Surface Finish Issues with CNC Lathe

It's a full CNC lathe. It was sold by South Bend with no gearbox and no apron, and an aftermarket company installed servos for the X- and Z-axis. The leadscrew is only driven by the servo motor, shown in the first picture from my last post. The servo drives the leadscrew through a zero-backlash poly coupler, so no misalignment of the servo can be translated to the leadscrew.

I'm not certain the leadscrew isn't bent. It may be that the apron/ballnut is holding it in alignment. I'll probably have to take it totally off to verify 100% that it is not the issue. Just trying things in the mean time.

Also, is it possible that one of the balls inside the ballnut has become damaged? It would stand to reason that each time a ball recirculates it may be forcing the ballnut off center?

I'm also going to try turning a test pass by hand with the z-axis servo disconnected, and I need to try a pass with the x-axis servo disconnected as well although that it much more difficult (it's mounted behind the apron).
 
On a side note, when I pried back the leadscrew covers I doscovered that this machine has a 1 1/8" leadscrew. The force required to bend it would be immense, and I would expect that whatever caused it would have to have damaged the apron and/or ballscrew covers (which are thin sheetmetal) as well. I can find no damage at all to the lathe, apron (which also is only sheetmetal), or covers. I'd bet that nothng short of lifting the lathe by the leadscrew, or tipping it over onto the apron, could bend it.

The other thing I'm struggling with is the fact that the previous owner never experienced this. I've been corresponding with him and he is as stumped as I am. It would seem that the most likely explanations, a poorly-machined or bent leadscrew for instance, or a gib adjustment problem or alignment issue with the LS mounts, should have been a consistent problem before and after the move. As well done as the conversion is, and the fact that the previous owner had no similar issues with it, I have to believe that it's not a problem with the leadscrew alignment, apron alignment, etc. If it's a mechanical problem then it has to be something I jarred loose during the move, or a part that has worn out/failed.

Of course I've been wrong before :)
 
Are you using the same bench as the prevous owner? Is the lathe properly leveled and the ways parallel? Are you using leveling pads? Mabye there's a twist in the bed.

Does the problem happen at all ranges of the carriage travel?

And finally, is it possible the servo motor is having a problem? I'm assuming it's highly geared and in those cases, minor fluctuations in motor RPM make big changes in the outcome at the final output.

I'm just tossing as many ideas as I can think of...

BTW: I think leveling pads are pretty useful in a lot of machinery -especially if you're on a concrete floor.
 
It's a floor-standing lathe (I'm attaching a pic of the overall machine). There are levelling feet built into it, and I levelled it as per the South Bend setup instructions. This is especially necessary in my garage as the floor is sloped approximately 1/8" per foot towards the garage doors for drainage. I have a Craftsman digital level, not a machinist's quality instrument by any stretch but it does read to tenths of a degree. I levelled the headstock end across the ways, then the tailstock end across the ways, and then the entire lathe lengthwise. I don't know if the previous owner levelled it, and in fact based on the position of the feet when I acquired it I doubt it (one was much higher than the other 3), but it's level now.

The problem happens for atleast the entire 16" nearest the headstock. I would like to cut a test bar the entire length of the lathe, but I don't have a follower rest so workpiece flex would probably negate the value in such a test. I could get a long piece of stock and chuck it at various lengths with a tailstock center and make a cut for a few inches at various points of the bed. Based on the 16" bar it looks like it is unchanged for the whole length, but it would be nice to prove that to myself.

I would like to disconnect the x-axis servo and lock the gibs and make a pass, I just haven't had time. I would think that a servo issue would be more random than this, but it pays to check everything of course.

SB Fourteen.jpg
 
Man, that's a big lathe... Anyhow...

I know this is a stretch and it's probably not the cause of the problem but, at some point, maybe some neoprene-type pads between the floor and leveling bolts will help. Case in point... On my 1200lb surface grinder, I would occassinaly get a strange pattern on fine finish passes. It would come and go. It drove me nuts. One day, I bought neoprene leveling pads and the problem went away and never came back. -Go Figure... It was a weird vibration.

Try those easy things first, even if they don't make sense. You might just stumble upon the solution. It's imporant thought to just try one thing at a time so you can learn what the problem was.

... How about the leadscrew bearings and tolerances? leadscrew lube the right kind? Is it very cold?

Again, just tossing more thoughts out there. I had the exact symptom on my old manual lathe but, the solution wasn't as hidden but these are things I tried.
 
If you have a long travel indicator, set it up and watch it during a cut, or even just moving in air in -Z- to see if there is a "jump" in the travel. A 1" indicator might do it, but longer would be better. Also I'd want to look at the shaft as it was being cut by running an indicator on it behind the tool. When the "jump" occurs, I'd expect to see it on the indicator. I don't mean as the indicator passed over the spot, but as a result of the cross slide being moved in some direction. Might want to run the indicator on the ways trailing the carriage too, so see if it is jumping up or something. All could be clues to help zero in on it.
 
I put a mag base and a DTI on the carriage tonight, referencing the flat way on the bed, and ran it manually up and down the ways. I noticed that when I ran the carriage towards the headstock I got a repetitive .0015" jump. Here's where it gets really weird. I also noticed that when I ran the carriage away from the headstock the DTI stayed steady. To prove this wasn't a fluke I put an identical tool in the toolpost that was a LH insert holder and took a pass away from the headstock. No surface variation!:thinking:


Now I'm thoroughly confused. The front edge of the carriage jumps in time to the leadscrew pitch when travelling towards the headstock, but not when travelling away from the headstock. How can that be? Atleast it seems I can rule out bent leadscrew, since that would have to give me trouble in both directions, but now I'm kind of stumped. My guess is an issue with the leadscrew bearings, although I'm not sure in what manner they could have failed to cause this issue in only one direction.
 
Sounds like a gear in the apron doing that jumping. I wonder if you put the DI on the right side of the carriage and move toward the TS, if you see that jump again. This would be a different problem than the other one because in auto mode, the half nuts are grabbing the LS. In manual mode, a gear off the handcrank is driving off the underside of the way.

Hmmmm... Q: Does your handcrank spin by itself when in autofeed mode? Pull that gear out of there that engages the handcrank to the LS and see what happens with an auto-fed cut.

Have you taken a real close look at the LS and is it clean as a whistle? How about that handcrank gear, is it packed with swarf?

Doubtful, but is that handcrank gear adjustable or out of adjustment by sliding off the shaft it rotates around?

Oh, if your handcrank rotates while in auto-feed mode, does it kinda plop over when it rolls past TDC? Maybe try balancing the handcrank.

EDIT: Had to use the word "handcrank" just one more time.
 
We're less than an hour apart. -Don't make come up there and start tearing your lathe apart :nono:
 
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