CNC Tip:

Disclaimer: CNC Concepts, Inc. accepts no responsibility for the use or misuse of techniques shown in this web page. We simply publish information we feel will be of interest to CNC users. In all cases, the reader is totally responsible for considering the implications, good and bad, of implementing one or more of the techniques we show.

by Michael Saylor of CNC Engineering and Service

For most of the Fanuc Series Controllers (FS) OT,16/18T,10/11T you can have a condition where upon every time you attempt a Zorn (Zero Return), a Soft Limit Over Travel occurs. This is especially troublesome with the Z axis of vertical machining centers and the X axis of turning centers. These axes tend to drift during the night. When this happens, here's what you do:

Release NC Soft OT alarm by Jogging away from the limit. Turn the control's power off. Depress the "P" key and the "CAN" (cancel) key and keep them depressed when you turn the control on (continue holding P and CAN until CRT Display comes on. Release "P" and "Can" keys. Attempt Zero Return again. Pressing P and CAN makes the control ignore the soft limit, and resets at Decel via prox. If the axis over travels again, more than likely you have a bad Zrn Switch. Usually a Proximity Limit Switch. Check the machine maintenance drawing for schematic and DGN associated. Check the DGN I/O bits or ladder while placing a metal object in "Proximity" of the switch face. Note: Even though an indicating led works, it doesn't always mean the switch is good.