Design of high-speed spindle

The spindle is the heart of a CNC machine tool, providing the torque, speed and precision required for machining tool applications.

The cutting work of a machine tool relies on the drive of the spindle, so the performance characteristics that must be present in the design of a high-speed spindle that will be used in a metal cutting machine include:

• Spindle power is required for machining
• Maximum spindle load, axial and radial
• Suitable speeds
• ATC tool type, size, and capacity

In addition to this, the machine tool will have certain limitations on the spindle, such as the amount of space in which it can be mounted. A complex combination of factors can influence the final spindle design. Often, the more factors considered, the higher the cost. Machine tools are very important to the production line, and if there is a problem with the machine, it will affect the entire production process, so the maintenance and stability of the spindle also need to be considered in the design.

Spindle type

The first factor to consider when designing a spindle is the type of spindle, whether it is a belt driven spindle or an electric spindle. Belt-driven spindles are generally less expensive than electric spindles. Belt-driven high-speed spindles are very similar in design to conventional speed spindle designs, but there are some significant differences. A typical belt-driven spindle assembly consists of a spindle shaft, supported by a bearing system, and supported by a spindle housing. The spindle shaft comprises a tooling system, including a tool taper, a drawbar mechanism, and a tool release system. The mechanism providing the tool release force is typically mounted externally.

Power and rotation are provided to this spindle by an external motor. The motor is mounted near the spindle, and the torque is transmitted to the spindle shaft via a gear or v-belt. The power, torque, and speed of the spindle will therefore depend on the characteristics of the drive motor and the belt ratio used between the motor and the spindle.

Advantages of belt driven spindles

• Lower costs: Since the spindle itself is made up of several basic components, the costs are relatively low compared to alternative solutions.
• Spindle characteristics: as the power, torque, and speed of the spindle are dependent on the drive motor, to a large extent, the final specification can be modified by using a different motor or belt ratio. In some cases, gears are used to provide multiple speed ranges in addition to fixed belt ratios.
• High power and torque possible: The spindle motor is mounted on the actual spindle shaft, so a very large motor can usually be used. A large motor, especially a large diameter motor, can provide very high torque and power for spindle use. This is much more difficult in motor spindle design as the available space is always limited.

Disadvantages of belt-driven spindles

• Maximum speed is limited: Belt-driven spindles will be limited to a maximum speed due to several factors. The mechanical connection that transmits the torque to the spindle, belt, and pulley system is limited in terms of maximum operating speed. If a poly v-belt system is used, high speeds on the pulleys tend to stretch and disengage the belts, reducing their ability to make contact and transmit torque. Plugged belts eliminate the problem of slipping, however, at higher speeds, these belts produce unacceptable levels of vibration. Cogs have a very limited maximum speed and also produce high levels of vibration and heat if run at very high speeds.
• Belts utilize load carrying capacity. In order to be able to transmit the necessary torque, belt-driven spindles utilize belts and pulley connections at the end of the spindle shaft. The tensioning of these belts will exert a constant radial force on the rear spindle shaft bearing set. As the power and speed of the spindle increase, the applied tension and subsequent force will increase, using a large number of bearings with available radial loading capacity. Furthermore, replacing the bearings with larger ones or adding additional bearing sets would not be feasible, as these methods would only further reduce the spindle's ability to achieve high speeds.

Electric Spindles

Electric spindles do not rely on an external motor to provide torque and power. The motor is an integral part of the spindle shaft and housing assembly. This allows the spindle to rotate at higher speeds as a complete unit without the additional limitations of belts or gears. Typically, a complete motor spindle consists of a spindle (including the motor element) and a tooling system. The spindle is held in place by a set of high precision bearings. The bearings require a lubrication method such as grease or oil. The spindle will then rotate to maximum speed and display the power characteristics of the type of motor used. The choice of particular components will, of course, depend on the requirements of the machine tool.

In addition, the motor spindle must use a motor as part of the rotor shaft. Therefore, the size and capacity of the motor will depend greatly on the space available. As we discussed earlier, bearing size is critical in high-speed spindle design, so the motor shaft will affect the size of the bearings available. Bearing size also affects loading capacity, stiffness, and maximum speed, so motor characteristics must be matched to bearing capacity.

Conclusion

It is therefore clear that belt-driven high-speed spindles will be limited to certain applications. Typically, belt-driven spindles will be used up to a maximum speed of 12,000–15,000 rpm. In order to achieve this, other methods must be used to allow for higher speeds, including different bearing types, settings, or bearing lubrication. These methods will be discussed further in subsequent sections of this report, as they are similar to those used in motor spindles. This type of spindle may be as high as 30 hp, however, it is sometimes difficult to provide high torque at top speeds, which will depend greatly on the characteristics of the drive motor.