Strutting its stuff

German researchers believe that by using parallel kinematics, their tripod mount provides more speed and precision than a serial machine. Stuart Nathan explains.

In the sci-fi classic War of the Worlds, HG Wells conjured up tripods that could conquer the world. While his Martian war-machines are impractical as transport (highly impressive to look at, but too unstable), a similar solution could be about to make major inroads into the machine tool market.

Many engineering sectors, particularly the tool, die and aerospace industries, need high-precision machining, and all of them place a premium on speed. But there’s usually a trade-off between the two — the faster you work, the less precise you are.

Engineers from the Fraunhofer Institute for Machine Tools and Forming Technology in Chemnitz, Germany, have devised their own tripod in the form of a piece of mounting equipment, which could get around this problem.

The usual type of design for industrial robots which carry out machining tasks uses what is known as serial kinematics, where each moving assembly is placed on top of the next. This, according to the Fraunhofer team, seriously limits the way the machine can move — particularly how parts of it can accelerate and jerk. These limits can only be surpassed through highly sophisticated engineering at correspondingly high cost, said research director Steffen Nestmann.

The team’s solution to this problem is to switch to a different mode of operation: parallel kinematics, where the elements that move the machine head are placed next to each other rather than in a line.

The team has developed a tripod mount which allows the head of the tool a large freedom of movement — much faster and with more precision than a serial machine.

‘We developed the concept out of different parallel kinematics, primarily for finishing tasks,’ explained Nestmann.

The tripod consists of three struts attached to a fixed frame platform via swivel joints at one end. The other end of each strut supports the moveable platform that holds the motor spindle for milling tools, mounted on ball-and-socket joints.

The length of each strut can be altered, independently and synchronously, by linear motors. This, according to the researchers, allows the tool head to be rotated continuously in the C-axis, to be inclined by ±30° in the A-axis, and moved 30cm in the Z-axis.

In conjunction with two further linear axes, this allows simultaneous five-axis machining, the team claimed.

The key to the speed and precision of the tripod is the fact that the mass of the machine tool is borne equally by the three struts. Each one, therefore, has less mass to move, rather than in a serial machine where each stage has to move the whole mass. Accelerations can be much faster, and there is less deformation.

According to Nestmann, this greatly improves the accuracy and speed of the machine. ‘The processing time for complicated parts with highly curved freeform surfaces is reduced to about 70 per cent compared with conventional five-axis machines,’ he said.