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Faster production with adaptronics

Today’s drills and milling tools are so precise that the machined workpieces hardly require manual correction. However, when operating at high speeds the machines begin to vibrate. Researchers can now control these unwanted vibrations with the help of adaptive materials.

A washing machine spinning at top speed may easily skip a few centimeters across the floor. Manufacturers therefore add weights to the appliance to keep it in place. In the case of car tires, small weights fitted to the rims compensate for any wheel imbalances. A similar vibration problem occurs during the fine-machining of metal parts. Precision tools such as drills and milling machines can produce objects of any shape and size, for example car bodies, with such a high contour precision that they hardly require any manual finishing. “However, today’s high-speed cutting (HSC) processes require spindles and tools to rotate at such high speeds that the machine begins to vibrate,” explains Dr.-Ing. Volker Wittstock, team leader at the Fraunhofer Institute for Machine Tools and Forming Technology IWU in Chemnitz and Dresden. “The spindles, which hold the drills and milling tools, run at up to 32,000 revolutions per minute. High speeds such as these result in slight imbalances of the rotating spindles, which are different for each tool.” Wittstock and his colleagues in the Adaptronics and Acoustics department have therefore developed an adaptive spindle support for drills and milling tools. It actively reduces vibrations, making it possible to mill and drill accurately at all speeds. The adaptive spindle support moves the HSC spindle in all three spatial directions and also enables it to be tilted. The support is based on a gimbal-type flexure hinge which ensures a basic level of rigidity. Attached to it are six piezo actuators – three above and three below. If voltage is applied to these actuators, they warp in a controlled manner and can move the hinge and mounted spindle in a highly dynamic way, thus immediately offsetting any undesired vibrations of the spindle. “The actuators must be triggered simultaneously. This works faster than in a modern real-time driving simulator in which the user can sit and is moved according to the angle of each curve. It is mainly thanks to the lightness of the piezo actuators and spindles that we have been able to achieve such efficient machine dynamics,” says Wittstock, explaining the principle. The spindle support is now on display at the Fraunhofer Institute in Dresden.

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