i've had a lot more time to experiment and nothing seems to help. I've tried tightening y axis belts a little and tightening them a lot. Tightening them a lot, 2 turns, on the screws actually made the circle off by .7 mm instead of the normal .35 to .5mm.

Are the 6 mm belts are too narrow for the dual extruder gantry?

The pullies are gt2 20 tooth 6mm wide.

the drive pulley is tight.

Am I expecting too much? Would a round object 1 inch in diameter be expected to be out of round 0.015" to 0.025"?

I did a threaded part but at that tolerance the nut would have to be 0.030" larger to fit and would be uselessly loose just to prevent binding in the tight dimension.

I just received a Sigma from matter hackers. The y axis was having some problems. First is that it crashes the end of travel at the left front of the machine. It turned out the gantry was 5mm off square. I fixed that by rotating the left pulleys.

How tight should the belts be? I can squeeze the y axis belts so they touch midspan with almost no force. Maybe an ounce.

Next is that every printed object is a little short on the y axis. Circles are out of round about 0.015". Circles have a flat spot front and back. I was trying to check the backlash by issuing g codes and monitoring a dial indicator. I installed pronterface. X axis G commands work as expected but Y axis commands will not move the stepper motor unless they are about 5mm or more. A G0Y1 from Y=0 does nothing. 10mm works though. This is not the case with the x axis where it responds as expected.