Commercial spindle kits simplify the design of electric spindles by combining all the necessary components in one package. The kits incorporate a permanent magnet motor, resolver, digital servo-amplifier, and software development tools. To limit thermally induced errors, the motors, which are available with solid as well as hollow shafts, employ low-loss permanent magnet rotors and water-cooled stators.

Take away their computers, lasers, and advanced structural features, and machine tools seem no different today than they were 100 years ago. They still move "metal" against "metal" over straight lines and curved paths, and their spindles still spin just like they always have. Well, almost.

Today's machine tool spindles still go 'round and 'round, but a lot faster and more precisely than the spindles of even a decade ago. The improvements are the result of water-cooled synchronous motors and digital servo-amplifiers that regulate motor flux. Together, the components form a new breed of high-speed electric spindles that are changing the face of an industry.

Electric spindles

An electric spindle is an electromechanical assembly consisting of a motor, a spindle shaft and its bearings, and some sort of tool holder. It may also include provisions for cooling.

Electric spindles are becoming common in high-speed machining and precision milling equipment because they make machines simpler, more accurate, and more compact. They also open the door to new machining techniques such as lathemilling and large-scale material removal.

One of the drawbacks associated with electric spindles stems from the use of induction motors. During heavy use, high rotor losses heat up the spindle shaft, causing it to expand. It's not uncommon for axial expansion to introduce as much as 50 μm of tool offset. Shaft heat also flows to the bearings, dramatically shortening their service life.

Though induction motors are the norm for electric spindles, a growing number of machine-tool builders are beginning to use permanent-magnet synchronous motors instead. Unlike induction motors, these special synchronous motors, designed specifically for spindles, are not subject to rotor losses. Their permanent-magnet rotors draw no electric power. All losses occur in the stator, where the heat can be easily removed using water cooling or other techniques.

In recent tests at the University of Darmstadt in Germany, independent researchers showed that low-loss synchronous spindle motors not only run cooler than asynchronous motors, but also are more precise, producing half the axial displacement. And it could have been less.

Most spindles, including the one used for the test, incorporate angular contact bearings, which have an inherent axial offset of around 10 μm. The offset is due to uneven centrifugal forces caused by asymmetries in bearing construction. With better bearings, synchronous spindle motors can achieve total axial displacements of less than 10 μm.

Why brushless

Not long ago, it would have been unheard of to even suggest putting a brushless motor on a spindle. Brushless motors were designed for something altogether different, primarily positioning or servo applications. The cost of the amplifier alone – for a spindle, it would have to be overrated by as much as a factor of 15 – was more than enough to keep brushless motors out of the picture.

But things changed. For one, machine tool builders started turning to lighter spindle motors in the pursuit of higher feed rates. Spindles are often conveyed along feed axes, so making them lighter is one way to pick up the pace.

While feed rates were going up, cycle times were coming down. All of a sudden spindle motors were thrust into the "hurry up" mode. Instead of being able to spin up and down at a leisurely pace, spindle motors now had to burst into action and often stop just as abruptly to get set for the next cycle. Some tool builders went so far as to close the position loop, so spindles could synch up with other machine axes as well as assist in tool changes. The line between spindle and servo motors was blurring.

Meanwhile, motor manufacturers were chipping away at the amplifier problem. It's not as if there was anything inherently wrong with early servo-amplifiers; they just weren't designed with spindle applications in mind. To meet the power requirements of a spindle meant that the amplifiers had to be significantly oversized.

Amplifiers, whether they're used on spindles or feed axes, have to deliver a certain amount of power. The requirements, in terms of current and voltage, are determined by the application, motor, and control method. Usually, the application dictates the motor, which, in turn, sets minimum current and voltage levels.

Using conventional control techniques, a brushless spindle amplifier must deliver enough current to meet the motor's low-speed torque requirements, and enough voltage to meet the maximum speed requirements at nominal flux. To achieve this output calls for an amplifier three to 15 times more powerful that what it really needs to be.

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