Lathe levelling

If the Headstgock and Tailstock are not correctly aligned, turning between centres will produce a taper.

(Deliberately offsetting the Tailstock centre is used as a means of turning a taper )

If the Headstock is correctly aligned with the bed, work held in the chuck, but without Tailstock support should be turned parallell, AS LONG AS THE BED IS NOT TWISTED.

If the bed is twisted, the Headstock and Tailstock are unlikely to be correctly aligned at all points along the bed. They may be centre to centre at the Headstock end, but a twisted bed will mean that they move out of alignment as the separation increases.

If the bed is twisted, the position of the cutting tool will vary, in both planes, as the Saddle (Which you assume to be perfect ) progresses along the bed, to the detriment of parallelism and probably surface finish.

If (1 ) is at the Headstock end, and (3 ) at the Tailstock end of the bed, your illustrations make the point very nicely.

In particular, Myford, who started making lathes, originally based on the Drummond (dating from just after 1900 ) have only been making lathes for some 80 or 90 years, would not advise levelling the bed to eliminate twist, unless it was necessary.. They do not mention using the saddle as a datum.

It does not matter if the level, and its supports, resting on the bed, are not horizontal, as long as unworn sections are used for consistency, (Immediately beneath the chuck, and behind the Tailstock are unlikely to be worn )

What matters is that the level reading is the same at both ends, showing that the bed is not twisted.

Working off the Saddle introduces further possible sources of inaccuracy, and error.

Whether the bed is hogging or sagging is a different matter!.

Howard

As a non trained mech engineer, I was networks and telecomms, I do wonder if it's really necessary to have the bed level in order to turn parallel, unless we are talking large lathes.

I can obviously understand it on larger industrial stuff, where headstock and tailstock ends are on separate pillars; I've even read of very long dockyard lathes whose accuracy depends on the state of the tide.

For most smaller stuff, built onto a rigid cabinet, surely it's really twist in the bed which is important, along with tailstock alignment.

How could a ship's lathe turn parallel unless it were in dock or the doldrums.

Bill

Why?

Level, from Headstock to Tailstock is only important if continuous flow coolant is is used, to ensure that the coolant flows to the drain before returning to the tank.

For the same reason, the lathe can be canted, front to back, AS LONG AS BOTH ENDS ARE THE SAME, to eliminate twist.

Howard

Auto correct!!!!!!!!!!!!!!!!!!!!!

Edited By Howard Lewis on 23/03/2020 13:46:44

Bill,

The term 'lathe levelling' refers to removing the effects of bed twisting. It's actual level is not important.

If you read the various articles on updating Myford ML7s from narrow to wide shear location, you will see pictures showing that in many cases, of used machines, the saddle does not make overall contact with the bed. In which case, it could be liable to wobble in a vertical plane.

Also, to enable the saddle to move along the bed, it must have clearance.

So if there is any clearance between saddle and bed, there is the chance that it it may change (very likely if the bed is twisted ) and so introduce inaccuracy.

Laying the level so that it rests on the bed minimises the number of parts involved and sources of error.

The more directly that the measurement can be taken reduces possible errors.

Not applicable in this case, but think in terms of drilling ten holes in a strip of steel, with Centre distance 1.000.

If you dimension every hole from one datum point, then every hole will be within 0.005" of the nominal position.

Dimension to a +/- 0.005" tolerance on the individual centres, and the last hole one could be as much as 0.045" more or less than the nominal 10 inches.

The moral is: minimise the number of dimensions involved. So work direct off the bed, because that is what you wish to measure, not in terms of thous or microns, but graduations on the level..

You would not try to measure the backlash in your cars transmission by measuring an angular movement between the road wheel and the flywheel

If you did, there will be clearances in both ends of the drive shaft, gears in the differential, gears and their splined shafts, gear to gear clearances, and between the splines between first motion shaft and clutch centre plate.

But you would not know if there was an excessive clearance at any one point..

As the amateur radio fraternity say, KISS "Keep It Simple Stupid"

As an Engineer in a Quality Department, I was trained to think logically, and to measure as directly as possible.

Howard.

Hi there, all,

I hope that I will not be censured for an off-topic post but this post isn't about 'lathe levelling' - it seemed like a good place to interject an observation about engineers' precision levels and their calibration.

Half of my offering is from Stan Zinkovsky's channel on YouTube (He of the Bar-Z Bash fame). The rest is my idea but probably not original.

An engineer's level is accurate if it reads level both ways round when on a level surface. How do we determine if a surface is level with a 'wonky' level? Not all of us have more than one level! But many of us do have a surface plate.

Stan pointed out that somewhere on a non-level plane surface, there is a line that is absolutely level - it is at right angles to the line of maximum slope. It obviously helps if the plane surface (e.g. surface plate ) is fairly close to level. So, how might we find this level line? If we can find the line of maximum slope, we're only a single step away.

One way to find it is to use an ordinary ball bearing. Put it gently in the middle of the plane surface, it will roll down the line of maximum slope. If the ball doesn't know which way to roll, that's useful information. We're nearly there but we'll get a better result if we repeat, placing the ball at the top edge of the plate. We know where that is now. Mark the path of the ball with a Sharpie and repeat a few times to get an average. Then use a square to draw a second line at right angles to the first.

The level to be calibrated can then be aligned with our level line and adjusted until it reads level both ways round.

Have I confused everyone? If not, let me know and I'll try again!!

Best regards,

Swarf, Mostly!

If the surface is off level, an accurate level will show the same deviation when rotated through 180 degrees.

i.e. if the surface is 1 degree off horizontal, the accurate level will show a deviation from horizontal. Roating the level through 180 degrees should show the same reading, but in the opposite direction.

If the level is out, it should be adjusted until it shows the same which ever direction it faces.

Think of a DTI on a base where it can swivel. Set Zero, on a line normal to the slope.

Swing DTI through 90 degrees so that it is now up the slope. Say it reads +1.

Swivel through 180 degrees so that the DTI is now down the slope. The reading will now be -1

So having the surface absolutely horizontal is not absolutely needed. The objective is that the level reads the same whether one end is up the slope, or the same, but opposite, reading when that end down the slope.

Howard

Hi there, Howard,

Thank you for your response to my post.

What you wrote is fine UNLESS the slope is so great that the level goes into saturation, i.e. it's so far beyond its range that its reading is meaningless.

If the position of the level line is known to be close to level, one can proceed as you wrote without having to iterate through many off-scale steps.

Best regards,

Swarf, Mostly!

One reason for actually checking and truing using turned diameters is that very few people will be in the possession of a sufficiently sensitive spirit level.