There are two of belts in the x axis that can affect thick and thin lines, one on the right, one on the left. Both need to have no wear on the teeth, and need to be tight. Also, make sure you are tensioning the rubber before each cut, not just at each roll change.
H19-018 for the X motor, $7.00
H19-011 for the Y motor $9.00
V40-452-00 for the serpentine belt $19.00

For replacement of the 536 cutter belts please cross reference chart attached. Make sure you check the diameter of the large black pulley on the right side before ordering, as there are different revisions of the 536. The part numbers are shown on the attached doc, one of which is the IMPROPER way to install the x axis motor belt
Attached are some pics and docs of our 536 cutter belts that should help

You can order these by calling our front desk at the number below.