Arrival Date/Feb., 05, 2003 /Grinding machines can now move with greater precision thanks to a new technology called Axtrusion—developed by the Massachusetts Institute of Technology and New York-based Overbeck Corporation. This actuator/bearing design is remarkably simple yet effective—allowing for extremely accurate linear motion. And precision is only one of its advantages.

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The configuration—which brings together linear electric motors and aerostatic linear bearings—also offers low cost and high modularity and flexibility. This means end users can obtain grinders completely customized for their applications in much less time than required by more conventional systems.

To get an idea of how the Axtrusion concept works, picture air bearings sitting on two intersecting perpendicular planes. A linear motor preloads these bearings, and this motor is placed in such a way—at a certain angle and position from the air-bearing pads—that it can distribute the preload force in any desired manner.

The Overbeck L/T Super Precision grinder is one of the first production grinders to utilize this linear electric motor/air bearing combination. Developed in the partnership between MIT and Overbeck, the L/T makes the most of the powerful magnetic force of open-faced linear motors.

This magnetic force can strain traditional rolling-element or sliding-contact linear bearings, causing them to overheat and rapidly wear out. To fix this problem, other builders arrange two open-faced motors back to back or employ an ironless core motor. But such solutions increase design complexity and in turn, the cost of the machine.

In comparison, the L/T grinder keeps it simple. It relies on the magnetic force to keep the bearings in place, making an underside “keeper” rail unnecessary. The magnetic force also allows the machine to be evenly preloaded and to remain stable through strong external forces that might cause undesirable effects such as pitch, yaw or roll.

The motor’s attractive force is so effective for preloading that even when the machine is severely loaded, it maintains air-bearing stiffness and damping.

The Axtrusion configuration also removes the need for expensive and fragile wraparound preload mechanisms, so the machine can be made from mostly simple planar surfaces. And this uncluttered design enables extremely accurate movement at low cost. Because this uncomplicated planar geometry practically allows an axis to protrude, ball screws and linear bearings no longer restrict machine design.

Aside from depending on its open-faced linear motors for damping, the L/T grinder also relies on its modular porous-carbon air bearings for stability. When affixed and pressurized correctly, these air bearings deliver near-critical damping for machine axes.

To optimize stiffness and damping, the machine employs six bearing pads per axis. The pads are suctioned onto the planes first and then secured into place through epoxy to produce a rigid, damped and elastically even configuration. Critical damping stops vibration in the machine carriage. For more damping, the machine relies on a layer of viscoelastic damping material, which can be found between its granite surface plate base and steel support frame.

The final feature in the modular makeup is the machine’s PC-based CNC, the MachineMate L2. This CNC allows for both linear and circular interpolation. In addition, it’s easy to program, and paired with the Axtrusion concept, it offers a great deal of design flexibility.

The first production version of the L/T was unveiled at IMTS 2002. This machine sustains up to a 1,000-lb. load on each axis and handles up to a 4-inch-diameter part. Its 20-hp spindle manages full grinding loads, allowing for only submicron deflections in the machine.

The L/T will be customized to support just about any load capacity, says Overbeck. End users can not only indicate what type of L/T grinder they want—internal, external, cylindrical, face, surface, center, centerless, or universal—but also ask for specific features, from certain wheel spindles and dresser types to coolant-delivery systems.