Probe cast iron surfaces without worrying about abrasive damage to your workpiece or stylus. These styli have a zirconia ceramic ball end, which minimizes friction on cast iron better than other stylus materials, such as ruby. When they touch a workpiece, they trigger your coordinate measuring machine (CMM) probe to send a signal and measure geometric features. The ball end is completely round to keep from touching your workpiece at more than one precise point, preventing inaccurate measurements. As a rule of thumb, you should use a stylus with the largest ball-end diameter that works for your application. Larger ball ends minimize the risk that the stem will accidentally touch your workpiece and interfere with the reading.