View Full Version : Using a Boring head for offset, when turning between centres.

05-04-2008, 03:35 AM
When using a boring head in the tailstock for turning between centres, how level does the slide of the boring head have to be?

If it isnít exactly in the horizontal plane does it alter the amount of taper?

Steve Larner

05-04-2008, 08:09 AM
Setting it level by eye or with a machinist's level will be adequate in almost all cases. A high precision level is not needed, just the Starrett 198 or equivalent or even the bubble in a square head.

It is difficult to calculate the exact setting when turning a taper, and some fiddling is usually required with most setups to get a dead accurate taper if that is required. If the attachment is a bit off level, it will have minimal effect on the outcome.

05-04-2008, 01:44 PM
If it's high or low, you'll be turning a barrel shaped taper. The smaller the diameter of the part, the worse this will be. It would probably suffice however to bring the tailstock up to the center in the chuck, and check the relative position of the center in the tailstock mounted boring head. I don't know how you'd be able to make an up-down adjustment, but I think I'd try to anyway. By the way, if you're turning a taper to fit the spindle or tailstock bore, you can turn down the central part a few thou so it doesn't make contact with the bore at that point. You don't really need a total line of contact for that, just a good support at two distances apart to keep the taper from rocking in the bore.

I am assuming that the turning will be close to start with- if it's obviously barrel shaped, then your two 'lands' won't fit the bore very well either, which won't be good.

If you're needing the taper to be absolutely straight sided, then you'll have to see about getting the point in the tailstock to the same height as the center in the chuck.

05-04-2008, 08:06 PM
If the boring head isn't level, the angle cut will be less than the angle computed by the offset because the out-of-level brings the center back toward the axis of the headstock.

At the same time the point of contact between the tool and the work will be moving up or down with respect to the centerline of the work, since the outer end is lower or higher than the headstock. The tool contact angle will be changing as you move away from the headstock. The contact point will also be getting further away from the centerline. The effect is greater on small diameters.

I'm having trouble visualizing how this results in a barrel shaped taper, but I've never tried it on the lathe. I'll take Darryl's word for it. I think it's more like to happen if the contact point is below the centerline of the work near the headstock and moves toward (or above) the centerline near the tailstock because the boring head is not level.

But, trig is on you side here. A small deviation from level will have little effect.


John Stevenson
05-04-2008, 09:06 PM
Don't forget to allow for wind and the curvature of the earth.


05-04-2008, 11:44 PM
ROFLMAO, Sir John, you just won't do.

J Tiers
05-05-2008, 12:19 AM
I'm thinking we have been through this before, and while it is true, it also is barreled by an amount that was calculated to be barely measureable for most settings.

05-05-2008, 03:08 AM
Looks like I will be getting a boring head and MT3 mount at the show on Friday.

Steve Larner

05-05-2008, 04:19 AM
I thought about this a bit more, and now it seems that a taper turned with a tailstock center that's out of height compared to the chuck center will have a narrow waist, not an enlarged one (barrel shape). My (updated) reasoning is this- if the tailstock center is high or low, the cutting edge will still remove material until it has 'cut out', in other words removed as much material as the adjustment of the crosslide has allowed. If the tailstock center is then adjusted for proper height, the cutting edge will dig away a bit more material, without adjusting its position. The height will be just right when the cutter cannot remove any more material without being dialled in more. So when it's not on center, it turns the small end larger than it should have. I'm leaving out the effects of springiness in the setup to keep this explanation simpler.

In the first case, the size at the tailstock end of the taper would be larger than what it should have been, therefore the center part of the taper would be smaller than what would be indicated, based on the size at both ends of the taper. Not barrel shaped, but just the opposite. Still not perfectly straight in any case. By what amount? Maybe not significant, and less so the larger the diameter of the workpiece, as suggested.

Here's a way to envision it- imagine the cutting edge is on center height, and both centers are on height. You cut the taper, and for now assume that the small end is 50 thou across. Raise the tailstock center by 25 thou, and the cutting edge would have to be dialled in 25 thou just to touch the workpiece again. Lower the center and the edge of the cutter can't cut anymore because the work is rubbing on the cutters face under the edge.

In either case the cutting edge would be farther away from the workpiece, and if the taper was cut like that, the tail end would be larger than intended. The taper starts off good from the chuck end, then gets progressively fatter than it should be toward the small end, thus it doesn't have straight sides.

If your center is high or low by say 10 thou, and the workpiece is to become say, 1 inch diameter at the small end, you can probably ignore the effect. I haven't done the math, but I believe at least one person has, if we've been through this before.

John Stevenson
05-05-2008, 04:27 AM
The main claim to fame for this method is the ease of which the taper can be dialed in as opposed to whacking the top slide with either a big hammer or a wet kipper.

Chances are you won't have the correct taper anyway so dialing it in will be needed and any slight error in centre hight will be compensated.

If you cant manage to get the boring head setup on centre hight or as close to as possible then you need to go back to basic turning 101 and start from there.

A 6" rule and the MKI eyeball can get you within 10 thou, if not try to get a braille rule.


Peter S
05-05-2008, 07:38 AM
I have one of these units - you can see they provide an in-built spirit level to set it up correctly. This is scanned from a c.1980's 'Verdict' catalogue, but it is the same as mine, made by Bowers in the UK. Not the same Bowers that makes the beautiful internal micrometers etc. though, (I contacted Bowers, they said there was some relationship many years ago). I think this company no longer exists.


I don't know why this isn't appearing as an image in the thread...I pasted in the correct link, but it seems to be ignoring it...