Advanced bearings for high-speed machining

Optimized spindle-bearing systems run faster without unwanted vibration or heat. Precision high-speed machining requires low-friction bearings that provide dynamic stability. Hybrid bearings with ceramic balls run faster and have higher stiffness than comparable all-steel bearings. In the world of precision machine tools, few topics receive more attention than high-speed machining. Cutting metal at higher speeds increases productivity. But to run faster without losing quality or reliability, the spindle, motor, and bearing system must provide the torque, rigidity, and accuracy for cutting while minimizing rotational friction for the tool and workpiece. As such, the bearing design must ensure high precision and address factors that limit high-speed capabilities. For instance, any loss in bearing dynamic stability can lead to vibrations, tool chatter, wear, and costly scrap. Higher speeds generate more heat because the lubricant shear rate increases. They also boost centrifugal force of the bearing rolling elements. Both factors can alter the bearing's design preload which, in turn, can generate even more friction and heat. Optimizing the bearing system lets machine-tool builders raise speeds and achieve "first part correct" running accuracy on a consistent basis. The latest designs improve dynamic stability for chatter-free metal removal while maintaining lower operating temperatures. They also contribute to longer machine service life. There is no hard-and-fast rule for segregating "high" from "standard" speeds, but a dN value >1 million is often considered to be in the high-speed realm. A well-established means of defining bearing speed, dN = (bearing bore diameter in millimeters) X (shaft rpm). Bore diameter normalizes the speed rating because larger bearings often run slower. Many factors come into play when specifying high-speed bearings, complicating the selection of the best bearing for a given application. Here are some important considerations. Annular Bearing Engineering Committee (ABEC) standards, which conform to ISO standards, define tolerances for several bearing dimensions, but they make no mention of several critical parameters. Raceway uniformity, waviness and surface finish of the rolling contact surfaces, cleanliness, ball precision, preload offset limits, and internal design are just a few of the unspecified critical parameters that can affect bearing performance, spindle stiffness,