Lathe levelling

This topic has deviatingly reared its ugly head in another thread. Rather than perpetuate the error I thought I'd start a new one.

My lathe sits on a metal cabinet on top of a carpet It wasn't "level" on first installation so I fixed it down solidly on large 4mm thick washers at three points. Under the forth foot (tailstock front) I inserted a wedging device along with the pull-down bolt.

Now when I want to do some precision parallel turning, I do a quick machining of my test piece. If there is a difference in diameter between the two ends I simply slacken the bolt on the adjustable foot and pull out or tap in the wedge a fraction. I then tighten the bolt and make a final check.

Its so simple and I often wonder why no one else (or manufacturer) does something similar ?

It's a good trick, but I feel there are some disadvantages.

Firstly, much depends on the lathe - not all of them are as bendy as others! A Chinese mini-lathe is stiffer than a Myford because it doesn't have a gap. Gap bed lathes are flexible in a good way because they allow bigger work to be squeezed on to the lathe, hurrah. But the gap makes them liable to be flexible in the bad sense too - and usually the more rigid a machine tool the better. A second issue - as Gerry has noticed - is that a deliberately twisted lathe is likely to come out of adjustment, which is undesirable if the machine becomes untrustworthy. Third point is that a correctly installed lathe that's cutting inaccurately probably has something wrong with it, like a worn bed, out-of-true headstock, bent spindle, damaged bearings or duff chuck. It might pay to fix the actual fault rather than hide it by twisting the bed.

Dave

With respect, is this about "levelling" the machine, or aligning it?

I would not be happy about doing anything that twists the poor thing, but strict level-ness is less important than strict straightness. If you need to test and adjust the lathe each time, there's summat not right!

A machine-tool should as far as possible be on a rigid surface, with all its own feet in natural contact, helped by levelling-screws to overcome imperfections in the bench, not to twist the machine (unless correcting existing warping).

The cabinet or bench itself might be on carpet if circumstances dictate, and does not need to be level to NPL standards, as long as that too is fully supported and cannot drift out of kilter. In fact the manual for my Harrison L5 advises using the levelling-screws in its base to tilt the whole assembly very slightly towards the suds-tray drain.

++++

{Only my L5 had the holes but lacked both the screws and the threads for them... Cutting threads for 1-1/8" BSF bolts, by hand-taps, in existing slightly too-small holes in 3/4" plate with a rather hard surface, in-situ, as I lay on the floor in a constricted space, is another story... The screws by the way, plus nuts, came from a unexpected stockist: Settle Coal, in the town of Settle!}

Edited By Nigel Graham 2 on 05/05/2019 00:35:43

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Nigel,

Please read my post [ 02/05/2019 22:25:49 ] on page 3 of the other thread **LINK**

https://www.model-engineer.co.uk/forums/postings.asp?th=140345&p=3

MichaelG.

Bwahahaha! Evidence please.

I think the OP is a good idea. Probably the same thing is achievable with Myford's theaded adjusters under each foot on the raising blocks. But an adjustable wedge is smart thinking.

Edited By Hopper on 05/05/2019 07:35:19

Even though the lathe test piece measure the same at both ends does not necessarily mean that the lathe is "level" (aligned). This may only be true when machining getween centres and at the end positions of your test piece.

See this video from a previous post (skip 1st minute)   Lathe Alignment / Levelling

Paul

Edited By Paul Lousick on 05/05/2019 08:29:18

[This posting has been removed]

I plead guilty. I've not actually tested it!

But in mitigation:

• The bed sizes are roughly proportional in terms of size and weight (39kg vs 84kg)
• The mini-lathe bed is a single internally braced box girder of modern prismatic design, stubbier than the Myford. (350mm vs 508mm)
• The Myford bed is flat, cantilevered and interrupted by a gap. Whilst the gap is a useful feature, every silver lining has a cloud, - the gap weakens the bed.
• The Myford is deliberately designed for maximum accuracy by allowing the bed to be de-twisted. After mounting the lathe level it is possible to tune out any residual manufacturing effects such as the bed de-stressing over time, or small installation errors.
• Although the mini-lathe can be bolted down, I don't think many owners bother. Minilathes are normally fitted with rubber feet and plonked loose on an ordinary bench-top. It does not appear necessary to level them, nor is it straightforward to twist a mini-lathe bed in the same way as can be done to a Myford.

A point that might be missed comparing hobby lathes with classic machines like the Myford is that current Far Eastern machines are based on Western 'best practice' that appeared 10 to 20 years after the Myford ML7 was designed. In comparison to the ML7 most modern lathes appear boxy and less solid, but they are simpler to make and strong where they need to be, especially the bed.

I'm only suggesting the mini-lathe is stiff compared with Myford 7s; a photo was published recently on the forum of a stripped Chinese lathe showing the bed severely weakened to accommodate the headstock. It looked likely to flex. And bigger machines tend to be of relatively spindly construction compared with small ones because weight doesn't scale linear with dimensions. I can confirm that a 250kg WM280 is bendier than a 40kg mini-lathe!

Hopper is spot on though. These are assertions rather than facts. What's really needed is measurements. Does anyone own a mini-lathe AND a Myford?

Dave

Edited By SillyOldDuffer on 05/05/2019 10:30:36

No, but I have used and worked on both. My experience is that a mini-lathe is not even close to as rigid a machine as a Myford, based on observation under cutting load. The Myford will take deeper cuts in steel without showing signs of flex, eg chatter, dimensional instability, finish etc. Ditto interrupted cuts such as facing square blocks of steel or iron and milling in the lathe, which really puts some stress on the whole shebang.

Shimming the feet of the lathe bed is not a process unique to Myfords. And certainly is not necessary to get a Myford to turn true. My own ML7 was for a long time set on two wooden raising blocks on the bench top (one of those temporary arrangements that almost became permanent.) With the mounting bolts barely nipped down , it turns parallel within less than a few tenths of a thou over six inches. I'm in the process of putting in some steel raising blocks -- so no doubt it will never be as good again!

But not relevant to most hobby lathes as they do not have the four headstock end mounting bolts that can be used to "tilt" the spindle up and down relative to the bed. They have only two mounting points headstock end and two tailstock end. Whole different ballgame.

And machining between centres is another whole ballgame again, depending entirely on tailstock alignment, not spindle to bed alignment. Your spindle could be 10 degrees out of line with the bed but you could set the tailstock so the lathe turns perfectly parallel between centres.

Now I am worried, a mini lathe more flexible as a Myford (presumably 7 Series)!

My ML7 would shift the bubble if I applied any torque to a fixing, once it was level.

In this context, "Level" means no twist in the bed, between Headstock and Tailstock to ensure turning parallel.

If the Tailstock centre is off the axis of the Headstock, in either horizontal, or vertical plane, turning between centres will not be parallel. As Apprentices, we were taught how to turn tapers, by offsetting the tailstock centre, in the horizontal plane.

If the centre is high or low, relative to the Headstock, then shimming, or machining / scraping of the tailstock foot is needed to put things back in order again. This assumes that the Headstock axis is parallel to the bedways, of course.

And all this depends upon the floor on which the bench / cabinet for the lathe, being stable, and not moving according to the weight upon it, or ground movement.

It all depends on how many microns you want to chase!

Howard

Gerry you missed the most important part of the set up of the lathe, it's on a metal cabinet, and sitting on carpet, that's OK, but what is under the carpet, is it a timber floor, or concrete, if it's timber the set up will change with the weather, and that may happen a number of times each day. My lathe is bolted to a 200 mm concrete slab with extra reinforcing, it's been through 2 major earthquakes and thousands of after shocks, and no adjustment has been required.

In my opinion the stand, or the floor that the lathe is mounted on would need to be many times stiffer than the lathe bed if reliable twist adjustment is to be obtained.

Ian S C

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Probably because [without your intimate knowledge of your lathe] we would struggle to know the size of 'a fraction'

... at a guess; to achive a good result, it would take me several iterations of what you describe.

MichaelG.

Edited By Michael Gilligan on 05/05/2019 12:53:59

I've recently bought a Harrison lathe. Im still in the process of cleaning it and all the attachments. It is being converted from three phase to single phase so I can operate it in my workshop at home. First time posting here. I expect I'll have plenty of questions.

Regards

Michael

Having a lathe level is less important than aligning it by taking out any twist that it has. After all there are commonly lathes on ships which are expected to perform despite never being level.

The facsimile manual (from Tony Griffiths) for my Harrison L5 tells you not to level the lathe, but give it and its cabinet a very slight tilt down to the chip-tray drain.

Bear in mind these lathes are designed to be on their own cabinets, a massive steel-plate fabrication with a planed top.

Logically, the only real advantage of having a machine-tool dead level by slide-ways or bed measurement is allowing work-piece setting by levels, and what really matters is that the machine is secured in a way that maintains its alignments and straightness without unfair strains.

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Nigel ^^^

I don't know exactly what your manual says, but; in the context of this discussion 'levelling' is not the same thing as 'setting the unit horizontal'.

MichaelG.