Hello guys,
I am new here, as the hobbist in CNC I use grbl few years now.
I have some problems and think I am not able to solve it by myself.
I have feeling that it must be somehow arduino or grbl related.

I can cut very nice smooth surface finish on X and dual Y.
Problem is when machine goes into something not straight, like 45° or Arc, nice surface
finish is gone and edgy. If I run program with spindle off, just cutting air,
I can feel by hand on non-straight lines these vibration shaking.
I have to mention that I checked everything mechanical, machine
is squared, I use preloaded C5 ballscrew, there is no backlash. As motors
I use Clearpath from Teknic with proper tuning. There is no tension on
machine, everything is super tight and free to move. If I push hard on gantries,
there is no measurable bending. As controller I use Arduino Uno with Grbl shield (no stepper drivers),
I do not need that shield, but it is easier wiring with it for clonning Y to A.
I have to mention that also changing feed rates does not help and does
not affect surface finish on these non straight lines, always edgy.
I tried to generate g-code in different programs - same result, also different g-code senders - same result.
I modified several times grbl setting, also tried different steps/mm, junction deviation... still same.

Does anybody know what causes it, I suspect there is something with grbl that cannot synchronize somehow XY movement ?

I add photo:
First two are cutting on X and on Y, its nice smooth cut. Other two are edgy, 45° and Arc on circle.
On 45° is visible repetitive pattern in waves.



I would be glad for any help.

thanks