Moving from process inspection to process control: CMMs take the lead.

Author:Card, Gary
Position:Quality in Manufacturing[R]: strategies * tools * solutions - Coordinate measuring machines

Over the past four decades, coordinate measuring machines have found increasing use as reliable devices lot gathering dimensional data used in measurement and inspection applications. CMMs are a flexible means of providing high-accuracy, high-throughput measurement, and are more cost effective than expensive custom gages used in many manufacturing operations.

Quality control and manufacturing engineers have found that while post-process inspection is critical to reducing the number of non-conforming parts that are shipped to customers, the power of dimensional data analysis is in the application of the results to the control of the machining operation. In response to that need, CMM manufacturers are focusing their development efforts on improving the speed, accuracy, and real-time data gathering and analysis capabilities of CMMs for process control applications.

On the shop floor

Improved process control reduces the probability of producing out-of-tolerance parts, resulting in less scrap and rework. The closer to real time the application of results is, the more value the results have in controlling the process.

Moving CMMs from the inspection room to the shop floor is a step toward the goal of real-time process control. Shop floor gaging with a CMM helps reduce production costs, improves inspection throughput and builds an historical database of how machines, parts, pallets, and fixtures behave during the actual machining process. The challenge, however, has been to protect CMMs from the effects of ambient temperature, vibration, and airborne contaminants found in the shop environment. Meeting that challenge has been one of the drivers in the evolution of CMM technology during the past few years.

CMMs designed for shop floor service are constructed of materials that resist temperature changes and vibrations. The recently introduced ONE CMM from Brown & Sharpe is built around the use of steel bearings. All of the other components of the machine have thermal properties similar to steel to minimize the effects of temperature variation on measurement accuracy. The X-axis carriage and Z-axis ram are constructed from an aluminum/ silicon carbide matrix material that exhibits a similar coefficient of expansion as that of steel. A high-density polymer composite base exhibits similar thermal properties, plus it stiffens the machine and provides ten times more vibration dampening than traditional materials. The base also absorbs high-frequency noise...

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