It's been a while since I visited this gcode post. For a bit of background, I created this post for a Bridgeport Series II, which is setup so that the uppermost quill travel is z=0, and then plunges into the work (where z <0) . So for example if I cut into the work 1" the resulting gcode would be G1 Z-1.0
Also, the Mastercam Machine and Control definition has the origin at 0,0,0 and all cutting is negative Z.
So to answer your first question, the search line is:
"G49", "G0", "Z0."
and change Z0. to whatever value you want.
Changing this Z0. to Z(whatever) may solve your problem if your machine allows a suitable positive Z value to give you the extra clearance. (mine doesn't).
If I may make a suggestion. Depending on your machine (and we know they are all different), it might be better to modify the Machine and Control Definitions so that your machine 0,0,0 "Home" has greater distance from your bed than it may have currently. On my bridgeport I simply lower the bed manually to be sure I have enough distance for tool retraction and tool change. On many machines the mill head moves and can give a wide range of max Z values, which can be set in Mastercam.
Once I have set the bed distance, I touch off the tool to the work and measure the distance (which is always negative, btw) and set this negative value as the height in Mach3.
Works like a charm.
Good luck!
Charles.
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