in mach when starting cutter comp is it necessary to have both an x and y move? or will just one of them suffice set cutter comp?
Just one should be fine, but make sure the lead-in move is at least as long as the tool radius, and preferably in a location where the comp can be easily applied.
Here's how I'd code a simple arc with comp.
G40
G20
M3
G0 Z0.1250
G0 X0.0000 Y0.0000 Z0.1250
G1 X0.0000 Y0.0000 Z-0.2500 F50
G42P0.125
G1 X0.0000 Y0.3750 Z-0.2500 F150
G2 X3.0000 Y0.3750 Z-0.2500 I1.5000 J-1.8670
G40
G1 X3.0000 Y0.0000 Z-0.2500
G0 X3.0000 Y0.0000 Z0.1250
G0 X0 Y0
M5
M30
Just a note. Mach's comp has been constantly evolving over the past few years. In it's current incarnation, it can behave differently in different situations. Notice in the pic and code that I posted, how comp is applied. The tool moves along the programmed path, until it's diameter is tangent to the arc. At this point, comp is applied. Then, notice that comp is turned off when the tool's diameter is tangent to the lead out move. At one point, comp was applied the same way it is turned off in this example. The tool would move from the programmed start point, to a point where it was tangent to both the lead-in move and the first comped move.
Now, I did a little testing. Basically the same code, but without the arc. Notice now, the the way comp is applied and removed is exactly the opposite of the way it is with the arc. Now, comp is applied during the lead in move, moving to a point where the tool is tangent to the lead-in and the first comped move. But during the lead out move , the tool actually travels along the programmed path. Here' the code:
G40
G20
M3
G0 Z0.1250
G0 X0.0000 Y0.0000 Z0.1250
G1 X0.0000 Y0.0000 Z-0.2500 F50
G42P0.125
G1 X0.0000 Y1.1045 Z-0.2500 F150
G1 X3.0000 Y1.1045 Z-0.2500
G40
G1 X3.0000 Y0.5698 Z-0.2500
G0 X3.0000 Y0.5698 Z0.1250
G0 X0 Y0
M5
M30
Now, the last pic is a drawing I did showing how comp used to work, and how I hope it works again at some point. I believe that the comp should be applied during the lead-in move, and removed during the lead-out move, as shown in the pic. And hopefully it works the same whether the first comped move is a straight line or an arc, NOT diffeent as it apparently does now.