All that glistens is not gold!

[davedfsme]

Maybe I haven't understood the setup Arnak is using. However I will try to explain more clearly what I mean.

My earlier post was written in the belief that Arnak's setup has

• the boring bar, with it's tool bit, rotating between centers,
• the work piece firmly mounted on the carriage,
• the carriage advancing along the lathe bed to produce the length of the bore.

With this setup one can be sure of getting a non-tapered bore. The axis of this bore is of course parallel to the axis of the lathe bed and not to the axis of the boring bar. Offsetting the tailstock (horizontally say):

• will not alter the diameter of the circle followed by the tip of the tool bit;
• will move (horizontally) the centre of the circle followed by the tip of the tool;
• will disalign the axis of the boring bar from the axis of the lathe bed; thus the projection of the circle followed by the tool tip onto the vertical section through the work piece will be an ellipse with the horizontal axis slightly shorter than the vertical axis.

If, after making a first (circular) bore, you offset the tailstock end of the boring bar and make a second pass, then you will still have a non-tapered bore but it's vertical section will be the sum (with overlap) of the original circle and this offset ellipse. While the ellipse effect is admittedly very small, to me it seems that the error in approximating a larger circle by two overlapping smaller circles is not small.

I hope this is clearer. I also hope that it makes sense; if not somebody please sort me out!

First Principles.

Romartin is absolutely correct, except perhaps on one point? If you make any adjustment to the tailstock between cuts in the usual sense, i.e. in the horizontal plane, you WILL end up with a non-circular "taper" in your bore. It WILL be larger in a horizontal plane, but will be the same size in the vertical, relative to the axis of the cuts. Viewed obliquely, all bets are off . Geometrically, the taper will be uniform from one end to the other, neither end will be circular, and you will be wearing out the centers. You would have to offset both towards yourself and away from yourself by the same amount if you wished the axis of this "hole" to remain normal to the block upon which you inflicted it, unless the block itself was not set square to the axis of the bed to start with. The geometry behind whatever your desired result is becomes staggering . . . when it should be simple.

The only way to effect a circular size adjustment by offsetting the axis of a normal pattern "between-centers boring bar" would be to shift the (usually female) center in the end of the boring bar off-axis. This would cause the outer surface to run out, and the effect on the radius inscribed by the tool tip for a given adjustment would depend upon the relative position of the tool tip between the effective centers. This is contrary to the very idea of a "between-centers boring bar". As Goldstar 31 pointed out, adjusting the effective radius of the tool tip in relation to the axis of the bar is the "correct" way of the "old masters". The only way that I can see that you could possibly develop a true taper with a "between-centers boring bar" would be with a "travelling-head" on the boring bar, with the bar itself running off-axis to the axis between centers.

All that being said, I get the impression that not everyone who commented has the same visualization of the set-up. The tool-tip is meant to follow a straight line path, and that path can be adjusted in relation to the workpiece's relative axis of rotation in a number of different ways. It does make a difference whether the tool-tip or the workpiece is rotating relative to the direction of travel, in the sense that the direction of travel relates to the axis of rotation. Do you want a truly cylindrical bore, or a truly tapered bore? Set up accordingly. That is not to say that you will end up with a perfect bore on a twisted machine, but think of what you want, and measure what you get. If you "know" your machine is straight, but your cut is not, there is a reason.

Wrap your head around it. It is an interesting puzzle!

DJD

 

[romartin]

Thank you Sage and DJD. Sorry my first post was confusing.
DJD: I still cannot see why the bore becomes tapered if I make a cut after after offsetting the tailstock. I reckon that offsetting the tailstock affects the section of the bore in the vertical plane orthoganal to the lathe bed axis, but that this section (whatever it is) will be identical all the way down the bore. This is what I meant when I said that the bore will still be parallel.

 

[Lykle]

Hi guys,
I see now, yes my original setup will generate an oval hole.
Nice to know if I need one.

The solution would be to keep the tail stock end centered but move the head end out of center. Then you will not get an oval hole as the offset will revolve around the center line.

Nice, now I know for sure how to do it if ever I need it again.

With my ball turner I only used it for the last few cuts, so I think that is why it worked for me. The bronze bearings do feel correct and the shaft turns nice and smooth.

I am glad we got that cleared up.

Lykle

 

[davedfsme]

Thank you Sage and DJD. Sorry my first post was confusing.
DJD: I still cannot see why the bore becomes tapered if I make a cut after after offsetting the tailstock. I reckon that offsetting the tailstock affects the section of the bore in the vertical plane orthoganal to the lathe bed axis, but that this section (whatever it is) will be identical all the way down the bore. This is what I meant when I said that the bore will still be parallel.

Ian,

Again . . . you are absolutely correct. There is no taper produced, because the block being bored is being traversed past the cutter rotating in a single plane.

You know, I started composing a completely different answer before giving my forehead a smack . . . I guess you could say I explained myself into a corner before I realized I was wrong.

Aren't geometry puzzles fun?

DJD

 

[romartin]

Thank you DVD. This thread has forced me too to sort out my ideas; I now have a better appreciation of the consequences of using a boring bar and also of the opportunity it gives for making bores which are perfectly parallel but whose section is not a circle; for example for the inside of the case of an exotic pump or IC engine. Almost certainly I think I would try to get the effect by moving, in the vertical plane, the work piece and not the tailstock!