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Sliding shock absorbers

The milling of micro system components such as tiny ear prostheses or miniature cogwheels demands the highest possible precision. If the machine rocks, sways or jogs, this precision is lost. The MiniMill milling machine puts a stop to such unwanted movements.

Tiny blood pumps made of plastic sit in patients' veins, helping their circulation. Minute ear prostheses replace the stirrup bones in their ears. And the smallest of cogwheels help clocks to tick accurately. Be it for medicine or mechanics, micro system components are becoming increasingly important. Their manufacture requires the utmost precision: Many of the milling tools used for metal molds and components are no more than 50 micrometers across – only half as thick as a human hair. And they must mill the surface accurately to within a hundredth of a hair's breadth in each direction. Although today's high-precision milling machines can perform this task reliably, they take their time – for if the machine accelerates the tool too quickly it can jolt the equipment, causing it to oscillate. The new MiniMill milling machine developed by researchers at the Fraunhofer Institute for Production Technology IPT absorbs these shocks, allowing small metal parts to be produced more quickly. "On each of the device's three axes there is a small slide which absorbs the momentum of the jolt and moves backwards," explains Rainer Klar, the IPT researcher in charge of development. "It's a little like the recoil of a cannon: In order to ensure that the cannon doesn't sink into the ground on being fired, it is mounted on a carriage, a kind of guide rail. The recoil causes it to roll backwards on this rail, thus absorbing the shock." So far, such a momentum decoupling system has only existed on one of the three axes on which the tool or work piece can move. This is not enough, as most of the geometries that have to be milled are more complex than that. The researchers have now for the first time succeeded in installing a decoupling system on all three axes. "This allows us to reduce the production time required for miniature metal components by up to 20 percent, while at the same time improving the processing quality. Conventional machines only achieve an efficiency increase of two to three percent," says Klar. A further advantage of MiniMill is that it only takes up one square meter of space, while conventional machines require about three times as much surface area. If production is flexible and there is a change in sites, for example, the compact MiniMill system can easily be moved with a forklift truck. Last month the researchers completed a MiniMill prototype, which will be presented at the MiNaT trade fair in Stuttgart (Hall 2, Stand 2A14) from June 12 to 14. The scientists' next step, together with their colleagues at Siemens, will be to optimize the machine's control system.

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