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Calibration of styli

Before you begin taking measurements it is absolutely crucial that you calibrate the probe precisely for all your measurement procedures.

Calibrating styli

The effective dimensions of the probe components have to be established if good results are to be obtained. These values are stored in the machine's data processor.

In the case of CMMs, this may involve calibrating several styli or sensors, often in multiple head orientations. Calibration of machine tool inspection probes is generally simpler.

How it works

The position of the individual ball styli and their diameters are established using a special probe calibration programme (see the machine manufacturer's user manual).

You contact a reference point with all the styli used, one after the other. The reference used is usually an extremely precise, manufactured ball with a known diameter (referred to as a datum ball). The exact dimensions of the ball being calibrated are input into the measuring software.

If the styli are to be used for measuring separate points, the stylus is calibrated using 5-6 points on the reference ball's high points. A far greater number of points are probed in scanning systems. The machine manufacturer's user manual will describe the precise probing strategies for calibrating the styli. Make sure, particularly if you are using more than one CMM, that you use the calibrated datum ball whose values have been input into the software.

Outcome

The probe calibration procedure establishes the stylus tip's effective diameters when measuring, and their positions in relation to one another and to the machine's co-ordinate system.

A special analysis program and the known diameter of the calibrated ball are used to establish the unknown diameters of the stylus tip.

The co-ordinates of the centre of the first stylus ball calibrated are stored in the CMM's data processor as reference co-ordinates. All remaining tip positions are established by generating differences with the first one, and then also stored as ball centre coordinates.

Once the various tips of a stylus configuration have been calibrated, their centre points are compensated for by the CMM software, so that measurements with all the styli appear as though they had been taken with a single stylus.

This means that whichever stylus you use to probe a point, you always get the same result.

Calibrating other shaped components

Apart from reference balls, stylus calibration can be done using other references such as end gauges, ring gauges and pin gauges. Typical examples are with cylinder and disc styli. The basic principle does not alter. The machine manufacturer's user manuals will describe the routines for these types of calibration.