I've got a CNC running Mach3.

When I home the x and y axis with the units set to MM, the gantry moves to the proximity sensor, then it moves away a small amount, and slowly moves in to around 2 mm away from the sensor.

When I home the x and y axis with the units set to INCH, the gantry moves to the proximity sensor, then it moves away much further, and VERY slowly moves to around 3/4 of an inch or more away from the sensor.

Is this normal?  I would think that regardless of the unit setting, the gantry would home right where the sensor triggers.

So My first question is how are you changing between inch and mm ? This should be done in the settings tab.
The second question is, what do you have the home off setting for inch mode?
The best thing to do if you have to switch between inch and mm is to make a mm profile and a
inch profile.

I am changing it in the settings tab by clicking the UNITS button to toggle it.

After talking with a few people with knowledge of Mach 3, I think I finally understand what is happening.

When I click REF ALL HOME, it sends a g-code command to move left until the sensor triggers, then sends a command to move right 0.5.  If units is set to mm, it moves right 0.5mm.  If units is set to inch, it moves right 0.5in.

I also learned that the units setting is simply the native units for the software interface, so there is no need to change that.  The toolpaths will generate the codes to set the units for the toolpath.

Therefore, I will be keeping the units on the machine set at mm so that I get an extra 0.5" of cutting area.