TL;DR

Rotary axis backlash causes positional errors on 5-axis simultaneous cuts. Measure actual backlash with a test bar and dial indicator, then update the backlash compensation parameter in the CNC.

What you might see

dimensional error on complex 5-axis surfaces
witness marks where the rotary axis reverses direction
part inspection shows angular position error on rotary features
vibration when the rotary axis is commanded to hold a position under cutting load

Likely causes

Tap to mark as ruled out

Rotary axis drive gear or torque motor backlash from wear
Clamp / brake system on the rotary axis not holding fully under cutting forces
CNC backlash compensation parameter outdated for current axis wear state
Thermal expansion of the rotary axis body shifting the zero position

Required tools

Precision test bar (suitable for the machine spindle taper)
Dial indicator with magnetic base
Torque wrench for clamping
DMG Mori CNC parameter access

Safety first

Heavy fixturing and the machine table represent significant stored energy during 5-axis moves. Ensure all tooling and fixtures are securely clamped before running any motion.
Do not adjust rotary axis preload or drive mechanisms without OEM guidance. Incorrect preload can damage the rotary axis bearing system.

Procedure

6 steps · ~60 min

1
Command the rotary axis to a known angle and clamp it if the machine has a hydraulic clamp. Mount a dial indicator on the table contacting a precision test bar installed in the spindle.[1]
2
Apply a light force in the positive rotary direction and note the dial indicator reading. Then apply force in the negative direction. The difference is the measured backlash.[1]
3
Compare the measured backlash to the DMG Mori specification for that axis. Typical allowable backlash on aerospace-grade 5-axis machines is under 2 arc seconds.
4
If backlash is within specification but parts are showing errors, verify the CNC backlash compensation parameter matches the latest measured value.
5
If backlash exceeds the specification, contact DMG Mori service. Rotary axis drive preload adjustment requires OEM tools and procedures.
6
Run a test piece in aluminum and inspect with CMM to confirm the correction is effective before returning to aerospace production.

Sources

DMG Mori DMG Mori DMU / DMC 5-Axis CNC Machining Center general technical documentation, DMG Mori
DMG Mori DMU / DMC machining center operator and maintenance documentation, rotary axis accuracy and compensation procedures (general)

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