You are wrong. 20 turns per inch = 1.27 turns/mm Mach3 doesn't care what pitch your screws are. You just tell it how many steps to take to move a given unit, regardless if that unit is inches or mm's.Please tell me if I am wrong, however: I believe that, since my machine has english (20 thread per inch) leadscrews, I need to have the native units set to inches, and then let Mach3 do the necessary math to convert metric coordinates as necessary for the machine?
The metric/english setting doesn't seem to persist as part of the profile. Am I wrong about this?
Mach3 will always start in it's Native Units.
Clone your existing profile, and create a separate shortcut for the metric profile. If you don't know how to use the /p switch in the shortcut, see the Mach3 manual.
Set your native units to metric, and divide your steps/unit by 25.4. You'll need to convert your velocity and acceleration values to metric as well.