage.

I am intending to mount the lathe on some levelling screws, as per Chris Heapy's site - large diameter, fine pitch bolts, and some large washers. Probably going to use M10 fine, easiest thing to get hold of.

My question is: how level do I need to get it? I don't have a (working) engineers level, and am reluctant to buy one at £20 off ebay, if I'm only going to use it once. I have my grandfathers old wooden 'boat' style level.

Should I 'eye' it up on grandad's woodwork level, or bite the bullet & buy the proper thing?

Cheers,

Ed

Reply to
Steve

Cheers for all your replies!

I'd never considered making it higher to make it more precise! Do I need a really big height gauge as well now? LOL

The bench is made of metal, 25 x 3mm box section. Bolted to the walls & floor, fair bit of cross bracing, probably more to come.

I shall go for grandad's level to start with - nice idea taping a scale to it, will do. And RDM for the tune up, as it were.

Also, if anybody ever needs to know, the centres of the feet on an SB

9" are not the nice sensible 6" one would ass-u-me they are. DAMHIKT.

Ed

Steve wrote:

Reply to
zedbert

This is an interesting point.

I had the same idea when I made the bench to carry my Boxford ME10. I reasoned that the bench top should, as nearly as possible, have the same expansion coefficient AND the same thermal time constant as the lathe bed.

With this in mind I made a sturdy wooden bench topped with three mighty box section steel girders laid side by side and welded together. This assembly was comparable in both weight and torsional stiffness to the Boxford bed.

While this achieved the original aim I hit an unexpected problem - extreme sensitivity of the hold down bolt adjustment. Because the welded benchtop was so torsionally stiff, a fractional thou movement of the bolts resulted in more than a thou taper turn which made adjustment pretty fiddly. With patience I got a good result and the setup has remained nicely stable over several years.

On reflection I realise that the heavy stiff bench top is the wrong way to go.

There is more than meets the eye in the comparitively flimsy sheet metal stands supplied by lathe manufacturers.Because the sheet metal top is torsionally so much less rigid than the lathe bed a relatively coarse hold down bolt adjustment results in a usefully fine bed twist adjustment. The thermal time constant mismatch is of little importance because the resultant bed twist is a result of the spring force of the deflected metal top and this is comparitively temperature insensitive.

Jim

Reply to
pentagrid

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