How do these design concepts play out in a real CMM design? The Brown & Sharpe Global CMM, a moving bridge, air bearing CMM manufactured by Hexagon Metrology, Inc., is a well known example of the application of modern CMM design principles. For Global, the main design drivers were accuracy, throughput, and environmental flexibility. Granite is used for the fixed base of the CMM that the moving bridge travels on.
Even with its poor thermal properties, granite is still an excellent choice for a high quality flat work surface with extreme durability. Improvements in granite machining processes permit the manufacture of a one-piece granite base with integrated guide ways for the air bearings. In addition, the Global granite base is heavy and, in combination with passive elastomeric isolation pads, this serves to isolate the machine from environmental vibrations which have a negative effect on accuracy and repeatability.
From a thermal point of view, the choice of granite is less favorable because the large granite base is prone to thermal bending resulting from temperature changes within the installation environment. However, this thermal bending is very predictable and well suited for software error compensation. Multiple temperature sensors on the top and bottom of the granite base are used to correct for the geometry errors in the CMM structure that result from the thermally induced granite bending, even when this bending is asymmetric. This compensation method has proven very effective in practice.
Aluminum and steel have dissimilar thermal expansion properties which can lead to problems if the design of the interface between the materials is not designed correctly. If two materials with disparate thermal expansion properties are rigidly joined, they tend to distort in difficult to predict ways. The Global design solves this problem by the way the steel scales are applied to the aluminum structure; the scales are fixed to the machine structure at one end and allowed to expand and contract along the scale's entire length independently of the aluminum structure. Thus, the two materials are permitted to expand and contract independent of one another in a well behaved and predictable manner and the error introduced by the well behaved linear expansion of the steel scales is easily corrected.In contrast to the static base, Global's moving bridge structure is made entirely of aluminum extrusions and castings. As we know, aluminum has good thermal properties with respect to CMM design. However, the scales, which ultimately determine the position of the probe within the CMM measurement volume, are made of steel.
Global's X-beam and Z-rail are aluminum extrusions, while the legs and XZ-carriage are aluminum castings. As we discussed earlier, granite is a good solution for the static base, but it is less than ideal for the moving structure. While it makes good design sense to compensate for thermal granite bending in the base — to take advantage of its desirable work surface properties — it's all downside when it comes to the moving structure. Aluminum for the moving bridge components provides stiffness, light weight, and thermal stability which are some of the key elements that make the Global CMM accurate, fast, and relatively insensitive to its thermal environment.
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