Good Practice Guidelines

Shop floor gear measuring machines can be very accurate with a measurement uncertainty (U95) of between 1.5 to 2μm achievable in a controlled environment. To ensure this is achieved, operators need to follow and apply good measurement practice as part of their daily routine.
The accuracy of the measuring machine should be verified with calibrated master gears at regular intervals. Without this, the financial investment in good facilities will not be realised and gears that are within tolerance may be rejected and gears that are outside tolerance accepted. This may result in sub standard gears, poor service records and unhappy customers.

Good measurement procudure includes:

  • A check that the machine is calibrated
  • Visual examination of the gear
  • Probe calibration
  • Measurement of gear mounting accuracy
  • Measurement of profile and helix error on 4 teeth
  • Measurement of pitch error on all teeth
  • Interpretation of measurement results to a standard
  • Identify the cause of manufacturing errors


Procedures for verifying the instrument measurement uncertainty are defined in ISO 18653, ‘Evaluation of instruments for the measurement of gears’ which, together with the supporting Technical Report ISO TR10064-5 is recommended reading for anyone involved with evaluating gear machine performance.

Most organisations apply these guidelines by implementing weekly checks as follows:

  • Complete a visual check on the machine, including centres, stylus and auxiliary equipment;
  • Measure runout of centres;
  • Use a calibrated master gear to compare profile, helix and pitch measurement accuracy with traceable calibration data;
  • At one month interval mount a calibrated mandrel between centres and check the alignment, straightness of the axis between centres and the lead axis.


Excessive differences should be investigated (most machines should be capable of results within 2 to 3μm of the calibration data).
We run training courses, with the BGA, on the implementation of ISO18653 and introducing good measurement practice into the workplace.