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High-speed spindle beats out belt.

The challenge for RS Precision Industries Inc, Long Island, NY, was to grind semicircular ball tracks of case-hardened bearing steel to tolerances of [+ or -]0.0004" and parallelism of 0.000 080". A fine finish without minute waviness or chatter was a must. A belt-driven attachment to their Brown & Sharpe surface grinder proved the validity of the concept of using a small-diameter abrasive wheel, but too much spindle power was lost and the auxiliary spindle could not be held perfectly square. This variation and changes in belt tension caused inaccuracies. Also a single-point dressing tool quickly developed a flat and needed constant adjustment.

The solution was based on installing a self-contained electric highspeed spindle from Precise Corp, Racine, WI. The Model SC 121-0 variable-frequency spindle delivers 8 hp and speeds to 40,000 rpm and is liquid cooled by an RCS 900 refrigeration system to maintain operating temperature.

Other elements in that solution were a diamond roll dresser and the use of tandem grinding wheels of SG ceramic, 1.5" dia, that run at 15,000 rpm. Tandem wheels require a rigid spindle and more power, and the Precise spindle delivered on both counts. The use of the diamond roll dresser also helps the wheels cut more freely, produces a fine finish, and reduces dressing time dramatically. Because the wheels are fixed and the dress rolls manufactured to close tolerances, spacing between the ball-track radii is now held consistently.

Before, with the belt-drive approach, reports Robert Savitsk president, RS Precision Industries, it took up to 3 hr to rough- and finsh-grind the four ball tracks in one set of workpieces. With the new spindle and dressing approach, a typical cycle is now completed in 10 to 15 min. "With the added spindle power and the diamond dressing roll, the operator is able to feed 0.0002" to 0.0003" per stroke, cut more freely, and rough out both sides of the part. He then dresses the wheel and finish-grinds both sides."

For more information on Precise spindles, contact Precise Corp, 3715 Blue River Ave, Racine, WI 53405 or circle 340.
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Title Annotation:Precise Corp. variable-frequency spindle used by RS Precision Industries Inc.
Publication:Tooling & Production
Date:Apr 1, 1991
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