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

Machine comp offsets the toolpath in the control using a register you set at the machine; CAM comp bakes the offset into the G-code itself during programming; wear comp is a small correction applied on top of whichever base method you use, to account for the actual tool diameter versus nominal. They solve different problems and mixing them up is how you get a part that's out of tolerance in a way nobody can explain.

Machine (control-based) cutter comp

This is G41/G42 with a diameter register (D-word) that lives in the control's offset table, not in the program. The program calls out a nominal path plus a comp command, and the control shifts the actual toolpath left or right by whatever value sits in that offset register. Change the register, and the same program cuts a different size part without touching the code.

The advantage is flexibility on the floor. If a part comes out 0.05 mm oversized, you adjust the offset and rerun, no edit to the program required. This is the standard approach for finish passes where you expect to tune the size after a first-off measurement.

CAM (programmed) cutter comp

Here the CAM software calculates the actual toolpath centerline for a specific tool diameter and writes it directly into the G-code coordinates. There's no G41/G42 call and no offset register involved; the path itself already accounts for the tool radius.

This is common on 3- and 5-axis surfacing work where the toolpath follows a complex 3D contour and true cutter comp isn't practical to apply in the control anyway. The tradeoff is that if the actual tool comes in a different diameter than what the program assumed, there's no easy offset to tweak. You either regenerate the toolpath in CAM with the corrected diameter, or you accept the size error.

Wear comp

Wear comp is a separate, usually small correction layered on top of whichever comp method you're using, meant to account for the difference between a tool's nominal size and its actual size, or for gradual dimensional drift as an insert or edge wears during a production run. It's typically a much smaller value than geometry comp, applied as a fine-tuning adjustment rather than the primary offset.

The distinction matters because some controls separate geometry offset and wear offset into different registers specifically so you can zero out wear comp between tool changes without disturbing the base geometry setting. Dumping a wear correction into the geometry register, or vice versa, is a common way to lose track of what a part actually measured versus what you think you set.

Why it's worth keeping straight

Mixing these up shows up as parts that drift size in ways that don't match the adjustment somebody thinks they made. Adjust a wear offset expecting a 0.01 mm correction, but touch the geometry register instead, and you get a much bigger shift than intended.

On a boring bar or a finish endmill near a tight tolerance, that's the difference between a good part and a scrap one.

Know which register you're touching before you touch it. Know whether the toolpath already has the tool radius baked in from CAM, or is expecting the control to add it. Those two things determine whether an offset adjustment does what you expect.