Theoretical Taper due to tailstock height misalignment.

Why does it suggest it must matter? Surely it suggests that although there is an effect it doesn’t matter for small deviations in height. It’s just good engineering to design something to wear about the optimum within acceptable tolerances.

regards Martin

I'd agree with martin, if head is high and tailstock low and tool at mid way then a concave part will be produced not a barrel

No need for anything more than holding a straight edge horizontally against something cylindrical which is at an angle and you will see the straight edge(toolpath) only contacts the cylinder in the middle so that is where material will be cut away

Cad model for thos einterested

First create your workpiece with head end low and tailstock end high

Draw a rectangle to represent material removed by a cutting tool moving along the bed at mid height

Simulate what happens when the part rotating on its angled axis meets the horizontal tool path

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As per Hopper’s comment: “FWIW and just to keep it going a bit longer”

This is a forum, and the best outcome is when everyone has considered all the opinions, and we come to a collective understanding.

MichaelG.

It’s looking good from here, Jason

Which makes one wonder about the dogma

MichaelG.

Or if someone with a lathe stuck some strips of 10 thou shim under their tailstock before clamping it down and then took a cut along some bar, all would be revealed.

To all of you ! I have spent my whole life learning, but never so much as after I joined this forum. Thank you Gentlemen. Noel.

Jason, shouldn't the simulation be head end on centre, tailstock end high, not head end low and tail end high?

I suppose by the time the angle has been reduced to what it would actually be if there were only a thou or two height difference and an allowance made for the work flexing away from the tool which would go some way to counter act the concave cut it may simply be so small a difference that it is simply not worth getting excited about

Best not get into the effects of having one end in a chuck and the other supported with a ctr vs, turning between ctrs as that introduces bending into the part

Hopper I was commenting on DC31K's post and Martin's subsequent comments

EDIT

Effect of setting the tool height (blue arrows) to height of axis at headstock end  just moves the deepest part of the convex cut to the point where the two axis cross (red arrow) So if one was to only measure an actual workpiece at each end you could be forgiven for thinking you have a constant taper particularly as any taper is likely to be very shallow the concave will not be visible to the eye

Edited By JasonB on 05/01/2023 10:18:50

Edited By JasonB on 05/01/2023 10:21:18

I suspect if they did then the only result would be a thread asking how to measure deviations of millionths of an inch from a true cone. (and it’s always going to be a cone because I defy anyone to parallel turn to those limits.)

regards Martin

Rather we would have a thread concerning 'why can't I drill a half inch hole with a half inch drill in my tailstock'.

Thanks Jason. I have caught up, finally!

Just checked Connolly's bible Machine Tool Reconditioning and he allows 0 to .001" high for vertical alignment of tailstock.

Edited By Hopper on 05/01/2023 11:20:49

Being a simple minded soul, and thinking in terms of a 1 inch bar in a lathe with the Tailstock set 0.002" high I visualised a right angled triangle.

The hypotenuse is 0.5, and the perpendicular is 0.002.

Applying Pythagoras,

0.5^2 = 0.25

0.002^2 = 4 x 10^ - 6

So 0.25 - (4 x 10^- 6 ) = 0..249996 (The Base ^ 2 )

The square root of that is 0.499996

So we seem to be looking at a difference of 4 x 10^ - 6 on a piece of material an inch in diameter.

This is the sort of dimension common in temperature and humidity controlled Calibration Rooms, when checking gauge blocks, rather than our workshops.

The local temperature generated by cutting will probably cause a much greater expansion than that, not to mention effects of the necessary clearances between Centre and Centre Drilling, Saddle and Lathe bed, and any deflection of cutting tool and workpiece.

Consequently, ignoring such errors seems to be acceptable to me.

Howard

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Yes, and that limit will be constrained by the effect it has on drilling from the tailstock rather than turning as has been alluded to.

regards Martin

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That’s why I suggested the use of a Dividing Head with Elevating Tailstock

One could then easily demonstrate the effect [whatever it might be] by doing things on an enlarged scale.

In less-blinkered places than here, that would perhaps be called a Model …

MichaelG.
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P.S. __ Jason’s use of 3D CAD probably obviates the need for a physical model.

No the hypotenuse is what you need to calculate. The long side is 0.5 and the short side is 0.002 for your example. The hypotenuse (the radial distance from the centre of the workpiece is root 0.5^2 + 0.002^2

what ever that is (don’t have a calculator handy) but it’s a tad bigger than 0.5” not smaller.

regards Martin

all that said I agree that the effect is not worth bothering about.

Edited By Martin Kyte on 05/01/2023 12:02:39

So we're worrying about a tenth of a micron on a 25 mm workpiece?

Don't even breathe on it. Even your radiated body heat might produce an error!

Howard

I don’t know if the moderators can edit the thread title maybe by adding the word(s) insignificant, theoretical, or minuscule. It may make it a little less misleading.

regards Martin

Well for those that doubt the new fangled CAD I thought something more practical may be of interest. Not fancying cranking my rotary table round and round to use Michaels adjustable tailstock suggestion I came up with teh idea of offsetting the tailstock ctr, not horizontally but vertically using a boring head.

First cut shows the effect of having the tool height mid way between the headstock and tailstock heights which as the CAD showed produces a concave waist. Second is with tool on headstock ctr height which just shifts the position of the narrowest part of the waist. Did get so tool rubbing on teh first setup as the tool was way too high and I think diameter plays a part a sthe second option is not quite so curved but dia is smaller