Quick Guide to Precision Measuring Instruments

PG70Coordinate Measuring Machines • Selection of a stylusIt is recommended that a stylus be selected on the basis of the following factors to ensure the highest accuracy of measurement.1. Choose the shortest stylus possible.The longer a stylus, the more it will flex, and lower accuracy will result. Also, the positional accuracy of the probe is inversely proportional to the distance from the probe pivot to the stylus ball, therefore the shortest probe gives the highest accuracy.2. Reduce the number of joints wherever possible.The combination of styli and use of extensions will increase the possibility flexure. Use the fewest possible components for any application.3. Use a ball tip as large as possible.The use of a larger ball increases the clearance between the ball and stem, thus reducing the possibility of contact between the stem and workpiece (shanking). A larger ball also reduces the influence of the surface finish of a workpiece on measurement accuracy.• MaterialA stylus uses an appropriate material for its shaft, ball and other accessories according to the application. The following introduces the fea-tures and merits of commonly used materials.1. StemTo minimize flexure, the stem needs to be as stiff as possible. Mitutoyo offers the following materials:• Tungsten carbideThis material provides excellent rigidity for small stem diameters, thus being optimal for most standard applications. Consideration should be given to the stylus mass in the case of large stem diameter and long stylus length.• Stainless steelNon-magnetic stainless steel stems offer the best stiffness to mass ratio.The choice of stylus has an important effect on the accuracy of measurement obtainable from a CMM. Here is a quick guide on how to select a stylus.The stylus is the part of a probe that makes contact with a workpiece, generally consisting of a stem and a ball tip. The probe func-tions by bringing the ball into contact with a workpiece to acquire a measurement from the resulting signal. The form and dimensions of a stylus need to be selected depending on the workpiece. In any case, it is important that a stylus has high rigidity and its tip shape is a practically perfect sphere.Quick guide to styli

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