ER Collet Runout Measurement: How 0.0005" Concentricity Errors Break Bits

The Physics of Spindle Runout

When a 2-flute or 4-flute end mill rotates with even 0.0005" (0.012mm) of runout, only one flute takes 100% of the chip load while the opposite flute barely grazes the workpiece.

This asymmetric loading creates:

• Uneven tooth wear and rapid micro-chipping.
• Spindle vibration and loud high-pitched chatter.
• Wavy, faceted wall surface finishes.

How to Measure Spindle TIR in 3 Steps

1. Measure Spindle Taper: Clean the internal spindle bore and place a 0.0001" dial indicator on the internal taper. Rotate by hand. (Target: < 0.0001" / 0.002mm).
2. Measure Precision Test Pin: Insert a certified ground gauge pin into your ER collet and torque to manufacturer specification. Measure TIR 1 inch below the collet nose. (Target: < 0.0003" / 0.007mm).
3. Inspect the Nut: Dirty threads or damaged clamping eccentrics can cock the collet during tightening. Always torque using a calibrated torque wrench.