touch off

To touch off is to locate the exact position where a cutting tool just makes contact with a workpiece or machine reference surface, establishing a known coordinate from which all subsequent movements are measured. In CNC machining, this is the critical step that anchors your entire job to the machine's coordinate system. Without an accurate touch-off, your part dimensions will be wrong or your tool will crash into the workpiece or vice.

The process usually works like this: you move the tool slowly toward the workpiece using manual controls (or a jog function on a CNC), watching for the moment of contact. Many operators use a feeler gauge, paper, or a piece of thin shim stock between the tool and workpiece to detect contact more precisely than by eye. Some machines have electronic touch probes that signal automatically when contact occurs, eliminating the guesswork and improving repeatability. Once contact is confirmed, you record or set that position as your reference point, or zero, in that axis.

There are different datum points depending on your setup. You might touch off the tool tip to the workpiece surface to set the Z-axis (depth), to the edge of a part to set X, or to a fixed machine reference like the machine table to establish absolute coordinates. Multiple touch-offs are normal on complex jobs: one for each tool if you are running several cutters, and often one for the part itself if it has been repositioned.

Common touch-off errors and prevention

The most frequent mistakes are touching off with too much feed rate (crushing the feeler gauge or damaging the tool), using an inconsistent reference surface, or forgetting to zero the DRO or control after touching off. Some operators touch off at the highest point of a part and fail to account for part runout or bending under load, leading to chatter or gouging once cutting begins. Modern CNC machines with automatic probing have reduced but not eliminated these issues; the probe itself must be calibrated regularly.

The term itself reflects the physical act: you are literally touching the tool to a surface to find the exact moment of contact. In manual machine work, this was always a tactile and visual skill. CNC automation moved it toward precision electronics, but the fundamental concept remains unchanged. Machinists distinguish between roughing touch-offs, where a few thousandths of error is acceptable, and finishing touch-offs, where repeatability within one ten-thousandth or better is expected, depending on the part tolerance and tool path.