Round objects in 'Z' plane coming out "squareish"

[mastory 39]

I am trying to print some round objects that need to be truly round as much as is feasible. I am getting parts that are consistent in diameter in the X and Y directions, but when measured on the diagonal they are somewhat larger.

Since I have stock motors, and stock x-y drive system and I am getting consistency in the x and y directions, I am lead to believe the axes are calibrated correctly. This is done in the firmware right?? My prints are coming out slightly over sized, but I think this is due to the outer contour widths. My current concern is shape, not size.

My belts are in my judgement snug, but not overly tight. If I tighten them any more, they will wear out prematurely. My 6mm guide shafts are as near to square as I could get them (should have little effect relative to my problem). I don't 'feel' any slop in the drive system.

Latest prints sliced with Netfabb and sent with Pronterface.

What else if anything should I look into?

Matt

[owen 18]

What firmware are you using. Marlin should help a bit I think. Other than that it's normally loose belts but could also be a loose pulley.