Production Machining

OCT 2017

Production Machining - Your access to the precision machining industrial buyer.

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• e best way to check lobing error is to measure a known diameter ring gage with 25 equally spaced points. Use software (or export individual data points to an Excel XY chart) to display the point data. Calculate the measurements' effectiveness using two factors: • Form error: Is it within the limits of how much percent of tolerance you are comfortable with? • Lobing error: In the diagram, is the triangle created by the point data visible? Examine the point data distances to ensure that too much of the roundness tolerance zone is not being used. Many people want to use the touch probe method to evaluate and grade roundness, but caution should be taken because of the inherent weakness in the probing and measuring system. ere are specific roundness systems that are designed to provide a more comprehensive and accurate evaluation of this tolerance. Concentricity Concentricity measurement is a common application for touch probing and is ideally suited as a probing method. Some general best practices should be followed when applying this tolerance type: • Make sure to measure the full circle of each hole. e concentricity tolerance uses a reference feature, so be sure to measure the diameters in their entirety. • Measurement of partial circles is difficult to use, because the circle segment calculation is subject to the repeatability issues described in the radius measurement section. Guidelines for Using Touch Probes The size of a stylus tip plays an important part in measurement with a touch probe. The stylus radius should not exceed the radius of the feature to be measured. Usually, the best practice is to use a spherical stylus, as this is the most common and cost-effective choice. There are a variety of styli available, including spherical, cylindrical and custom configurations. Here are a few tips for stylus selections: :: Choose the shortest stylus possible. The longer the stylus, the more it will flex, resulting in lower accuracy. Also, the positional accuracy of the probe is inversely proportional to the probe pivot to the stylus ball; therefore, the shortest probe gives the highest accuracy. :: Reduce the number of joints wherever possible. The combination and use of extensions will increase the possibility of flexure. Use the fewest possible components for any selection. :: Use a ball tip as large as possible. The use of a large ball increases the clearance between the ball and stem, thus reducing the possibility of contact between the stem and the workpiece (shanking). A large ball also reduces the influence of surface finish of a workpiece on measurement accuracy. • Feature form errors can influence concentricity, so keep a watchful eye on the features' roundness. Thread measurement read measurement is difficult for the probe evaluation method. Typically, the stylus radius exceeds the size of the thread area. reads have distinct evaluation parameters beyond the general measurement scope of CMM software. It is possible that with a micro stylus (or point stylus) that thread evaluations can be accomplished. Some general rules to consider: • To measure thread profiles, the stylus radius must be small enough to capture the thread detail below the pitch or root diameter. • Scanning is the proper method because data details must be acquired for angles, root, minimum/ maximum and pitch diameter. Many customers will use alternate methods for thread measurement such as contour and video systems, which are better suited for this type of feature evaluation. Touch probe measurement handles a variety of applica- tions and is appropriate for round, circular parts. But the method has some limitations when it comes to technology and applications. Consider specific feature types before over-extending the system. For more information from Mitutoyo America Corp., call 888-648-8869 or visit mitutoyo.com. CONTRIBUTOR Mike Creney is the vice president of measuring instruments sales and support at Mitutoyo America Corp. SPECIAL COVERAGE 28 PRODUCTION MACHINING :: OCTOBER 2017

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