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2840 |
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Prototype of a Planar Guided Milling Machine for High Precision Applications |
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Design of Precision Machines and Instruments |
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| Content |
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In the field of machine tool development there is a strong need for improved accuracy, stiffness and the other aspects of physical performance. Beside this, the cost for achieving these improved properties should be reduced compared with the actual values. In order to improve accuracy, physical performance and to reduce the cost at the same time for milling applications, a new machine concept is proposed here. Planar guided X-Y-tables are well known in the IC-industry where no process forces have to be withstood.
The X-Y-stage (stroke 280 x 280 mm) which has been realized consists of a table which is aerostaticly guided on a granite surface. As aerostatic bearings a proprietary concept [1] is applied. Four groups of eight inserts containing micro nozzles are used. Due to the fact that the micro nozzles are directly integrated into the lower surface of the table, no alignment elements are required.
For the guide-ways of the gantry axes, roller bearing elements are used. The vertical preload is established using elastic elements between the table and the Y-axis beam, while in horizontal direction the coupling is designed to be stiff.
For the X-direction of motion the table is moving along a beam, which is moved along Y-direction in a gantry configuration. As drives, ball screws in combination with linear scales are used in order to minimize internal thermal loads. As design parameters a maximal feed-rate of 60m/min, an axis acceleration of 2.5g and available feed forces of 2.5kN were used. Due to the alignment of the centers of mass and the drive locations, no considerable inertial or frictional cross-talk should appear.
Using such a concept, the vertical straightness and the tilt error motions around X- and Y-direction as the stiffness are substantially improved in comparison with standard cross-table configurations, while the assembly is strongly simplified.
Actually the prototype which was presented at the Prodex industrial fair in Basel last November is undergoing various performance tests. At the moment the vertical Z-axis carrying the 8kW, 60’000 rpm spindle is in final design. First milling tests are foreseen for early summer. In order to have a model range of machines which can be used on the shop floor some additional elements such as covers appropriate for the given requirements are under construction. A project in the field of micro machining where the machine will be used in shall start soon.
In the paper the design of the prototype is explained in detail. The uncertainty budget is presented as are measurement results covering thermal, static and dynamic properties.
[1] A new production technology for aerostatic bearings; K. Wegener, S. Jaumann, S. Weikert; Proceedings of the 8th euspen International Conference; 2008
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