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In response to demands for greater speed, better accuracy, increased durability, and more versatility, spindle makers are looking to old and new technology to find the answers. JIT and two and three-shift operation are now common in many plants and shops. Temperatures and loads developed in near-continuous operation put a heavy strain on spindles. Because job lots are smaller and inventories minimized, there is more need for quick tool changes because changeover time is a larger fraction of total production time. With more agile manufacturing cells replacing the fixed transfer line, one spindle must operate with a different tool at a variety of speeds and feeds. To meet these increased demands, there has been significant improvement in spindle bearings, coolants, motors, and tool-holders, as well as overall spindle durability and accuracy.
Better Basic Bearings
The vast majority of spindles use rolling-element steel bearings. Changes are continuously being made in metal composition and race configuration, but the basic elements remain the same.
To meet higher speed requirements, many spindle makers are now using hybrid or ceramic-ball, steel-race bearings. The ball's light weight can allow up to a 50% increase in spindle shaft speed. The typical angular contact bearing encounters its speed limit as centrifugal force causes the balls to move up the curvature on the inner ring and deeper into the groove on the outer ring. "For example," explains Dr. Russell Kulas, Bryant Grinder Corp. (Springfield, VT), "in conventional steel 15deg. angular contact bearings, the contact angle may increase to 20deg. or more on the inner ring and drop to under 10deg. on the outer ring. The discrepancy between contact angles causes the balls to skip. This, in turn, causes heat build up and lubricant breakdown."
A solution is ceramic balls that have a lower mass and are not as influenced by the centrifugal effect. They can, therefore, operate at higher speed and have a longer service life. Because inertia is also lower, there is less skidding during high acceleration and deceleration. This is important in today's high-speed machining environment which requires frequent tool changes.
On the negative side is cost. "Initially, they were five times more costly than steel bearings, but today that difference has dropped to one or two," says Kulas, "you can easily...