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Inverted struts improve hexapod rigidity

By Roy O'Connor -- Design News, February 16, 1998

Stuttgart, Germany--Although the speed, accuracy, and flexibility of conventional machine tools have improved over the years, the basic design remains unchanged--tool and workpiece are still constrained to move along linear axes. Now, this long-standing configuration has competition from hexapod (Greek, for six-footed) designs.

Instead of the usual linear axes, hexapods have six struts, arranged in pairs and comprising linear actuators. Since with the hexapod design the only forces on the machine's structure are tension and compression, it is less susceptible to bending than a conventional machine design.

At Stuttgart University's Institute for Machine Tools, researchers have patented a hexapod design which goes one step further to provide even better stiffness behavior. Although it looks complicated at first sight, the HEXACT simply inverts three struts. These react with the other three, pulling against each other so that the struts are always under tensile loading.

The level of prestressing, therefore, can be optimized to suit the machine. This configuration reduces play, backlash, and non-linearities in the system. The design is also more compact than the usual hexapod, and with the tool head held at both ends, head stability under high acceleration is improved.

Because of the mutual prestressing effect in the struts, the axis drives are less prone to inaccuracies from thermal deformation. Better dynamic characteristics and more accurate path guidance result.

The HEXACT configuration enables five-axis machining of cubic parts, says institute professor Uwe Heisel. "HEXACT is very versatile. It can operate in both vertical and horizontal orientations and can be combined in multiple units." Apart from various types of machining operations, Heisel envisions robotic applications such as welding and assembly benefiting from this technology.

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