Gerber HS750 Plus User Manual page 26

plus
20 HS750
To prepare the plotter
1. Center the carriage spindle preferably by using the slew keys
(see "Front Panel" section) or manually (with certain
precautions, as noted in "Loading Material"). Be sure that the
carriage spindle is not located at either extreme end of its
travel and that both ends of the rubber drum can be reached
easily.
2. Open the bail arms at either end of the drum by pulling them
away from the drum.
3.
Using the slew (arrow) keys, rotate the drum until three closely
spaced pins are visible. These three sprocket pins are used to
align the material.