How Does Probe Pretravel Effect Feature Measurements

The mechanics of any touch trigger probe, machine tool probe included, are based off a concept called pre-travel.   Pretravel is defined by the amount of machine movement that occurs between when the probe contacts the probing surface and when the probe mechanism is triggered.

Probe Pre-Travel is dependent on the following items:

1. Machine Speed when surface contact occurs.

   1. Pretravel can be larger when a faster speed is used or smaller when a slower speed is used.

2. Stylus length

   1. A longer stylus will have more deflection leading to a larger pretravel while a shorter stylus will have less deflection leading to a smaller pretravel value.

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One of the reasons for performing a probe calibration is to determine the probe pretravel based on the current stylus length and machine speed. This is why a calibrated probe always shows a stylus diameter smaller than the stated nominal value. This is known as the ECD (Effective Calibrated Diameter) for the calibrated probe. Once calibrated, this ECD is used as probe compensation feature calculations as all measurement data from the machine is reported at stylus center.

If a probe is not calibrated, therefore the ECD is not known, the software will use the nominal diameter in all feature calculations. If the nominal diameter is used, then the pretravel value will not be compensated for and will appear as feature size deviation from nominal since it has not been compensated for by application of the calibration. This will report the feature size as less than nominal.