Cutter Compensation
Does cutter compensation change the program or the machine offset table?
Cutter compensation adjusts the machine's interpreted toolpath at run time; the programmed part geometry remains unchanged, while the control internally applies the stored cutter radius or diameter offset.
Why is cutter compensation valuable for tolerance control?
It decouples tool diameter variation, wear, and deflection from final part accuracy, so finish size can be corrected with offsets rather than a full repost.
When should cutter compensation be used instead of pre-calculating the tool centerline?
It is preferred for contouring operations where the final wall or edge dimension must be adjusted on-machine, especially during prove-out, tool wear correction, or when tooling changes late in production.
Cutter compensation is a CNC control function that offsets the tool centerline from the programmed part contour by the cutter’s radius, allowing the control to machine the part geometry directly. In G-code, G41 offsets left of travel, G42 offsets right, and G40 cancels. This lets programmers use the same profile with different tool diameters or adjust for wear by changing offset values in the control rather than rewriting code.
On a CNC mill, cutter compensation is essential for 2D profiling, slotting, pocket walls, and finish passes where final dimensions rely on cutter radius and wear. The control automatically shifts the toolpath from center to edge, so programmers code the nominal part line and let the machine calculate offsets. This is critical when an end mill isn’t the exact diameter planned or when wear offsets bring parts back into tolerance without reposting. In practice, cutter comp is engaged on a lead-in move away from the finished contour, maintained through the cut, then cancelled on a lead-out move before retracting. In millwork and edgebanding, the same principle applies: operators use cutter diameter control to achieve drawing dimensions despite tool wear or diameter variations.
Turning comp on too close to the part: Activating G41/G42 without enough lead-in distance prevents the control from resolving the offset cleanly, causing alarms, gouging, or a bad first cut into the finish edge.
Canceling comp in the wrong place or forgetting G40: Leaving cutter compensation active after the contour causes unintended offset motion on the next move, potentially triggering alarms or driving the tool into clamps, stock, or air.
Wrong side selection or wrong offset value: Using G41 when G42 is needed, or entering diameter/radius incorrectly in the offset table, shifts the tool the wrong way and immediately produces oversized or undersized parts.