In many cases, it's difficult to determine the exact drive technologies used in a machine just by looking at it. However, when you need to perform a specific action, you can typically choose between the main drive, feed drive, or auxiliary drive systems. The main drive is usually responsible for the primary motion, such as rotating the spindle, and is typically controlled through a closed-loop system. Most main drives use either synchronous or asynchronous motors. Common applications include lathes, milling machines, and grinding machines, as well as machining centers that use kit motors or housed motors. A conventional spindle drive with a main motor is widely used and mostly air-cooled. Compared to motor spindle systems, this approach is often more cost-effective when considering indirect or long-term costs.
Adding a gearbox to the spindle allows for the conversion of angular velocity and torque to meet machining requirements, but it also introduces additional radial forces, noise, and wear. On the other hand, main drives that integrate the motor directly with the spindle are technically mature. By eliminating the need for a gearbox and clutch, these drives can rotate smoothly around the center axis without shear forces. This design ensures long-term smooth operation and minimal wear, especially at high speeds, making them ideal for precision machining tasks.
Currently, generating higher torque remains expensive, as it often requires planetary gears within the crankshaft or the use of a more powerful motor. To support regular maintenance and repair, integrating monitoring sensors into the spindle to collect real-time data is becoming a standard practice. Oil, air, or glycol cooling remains essential for maintaining optimal performance.
Feed drives typically involve a choice between electromechanical and hydraulic systems. Making the right decision requires careful evaluation of each system’s unique advantages and disadvantages. Electromechanical feed drives, which use servo motors with ball screws, are currently dominant. These systems convert rotational motion into linear motion, and synchronous motors are preferred due to their superior positioning accuracy, synchronization, and dynamic performance compared to main drives.
The high static stiffness of electromechanical feed drives makes them suitable for a wide range of applications and has made them a traditional choice. However, they do have some drawbacks, such as susceptibility to wear. Depending on installation conditions and required torque, the servo motor can be connected directly or indirectly to the spindle, such as via a synchronous belt. Although the concept of linear motors dates back to the 19th century, it wasn’t until the early 1990s that they were applied in machine tools. Rexroth introduced the first series-excited linear motor during that time. These drives offer benefits like reduced wear, high stiffness, and excellent dynamic performance, leading to long-term, precise, and reliable operation compared to ball screw systems with indirect position detection.
Load capacity is another important factor that influences drive selection. While ball screw assemblies and hydraulic systems can still be used under high resistance, they may not always be the best option. Another key consideration is how well the motor supports machine components, such as swarf covers with maximum allowable sliding speed or rack rails with damping characteristics. Despite the advantages of linear motor drives, their higher initial investment cost has limited their widespread adoption globally.