Trying to prevent ribbing on tapers

[RSG]

I have a problem that has plagued me for years and I don't know how to fix it....aside from getting a bigger lathe with more rigidity.



If you look at the pic you'll see very faint ribs which occur on the tapers only. It's very difficult to capture on camera and not easy to see without looking at the part in good light. I think it's caused, from dialling the top slide in or out during the cut while the top slide wiggles (so to speak) with vibration. I have tightened the gib as much as I dare without making it too tight to turn or cause premature wear. I've tried dialling very slow and as consistent as I can but have not found a way to reduce this issue. It only shows itself once polished and most of my stuff requires that finish so it's unavoidable. I've found I can reduce it greatly by hand sanding prior to taking it out of the lathe but no matter how much time I spend before going to the vib finishing stage it's still there.

I would appreciate any suggestions to help mitigate this problem.

RSG

[tornitore45]

My first thought is that the cranking of the compound slide add some force that cause movement. If the compound gib is tight, then do you have the carriage locked? that can move too.
I can not see the ridges on the picture. Are the ridges uniformly spaced one turn of the compound? That can be a clue.
Is very difficult to have a constant feed by hand. If your compound crank has the classic balanced one handle wheel then is just natural to have different angular speed across the circle. When I am roughing a taper on my late I see cyclical ridges because I crank with one handle but when finishing I use a different method. Insert the second handle (180* apart) and with two hands work the feed screw by a combined motion of the nob driven by the handles (thumb and index finger) carefully transitioning from one hand to the other. Is difficult to explain but comes natural. That method provide an order of magnitude improvement on the turning uniformity and assure the torque applied has no side force component.

[spro]

We need to look back to how it was done in production. It had to go quickly. Those machinists didn't own those lathes. Inspector says make sure the cutting tool is dead center, across the slides' travel. Industry ramps the speed so it isn't one object but 100. All the stresses and sliding members are checked and tweaked to produce many of the correct size and taper. Interruption of cuts is bad.

[Harold_V]

Mauro addressed the issue correctly. It is not easy to use a compound and achieve a uniform cut, due to deflection of the slide as you apply pressure. You may find that loosening the gib so the slide moves easily may improve the finish, but don't count on it. Best way to eliminate the problem is to apply power feed to the slide. Some creative thinking could prove to be a solution; a simple speed control of some kind with a flex coupler attached to the screw, with no handle present. Might be worth the effort.

You may enjoy success by simply spin polishing the area in question, using a flat surface covered with abrasive cloth. A file, for example. You'd have to spin polish until all traces of irregularity have been removed, and that can be challenging.

H

[Russ Hanscom]

Would a larger radius on the cutting tip have an impact? It would not address the basic problem, but could possibly spread out or reduce the rib height.

[John Evans]

rig up a cordless drill to drive the compound but steady feed will still be a issue with the hand trigger.

[BadDog]

A few years back I had to make something like 28 ~5" tapers, right at the limit of my compound travel. And the first prototypes looked like there were old school wax phonograph grooves. But I noticed my compound (balanced ball) handle is held on with a SHCS in the center. So, I grabbed my old school "speed handle" (square drive sockets), added a hex drive (allen) socket to fit the SHCS, and smoothly cranked out all the tapers needed with almost imperceptible feed grooves. Mine was plenty good enough without trying too hard, but with careful cranking, I think this might be an answer. To get more uniform would require rigging a power feed of some sort to the compound.

Then there are the tracer options...

Wild thought, but I wonder if burnishing would help?

[SteveM]

rig up a cordless drill to drive the compound but steady feed will still be a issue with the hand trigger.

One of those handheld powered screwdrivers (not a screwgun) has a much lower speed than a drill.

They have small motors, but are very geared down to get torque.

It might have enough power. Doesn't look like you are taking a heavy cut.

Steve

[RSG]

Thanks for the thoughts guys....

Yes Mauro, they are perfectly spaced one handle crank apart.

Spro, While I am doing this sort of under a production format I can afford to take to my time....and it still manages to happen

Harold, I remember seeing a guy on YT add power feed to his top slide, You feel it would eliminate the problem, or at least reduce it? I'm game if that's the case. Currently I take sanding blocks to the parts at 240 and 600 grit and look great on the lathe but the brushed finish hides the ribs........until vib finishing. The Synthetic cone machine does a great job reducing the issue but once polished you can still see remnants of it. I simply cannot have that.

John, good idea if I could control the speed.

Russ, I try to use an 1/8" radius tool for those cuts currently, more for the inside corners to have a radius but it still doesn't seem to help the surface finish.

BadDog, even using two hands to try to keep the dial tuning smooth doesn't cut it but your hand crank idea might work. And as for burnishing, it might work but is a no no due to it's work harding nature which doesn't go well with the anodising phase.

Steve, I just happen to have the guts of one of those geared motors sitting in the shop. I'd just need a way to control the speed.

[NP317]

Might be time to incorporate that ribbing into your final finish design.
Call it Blng...

RussN

[Bill Shields]

i had my life saved 40+ years ago by a Graziano lathe that had a power feed built in on the compound.

all of the above suggestions are worth chasing...including a cheapie 12 VDC gearmotor with variable voltage supply.

[Harold_V]

Harold, I remember seeing a guy on YT add power feed to his top slide, You feel it would eliminate the problem, or at least reduce it?

Yes! I'm sure of it. It's caused by the uneven pressure applied to the handle as it is turned, plus the varying cutting pressure due to uneven feed. Hands will never rival the consistent rate one can feed with power.

Loosening the gib may sound counterproductive, but the fact that the slide will move easier may limit the amount of pressure required, in turn, eliminating the deflection. May or may not work. I can attest to the idea of loosening the gib when I've struggled with the same problem. Achieving consistent feed rate is critical to the end result.

Currently I take sanding blocks to the parts at 240 and 600 grit and look great on the lathe but the brushed finish hides the ribs........until vib finishing.

That's likely due to the fact that the sanding blocks are not rigid enough, so all you're doing is blending the error. By using a rigid backing, like a file, and pursuing the effort long enough, you should be able to flatten the surface entirely. Alternately, if you apply power feed and keep the gib properly tightened, you won't introduce the side to side movement (deflection) that is the cause of the problem.

H