Hexagon Metrology, Inc.

Intro to Coordinate Metrology - Using Effective Probe Techniques

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Using Effective Probe Techniques

You can reduce the likelihood of shanking by using a larger diameter tip to increase the clearance between the ball/stem and the workpiece surface. Generally, the larger the tip diameter, the deeper the stylus can go before it touches the workpiece feature. This is called the effective working length of the probe (Figure 21). Also, the larger the tip, the less effect it has on the surface finish of the workpiece since the contact point is spread over a larger area of feature being measured. However, the largest tip that can be used is limited by the size of the smallest holes to be measured.

figure21.gif

Measurement points taken with an electronic probe are recorded when the stylus is deflected enough to either break mechanical contacts or generate enough force to trigger pressure-sensitive circuitry. The physical arrangement of the contacts causes slight errors in accuracy, although these are reduced during probe qualification. However, the longer the probe tip extension, the larger the pre-travel error and the more residual error is left after probe qualification. Longer probes are not as stiff as shorter ones. The more the stylus bends or deflects, the lower the accuracy. You should avoid using probes with very long stylus/extension combinations.