One of the most common questions I encounter with customers is: What is tool compensation on a CNC machine and in CAD/CAM, and how exactly does it work? Understanding this mechanism is absolutely critical for achieving dimensional accuracy on parts and extending tool life.
What Is Tool Compensation?
When milling a contour, the CNC control system needs to know whether the tool centre should follow the exact geometry of the programmed path, or whether it should offset the path by the cutter radius (to the left or right of the direction of travel). Without compensation, the tool centre would run exactly along the edge of the part, reducing it by the cutter radius.
Compensation Types in Autodesk Fusion 360
Fusion 360 offers several compensation types in the Passes tab for 2D and 3D strategies:
- In Computer: Fusion 360 calculates the offset path directly. The output G-code coordinates already correspond to the actual tool centre path. You cannot easily adjust the dimension at the machine without regenerating the CAM toolpath.
- In Control: Fusion 360 outputs the exact part contour coordinates and inserts compensation commands (
G41for left,G42for right). The operator enters the tool radius into the machine’s offset table and the control calculates the adjusted path. - Wear: The ideal combination. Fusion 360 calculates the path the same as In Computer but also activates
G41/G42. Only the deviation/wear value (e.g. -0.05 mm) is entered at the machine if the part is running oversize. - Inverse Wear: Similar to Wear but for specific control systems that work with the opposite compensation sign.
Important note for post-processing: For In Control or Wear compensation to work correctly, your post-processor must properly generate the lead-in and lead-out moves. The machine needs a linear move to activate compensation before touching the material.
Conclusion
For typical custom production with high accuracy requirements, the Wear method is the most proven approach. It lets the operator fine-tune dimensions directly on the shop floor without constantly running back to the computer to regenerate CAM projects.