When Z height drifts or drops during a job, preserve where it happened and whether the axis returns to its reference. A sudden loss, a gradual change, and an error tied to reversal lead to different checks of coupling, binding, load, heat, and control.
Map Where Z Height Begins to Drift
Mark whether Z changes suddenly or drifts gradually. A sudden step suggests tool slip, coupler or drive slip, missed motion, collision, or abrupt overload. A slow change can involve loosening, thermal behavior, stock movement, accumulated reference error, or a surface that was never stable. Those patterns require different tests.
Inspect the Tool and Holder First
Before moving the machine casually, record the program location, displayed machine and work coordinates, tool projection, and a reference check if it can be made safely. Inspect the failed feature and tool for a discrete witness mark that identifies when load or height changed.
Check the Z Force Path
With power isolated as required, check collet and tool seating, spindle mount, Z coupler, screw or belt, guides, fasteners, cable drag, and any brake or counterbalance described by the manufacturer. Do not tighten every component blindly; preserve evidence of a loose interface or slipped mark.
Compare Commanded Motion With Unloaded Motion
Separate commanded load from mechanical motion. Review stepdown, entry, plunges, retracts, acceleration, tool engagement, and chip evacuation at the first bad location. Then use a low-risk non-cutting movement or coupon to see whether Z returns correctly without cutting force. Use CNC Backlash, Flex, and Lost Steps to distinguish a sudden lost position from elastic Z movement that recovers after load.
Review Entry Load and Vertical Acceleration
Check workholding and the reference surface. A bowed board, compressed spoilboard, lifting part, moving fixture, or incorrect probe contact can make the cut appear to lose Z while the axis follows commands. Compare machine position evidence with physical stock movement instead of assuming a motor problem.
Verify Recovery on a Depth Ladder
After a verified correction, repeat the same reference motion and a representative coupon through the original risk point. Record before-and-after tool projection, axis return, load condition, and feature depth. Do not increase current, force, or limits as a first-line remedy for unexplained position loss.
