Go to Settings ( Alt-6), in lower left side of screen you will find Axis Calibration, click the Set Steps per unit button,
select the axis and click ok, input how far you want the y axis to move, then click ok.
The axis moves and then the screen asks you how far it moved, you input how far the axis actually moved. It then asks
you if you want to accept the steps per, click yes.
The steps per value it calculated will be saved to the motor tuning.
Now test that same axis again the same way. You may need to do it a few times and each time Mach's calculated value will get closer to as perfect as you can measure the actual move.
When done ie; you commanded say 8.0000 and it moved 8.0000, then there is no need to continue doing it.
So then Config> save Settings, open the motor tuning and note the step per unit value for the axis.
Go out and back into Mach. The steps per unit should be correct to what you noted.
The input move should be as long as possible. How repeatable or accurate things are totally depends on your machine and how well you measured the actual distance moved.
NOW....one last thing
Go read the MACH 3 manual!
RICH