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Spindle design boosts appetite for aluminum

By Design News Staff -- Design News, September 21, 1998

Wales, UK--Amidst large aircraft backlogs, British Aerospace is betting on a new high-velocity profiler from Ingersoll Milling Machine Co. (Rockford, IL). Executives believe the 5-axis, linear-motor-driven machine will quadruple wing-spar production. The 4 million-lb machine measures 107 3 48 ft, stands three stories tall, and carves large aluminum billets into "monolithic," or one-piece, wing spars. Oddly enough, it is four comparatively small horizontal spindles that actually buzz away, creating much of the clamor and doing the bulk of the work.

The motors powering Ingersoll's hydrostatic fluid-bearing spindles deliver 100 hp at the tool's tip, says John Osborn, Ingersoll's manager of engineering for the company's Heavy Machine Division. But it takes high power in a compact package to access and carve the deep, complex, thin-walled pockets. Simon Sparrow, general manager at Nelco Systems Ltd. (Watford Herts, UK)--Ingersoll's spindle-motor supplier--says although the motor fits in a 245-mm-diameter, 240-mm-long space, it puts out a constant 150 kW from 10,000 to 20,000 rpm.

The motor manufacturer specifies API Harowe (West Chester, PA) resolvers because they are large enough to fit on the 75-mm-diameter spindle-shaft bore, robust enough to handle high rotational speeds and temperatures, and accurate enough for precise motor commutation and positioning. API Harowe's resolver uses a high carrier frequency to improve noise immunity, and has a custom housing and rotor bore to suit the spindle. To eliminate resonance associated with key ways, motor and resolver rotors are press fit at tight tolerance to the spindle shaft, explains Sparrow. This design requires pumping hydraulic oil into the interface for removal.

Nelco's digital-vector-controlled amplifier is performance matched to the motor characteristics to provide maximum performance and fast dynamics. Due to the 50m cable run from the motor back to the amplifier, Nelco provides an interface card that converts analog resolver signals into digital form for even higher noise immunity. The result is highly accurate control that enables active dimensional probing between machining operations.

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