An Evaluation of the Design and Performance of Three Advanced Spindles

Final Report of NCMS Project No. 150306 - Strategic Machine Tool Technologies: Spindles (June 2001)

This technology report is the first public release of data from a four-year $7.5 million development effort by an NCMS-led project team that designed, built, and evaluated the performance of three advanced spindle prototypes. One spindle is supported on jet-lubricated, tapered-roller bearings; the second on air/oil-lubricated, hybrid-ceramic bearings; and the third on water-lubricated hydrostatic bearings.

The report details the design, manufacturing and testing of the three spindles. In comparing how the test results fared against performance expectations, discrepancies were found that led to a discovery of the root cause of a previously unexplained motor heating source that is responsible for the failure of many high speed spindles. This led to the invention of means to eliminate the heat source by changing the wiring layout. This simple change allowed a 30% increase in low speed torque without overheating. The performance of the modified spindles was tested by cutting cast iron engine blocks and aluminum heads at metal removal rates that were twice rates use the current production.

The hydrostatically supported spindle is a unique design that substitutes self-compensation for the orifices or capillaries more commonly used in hydrostatic spindles. Water was used as the lubricant to avoid the problems inherent with keeping oil separated from coolant. Hydrostatic bearings were selected in place of ball bearings so the stiffness required for single pass machining could be achieved when a cluster of precision holes are required and the holes are too closely spaced to fit the optimum sized ball bearings on the shafts. The test required a set of four in-line holes to be produced in the upper and lower decks of a cast iron gear carrier. Result showed consistently high values of the statistical parameters use to gage quality.

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