Should You Cancel Cutter Comp Mid-Program, or Always Run It to a Clean Exit Move?

Always run it to a clean exit move. Canceling cutter compensation mid-toolpath, especially inside an arc or right at a corner, is one of the more reliable ways to gouge a part or crash into the fixture. The control has no way to know where the tool center needs to go the instant comp drops if you cancel it somewhere ambiguous.

Why the exit move matters

Cutter compensation offsets the programmed path by the tool radius so the cutting edge, not the tool center, follows your drawn geometry. The control builds that offset move by move, looking ahead at upcoming blocks to figure out how to blend the compensated path around corners.

Cancel comp (G40) in the middle of that lookahead, say right after a tight corner or inside an arc, and you're asking the control to snap the tool center back onto the nominal path from wherever the compensated path happened to leave it. On some controls that produces a straight-line move connecting the two points. That move can cut straight through material that was never supposed to be touched.

A proper comp cancel needs a straight, linear exit move of adequate length, away from the part, on flat unobstructed travel. That gives the control room to unwind the offset cleanly.

Skip that room and you're gambling on how the specific control handles the transition.

What actually happens when you get it wrong

Two failure modes show up. A gouge on the finished surface, where the tool center's path back to nominal clips a corner or wall that was supposed to stay clean. And a crash, where the unwind move travels through a clamp or vise jaw that wasn't in the tool's compensated path but sits right on the nominal one.

Both tend to happen without warning in a simulation that only checks the compensated path and doesn't specifically flag the cancel move's behavior. There's a subtler version too: canceling comp with the tool still engaged in an arc leaves some controls to interpolate the exit differently than others, so a program that runs fine on one machine can crash on another with a different control brand, even with nearly identical code.

What to do instead

Program every comp-enabled toolpath to end with a straight linear move that clears the part and any fixturing before the G40 cancel. Start it the same way, straight approach, then G41/G42 engaged on that straight move, never engaged directly into an arc or corner.

Treat the entry and exit moves as part of the toolpath, not bookkeeping to trim off in editing.

If a program came from a CAM system that already handles this correctly, leave the entry and exit blocks alone. That's usually the exact lookahead room the control needs to avoid gouging.