Hi,
the only way to zero machine coordinates is to reference (home) your machine.
You can change and/or set work coordinates at will but not machine coordinates.
Note that if you do not have home switches installed and assigned a machine will 'HomeInPlace', that is when you
hit <RefAll> Mach will assume that its current axis (axes) position is in the Home position and zero (or set to the
axis offset if programmed) where it is. If however you have a home switch assigned the machine will drive that
axis until the home switch activates and then zero (or set to the axis offset if programmed) the machine coordinates
of that axis.
Craig