What's the Difference Between Machine Cutter Comp, CAM Comp, and Wear Comp on a Mill?

Machine cutter compensation (G41/G42) offsets the toolpath by a radius value the control applies in real time as it runs the program. CAM comp bakes that offset into the toolpath during programming, so the G-code already contains the compensated path and the machine just follows it. Wear comp is a small correction layered on top of either one to account for a tool that's slightly off its nominal size. It's not a separate compensation method. It's a fine-tuning value.

Machine comp: G41/G42

With G41 or G42 active, you program the actual part profile, and the control offsets the tool by whatever radius value sits in the tool's offset register, live, as the program executes. Change the register value and the toolpath shifts without touching the program itself. That's the appeal: one program, and the actual cut dimension gets dialed in at the machine by adjusting a number, not by re-posting code.

The tradeoff is that comp doesn't always behave cleanly through every kind of motion. Older controls in particular can struggle to carry compensation correctly through arc moves, especially G02/G03 blocks that were programmed without accounting for how the control calculates the offset direction on curved paths. If comp only seems to apply properly on straight lines and gets weird on circular interpolation, that's usually a program or post-processor issue, not a machine limitation. The entry and exit moves into the arc need to be set up so the control can resolve the offset direction unambiguously.

CAM comp

CAM comp means the toolpath in the post-processed G-code already accounts for the tool radius. The program moves the cutter center along the true offset path, with no G41/G42 in the code at all. This is common on CAM systems that generate the compensated path directly rather than relying on the control to do it live.

The tradeoff runs the other way from machine comp. If the actual tool is a slightly different diameter than what CAM assumed, there's no live register to nudge. You either edit the CAM model and repost, or skip pure CAM comp on a feature where tool size might drift. That's most production work, since inserts wear or get swapped.

Wear comp

Wear comp is a small delta applied on top of whichever comp method is active. It corrects for the tool cutting slightly big or small relative to nominal: insert wear, a slightly different tool than assumed, thermal growth over a long run. It's a fine adjustment, typically a few thousandths, dialed in after measuring a part rather than guessed ahead of time.

Some controls distinguish wear comp from geometry comp. Geometry offset moves the whole tool position and is used for initial setup. Wear offset is meant specifically for that ongoing size drift during production.

Mixing the two up, putting a permanent setup correction into the wear register, works fine right up until someone resets wear offsets between jobs. Then the part comes out wrong for a reason nobody remembers.

The practical takeaway

Use machine comp (G41/G42) when you want dimensional control to live at the machine, adjustable without reposting. Use CAM comp when the tool size is fixed and known and you want the program itself to carry the true path. Either way, keep wear comp for what it's named for: small, ongoing size correction. Don't let it quietly become the place where a real setup error gets hidden.