| By :
Nadine Davis
Copyright (c) 2010 Nadine Davis The ability of modern engineering techniques to manufacture components to an astounding degree of accuracy has led to the development of products that enhance every aspect of life. It is now possible with the use of computer technology to set machines to microscopic tolerances, and this has allowed the range of manufactured products to be limited only by human imagination. However, in so doing, industry recognises that the margins for error are also greatly reduced. What may have been an acceptable degree of tolerance twenty years ago is now considered to be an error that must be corrected and is the basis of the continuous improvement philosophy. 5 axis machining is a technique that provides intricate and precise cutting abilities. The complexity involved in operating around five different axes of movement requires a computer program, but of necessity to get the required level of accuracy, 5-axis deviations must be taken into account and the measurement process needed is complex and time-consuming. To counter this, some companies are using a measurement system called the "R-test" which accurately locates a rotary axis using a measuring head in conjunction with a precision ball. The deviations measured in this way show the location errors of the rotary axes, and the machine coordinate system can compensate for these errors, thus producing microscopic degrees of accuracy. There are two purposes for this test: 1. To identify axis location errors - the test finds the centre point of rotation and the parallelism of a rotary axis with respect to the machine's other axes. The machine's controller manages the centre-point location error, while the parallelism deviation is usually done by manually adjusting the table. When machine tool users know the axis location error they can improve machine accuracy by compensation and/or adjustment to reduce those errors. 2. As a total acceptance test - When the machine is programmed to move all axes around the precision ball when placed in an eccentric position on a table, it records all error parameters, including geometric errors and dynamic, controller errors eg. following or reversal errors. To get the test set up correctly, a probe is affixed to the stationery part of the machine. A master ball mounted on a rotary table fixed to the machine's trunnion, makes contact with three planar elements of the probe. Three measuring systems in the probe monitor the displacements of contacting elements during this motion, allowing the centre point coordinates to be pinpointed with sub-micron accuracy. One method designed to create quality and accuracy at the same time for industries such as aerospace, mining, and medical is the R-test. This test is also used in energy production and other industries. CNC clients can be confident that the best technology is available here to produce the accurate results they require.
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