Heath Robinson?, hey it works. Actually the ball-screw adaptation of the XY table would be an interesting write-up for anyone here looking to produce a CNC mill on the cheap.

I guess you're manually writing the G-Code?, many/most here will use a CAM processor (e.g. CAMBam on PC) to generate the tool paths, including the decreasing Z depth, and the output from the CAM processor will be the G-Code to feed into mach3,or similar. I'd be going this route, personally.

To answer your question - I've never tried this with Mach3 (though have done similar with LinuxCNC), you *could* (many ways to skin a cat), use a subroutine to perform your pocket operation at a single depth, and use a loop and G-Code incremental mode to create the increasing depth.

Something like (and this will be full of errors - so use it to understand the concept, not to test, -- prefix = my comments, not valid G-Code) It's nasty, it's probably naughty but it might give you ideas. But see (a) above - look at CAM processing instead! Again, don't run this at all!!


O0001
G90 -- Absolute mode
G0 X0Y0Z10 -- Move to 0,0,10mm
M98 P02 L60 -- 60 Loops, (60 x 0.1z = 6mm) ... Z will start at 10mm and ultimately drop to 4mm safe clearance height.
M30 -- program end



O0002 -- subroutine to perform one pass on the pocket
G0 X0Y0 -- reset to X=0,Y=0 - if your G-Code returns the spindle to the start position of the pocket then you don't need this, but you must be positioned to XY that's safe to plunge into the work piece
G91 -- incremental mode
G1 Z-10 F100 -- plunge down 10mm
G90 -- absolute mode

-- << do your pocket stuff here... NO Z movements!!!! >>

G91 -- incremental mode
G1 Z9.9 -- retract 9.9mm, the difference between this and 10mm is your plunge depth, here, 0.1mm
G90 -- absolute mode

M99 -- return from subroutine