Posted in | Materials Processing

Three Legs Go Faster

Depending on their design, machine tools carry out their movements sequentially or simultaneously. Parallel kinematics are less easy to control, but faster and more precise. Fraunhofer engineers will be presenting their new tripod tool at the EMO fair in Hanover from September 14 to 21 in Hall 12.

The terms “serial” and “parallel” are probably familiar in the context of PC interfaces: Serial means transmitting a stream of data one bit after another and parallel means sending several bits of data at once, side by side. In the world of machine tools, a serial machine structure is one where each moving assembly is placed on top of the next – as in the usual type of industrial robots for instance. The kinematics of this design places tight limits on acceleration and jerk that can only be surpassed through highly sophisticated engineering at correspondingly high cost.

A completely different type of design provides a better solution: parallel kinematics. An example is the tripod developed, designed and built by engineers at the Fraunhofer Institute for Machine Tools and Forming Technology IWU in saxonian Chemnitz. The three struts of the tripod are attached via swivel joints to a fixed frame platform. At the opposite end the moveable platform is mounted on ball-and-socket joints and carries the motor spindle for milling tools. Linear motors enable the length of the struts to be adjusted synchronously. This allows the tool head to be rotated continuously in C-axis, to be inclined by ± 30 degrees in A-axis and moved 30 centimeters in Z-axis. In conjunction with two further linear axes, this arrangement allows simultaneous 5-axis machining. To be handled by numerical control system the orthogonal work piece coordinates have to be transformed into parallel strut coordinates.

Particularly in the tool and die as well as the aircraft industry, there is a need for high effective processing speeds coupled with high precision – even when working on complex-shaped parts. Since parallel kinematics machines have less mass to move, they can accelerate much faster. The loads and forces can be borne simultaneously by the parallel operating struts. This leads to less deformation, which ultimately improves the accuracy of the machine tool. “We developed the concept out of different parallel kinematics primarily for finishing tasks,” stresses IWU department head Steffen Nestmann. “The processing time for complicated parts with highly curved freeform surfaces is reduced to around 70 percent by comparison with conventional 5-axis machines.” Nestmann and his colleagues will be presenting their tripod tool and its design principles at the EMO fair, in Hall 12. The “world of machine tools” takes place in Hanover from September 14 to 21.

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