Worn leadscrew?

[Doddy]

I've a couple of questions - really out of curiosity (and boredom - 2 weeks away from my machines) than progression to a solution, and I have zero experience of servos. But you should be able to answer the following with only a short time in the workshop.

Firstly, what, if anything, has changed since the machine worked well? I'm assuming that you'll reply will you have not deliberately changed anything.

Presumably you can swap the drive/positional feedback between the X/Y servos?, have you tried this?, what's the result? Then repeat with swapping the X/Y servo drivers. You should be able to isolate to either servo or driver in error... if not, then it's likely mechanical.

You don't mention: Is your part a progressive multi-depth cut?, If you was losing steps then you would expect any error to accumulate and might expect to witness this as you cut through the material. I'm inferring from your post that this (accumulated error) is not the case, and the error is repeatable.

You don't mention: If you move the workpiece centre elsewhere on the X axis and repeat the cut does the problem remain? If it did this would tend not to support the leadscrew argument.

Have you tried any measurement of x-axis positioning? get a sheet of material on your bed and engrave a rule (Y stroke every 10 mm across the X-axis, say) - from X=left though X = max. Cut one rule in the left-to-right direction, and a second in the right-to-left direction. In each case ensure that the X-axis velocity is expected to be zero when you're engraving the Y stoke (this is just to test the X-positioning). Are these the same? (If not I'd be checking the fasteners on the ball nuts, ball-screw supports, spindle and even machine bed).

Repeat but with a hand-crafted sinusoid (y = scale * sin (x * some-fettle-factor)) (cutter permanenting engaged with material) so that you're now testing the positional accuracy with the x-axis under load/ and not coming to rest) - measure the positional accuracy of the Y=0 crossing point. I'm not sure what this would prove other than ensuring that there's no significant time-constant in the servo positioning (is this the servo controller gain?).

Is this with your DIY spindle?, if so you should be very intimate with the build of the spindle - are you confident there's no play in the spindle under load? Again, check fasteners also.

[JAZZCNC]

Presumably you can swap the drive/positional feedback between the X/Y servos?, have you tried this?, what's the result? Then repeat with swapping the X/Y servo drivers. You should be able to isolate to either servo or driver in error... if not, then it's likely mechanical.

Doddy it's not that simple with Servo's Esp if running analog (And if Husky is who I think(Mat) then sure he is running analog) because each drive is tuned to the motor and it's load, so it's a lot of work not just a simple switch over like steppers.

You don't mention: Is your part a progressive multi-depth cut?, If you were losing steps then you would expect any error to accumulate and might expect to witness this as you cut through the material. I'm inferring from your post that this (accumulated error) is not the case, and the error is repeatable.

He's using AC brushless Servos so there are no steps involved. The encoders deal with position and feed it back to the drive so corrections can be made. You get what's called following error so if drives not tuned correctly to the motor and it's load then the error becomes larger, up to a point then drive will fault.
A correctly tuned setup will keep this following error to a minimum which doesn't affect part quality. But if anything changes like start putting much heavier than usual loads on table or binding or mechanical issues then the following error becomes greater and will show in the part. If the following error is on the edge of acceptable or binding etc is isolated just to one spot then because of how servos work the servo loop always allows the motor and position to catch back up so don't get a fault. The allowable following error can be adjusted in the drive so it could be set to a lower value which will help catch any trouble before the part is ruined, but it's a fine line between right and wrong.! I've also given a very simplified explanation of servo tuning , in practice, there's more to it.