I have a 30" x 48" cnc router built from a kit. It has aluminum extrusions framework and aluminum panels for gantry and two ball screws on the Y axis, second one slaved to first one in Mach3. And I just installed new couplings. My problem now is that I get the machine calibrated to within .002 to .005 inches. I use a long ruler with a pointed engraving bit. I have a test program to check it. It's a square made with a 1/4" bit x 1/8" deep. The outer perimeter of the square is 5.875" (done in Fusion360). When I run the program, and measure with calipers, both X and Y directions are off about between .012 to .014". I check the calibration after the test cut and it's within a few tousands, not .012 to .014. Has anyone ever run into this situation? Or can someone direct me in the right direction as to what could be going on? Thanks in advance for any help or suggestions!