Temporary change of units?

I run a CNC I built for our local community shop, with a wide user base. As such, I need to keep operations really simple and avoid complex special cases. No ATC, just manually changing bits and either eyeballing the Z=0 or using a wireless tool setter.

Recently someone noted the tool setting probe wasn't probing. I found it technically was- prob being, the probing subroutine's .ngc file is in inches, the user's code had put it in mm with G21. OK, fine, add G20 to the top of the machine's subroutine file.

But, what if a user with a metric file breaks the bit mid-job, installs a new one, uses the tool height setter for the replacement, manually restarts the spindle, and does the standard Run From Here? Well, that seems like the file would then be misinterpreted in inch units unless you MDI in a G21. Which I can't explain to a wide pool of users, 95% of whom never touch the MDI window and don't understand G-code commands. I can't just add G21 to the bottom of the .ngc probing sub, because that would then misinterpret the inch files as mm and break most jobs.


This does bring some other questions to mind. The .ngc probe subroutine also cancels feedrate overrides. Is there a general purpose solution for saving and restoring modal states that I don't know about?

how about this?

linuxcnc.org/docs/2.8/html/gcode/m-code.html#mcode:m70