I converted my Bridgeport j-head mill to cnc some time ago. Use an original Dan Mauch servo system and a ball screw kit from Mcmaster Carr. I use Mach II for the controller.

I recently noticed contoured circles are not turning out round in the diagonal direction. They're the right size in x and y but an 1.500 one is about .020 small diagonally. My backlash is under .001 in both x and y.

I'm mystified and looking for suggestions.

BTW, I recently added 2-200lb x 16 inch gas springs to the knee of the bp, also from mcmaster carr. They allowed me to triple the z speed. 

Thanks,

Jim

Have you checked to see if you have any movement in the gibbs :-

can you twist the table pulling it back and forth from the front of the machine.

People nearly always use the middle part of the X axis so it wears more in the middle.

This would not show up in a backlash test.

Graham.