I've worked around this issue but I've never completely understood it so I'm hoping someone can provide a lucid, explanation.
For example, if I have the following G code:
M6 T6 (selects a tool)
G0 G90 X0.75 Y-2.75 E1 (positions my table/fixture precisely under the tool)
S3500 M3 (spindle speed @ 3500 RPM CW)
H6 Z0.3 M8 (selects tool length offset, sets I-plane [tool position] 0.3 above part, turns on coolant)
I'm working on a flat surface, no steps, no clamps to clear.
Now, I use G82 for my spot drill because I like the option of the dwell before retract. So if I program as follows:
G82 G98 R0+0 Z0.03 F12. P100 X0.75 Y-0.5 E1
Why can't I set R0 at zero? The Fadal manual states "The I-plane must always be equal to or above the R-plane" and "In the absolute mode the value for this location is relative to Z zero". The explanation and drawing in the Fadal manual clearly suggests that R zero (R0+0) would be at the surface of the part which is 0.3 below the I-plane.
Maybe I'm having a bad day but darn this R-plane concept baffles me.
The confusion arises because of two things....nowhere in the Fadal manual does it indicate that R0 is actually relative to the "zero-plane" or what would be referred to as Z-zero (vise top?). Also, the only definition of "R" that I came across some time ago is that it referred to "retract"..which is obviously wrong (and in fact compounded by the horrible drawings in the Fadal manual). Unfortunately that definition stuck in my almost hairless head and created a great deal of confusion (and the loss of even more hair!). Your explanation is "lucid"...therefore bonus points!!! I realize that the R plane concept was created to minimize G code when having to clear a feature or clamps but I haven't had to deal with that yet and if I do I'll probably just add a couple of lines of more code.