ok here is my problem, I load mach 3 up with a program and then turn my cnc controller on. Both current position and machine coordinates are all zero, i then machine a part. If i move the axis to set the datum for another part and zero x & y then the machine coordinates are no longer zero and when it is done it returns to machine zeros which is a pain?? how can i zero the machine coordinates without rebooting everything? cheers

You Can't reset machine coordinates as you describe.  You can however, use work/fixture offsets to rezero and deal with the problem you are having.

Check out the Coordinates/Homing video tutorial

HTH
Sid