I am looking for opinions from CNC machine operators on a Fanuc CNC function called "Manual Handle Interrupt" (MHI), or similar function on other CNCs.
Some background: I am a PLC developer for a machine tool builder; I've been asked by a potential customer to add MHI to a machine. I'm a "hands on" developer, so while I may not be a machinist, I'm really focused on making the machine work well and spend a lot of time on the CNC ensuring so.
I have a 31i-B5M CNC with the necessary Fanuc option, and have done the needful to set the necessary signals to the CNC when the operator presses the newly added MHI button. The MHI function works, in that when I turn the MPG wheel, the selected axis moves according to the MPG pulses. Even works while the selected axis is already moving due to a program command. This all matches what the Fanuc documentation explains.
One thing that really worries me is the messing with the work offset; all the motion done with the MPG while in MHI mode is added to the current work offset. Not by directly changing the work offset; instead, the option adds a HANDLE (or HNDL for small screens) softkey to the POS screens (see attached); this screen lists the axis-specific total motion added by the MPG motions. So the effective work offset becomes: commanded (G54..G59) + EXT.OFFSET + any distance moved by MPG in MHI mode. Since this additional "offset" isn't as "visible" as the others, it seems ripe for disaster (wrong offset makes bad parts or crunchy tool
).
My next step is to write the "operator manual" entry for this function. As many of you know, Fanuc documentation can be somewhat... obtuse. It explains the technical aspects, but rarely does it give a good sense of why this function is used. So I'll looking for opinions from people that have actually used this (or similar) function. For instance:
1. Why do you use this function? In other words, what machining scenario leads you to push that MHI button and move the tool around with the MPG wheel?
2. How do you deal with potentially disastrous effect on work offset? Does your CNC always clear the MHI addtional motion on RESET (this is optional, as set by parameter). I feel like I want to add a Big Red Flashing Light to my OCP that says "CHECK MHI OFFSET" if there is any MHI motion.
Thanks for listening...