Question: I have a Xermac CNC EDM. I am starting to see some slight position errors. Can this be related to backlash? How do I do backlash compensation?

Answer: This is a question that we get very often, especially with older machines. There could very well be backlash that has developed on the ball screws. But the backlash usually manifests itself as severe cut instability long before anyone would notice positional errors. This is particularly true with low power finishing. A more likely problem is a dirty linear scale. Most Xermac CNC machines are equipped with Acu-Rite scales. With time and exposure to EDM mist, the seals around the scale will harden and the glass scale will become dirty – leading to position variations.

The ball screw and fixed end support bearing are both preloaded. There should be no backlash or axial lost motion. If the EDM machine will be used for orbiting or cutting on angles, EDM Zap highly recommends not implementing backlash compensation. Unlike a CNC milling machine that stays in one direction for a long time, it is the nature of EDM to require a high number of direction reversals – often, many times per second. Adding backlash compensation will lead to instability and extended cutting times. However, for small amounts of backlash (less than 0.0003 inches), it is possible to add backlash compensation with reasonable results. Xermac CNC machines use the Delta Tau PMAC for motion control. Backlash compensation is handled inside the PMAC through parameters. Parameter IX86 determines the backlash amount. The units are 1/16 scale counts. The speed at which backlash is taken up is parameter IX85. The units are scale counts per millisecond.