is there a buffer memory on stepcraft D series ?

I answer Myself : No. There’s no buffer memory on Stepcraft D series. BUT int the usb module (I own an AxeMotion PulseBox NXTG), there is a 400 vectors max buffer. When you activate it, it allows you to mill perfect circles and perfect curves.
So if you have a buffer memory on your controller, use it. It increase accuracy.

There should be no reason to see any accuracy issues related to buffer or software related communication when using the UCCNC

most likely its backlash

thanks for your reply Rory. I have near zero backlash with my stepcraft. once, accuracy issue was caused by poor tension of the belt. i fixed it, checked all axis for no backlash, and my stepcraft was still unable to cut perfect circles...

Remember smaller cutters will also deflect under load.

we see commercial machines with better than 10 micron accuracy show weird circles if you drive the smaller cutters hard. cutter deflection is a real issue with a stiff machine. Not sure if this could be it.. but worth a try.

remember backlash is not just active when the machine is static, Its defined as any rotation of the motor in the drive system and loss of this motion in translating into the linear motion.

are you sure you have no pulley rock on the shafts?