Runout Tolerance
Runout tolerance is a GD&T rotational tolerance that limits surface variation when a part rotates about a datum axis. It controls wobble, eccentricity, and surface deviation on shafts, bores, flanges, and rotational features. In CNC machining, runout is measured as Total Indicated Runout (TIR) with a dial indicator during rotation, with acceptable values typically ranging from 5 to 25 micrometers depending on machine precision.
In practical CNC application, runout tolerance controls tool centering on mills and concentricity on lathes. On a mill, uneven flute loading from off-axis tools raises chip load and heat, accelerating wear. On turning cells, runout protects bearing journals and locating diameters from vibration and assembly misfit. During inspection, circular runout checks a single cross-section per revolution, while total runout traverses the full length for tighter control of long shafts. In millwork and edgebanding, excessive runout manifests as edge chatter and scalloped cuts. Small-diameter cutters and high-speed spindles demand tighter runout, often 2–10 μm, whereas standard VMCs accept 15–25 μm. Maintaining clean spindle tapers and collets is critical to achieving these tolerances.
- Dirty taper or bad toolholder seating: Chips or burrs in the spindle taper prevent full seating, making the tool run eccentric. This results in high TIR readings that mimic spindle problems but are actually contamination.
- Wrong measurement setup: Checking runout on a damaged shank or at an incorrect gauge distance gives misleading readings. Runout increases with distance from spindle face, so a taper reading underestimates cutting-end error.
- Confusing runout with size tolerance: A part can meet diameter size limits yet fail runout because the surface is off-axis. This causes bearing noise, seal leakage, or vibration despite acceptable micrometer checks.
Is runout the same as concentricity?
No. Runout controls surface variation during rotation relative to a datum axis; concentricity is a different, much more specialized control of median points and is far less commonly used in production inspection.
What is the difference between circular runout and total runout?
Circular runout limits variation at a single cross-section during one full rotation, while total runout limits variation over the full surface as the indicator traverses the length of the feature; total runout controls both form and coaxiality more tightly.
Why does runout matter so much on small-diameter cutters and high-speed spindles?
Small cutters have little margin for eccentricity, so a few microns of runout can create major flute load imbalance, poor hole quality, and shortened tool life; high-speed spindles therefore require much tighter runout control than standard VMCs.