A G12 or G13 when used in constant velocity mode can leave a little uncut material on a circle where the tool moves in a straight line to and from the circle edge because it accelerated away from the end point before reaching it.
If G12/G13 are changed to arc in and out rather than use a straight line there would be no uncut spots on the bore or circle. That and some of the drilling cycles when used in G98 retract mode dont use the R value to stop feeding when backing out of a hole. IE: they rapid down to R then feed down into the hole but then feed all the way back to the Z starting height rather than the R height.
Of course not a big deal but these are old G code issues from nearly 15 years ago when I was programming CNC's. I don't recall how but someone there modified the G12/G13 code on a Fanuc OM-C controller to make sure the constant velocity mode can be used with them to cut circles by using arc moves rather than a straight line move to get to/from the circles edge. Irrelevant for mach3 I would assume => The "problem" with canned drilling cycles was never encountered because that drilling mode wasn't used.
Not 100% sure but it may have been an M code that worked like a G code.
No, G12/G13 can not be changed.
Mach3 2010 Screenset
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)