Leadwell LTC-20BP intermittent accuracy issue

I am trying to diagnose an intermittent issue with a Leadwell LTC-20BP with Fanuc 0-T where when cutting a bushing, an OD and ID error appears - ID and OD become too large.

The operator adjusts offsets to get the correct dimensions, then after a period of time in the same shift the issue will reverse and the the ID and OD will be too small.

The amount of error is consistent during the period it has changed, but each time the error appears, the magnitude of error is not consistent. Error will range from 0.006"dia to 0.016"dia.

I have not been present when the error is evident to run through indicator tests, but the machine has been taken out of service to fix the problem.
I have looked at the turret mounting, it looks solid, no crashes reported recently(problem comes and goes anyway). The indexing is very clunky and sometimes will not complete the rotation(not clamped).
The turret is not well balanced, but the tools are not heavy. Clunkiness is still evident with the turret face removed.

I have started disassembly of the turret to get to the curvic, but I'm hoping to get some advice on things to look for.
The turret has a solenoid activated stop pin that appears to stick sometimes. Should I pull apart the stop mechanism and shock dampener?

I was told the operator did some indicator checks already, I will do my own in the morning.

Your help is appreciated.

I understand that the turret is making noise and having intermittent issues with indexing, but I think that you might be barking up the wrong tree by looking there for your accuracy issue. If there is a turret problem, the turret would be moving above or below center, which would cause issues, but I don't think it's likely that it would cause a shift of .016" in diameter... at least not without seeing other issues like poor surface finish, broken inserts, vibration, etc.

I would be looking at the X axis components- ball screw, ball screw bearings, belt. Is there a belt between the servo motor and ball screw? That's the first place I'd look. Look at it for any signs of it stretching, coming apart, losing teeth, etc.

Before anything, though, I suggest you put an indicator from the turret to the spindle and start pushing on the turret and looking for movement. Push in the direction of X travel and also perpendicular to that in the Y direction to try to see what's going on with the turret. Remember that you have to push hard enough to lift the entire turret, so it might be helpful to use a 2x4 as a lever against the tailstock.

Naturally, be very careful when working on the X axis as it can come falling down if you disconnect the ball screw from the brake.

Good luck, and report your findings back here.

It's a direct drive screw. Finish is coming out fine on all parts, only dimensions seem to be affected which drove me to check the curvic. The coupling was replaced 8 years ago. and is rotating very rough.

I haven't seen many ballscrews that would shift in one direction for a period of time then move back, most screw problems I've seen show with measurable backlash(like you're suggesting). These bushings are small and there is almost no load on the X.
I will perform some checks on the X though as you suggest, taking the full mass of the X off of the nut and end bearings using an indicator to quantify.

I'll share what I find today.

I would run a simple program with the X axis moving up and down in rapid a few times (many actually) against a dial indicator. Let it run when it is cold and look after a few minutes and see if it zero's out every time.
Then you would know if it is a turret problem or an axis problem.

I am sure you have checked this, BUT, are you 100% sure your tooling is solidly mounted? I had a similar issue once and it ended up being the clamping wedges not being tight enough allowing the cutting tool to move in the turret. Does the issue happen at a certain point in the cut or randomly? Does it happen more under heavy load than under light load? Are there any other common factors (always happens early in the day, or only happens after machine running for a long time, etc)?

I did a backlash check: 0.0005" lag in both directions. Loaded backlash is 0.0005" in X-, and I can lift the X+ by 0.0026", so 0.0031" overall. This always returns to zero +/- 0.0005 so I don't see it being the culprit. The part does not use much load in any servo axis or spindle.

Tools were firmly fixed, but definitely not balanced well. It's a small turret though and the tooling is light duty.

Full stroke repeatability looks good to within the backlash error.
The phenomenon will appear a few hours into a shift and change back sometimes an hour or 2 or hours of running later(but not specifically these times). Although this is not 100% as I did get from the operator that there was a "light" crash a few weeks ago.

When I got to the curvic today, it has 4 alignment tapered dowels, one is broken, the other 3 took a 3/4ton truck to pull out, including breaking 2 slide hammers. The 3 that came out were a little misshapen and clearly gouged from being rammed in too hard on the last rebuild.

The stop pin(solenoid activated) shows about 6thou of wear on the CW side, and one of the return springs was broken. I managed to fix the second spring, but I don't see how the pin not returning and the wear on the pin could cause a position error if the coupling is still engaging fully.

But maybe it isn't. Sometimes after a tool change the error comes up turret not ready, which is based on the clamped status of the mechanism. This is also intermittent, happening a few times a shift.

The broken pin is being drilled out tomorrow.

I am suspicious of the broken 10mm hardened pin (I've never seen one broken on a little machine like this), in combination with the turret stop pin not always returning easily. With the symptom of intermittent "clamped" status I suspect the curvic is not meshing completely(closing fully in the Z direction), but the turret has no wiggle due to the stuck stop pin.

I will inspect the coupling when it comes apart, but if it has slipped, the coupling may not seat properly regardless of the stop pin being stuck.

Thanks for the tips everyone, I will hopefully know more when I get it open.

Just some update on this: the operator attempted drilling out the broken pin but was unsuccessful. I began reassembly and took some additional measurements and found that in the 8 position turret, there was some movement in each position when push/pull, but in a few positions there was excessive backlash up to 10 thou(when grabbing the turret to force rotation). With an indicator on the tool slot near the outer edge, I would see this error, and when forcing it by hand I could see an additional 7 thou in each direction.
Bringing this up to the operator, he seemed to accept this as the likely cause of the problem. In the future I will be doing checks like this before tearing anything apart or taking the operator's word that they checked things.

The lathe will be used only in emergency, low tolerance situations, but it will be replaced when possible.

Thanks for the suggestions here.

Cheers