In the past ten years, due to the continuous advancement of cutting tool, drive, control and machine tool technologies, high-speed machining and high-efficiency machining, especially high-speed hard milling, have been widely used and promoted in the mold manufacturing industry. Many occasions have been replaced by high-speed hard milling. The comprehensive processing of the mold blanks under one clamping through high-speed hard milling not only greatly improves the processing accuracy and surface quality of the mold, greatly reduces the processing time, but also simplifies the production process, thereby significantly shortening the mold manufacturing The cycle reduces the production cost of the mold.
The continuous improvement of the working performance of the high-speed machining center is an important prerequisite for the mold manufacturing industry to efficiently and accurately process molds. In recent years, driven by drive technology, many different types of high-speed machining centers with innovative structure and excellent performance have emerged. Three-axis high-speed machining centers (such as the HSM700 high-speed machining center launched by Swiss Mikron company at the end of 1996) that have emerged in the middle and late 1990s have now been developed into five-axis high-speed machining centers. In terms of driving mode, it has developed from linear motion (X / Y / Z axis) servo motor and ball screw drive to the current linear motor drive. The rotary motion (A and C axis) uses a direct drive torque motor. Some companies have developed the machining center into a five-axis machining center that uses direct drive through linear motors and torque motors. Significantly improve the travel speed, dynamic performance and positioning accuracy of the machining center.
Structural characteristics and advantages of high-speed machining center
A common structural feature of high-speed machining centers for mold processing is the gantry frame structure, which enhances the rigidity of the machine tool and facilitates the full use of the space in the machining area. The material of the machine bed is mostly made of polymer concrete, because this material has better damping performance and lower thermal conductivity, so it is conducive to improving the processing accuracy of the mold.
At present, according to the configuration of the coordinate axis, the five-axis machining center can be basically divided into two structural types. One is that three linear axes (X / Y / Z) are used for tool movement and two additional rotary axes / (A and C /) are used for the rotation and swing of the workpiece. This type of high-speed machining center, such as RXP500DS / RXP800DS of German Röder company, GS1000 / 5-T of German Alzmetall company, HSM400U / HSM600U of Mikro of Switzerland and XSM400U / XSM600U called ultra-high-speed machining center, and Germany Hermle’s C30U / C40U / C50U etc. The other is a structure in which one swing axis (A) of the five coordinate axes is set on the spindle head. The fork-shaped spindle head realizes the swing of the spindle tool, and the swing spindle head can also be firmly clamped to make it Position at any position within the swing angle range. This type of machine tool is such as DMC75V linear / DMC105V linear of German Demag company, HPM1850U of Mikro and GAMMA605 / 1200 high-speed milling machine of Rolf Wisser of Germany. There are individual machine tools that have both the swing axis and the rotary axis set on the spindle head, such as the G996V / BSH / 5A high-speed milling center from Parat, Germany, and the five- or six-axis gantry milling machine from Edel, Germany.
The five-axis high-speed machining center is much higher in price than the three-axis machining center. According to the price comparison between the five-axis machining center of Demag DMC75V series and the three-axis machining center, the price of the five-axis machining center is about 50% higher than that of the three-axis machining center. Although the price of the five-axis high-speed machining center is high, this high-end machine tool is particularly suitable for processing molds with complex geometric shapes. The five-axis machining center can create the best process conditions for the processing of end mills through the additional rotation and swing of the workpiece or the spindle head when processing deep and steep cavities, and avoid the tool, the tool bar and the cavity wall Collisions occur to reduce the risk of jitter and tool breakage during tool processing, thereby helping to improve the surface quality of the mold, processing efficiency, and tool durability. When purchasing a machining center, whether a user chooses a three-axis machining center or a five-axis machining center should be determined according to the complexity and accuracy of the mold cavity geometry.
It can be seen from the continuous innovation of high-speed machining centers that making full use of the latest achievements in today’s technical field, especially the latest achievements of driving technology and control technology, is the key to continuously improving the high-speed performance, dynamic characteristics and machining accuracy of machining centers. .
High-speed electric spindle is the core component of high-speed machining center. In the processing of freeform surfaces and complex contours of molds, end mills with a smaller diameter of 2 to 12 mm are often used. However, when machining electrodes for copper or graphite materials for EDM, high cutting speeds are required. The spindle must have a very high speed. At present, the spindle speed of the machining center is mostly between 18000 ～ 42000r / min. The high-speed machining center XSM400U / XSM600U of Mikro in Switzerland has reached 54000r / min. For micro-milling of molds (milling cutter diameter is generally 0.1 ~ 2mm), higher speeds are required. For example, the five-axis high-precision milling machine of the German Kugler company has a maximum spindle speed of 160,000 r / min (using air bearings). Such a high speed can reach 150m / min when a 0.3mm diameter milling cutter is used to process the steel mold Cutting speed. At present, the German Fraunhofer Institute of Production Technology is developing a main shaft supported by air bearings with a rotational speed of 300,000 r / min.
When processing molds, high speeds are always used, and the heat generated by high speeds and the vibrations that may occur during cutting are important factors that affect the accuracy of mold processing. In order to ensure the stability of the high-speed electric spindle, the spindle is equipped with sensors for measuring temperature, displacement and vibration, so as to monitor the temperature rise, axial displacement and vibration of the motor, bearing and spindle. This provides correction data for the CNC system of the high-speed machining center to modify the spindle speed and feed speed to optimize the machining parameters. When the spindle produces axial displacement, it can be compensated by zero point correction or trajectory correction.
At present, most of the high-speed machining centers or milling machines used for mold processing still use servo motors and ball screws to drive linear coordinate axes, but some machining centers have used linear motors, such as RXP500DS / RXP800DS high-speed milling machines and Degema’s DMC75V linear high-speed machining center (its shaft acceleration reaches 2g and fast travel speed reaches 90m / min). Because this linear drive eliminates the transmission element that converts rotary motion into linear motion, it can significantly improve the dynamic performance, moving speed and machining accuracy of the shaft.
Machine tools driven by linear motors can significantly increase productivity. For example, when machining electrodes for EDM, the machining time is reduced by 50% compared with the use of traditional high-speed milling machines.
Linear motors can significantly improve the dynamic performance of high-speed machine tools. Since most of the molds are three-dimensional curved surfaces, the tool axis must be continuously braked and accelerated when the tool is processing the curved surface. Only with a high axis acceleration can a very high trajectory speed be ensured to track a given contour with a constant feed per tooth on a short trajectory path. If the curvature radius of the curved surface profile is smaller and the feed rate is higher, then the required axis acceleration is higher. Therefore, the axis acceleration of the machine tool greatly affects the machining accuracy of the mold and the durability of the tool.
In the high-speed machining center, the swing of the rotary table and the swing and rotation of the fork-shaped spindle head have been widely implemented by torque motors. The torque motor is a synchronous motor, and its rotor is directly fixed on the component to be driven, so there is no mechanical transmission element, and it is a direct drive device like a linear motor. The angular acceleration that the torque motor can achieve is 6 times higher than the traditional worm gear drive, and the acceleration can reach 3g when swinging the fork-shaped spindle head. The torque motor can achieve extremely high static and dynamic load rigidity, thereby improving the positioning accuracy and repeatability of the rotary axis and the swing axis.
At present, some high-speed machining centers of some manufacturers have used linear motors and torque motors to drive linear axes (X / Y / Z) and rotary swing axes (C and A), respectively. Such as Rsher’s RXP500DS / RXP800DS, Demag’s DMC75V linear and Edel’s CyPort five-axis gantry milling machine.
It should be mentioned that the combination of the directly driven linear axis and the directly driven rotary axis allows all the motion axes of the machine tool to have high dynamic performance and adjustment characteristics, so as to freely mold for high speed, high precision and high surface quality Surfaces provide the best conditions.
The CNC control system is an important part of the high-speed machining center, which largely determines the speed, accuracy and surface quality of the machine tool processing. Therefore, for high-speed machine tools for processing freeform surfaces of molds, the performance of CNC systems is of particular importance.
When machining high-precision freeform surfaces, the tool trajectory composed of micro-section straight lines and arcs results in huge part programs. These data streams need to be stored and processed by the machine tool control system. Therefore, the length of the program segment processing time determines the CNC control system An important indicator of work efficiency. At present, the processing time of the program segment of high-end CNC control system is generally up to 0.5ms (such as HEIDENHAIN’s iTNC530 CNC system), while the processing time of the program segment of individual CNC system has been shortened to 0.2 ~ 0.4 ms.
The modern CNC control system used in high-speed mold processing, in addition to the very short program processing time necessary to ensure high-speed feed speed, should also have Nurbs and spline interpolation functions, and can work with nanometer resolution. In order to obtain high processing accuracy and surface quality in the case of high-speed processing.
At present, high-end CNC systems can also be connected with CAD / CAM systems of different manufacturers, and data is transferred from the CAD / CAM system to the control system at a high speed via Ethernet. CAD / CAM is integrated into the control system, to a large extent, it can achieve good results in the processing of mold complex contours, and makes a very important contribution to shortening adjustment time and programming time.
In the five-axis high-speed machine tool quoted above, in addition to the Rsher company using its own developed CNC system, the other is mainly used Siemens 840D and HEIDENHAIN iTNC530 CNC system.
In the past decade, the great progress of drive technology and control system has promoted the continuous innovation of the machining center structure and the continuous improvement of performance. The application of electric spindle, linear motor, torque motor and fast numerical control system has played a decisive role in improving the high-speed, high dynamic and high machining accuracy of the machining center. Among the various structural innovations of mold processing machine tools, torque motors play a particularly important role. It is not only applied to the swing and swing drive of the rotary table, but also to the swing of the fork-shaped spindle head or the swing and swing drive of the spindle head, thereby forming various types of five-axis machining centers. The application of rotary and oscillating spindle heads provides technical support for the development of five-axis gantry high-speed precision milling machines for processing large molds.
In the future, further improvement of the spindle speed, dynamic performance and stroke speed will remain the focus of the development of high-speed machining centers. This will not only depend on the further development of drive technology and CNC technology, but also on the development of lightweight machine tool components and Development. It is expected that in the next 5 years, the axis acceleration of high-speed machining centers or high-speed milling machines is expected to reach 3 to 4g, and the rapid travel speed of the coordinate axis will reach 100 to 140m / min.