Hysteresis
How is probe hysteresis different from repeatability?
Repeatability is the scatter of repeated measurements under the same conditions, while hysteresis is the systematic difference caused by changing direction of approach; a probe can be repeatable and still hysteretic.
What physically causes hysteresis in a touch-trigger probe?
The cited measurement literature attributes it mainly to mechanical friction and compliance in the probe’s kinematic elements, including sockets, mating arms, shank deformation, and movable transducer parts.
Why does hysteresis matter more on tight-tolerance CNC work?
Because directional error in the micrometre range is large relative to tolerance bands on precision machining, especially when probing is used to establish offsets or verify final geometry.
Hysteresis is a history-dependent lag in a system where the output at a given input depends on whether the input is increasing or decreasing. In CNC metrology and machining, it appears as directional differences in position, force, or measurement readings during approach versus retreat, often caused by friction and compliance in probes, slides, and drive components.
On a CNC cell, hysteresis matters most when a touch-trigger probe establishes work offsets or checks bores/locations, because the trigger point can shift with approach direction and inject micrometre-level error into part-to-part variation. When an axis reverses direction, the table or head may lag behind commanded motion, leaving a deadband that causes contour error, corner rounding, and oversize/undersize dimensions during finishing. The same effect appears in millwork feed mechanisms and pressure rollers: any load-path reversal can make the machine repeat differently forward versus backward, affecting edge alignment, cut length, and hole position. Before applying compensation tables or probe offsets, shops must characterize directional lag in both directions; compensation tuned for one direction can worsen surface quality and dimensional error on a machine with significant hysteresis.
Mixed-direction probing: When a bore or edge is probed from different approach directions without control, the touch-trigger probe returns different trigger radii and stores a biased offset, causing false fixture correction and bad work offsets.
Reversal near finish dimensions: When a toolpath changes direction on the final finishing pass, axis hysteresis or backlash leaves a deadband, so the machine lags through the reversal and cuts a dimension that is consistently off, often appearing as taper or corner mismatch.
Overconfident software compensation: When a compensation map is applied without accounting for direction history and friction, mechanical hysteresis makes the correction nonlinear and unstable, degrading surface quality and dimensional accuracy instead of improving them.