walk
To tend to move radially while feeding axially, whether tending toward on-center or tending toward off-center. Walking may be desirable (e.g., when a reamer walks into concentricity) or undesirable (e.g., when a twist drill walks into eccentricity.)
walk: radial drift during axial cutting motion
In machining, walk describes the radial movement of a cutting tool as it feeds axially into the workpiece. The tool drifts sideways from its intended path, either toward the spindle centerline or away from it, while simultaneously advancing into the material. This happens because the cutting edges encounter unequal resistance or because the tool geometry itself favors lateral displacement under load.
Walk is common in drilling and reaming operations. A twist drill walking off-center creates an oversized, eccentric hole; the tool point migrates progressively toward one side as the flutes cut. Conversely, a reamer often walks toward on-center as it enters a pilot hole, self-correcting its position because the multiple flutes and larger body provide guidance. The phenomenon is a function of tool geometry, feed rate, spindle speed, cutting fluid, and workpiece material hardness and grain structure.
When walk helps and when it hurts
Controlled walk in reaming can improve hole accuracy. The reamer's design exploits the tool-workpiece interaction to settle into concentricity, reducing runout in the finished bore. Uncontrolled walk in drilling ruins tolerances: a 0.250 inch drill walking 0.020 inch off-center produces a hole 0.290 inch across at its widest point, making the part scrap. Walk is worst in deep holes, thin-walled tubing, and materials prone to work-hardening like stainless steel or cast iron.
Machinists manage walk through technique: using sharp tools, maintaining optimal feed and speed, applying adequate coolant, and choosing rigid setups. Drill bushings and through-spindle coolant delivery reduce walk by providing additional support near the point. In production, walk tolerance is specified as total indicator runout (TIR) on the finished hole; a typical specification for a reamed hole is 0.0005 inch TIR or better.
The term walk refers to the visible movement of the tool as it wanders across the workpiece surface during entry, similar to a person walking across an uneven floor. It is distinct from chatter (vibration) or deflection (elastic bending), though all three can occur together and degrade surface finish and hole geometry.