Over the past decade or so, thanks to continuous advancements in cutting tools, drives, controls, and machine tools, high-speed and high-efficiency machining, especially high-speed hard milling, has been widely applied and promoted in the mold manufacturing industry. Traditional electrical discharge machining (EDM) has been largely replaced by high-speed hard milling in many applications. High-speed hard milling allows for comprehensive machining of mold blanks in a single setup, significantly improving machining accuracy and surface quality, drastically reducing machining time, and simplifying the production process. This, in turn, significantly shortens the mold manufacturing cycle and reduces production costs. The continuously improving performance of high-speed machining centers is a crucial prerequisite for the efficient and high-precision machining of molds in the mold manufacturing industry. In recent years, driven by drive technology, numerous different types of high-speed machining centers with innovative structures and excellent performance have emerged. Three-axis high-speed machining centers, which appeared in the mid-to-late 1990s (such as the HSM700 high-speed machining center launched by Mikron in Switzerland at the end of 1996), have now evolved into five-axis high-speed machining centers. In terms of drive methods, the technology has evolved from servo motors and ball screw drives for linear motion (X/Y/Z axes) to linear motor drives, while rotary motion (A and C axes) utilizes direct-drive torque motors. Some companies have even developed five-axis machining centers entirely using direct drives through the use of linear and torque motors. This significantly improves the travel speed, dynamic performance, and positioning accuracy of the machining centers. Structural Characteristics and Advantages of High-Speed ​​Machining Centers High-speed machining centers used for mold processing commonly employ a gantry frame structure to enhance machine tool rigidity and facilitate full utilization of the machining area. The machine bed is mostly made of polymer concrete, which, due to its good damping properties and low thermal conductivity, is beneficial for improving the machining accuracy of molds. Currently, based on the configuration of the coordinate axes, five-axis machining centers can be broadly classified into two structural types. One type uses three linear axes (X/Y/Z) for tool movement and two additional rotary axes (A and C) for workpiece rotation and oscillation. Examples of this type of high-speed machining center include the RXP500DS/RXP800DS from Röder (Germany), the GS1000/5-T from Alzmetall (Germany), the HSM400U/HSM600U and the XSM400U/XSM600U (considered ultra-high-speed machining centers) from Mikro (Switzerland), and the C30U/C40U/C50U from Hermle (Germany). Another type features a structure where one of the five axes (A) is oscillating on the spindle head. This oscillation of the spindle tool is achieved through a fork-shaped spindle head, which can also be firmly clamped to position it at any point within the oscillation angle range. Examples of this type of machine tool include the DMC75V linear/DMC105V linear from DMG MORI (Germany), the HPM1850U from Mikro, and the GAMMA605/1200 high-speed milling machine from Rolf Wisser (Germany). Some machine tools have both the oscillating and rotary axes located on the spindle head, such as the Parat G996V/BSH/5A high-speed milling center and the five-axis or six-axis gantry milling machine from Edel. Five-axis high-speed machining centers are significantly more expensive than three-axis machining centers. A price comparison of the DMG DMC75V series five-axis machining centers with three-axis machining centers shows that the five-axis is about 50% more expensive. Despite the higher price, these high-end machine tools are particularly suitable for machining molds with complex geometries. When machining deep and steep cavities, five-axis machining centers can create optimal machining conditions for end mills through the additional rotation and oscillation of the workpiece or spindle head, avoiding collisions between the tool and tool holder and the cavity wall. This reduces tool vibration and the risk of tool breakage, thereby improving the surface quality of the mold, machining efficiency, and tool durability. When purchasing machining centers, users should decide whether to choose a three-axis or five-axis machining center based on the complexity and precision requirements of the mold cavity geometry. The continuous innovation in high-speed machining centers demonstrates that fully utilizing the latest advancements in technology, particularly in drive and control technologies, is key to continuously improving the high-speed performance, dynamic characteristics, and machining accuracy of machining centers. Electric Spindle The high-speed electric spindle is the core component of a high-speed machining center. In machining free-form surfaces and complex contours of molds, end mills with smaller diameters of 2–12 mm are often used. However, when machining electrodes for EDM of copper or graphite materials, very high cutting speeds are required; therefore, the electric spindle must have a very high rotational speed. Currently, the spindle speed of most machining centers is between 18,000 and 42,000 r/min, with the Swiss Mikro XSM400U/XSM600U high-speed machining centers reaching a spindle speed of 54,000 r/min. For micro-milling of molds (where the end mill diameter is generally 0.1–2 mm), even higher rotational speeds are needed. For example, the five-axis high-precision milling machine from the German company KUGler has a maximum spindle speed of 160,000 r/min (using air bearings). This high speed allows for a cutting speed of 150 m/min when machining steel molds with a 0.3 mm diameter milling cutter. Currently, the Fraunhofer Institute for Production Technology in Germany is developing an air-bearing-supported spindle with a speed of 300,000 r/min. Machining molds always involves very high speeds, and the heat generated by these high speeds, as well as the vibrations that may occur during cutting, are significant factors affecting the machining accuracy of the molds. To ensure the stability of the high-speed electric spindle, sensors are installed on the spindle to measure temperature, displacement, and vibration, monitoring the temperature rise, axial displacement, and vibration of the motor, bearings, and spindle. This provides correction data to the CNC system of the high-speed machining center to modify the spindle speed and feed rate, optimizing the machining parameters. When the spindle experiences axial displacement, it can be compensated for through zero-point correction or trajectory correction. Linear Motors Currently, most high-speed machining centers or milling machines used for mold making still employ servo motors and ball screws to drive linear axes. However, some machining centers have adopted linear motors, such as the RXP500DS/RXP800DS high-speed milling machines from Röders in Germany and the DMC75V linear high-speed machining center from DMG MORI (with axis acceleration up to 2g and rapid travel speed up to 90m/min). Because this linear drive eliminates the need for transmission components to convert rotary motion into linear motion, it significantly improves the dynamic performance, travel speed, and machining accuracy of the axes. Machine tools driven by linear motors can significantly increase productivity. For example, when machining electrodes for electrical discharge machining, the machining time is reduced by 50% compared to using traditional high-speed milling machines. Linear motors can significantly improve the dynamic performance of high-speed machine tools. Since most molds are three-dimensional curved surfaces, the tool axis must constantly brake and accelerate when machining curved surfaces. Only with high axis acceleration can a constant feed per tooth be ensured to track a given contour on a short trajectory path at a high trajectory speed. The smaller the radius of curvature of the curved surface profile and the higher the feed rate, the higher the required axis acceleration. Therefore, the axis acceleration of a machine tool greatly affects the machining accuracy of the mold and the durability of the cutting tool. Torque Motor In high-speed machining centers, torque motors are widely used to achieve movements such as the oscillation of the rotary table and the oscillation and rotation of the fork-shaped spindle head. A torque motor is a synchronous motor whose rotor is directly fixed to the component to be driven, so there are no mechanical transmission elements; it is a direct drive device like a linear motor. The angular acceleration achievable by a torque motor is 6 times higher than that of traditional worm gear drives, and the acceleration can reach 3g when oscillating a fork-shaped spindle head. Because torque motors can achieve extremely high static and dynamic load rigidity, the positioning accuracy and repeatability of the rotary and oscillating axes are improved. Currently, some manufacturers' high-speed machining centers use linear motors and torque motors to drive linear axes (X/Y/Z) and rotary oscillating axes (C and A) respectively. Examples include Röder's RXP500DS/RXP800DS, DMG MORI's DMC75V linear, and Edel's CyPort five-axis gantry milling machines. It should be mentioned that the combination of directly driven linear axes and directly driven rotary axes gives all motion axes of the machine tool high dynamic performance and adjustability, thus providing optimal conditions for high-speed, high-precision, and high-surface-quality machining of free-form surfaces of molds. The CNC control system is a crucial component of high-speed machining centers, largely determining the machining speed, accuracy, and surface quality. Therefore, for high-speed machine tools machining free-form surfaces of molds, the performance of the CNC system is particularly important. When machining high-precision free-form surfaces, the tool path composed of micro-segments of straight lines and arcs creates a massive part program. These data streams need to be stored and processed by the machine tool control system; therefore, the processing time of program segments is a crucial indicator of the CNC control system's efficiency. Currently, the program processing time of high-end CNC control systems can generally reach 0.5ms (such as Heidenhain's iTNC530 CNC system), while the program processing time of some CNC systems has been shortened to 0.2-0.4ms. Modern CNC systems used in high-speed mold machining, in addition to having the very short program processing time necessary to ensure high-speed feed rates, should also have NURBS and spline interpolation functions and be able to operate at nanometer resolution to achieve high machining accuracy and surface quality under high-speed machining conditions. Currently, high-end CNC systems can also connect with CAD/CAM systems from different manufacturers, with data transmitted from the CAD/CAM system to the control system at high speed via Ethernet. The integration of CAD/CAM into the control system greatly enhances the machining of complex mold contours and makes a significant contribution to reducing adjustment and programming time. Among the five-axis high-speed machine tools mentioned above, except for Röer, which uses its own developed CNC system, the others mainly use Siemens' 840D and Heidenhain's iTNC530 CNC systems. Conclusion Over the past decade, significant advancements in drive technology and control systems have driven continuous innovation in machining center structures and performance improvements. The application of electric spindles, linear motors, torque motors, and high-speed CNC systems has played a decisive role in enhancing the high speed, high dynamics, and high machining accuracy of machining centers. Among the various structural innovations in mold-making machine tools, torque motors have played a particularly important role. They are not only used for the rotation and oscillation drives of rotary tables but also for the oscillation of fork-shaped spindle heads or the oscillation and rotation drives of spindle heads, thus forming various types of five-axis machining centers. The application of rotary and oscillating spindle heads has also provided technical support for the development of five-axis gantry-type high-speed precision milling machines for machining large molds. In the future, further improving spindle speed, dynamic performance, and stroke speed will remain the focus of high-speed machining center development. This will not only rely on further developments in drive technology and CNC technology but also on the development of lightweight machine tool components and the development of parallel machine tools. It is foreseeable that in the next 5 years, the axis acceleration of high-speed machining centers or high-speed milling machines will reach 3-4g, and the rapid travel speed of the coordinate axes will reach 100-140m/min.