Facing parallel between centres.

I've posted recently about turning risers for VMC mills. The biggest new challenge for me was getting the ends of a 4-5 inch long 6 inch diameter round bar parallel using a lathe.

I got to about half a thou over 6" diameter / 4" long by bolting to a faceplate - I was quite pleased with that, and I expect it'll be fit for purpose.

However, the advice I had here was that the best way to do this is between centres. I tried that on a 4" diameter test piece and it came out about 1.5 thou wrong over the diameter.

I'm confused! I'm pretty bad at visualising things in 3D, but I think that even if the head and tail stock centres are misaligned the faces should end up parallel. That is turning the work end for end and facing at the tailstock end. I might well be wrong though!

Any advice would be welcome - I'd like to track this down.

Robin.

Edited By Robin Graham on 13/09/2020 00:04:23

Edited By Robin Graham on 13/09/2020 00:05:00

4 inches, between centres

Did you use a gigantic lathe dog?

edit: if you used chuck jaws this can introduce errors

very light cuts with a very sharp tool reduces distorting forces at the tailstock end

Between centres work can be exceptionally difficult on a hobby machine

Edited By Ady1 on 13/09/2020 00:15:34

Well, I do have a gigantic dog who sometimes who watches me me turning, but I've yet to put her onto workholding duties. Luckily the nature of the work allows a drive pin in this case.

Robin

Edited By Robin Graham on 13/09/2020 00:20:05

Are you talking about the top and bottom (end) faces being out of parallel or a variation in diameter between each end?

If it's the diameters that differ, then you need to adjust the tailstock to correct the slight error you have. However, if you can reverse the workpiece end for end between cuts at the tailstock end only, then you should be able to get a better result, as long as you don't disturb the cross slide setting between those cuts.

How is the parallelism over the 4" length?

Martin.

If your faceplate work was right on the money with a tailstock centre and the only change was to place the work between centres then it must be your positioning between centres and/or the cutting forces used

For really accurate hobby stuff I always use a well lubricated dead centre in the tailstock

The faceplate job only needed pushed and relatively little support but the between centres job will need a decent support hole at each end

Edited By Ady1 on 13/09/2020 00:34:31

Can you explain that further Ady? What are the problems?

Robin.

You need to set your tailstock offset adjustment so that it is turning dead parallel along the outside diameter of the job or a test peice before facing the ends. Otherwise, the ends will be conical to some extent. Trouble with old/cheap lathes is the tailstock can go out of line every time you move it or even tighten the barrel clamp.

The other factor with old/cheap lathes is the cross slide may well be cutting out of square to the main lathe axis, also giving a conical face to some extent.

Definitely +1 on use a dead centre in the tailstock, not revolving, for high precision work. Hobby grade revolving centres can be all over the place.

If you have already got with in half a thou over six inches, that is about as accurate as you can expect on a typical lathe. In industry, tolerances of less than a thou are usually done on grinding machines which are of a next-grade accuracy.

Edited By Hopper on 13/09/2020 00:50:06

Any error whatsoever will be reflected in the finished workpiece, from setup to completion

Proper between centres turning if done right allows you to remove the workpiece for other work to be done and then return it to the lathe, swapping the bar end for end if you want to, it gives accurate repeatability

but everything has to be really spot on, the man the machine and the setup, it's a highly skilled man who can knock out good between centres stuff (not me lol)

Edited By Ady1 on 13/09/2020 00:59:38

Robin

Did you turn and face both ends between centres, or just face one end ?

You did well to achieve such accuracy with the 6" diameter piece, did you check to see if the end faces were square to the length, as a cylindrical square ?

Emgee

Edited By Emgee on 13/09/2020 01:01:43

As an example, one of your posts mentioned the workpiece was up to 68 degrees when finished

So the workpiece expands and contracts between the centres if it gets too hot

.

Your visualisation of this process seems fine, Robin ... but I fear that what you are physically doing may not meet its rules.

It is theoretically possible to achieve perfect parallelism of the two end faces by swapping the workpiece between dead centres [i.e. like the bodger’s pole-lathe, or the watchmaker’s turns] ... it is ‘simply’ a matter of taking a ‘perpendicular to the axis’ cut twice.

In practise, however, there may be variation from that idealised situation:

• Your cross-slide action may not be precisely repeatable
• Your centres may be constraining the work to not rotate on two hypothetically pure points !

On that second bullet ... I have previously been ridiculed for suggesting that someone should consider the action of ball-ended centres, in comparison to cones, but it is a useful ‘thought-experiment’

MichaelG.

Edited By Michael Gilligan on 13/09/2020 06:50:45

One way to face two sides perfectly that has worked for me in the past is to:

First face the work piece both sides in the three or 4 jaw it does not mater. then drill and centre one side of the workpiece.

When you take the workpiece out of the lathe chances are that it will be a little out of parallel even if you set it up with a dial indicator, it will probably be a few tenth's out.

The next step is to place a piece of scrap in the lathe and then face the outer edge say 10mm then turn a recess in the centre say 10 thou deep. leave this piece in the lathe. It will be perfectly true.
ithin the limits of the spindle bearings If you want to be technically correct.

Then hold the non centred side of the workpiece and centre it up with the tailstock centre pressed firmly against it and locked. This process works better with a ball bearing centre. Another possibility would be to bolt it through the spindle to keep it pressed against the fresh turned surface.

The last step is using a fine light feed face the centred side, you will leave a small unfinished section in the centre, this can be removed with a file if need be. You should not need a dog if the workpiece is a reasonable diameter. friction will hold it

The method also gives limited access to the back and all the side of the disk. Probably enough to do a riser in one sitting. if you plan it well.

Regards
John

A plane is defined by three points in space. Two points make a line.....

Getting the centers sooooo close that one could 'flip' the part between the centers and have exactly the same plane????

That's what face plates are for......

Pete

No. The below exaggerated view shows how if the axis of the job is offset at an angle to the axis of the lathe, the previously faced ends will now be also at an angle. Imagine, if you will, now taking a facing cut on the pictured example, a cut that is square to the lathe axis. The end of the job will be conical as the cut is square to the lathe axis but not square to the job axis.

.

Edited By Hopper on 13/09/2020 07:36:56

But the ends will still be parallel within the vagaries of machine 'slop' and any possible errrors in flipping the piece?

Since this part is a bearer that only has to be parallel on it's circumferences then why not just stick a hefty stud into the center of each end and set it up between a chuck and tailstock such that both ends can be faced inwards as much as needed for the circumferential bearing surfaces without flipping the part and then remove studs, bore outwards on each face just below that bearing depth. The only variation you have is slop between left and right facing actions.

pgk

Whilst that is true the practicalities are that for bringing two clamping faces of a riser parallel, eye-balling the points will be sufficient.

The most likely source of error is that the live centre was not turning on axis.

Robin you should put your centre in the spindle and set a DTI on the side near the point. It must not have any runout.

No. The ends will  be conical if you cut straight in square to the lathe axis while the job axis is at an angle to the lathe axis. The ends in the illustration are parallel, but they were turned BEFORE the tailstock was offset in this taper turning example. If you now face the tailstock end of the job, it will be a convex cone. If you then flip the job end for end and face the other end, it too will be convex. Two convex surfaces are never going to be parallel.

Edited By Hopper on 13/09/2020 08:32:48

The faces will not be parallel but the ends of the part will be co-planar which is the desired result.

The problem the OP had with the face plate thread was that he did not give the full details for the end product.

Drilling holes in the ends of the part and bolting the part to the faceplate would have been the easiest and most obvious way to do it (as opposed to one thin bolt through the centre). Same as facing the ends - there is only about 3cm around the circumference that actually needs to be parallel - and (clearly now) one of those was not needed to be done until the spigot was completed.

The boring, for the spigot, certainly could not be carried out between centres, for sure.🙂

Order of operations and any later covered-over ‘setting up for machining’ holes were simply not considered before starting the machining - as far as I can tell. Like there are three holes, for bolts on a PCD, required at some point? I would likely have made those after facing the first end!

Tailstock does not need to be moved between turning the part between centres - just wound back a cm or so to clear the centre at one end. We don’t even know how all these measurements were taken, and the precision is likely as good, or better, than the original machine anyway.🙂

That leaves three possible positions for the spacer to be fitted - meaning that should there be the tinyist allignment out-of-true, there would first be a choice of turning the spacer by 120 degrees or Possibly skimming one face by a hundredth of a mm or so. No real sweat on the whole job ,whose length is not actually a critical dimension.

This lot, above, is the result of hindsight. But unnecessary if some fore-thought had been given to the job before starting, I feel.

Both faces will be convex. Call it what you like. Try perching your mill column on top of a disc with convex faces top and bottom and see if it acts like a ball joint or not.