When using a Jig with four squares and using from center to start, three of the slate coasters come out perfectly, consistently, however when engraving on the top left (see arrow), there is always an issue, either light or in this case extra lines through the engraving. Could this an issue with the gantry? I have oiled all the rails. Thanks so much!
Check the focus distance in the center of each slate. They should all be identical. A 1 or 2 mm difference may not be obvious, but that is a big difference to the laser.