I have a USB 6040 Chinese machine. I have cut several things with it and they have turned out ok, but they have all been small. Yesterday, I cut something that was 17.5 inches long. I had a box that was supposed to be centered in the material, but it was off. Then I noticed that although my piece measured 17.5 inches, if I jogged the axis from one end of the material to the other, it measured 20 inches in Mach3. I went back and calibrated my motors again and they were off very slightly. I fixed that, but my measurements are still far off by the same amount. I am telling Mach3 to move the axis one inch. I use my micrometer to measure. I am sure it is not NASA quality measuring, but it is very close. I don't think it could be enough off to cause 2 and 1/2 inches in difference in a 17.5 inch span. Should I go back and tell the machine to move 20 inches and see tell the machine it moved 17.5 and see if it fixes it? Is there some other variable I am unaware of? Any help would be appreciated!