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Behind the high-speed spindle: they are complex, use complicated and fragile bearing designs and demand more sophisticated systems. But the payback can be handsome. (Technology Notebook).

Until recently, the standard machining center spindle was mostly taken for granted.

It offered anywhere from 2,500 rpm up to 7,500 rpm, often using a heavy gearbox and belts. The power came from a large, externally mounted DC or AC motor. Large diameter, multiple-flute cutters were the norm, resulting in heavy cutting forces on the spindle bearings. A heavy flood coolant system removed heat and washed away the chips. Unless the spindle was abused, bearings would easily perform for ten years or longer.

The typical spindle design used sets of large, tapered roller bearings. Bearing lubrication was most often grease. Gears or a cogged belt setup transmitted power from the external driving motor to the rotating spindle shaft. The cutting tool was held by a tooling system, most typically CAT, BT or even NMTBA. In the case of automatic tool changing machines, the spindle required a power draw bar mechanism to grip the removable tool holder. High holding force was needed, as the cutting forces could be severe. Location and accuracy of the system relied upon the contact of the tool holder taper into the spindle taper. Balance was not an issue, as the rotating speed was not high enough to generate significant vibration.

Today's challenge is to produce parts faster, with greater accuracy, often in smaller batches. Thus, the manufacturing process must be flexible and dynamic. To accomplish this, it is necessary to utilize faster and more modern machine tool designs and higher spindle speeds. As such, a different spindle design must be used.

Inside the spindle

A modern milling spindle, running at 20,000 rpm and higher, cannot use an external drive motor with belts or gears. They create unacceptable levels of vibration and heat. The solution is an integral motor within the spindle itself. An integral motor is built into the spindle, consisting of an external winding (stator) and the rotating component (rotor). This motor may be driven by an AC flux vector drive, or in some cases, a brushless DC drive. Encoder feedback is used to measure and control the spindle position and allow for spindle orientation and rigid tapping. The spindle will have a typical AC motor curve, which has low torque at low speed and higher power available at higher RPM. Such spindles are not designed to perform high-power, low-speed operations. This design assumes the spindle will be used primarily at 50 percent or higher maximum spindle speed.

To achieve high speeds and deliver high power, bearing design will be different from the conventional spindle. The bearing needed will be a high-precision, angular-contact type as it allows very high speed, high accuracy and reasonable bearing life. These bearings probably will be hybrid ceramic ball bearings, a new type using silicon nitride balls. Hybrid ceramic bearings run faster, cooler and provide a longer life--but are more expensive. Lubricating the bearings becomes more complex. The lubrication system will be oil-air, using very small quantities of oil directed into the bearing. The quantity of oil is quite critical, as too much oil causes excessive heat and too little causes unacceptable wear of the ball race. In many cases, each individual bearing is supplied with oil mist from a controlled valve. A new technology becoming more common is through-the-race lubrication, where the oil is fed through a small bore made in the bearing race. This concept minimizes the heat produced by the bearing, which al lows higher speeds and longer life.

The pre-loading Issue

Bearing pre-loading was never an issue with slower spindles. The pre-load was fixed and high, resulting in a very stiff and rigid spindle. Ideally, a high pre-load is desired during low-speed operations. At high speeds, however, a high pre-load generates excessive heat and rapid bearing failure. High-speed spindles, utilizing lightly pre-loaded bearings, are by nature less stiff and less rigid. Variable pre-loading methods are sometimes used, including the use of springs, hydraulics and even electric servos to control pre-loading of the bearings.

The conventional spindle relied upon a CAT or BT tooling system to produce parts at low cutting speeds. High-speed spindles, however, cannot use this tooling design. At high speeds, the spindle nose will "bell mouth," opening up slightly because of centrifugal force. This causes the tool holder to be pulled into the spindle nose, resulting in loss of accuracy, reduced rigidity and even mechanical damage to the spindle. Therefore, a more advanced tooling system, HSK, must be used. The HSK standard incorporates a two-point contact system, as well as other features, that permit it to be run at very high speeds with safety and precision. The HSK design also incorporates a self-locking mechanism that guarantees very high tool retention force while the spindle is turning.

Balance is also more critical at higher speeds. As the force due to imbalance increases with the square of speed, imbalance must now be eliminated or at least carefully measured and controlled. A balance grade system is used to rate balance for spindles, specifying the maximum allowable imbalance, which is determined by tool mass, size and speed.

While the pursuit of speed is important, it cannot be achieved without maintaining accuracy and precision. Spindle runout and thermal growth must be minimized. Since the high-speed spindle is an integral motor, some heat is generated causing unacceptable levels of thermal growth. To control this heat, high-speed spindles are liquid-cooled. In addition, thermal growth can be measured with laser gages and compensated for in the CNC. A new feature available uses built-in sensors in the spindle that measure and report thermal growth, real-time, to the CNC.

At higher speeds, vibration becomes a more critical factor. Excessive vibration, caused by improper cutting tools overloading or other process problems, can quickly damage spindle bearings. It is not uncommon to have vibration sensors built in to advanced spindles to monitor and control vibration.

The process of high-speed machining is quite different from simply running a conventional machine faster. New cutter technology provides coated carbide cutters that can be run at significantly higher surface speeds. Chip loads are vastly reduced, as are actual cutting forces. The net result is a manufacturing process that produces components faster and with better accuracy and quality.

Consequently, the spindles required to perform at high speeds are quite different--more complex, using more complicated bearing designs, along with more sophisticated lubrication and tooling systems. Spindles must be temperature-controlled and monitored very closely for overloading, vibration and temperature. They are much less forgiving and more sensitive to misuse. Useful life of the bearing will be shorter and maintenance will be much more important. And high-speed spindles will cost more. Still, considering the tremendous increase in productivity resulting from the use of high-speed spindles, the increased investment can pay handsome dividends. IBAG North America, North Haven, CT, or circle 333
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Author:Popoli, Bill
Publication:Tooling & Production
Date:May 1, 2002
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