An Overview of High-Speed Machining Centers
High-speed cutting theory was proposed over 80 years ago. In recent decades, key machine tool technologies advanced rapidly. For example, electric spindle and high-precision CNC technologies improved significantly. Consequently, manufacturing capabilities of high-speed machine tools increased. Thus, high-speed cutting technology became widely adopted. Moreover, its applications span automotive, aerospace, mold making, and precision machinery industries.
Discussion on High Speed Machining
PPerformance advantages of high-speed machining centers include high speed, which improves processing efficiency. In addition, shallow cutting depth achieves higher processing accuracy. Moreover, dry cutting promotes environmental friendliness. They also feature low cutting force and low cutting temperature. Furthermore, these centers can directly process hardened materials. They also process thin-walled parts, reducing part deformation. Finally, process concentration enhances overall productivity.
Key technologies of high-speed machining centers involve hardware and software. Hardware technology includes CNC machine tools and cutting tools. Meanwhile, software technology involves CNC programming technology such as CAM systems.
High-speeds machining requires machine tools with high spindle speed and power. Currently, spindle speeds generally exceed 10,000 rpm. This is approximately 10 times faster than general-purpose machine tools. Moreover, main motor power typically reaches 22 kW or higher. This ensures high efficiency and heavy-duty cutting. In addition, high feed rate and rapid traverse speed increase productivity. These speeds are about 10 times those of general-purpose machines. The spindle and worktable also have high acceleration and deceleration, ranging from 1 to 8 g.
Furthermore, excellent static, dynamic, and thermal characteristics are essential. High-speeds cutting causes severe friction and heat generation between moving parts. Also, high acceleration creates significant dynamic loads. Therefore, designers adopt special processes in transmission and structural design. This ensures sufficient static, dynamic, and thermal stiffness. Lastly, auxiliary components must match the high speeds of main components. Thus, the CNC system requires excellent functionality.
Requirements for CAM software in high-speeds machining:
1. Basic requirements:
1) Safety;
2) Verification mechanisms;
3) Multiple machining strategies;
4) Trajectory editing functions;
5) Rich data interfaces.
2. Special requirements:
1) Automatic generation of high speed machining process parameters: The system automatically generates process parameters based on the material being machined, process characteristics, machine tool performance, tool parameters, etc., and allows programmers to optimize them based on experience;
2) Generation of smooth tool paths: High-speed machining involves high feed rates, requiring tool paths to be as smooth as possible to avoid sudden direction changes, which could cause the tool to deviate from the intended path and result in overcutting;
3) Feed rate optimization: The CAM system can adaptively optimize feed rates based on the size of the instantaneous material allowance, enabling the tool to machine parts at continuously varying cutting speeds;
4) Reduce processing data volume;
5) Rough stock knowledge: The system automatically records the rough stock remaining after each machining step.
Main components of high-speed machining centers: High-speed spindle unit (electric spindle); high-speed feed drive and transmission system; machine tool CNC system capable of providing high-speed feed control; including installation,clamping, rapid exchange, and dynamic balancing; workpiece clamping devices suitable for high-speed cutting; efficient cooling and chip removal devices; reliable safety protection and monitoring devices; and bed, column, and worktable components with good dynamic, static, and thermal characteristics.
The sole purpose of improving domestic high-speed machine tools: to adapt to faster,more accurate.
High-speed Spindle Technology
An electric spindle is a set of components, including the electric spindle itself and its accessories: electric spindle, high-frequency variable-frequency drive, oil mist lubricator, cooling device, built-in encoder, tool changer, etc. The rotor of the electric motor directly serves as the machine tool’s spindle, and the housing of the spindle unit is the motor base, which, in conjunction with other components, achieves integration between the electric motor and the machine tool spindle.
The machine tool spindle is directly driven by an internal electric motor, thereby reducing the length of the machine tool’s main transmission chain to zero and achieving “zero transmission” for the machine tool. This integrated transmission structure, where the spindle motor and machine tool spindle are combined into one unit, allows the spindle assembly to be relatively independent from the machine tool’s transmission system and overall structure. As a result, it can be manufactured as a “spindle unit,” commonly referred to as an “electric spindle” (Electric Spindle, Motor Spindle). Electro spindle, Motor Spindle, Motorized Spindle, or “Direct Drive Spindle” (Direct Drive Spindle)
High speed spindle system components: spindle motor, support bearings, lubrication system, cooling system, preload system, and electrical drive.
Brief Introduction to the Working Principle of the Electric Spindle
Electric spindle support bearings significantly impact functionality and service life. Therefore, they must have high-speed rotational accuracy, high radial and axial stiffness, low temperature rise, and long service life. Typically, support methods include ball bearings, rolling bearings, magnetic levitation bearings, air static pressure bearings, and hydraulic pressure bearings. Moreover, the electric spindle motor usually uses variable frequency drive and control.
The drive control features constant torque drive, meaning output power is proportional to speed. However, at low speeds, output power becomes unstable and fails low-speed, high-torque needs. Furthermore, it lacks precise stopping and C-axis control functions. These functions allow spindle positioning at any circumferential position. On the other hand, this method is relatively inexpensive.
Vector control drive offers constant torque at low speeds and constant power at medium and high speeds. It also has two control modes: open-loop and closed-loop. In closed-loop mode, the system obtains position and speed feedback via a spindle position sensor. Consequently, it achieves better dynamic performance and enables precise stopping and C-axis control.
Feed System for High-speed Machine Tools
Requirements for high-speed machine tool feed systems: high speed (60 m/min; sometimes 120 m/min); high acceleration; high static and dynamic accuracy; high reliability and safety; low cost.
The main feed systems used in high-speed machine tools include: high-speed ball screw drive systems; linear motor feed drive systems. Among these, linear motors offer superior acceleration performance compared to ball screw drive systems and are increasingly becoming the basic transmission method for feed systems in modern high-speed machining machine tools. However, ball screw drive systems have poor dynamic characteristics and severe thermal deformation at high speeds, making them unsuitable for direct use.
Methods for achieving high-speed operation in ball screw drive systems:
Increasing system stiffness; increasing the lead and thread pitch of the ball screw nut; forced cooling to reduce thermal deformation; adopting new nut structures; manufacturing balls using new materials such as ceramics; controlling the preload of the nut; adopting a motion scheme where the nut rotates while the ball screw remains stationary; adopting a dual motor drive structure; and using linear motor drives.
Numerical Control Systems for Machining Centers
Factors affecting high-speed machining accuracy: Position lag error; lag error caused by acceleration and deceleration; interpolation cycle size; contour error.
Technical requirements for high speed machining centers: High speed processing of program segments; rapid and accurate processing of control information flow to minimize machining errors; sufficient capacity or ability to transmit large amounts of data via a network; high-speed spindle motors, feed servo motors, and sensors; high reliability and safety.