Maintenance & Reliability · SOP

CNC Milling Machine Spindle Runout Verification

Standard operating procedure for measuring and verifying spindle runout on CNC milling machines to ensure precision, prevent tool failure, and maintain part quality.

Estimated Time

30 min

Difficulty

Intermediate

Regulation

ISO 230-1

Version

1.0

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Purpose

Ensure spindle concentricity is within manufacturer tolerance to maintain part accuracy, extend tool life, and prevent vibration-induced damage to the machine and workpiece. This verification must be performed after any spindle service, collision event, or as part of a scheduled preventive maintenance program.

Scope

This procedure applies to all vertical and horizontal CNC milling machines (3‑axis, 5‑axis) on the production floor. It is intended for operators, maintenance technicians, and quality inspectors who have completed machine‑specific lockout/tagout training and are familiar with dial indicators and precision measurement.

Safety Precautions

LOTOComplete machine-specific lockout/tagout per site LOTO procedure. Verify zero energy state before cleaning the spindle taper or mounting indicators.
PPEWear safety glasses and cut‑resistant gloves. Remove rings and loose jewelry that could catch on rotating parts.
SPINUse the spindle lock or brake per manufacturer instructions when manually rotating the spindle. Never force rotation with a wrench on the test bar.

Required Tools & Materials

Dial indicator (0.0005″ or 0.01 mm resolution) with magnetic base
Precision test bar or known‑good tool holder (sk10, bt40, cat40, etc.)
Clean lint‑free cloth (microfiber)
Digital caliper (for confirming gage distance)
Manufacturer‑approved calibration standard (optional, for audit‑grade data)
Lockout/tagout kit (padlock, hasp, tag)

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Procedure: Step-by-Step

1 Perform lockout/tagout on the machine. Remove any tool from the spindle. Clean the spindle taper bore and the test bar shank with a lint‑free cloth. Inspect for nicks or debris.

2 Mount the dial indicator base to a rigid surface (machine table or column). Position the indicator stem perpendicular to the test bar axis, typically 100 mm from the spindle nose as per ISO 230‑1. Zero the indicator after applying 0.1–0.2 mm preload.

3 Manually rotate the spindle 360° using the spindle lock or brake at JOG speed. Observe the dial indicator’s maximum and minimum readings. Record the total indicated runout (TIR = max minus min).

4 Repeat the rotation three times to verify repeatability. Average the three TIR values. Compare to the manufacturer’s tolerance (typically ≤0.01 mm for precision spindles, ≤0.02 mm for standard spindles).

5 If TIR exceeds the allowable limit, flag the spindle for deeper inspection (taper wear, bearing condition, drawbar force). If within tolerance, document the value and proceed.

6 Remove the dial indicator and test bar. Reinstall the spindle cover or drawbar. Release lockout/tagout. Log the result in the machine PM record or CMMS (e.g., ServiceGrid).

CNC Milling Machine Spindle Runout Verification

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