Hi,
before you commit to the expense of a controller.....
A g31 move is a special example of an ordinary g1 move. For example:
g31 z-5 f100
means 'move from the current z location to z=-5mm at a rate of 100 mm/min. It moves as if it were a g1 move.
Where it differs is that if the motion device detects a probe event while the move is in progress the machine stops,
aborts (clears) the remainder of the motion buffer and reports its X,Y,Z coordinates to Mach.
Firstly note that the Z axis coordinate is in 'work coordinates' not 'machine coordinates'. Lets say for example that the
current work coordinate is -10mm. When Mach encounters the g31 command above it will move upwards form its current
z=-10mm to z=-5mm and it wont encounter a probe event while doing so.
The point being that if the work coordinate system is out of kilter with the work surface (or touch off surface) unexpected
results can occur. Given that you don't have home switches it would be easy to have the work coordinates out of kiliter
along the lines I have suggested or some other combination that produces unexpected and failing results.
May I suggest homing your machine as normal. Jog to the touch off plate and lower until your probe is just or nearly
touching the plate. Then zero the work coordinates using the DRO buttons. Jog the machine upwards a mm or two.
Now try an MDI of:
g31 z-10 f100 (metric)
g31 z-.5 f4 (inch....bloody queer Yankees)
and see what happens.
Craig