[nicanor76]

When I circles program my DIY CNC (Mach 3, open table/moving gantry), they turn out to be ellipse; longer towards the top-left and bottom-right sides of the circle. It still does this even when I turn the speed way down, so I don't think it's missing steps.

I tried messing with the bottom legs (X axis), moving one of them more forward or backwards, more/less angled towards each other, etc. But no matter what it's still cuts ellipses instead of perfect circles.

Any ideas what it can be specifically? I'm guessing one of my alignments on the table is off??? Anyway I can fix this through the software in Mach3?

[YZF]

It sounds like the orthogonality between your X axis and Y axis is off. I.e. they're not exactly 90 degrees to each other.

[TOTALLYRC]

Try cutting a square and then checking the diagonals. a perfect square would have identical diagonals. also measure across oppostite faces. If the across the opposite faces you get the same dimensions, your are cutting a parallelagram and it is not the mach3 program or g-code, it would be that the x and y are not exactly 90 degrees from each other. Cut the largest square that you can comfortably measure, errors are easier to detect with a bigger object.
even if you could fix this in software, which I doubt, the machine should be set as close as possible to perfect, this way the software can worry about the cutting and not have to do other things at the same time.

[ger21]

Could be your steps/unit is off on one axis. A picture would be helpful.