It is entirely possible to run the CNC without starting the spindle. LinuxCNC does not start the spindle with the “Run” button. It is controlled by G-code, the Spindle button on the GUI, or the Spindle button on the MPG. The G-code line “M3” is the spindle start command. If it is not encountered during interp, the spindle does not start.
The “drag knife” toolpath in VCarve is used to do something a little different. A drag knife literally drags the blade behind the center of the rotating axis, right? Like the tip is trailing 1/4" behind the spindle’s center, and the knife self-aligns itself with the direction of travel
This works great for the middle of a straight line. However, consider a 1" square. If the knife tip’s dragging 1/4" behind the spindle, the spindle’s coordinates hit the corner and starts going 90 deg in another direction, then the knife tip turns mid-side, rounds, and never reaches the corner.
So, “drag knife” with a 1/4" tip offset would have the spindle overshoot the corner of the square by 1/4", then loop around (to steer the swiveling blade) and start the next side with the blade in a predictable orientation.
Is there a way to absolutely ensure the spindle can’t start with the drag knife in it, if something goes wrong and there’s an M3 spindle start command in the g-code? Well, yeah- just don’t power up the VFD. The machine will interpret G-code just fine, if an accidental M3 is encountered, the interpreter will wait for the spindle to spin up to the specified speed which is impossible without VFD power, so the interpreter mode is stalled indefinitely