just replaced stock 300 watt spindle for new 710 watt. When I was cutting my circles were ovals. Ive done different tests, 2 different bits 1/4 and 1/8. drew a few different circles and squares to cut, rebooted computer and still all the circles are a bit oval and the squares are rectangles. what should have been a 60 x 60 mm square is 59 x 56 mm. checked the parts that screw onto the lead screws and they were fine. Not long before I had to replace the spindle I had cut a larger circle for a project and it was perfectly fine. Only change is upgrading spindle when old one died. when just moving the spindle on x y it seems to move just fine with candle.
It seems something is not working properly now. My first suggestion would be to check whether or not each axis X and Y moves the right distance when moving independently. I would speculate that after changing the spindle perhaps one of them is not. Is the new spindle heavier?
Does the spindle use the same power supply as the steppers and controller? Or does it have an independent supply?
I had a similar situation recently, caused by interference from the spindle causing motors to loose step.
The thing is, I’d expect interference like that would prevent the start and end points of your rectangles and circles from lining up. You can see in my pictures how it produced really wandering paths… But your description doesn’t mention anything like that. Which makes me doubtful if this is the issue here
If you have dual motors on Y and they are not always in sync that sounds more like an interference problem. As you did not have this before it seems the cause is the new spindle. You can try the same problematic code with a pen in the spindle and the spindle off ( or any other you can measure the travel without the spindle on)
I could imagine the spindle carriage being a bit loose relative to the X and Y axes to do this, because of the bit digging into the work and pulling the spindle toward it. Then you’d get different impact in X and Y because those are usually fairly independently adjusted.