To accomplish the patterns typically imparted onto metal with guilloché, the cutter needs to be moved a repeatable distance each time the movement is made. (Such movement is typically in the Y axis.)

A Ratchet Stop is used on the cross slide to accomplish this.

This is a YouTube video from Frédéric Verschoore showing a ratchet stop at around 9:30 minutes into the video, so it is set to jump ahead to that.