In many cases, it's difficult to determine the exact drive technologies used in a machine just by looking at it. However, when aiming to perform specific actions, you can typically choose between the main drive, feed drive, or auxiliary drive systems. The main drive is usually responsible for the primary motion and is generally controlled through a closed-loop system. Most commonly, it uses either synchronous or asynchronous motors. These are widely found in turning, milling, and grinding machines, as well as in machining centers that use kit motors or housed motors. A conventional spindle drive with a main motor is a common solution, often air-cooled and more cost-effective compared to direct motor spindle systems when considering indirect or long-term costs.
Adding a gearbox to the spindle allows for the conversion of angular velocity into torque, which is useful for various machining tasks. However, this also introduces additional radial forces, noise, and increased wear on the system. In contrast, main drives that integrate the motor directly with the spindle are technically mature and offer significant advantages. By eliminating the need for a gearbox and clutch, these drives can rotate smoothly around the center axis without experiencing shearing forces. They are known for their smooth operation over time and reduced wear, especially at high speeds, making them ideal for precision machining.
Currently, generating higher torque remains expensive due to the need for planetary gears within the crankshaft or the use of more powerful motors. To ensure proper maintenance and repair, integrating monitoring sensors into the spindle to collect real-time data is becoming standard practice. Oil, air, or glycol-based cooling methods are still essential for maintaining optimal performance.
Feed drives, on the other hand, typically use either electromechanical or hydraulic systems. Choosing between the two requires careful evaluation of their respective strengths and weaknesses. Electromechanical feed drives are currently dominated by servo motors paired with ball screws, which convert rotational motion into linear movement. Synchronous motors are preferred here because they better meet the higher demands of positioning, synchronization, and dynamic performance required for feed drives.
Due to their high static stiffness, electromechanical systems are suitable for a wide range of applications and have long been a traditional choice. However, they are prone to wear over time. Depending on installation conditions and torque requirements, the servo motor can be connected directly to the spindle or indirectly, 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 the technology was applied in machine tools. Rexroth was among the first to introduce a series-excited linear motor. These drives offer benefits like reduced wear, high stiffness, and excellent dynamic performance, ensuring longer-lasting, precise operation compared to ball screw systems with indirect position detection.
Load capacity is another important factor that influences the choice of drive system. While ball screw assemblies and hydraulic drives can handle high resistance, they may not always be the best fit. Another key consideration is how well the motor supports the machine components, such as a swarf cover with maximum allowable sliding speed or a rack rail with damping characteristics. Despite the advantages of linear motor drives, their higher investment costs have limited their widespread adoption, preventing them from achieving global breakthroughs so far.