Spinning Speedier Spindles

"Motorized bearing" facilitates faster feeds, cuts harder materials

The art and science of material cutting relies on not only the quality of the cutting tool, but the mechanism that holds the tool, as well. In an industrial environment, machines spin tools into materials for milling, drilling, dicing and other operations. Increasing the tool spindle speed permits faster feed rates and the ability to cut hard materials, while leaving a smoother cut. In semiconductor dicing, these speeds approach the 60,000-rpm range.

Bred by modern technological developments, the demand for rotation systems with lower vibration yet higher maximum rotation speed has led to the design of air-bearing spindles. Colibri Spindles Ltd, MP Bikat Bet Hakerem, Israel, has developed the BOH 80 2.1 D 60 spindle with a modification they call “motorized bearing technology.”

Traditionally, a spindle is constructed from two axial ball bearings positioned ahead of two radial ball bearings. While the shaft distance between the two axial bearings is reduced, fixed and accurate gaps must be kept, regardless of possible thermal influences. In order to reduce axial expansion at the shaft end, where the tool is mounted, the axial bearing is usually placed at a distance from the spindle motor. Thermal changes due to electric-eddy currents in the motor system and friction forces operating on the bearings may result in axial expansion.

In an air-bearing system, the stiffness of the spindle depends on the geometric size of the bearing. However, producing longer and bigger bearings is far from simple or efficient, nor is it cost effective. The distance between the front and rear radial bearings is influenced by the shaft-end stiffness, bearing stiffness and self-frequencies.

In Colibri's design, bearing and motor are integrated together into a single unit. The motor actually serves as the radial bearing, allowing a shorter overall length of the assembly. The 150V, 1200W unit packs 2.1 N-m of torque into a package just 80 mm in diameter and 195 mm long, with a weight of 6.7 kg. Load capacity is 60 N radial and 100 N axial, with a stiffness of 14 N/micron radial and 9 N/micron axial.

MBT can be implemented in either the radial or axial bearings, depending on the type of motor. The design technology offers the following advantages: smaller geometric dimensions; reduced spindle weight; extended shaft cycle lifetime; higher rotation speed; higher self-frequencies with lower vibration amplitudes; increase in bearing stiffness and maximum load capacity; lower maintenance cost; simplification of automatic tool replacement system; and lower production costs. These advantages are significant for small, precision systems requiring the use of an accurate and reliable spindle, such as the hard disk of a home computer or portable computer. The current demand for increased disk rotation speed dictates a transition from ball bearings to aerodynamic bearings.

Another feature of this spindle is a dynamic contactor for determining the height of the cutting tool during a tooling setup. Electrical current is fed through the spindle shaft from the control system via brushes held in place with air pressure. Once machining commences, the air pressure is reversed to lift the brushes off the shaft, preventing brush wear and contamination of the motor from particles.

This arrangement facilitates a miniaturized bearing and motor system, enabling integration of the spindle into other compact systems. The MBT design also permits the use of Automatic Shaft Change (ASC) technology, whereby the shaft and integral tool can be changed automatically during a machining process with no loss of precision or tool balance. Besides dicing hard, brittle semiconductor materials, these spindles have also found use in micromachining and fine milling, fine drilling, automated semiconductor wafer testing, and the spinning of optics or drums for laser etching of printing plates.

—RM