Help Please! arcs in Mach turn

[theshed]

Hi
I have retrofitted my lathe and all is fine. Simple x and z cuts seem very accurate. But...I have hand written code to produce a pawn shape (No that was spelt correctly!) I am using a series of subroutines with G2 and G3 cuts using the I K system. My all dimensions are in mm to 4 decimal places. After 30 loops the position has drifted significantly by around 0.5mm per axis. What am I doing wrong??

I am on Servo motors with 2000 resolution encoders.

I have adjusted Gibs, backlash in the belts, the ballscrews seem fine.

I have slowed down velocity and acceleration to a fraction of what the servos can cope with.

I have broken each arc down into a separate subroutine in order that the errors do not add up - minor improvement

I have broken arcs that intersect axis lines (causing the motors to reverse) into separate sections.

I could get around the error multiplication by resetting zeros before a final cut but that seems very crude.

Are arc moves generally inaccurate or is it me?

[Hood]

Arcs are fine for me on my lathe. Can you monitor the encoders in your drives software?
Hood

[RICH]

I never found any problem with ARC's on my lathe.

Any cutting on the lathe will only be as accurate as your machines system.

Hood can help more relative to setup since you use servos.

RICH

[theshed]

Everything I know about electronics has been learned on this project so I am a little bit slow! I am using a dual servo drive from Granite Devices. There is software to monitor the encoder although I think that requires me to disconnect the BOB. If I can manage that what should I be looking for?

Many thanks for your prompt responses. This is my first experience with Mach3 - previous and very dead control was 1980s Anilam - its quite a jump up.

John

[Hood]

What I am thinking is it may be a tuning issue.
If you can monitor the encoder position whilst Mach is running then you could run some code and have some M1's in the code and take a note of the encoder reading at each M1, Run the code over and over and see if the encoder readings are the same on each run.
Hood

[theshed]

I think you are right. I tuned the motors carefully but only thereafter noticed that the gibs were loose in Z and belts loose in X and Z. I will retune and see what happens. Many thanks for your help. I can see that you are a great supporter of the forum - many thanks.