AUG 2018

Techspex provides metalworkers free research and analysis tools to help them find the right machine for their job.

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W H A T T O C O N S I D E R W H E N P U R C H A S I N G A C M M 6 QC labs, shopfloor conditions rarely can maintain a constant 68°F temperature considered optimal for CMM performance in a lab. When the ambient temperature varies, measurement results also vary. The laws of physics underlie these variabilities. Furthermore, holes in air bearings tend to clog with oil, dust and debris floating in the air where machine tools are nearby. In these conditions, a CMM will eventually stop working properly. Today, shop-hardened CMMs have emerged to resist these problems. For these devices, air bearings have been replaced with mechanical bearings and linear guideways. Machine components manufactured from thermally stable materials have resulted in stiffer, lighter structures and thermal compensation for use outside temperature-controlled settings. Machine footprints are smaller to take up less valuable manufacturing floor space. Cantilever designs, which offer the most effective measuring volume, are making a comeback for shop-hardened applications. Interestingly, cantilever designs were once the most common type of inspection machines in the early years of CMM technology. CMM Calibration Producing measurement data about a part to verify its accuracy and validate the integrity of its manufacturing process is a prime function of the CMM. However, to meet this function as well as to satisfy traceability requirements, CMMs must be properly calibrated periodically at a frequency set by the user. The two most common standards by which most CMMs are calibrated are ASME B89.4.1 and ISO 10360-2. The two standards are very similar. The ASME B89 standard involves a ball bar artifact. Its use is generally confined to North America, whereas the ISO 10360 is favored in Europe, where it originated. This ISO standard is based on the measurement of certified length standards. The B89 standard uses a ball bar of an uncalibrated length and is essentially a length measured repeatedly throughout the full working volume of the CMM. The axis scales are first calibrated by a separate test (Linear Displacement Accuracy) using a calibrated artifact such as a step gage or laser interferometer. Once the axis scales have been adjusted to measure properly, the ball bar is then used to ensure that the measurement of this length parallel to any axis will repeat anywhere within the volume of the CMM. This ball bar test yields the maximum range of results from all measurements. The ISO 10360 standard applies to articulated- arm CMMs using tactile probes and optionally optical distance sensors (also referred to as laser line scanners or laser line probes). It specifies the acceptance tests for verifying the performance of an articulated-arm CMM based on a calibrated test length as stated by the CMM manufacturer. It also specifies the tests that enable the user to periodically re-verify the performance of the CMM. Details on tests for scanner accessories are also given. Part of ISO 10360 specifies performance requirements that can be assigned by the manufacturer or the user of the articulated-arm CMM. It also specifies the manner of execution for the acceptance and reverification tests to demonstrate the stated requirements. The Future of Production CMMs Almost all CMMs will be fully integrated into the manufacturing process as manufacturing companies continue to demand in-line measurements for immediate feedback. This will enable timely, in-line adjustments to eliminate scrap and improve quality. CMMs will be treated like any other machine tool as an integral part of the manufacturing process. The same operator machining the

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