High speed milling technology of the hottest die a

2022-07-23
  • Detail

Mold high-speed milling technology and its NC programming example application

high-speed machining can not only double the production efficiency, but also further improve the machining accuracy and surface quality of parts, and solve some high-efficiency machining problems of special materials that are difficult to be solved in conventional machining. Therefore, high-speed machining technology has attracted great attention in the world. This paper expounds the key technologies of high-speed machining from the aspects of machine tools, cutting tools, materials and cam NC programming. Finally, two examples of high-speed machining are given

I. preface

as the key tooling for the production of molded products, the design and production cycle of molds have increasingly become the decisive factor to determine the development cycle of new products. At present, the aerospace, automobile, machinery, mould, machine tool and other industries in industrialized countries benefit from this new technology first, which significantly improves the product quality of the above industries, significantly reduces the cost, and obtains the market competitive advantage. In the automotive industry, in the past, the development cycle of new models was generally 10 years, but now it is shortened to 2 ~ 3 years. The development cycle of new models of Ford, general motors, Toyota and other companies is only one and a half years, all of which benefit from the improvement of the modernization level of enterprise mold design and manufacturing means. High speed cutting technology has been gradually applied to machining cast iron and duralumin alloy, especially to machining stamping die, forging die, die-casting die and injection die for large panel parts. The purpose is to improve dimensional tolerance and surface consistency while reducing processing time and development time. At present, the high-speed machining technology in the world is mainly used in the automotive industry, mold industry, aerospace industry, especially in the field of machining complex surfaces, and in the machining field where the workpiece itself or tool system has high rigidity requirements, showing a powerful function. The research and application of domestic high-speed machining technology began in the 1990s, and it is also mainly used in mold, aviation, aerospace and automotive industries. However, the high-speed cutting CNC machine tools, high-speed cutting tools and cad/cam software are mainly imported

II. Key technology of high-speed machining application

NC High-speed machining, as the most important advanced manufacturing technology in mold manufacturing, is an advanced manufacturing technology with high efficiency, high quality and low consumption. A series of problems in conventional machining have been solved by the application of high-speed machining. Compared with the traditional cutting, the cutting speed and feed speed are improved by stages, and the cutting mechanism has also undergone fundamental changes. Compared with the traditional cutting, the cutting process has undergone an essential leap. The metal removal rate per unit power is increased by 30% ~ 40%, the cutting force is reduced by 30%, the cutting life of the tool is increased by 70%, the cutting heat left in the workpiece is greatly reduced, and the low-order cutting vibration is almost eliminated. With the increase of cutting speed, the removal rate of blank material per unit time increases, the cutting time decreases, and the machining efficiency improves, which shortens the manufacturing cycle of products and improves the market competitiveness of products. At the same time, the small amount of fast forward of high-speed machining reduces the cutting force, and the high-speed removal of chips reduces the cutting force and thermal stress deformation of the workpiece, and improves the possibility of machining poor rigidity and thin-walled parts. Due to the decrease of cutting force and the increase of rotating speed, the working frequency of the cutting system is far away from the low-order natural frequency of the machine tool, and the surface roughness of the workpiece is most sensitive to the low-order frequency, which reduces the surface roughness. In the process of machining high hardened steel parts (hrc45 ~ 65) of the die, high-speed cutting can replace the processes of electric machining and grinding and polishing, avoid the electrode manufacturing and time-consuming electric machining time, and greatly reduce the amount of grinding and polishing of fitters. High speed milling can be successfully completed for some thin-walled die workpieces that are increasingly needed in the market. Moreover, on the high-speed milling CNC machining center, the multi-step machining can be completed by clamping the die at one time. These advantages are very suitable in industries such as mold, which require fast capital turnover, urgent delivery time and fierce product competition

1. High speed machining

high speed machining and cutting system is mainly composed of high-speed machining center that can meet high-speed cutting, high-performance tool clamping system, high-speed cutting tools, safe and reliable high-speed cutting cam software system, etc. Therefore, high-speed machining is essentially a large system engineering. With the development of cutting tool technology, high-speed machining has been applied to the processing of alloy steel (hrc>30), and is widely used in the processing of stamping dies, injection molds and other parts in automotive and electronic components. The definition of high-speed machining depends on the type of workpiece material being machined. Figure 1 shows the cutting speed commonly used for different materials when high-speed machining is adopted. For example, the cutting speed used for high-speed machining of alloy steel is 500m/min, which is the conventional forward milling speed when machining aluminum alloy

with the expansion of the application scope of high-speed machining, the research on new tool materials, the improvement of tool design structure, the generation of new NC tool path strategy and the improvement of cutting conditions have also been improved. Moreover, the computer-aided simulation technology of cutting process has also appeared, which is of great significance to predict tool temperature, stress and prolong tool life. The application of casting, stamping die, hot pressing die and injection mold processing represents the expansion of the application range of high-speed cutting of cast iron, cast steel and alloy steel. As shown in Figure 1, most of the development time of the industry-leading countries in the manufacturing of stamping dies and casting molds is spent on machining and polishing processes. The machining and polishing of stamping die or casting die account for about 2/3 of the total processing cost, and high-speed milling can be just used to shorten the development cycle and reduce the processing cost

Figure 1 high speed milling manufacturing cycle and cutting speed of common materials

2 High speed milling machine tool

ultra high speed cutting technology is the development direction of cutting and the product of the development of the times. High speed machining technology is one of the main development directions of machining technology. With the development of basic technologies such as CNC technology, microelectronics technology, new materials and new structures, it has stepped to a higher level. However, the high-speed cutting technology itself also has some urgent problems to be solved, such as the cutting mechanism of high hardness materials, the damage of cutting tools in the process of load change, the establishment of high-speed cutting database, the development of monitoring technology and green manufacturing technology suitable for high-speed cutting. CNC machine tools, cutting tools and cad/cam software used in high-speed cutting have high technical content and high price, which makes high-speed cutting investment, which restricts the popularization and application of high-speed cutting technology to a certain extent. The high-efficiency application of high-speed cutting requires that the components in the machine tool system must be advanced, which is mainly reflected in the following aspects:

(1) the rigidity of the machine tool structure

requires the driver to provide high-speed feed (the fast forward speed is about 40m/min, and the 3D contour processing speed is 10m/min), which can provide acceleration and deceleration from 0.4m/s2 to 10m/s2

(2) the rigidity of the spindle and the tool handle

shall meet the rotating speed of 10000r/min to 50000r/min, and the axial clearance between the tool handle and the spindle shall not be greater than 0.0002 inch through the spindle compressed air or cooling system

(3) the control unit

requires 32 or 64 bit parallel processors with high data transmission rate and automatic acceleration and deceleration

(4) reliability and machining process

can improve the utilization of machine tools (6000h/y) and the reliability of unmanned operation. The process model is helpful to understand the relationship between cutting conditions and tool life

the representatives of common domestic and foreign high-speed machining centers are shown in Table 1. Compared with conventional CNC machine tools, its machine tool structure, processing speed and performance are more excellent. For example, dmc85 high-speed machining center in Germany adopts linear motor and electric spindle. Its spindle speed reaches 30000r/min, feed speed reaches 120m/min, and acceleration exceeds 1g (gravity acceleration). High speed machine tools require high-performance spindle unit and cooling system, high rigidity machine tool structure, safety device and monitoring system, as well as excellent static and dynamic characteristics. They have the characteristics of high technology content and great difficulty in machine tool manufacturing. At present, the performance of domestic high-speed machine tools still lags behind that of foreign countries

Table 1 high speed machining centers at home and abroad

3 Due to the influence of centrifugal force and vibration during high-speed machining, the tool handle and tool for high-speed machining require high geometric accuracy, repeated clamping and positioning accuracy, high stiffness and safety and reliability of high-speed dynamic balance. Due to the large centrifugal force and vibration during high-speed cutting, the traditional 7:24 taper tool holder system shows obvious lack of rigidity, low repeated positioning accuracy and unstable axial dimension during high-speed cutting. The expansion of the spindle causes the deviation of the mass center of the tool and clamping mechanism, which affects the dynamic balance ability of the tool. At present, HSK high-speed tool shanks and foreign popular expansion and contraction fastening tool shanks are widely used. The heating system of the thermal expansion and cold contraction fastening tool handle has good rigidity, but the tool interchangeability is poor. Only one tool with a connecting diameter can be installed on a tool handle. Since this kind of heating system is relatively expensive, HSK type tool handle system can be used in the initial stage. When the number of high-speed machine tools in the enterprise exceeds 3, it is appropriate to use the thermal expansion and cold contraction fastening tool handle

tool is one of the most active and important factors in high-speed machining. It directly affects machining efficiency, manufacturing cost and machining accuracy of products. In the process of high-speed machining, cutting tools have to bear loads such as high temperature, high pressure, friction, impact and vibration. Therefore, their basic properties such as hardness and wear resistance, strength and toughness, heat resistance, process performance and economy are one of the key factors to achieve high-speed machining. At the same time, for high-speed cutting of workpieces with different materials, attention should be paid to the matching between the cutting tools and the workpiece materials. Table 2 shows the adaptability of common high-speed cutting tools to different workpiece materials. The cutting tool technology of high-speed machining is developing rapidly, and it is widely used, such as diamond (PCD), cubic boron nitride (CBN), ceramic cutting tools, coated cemented carbide, (carbon) titanium nitride cemented carbide TiC (n), etc. At present, high-speed machine tools and tool materials are expensive, which is one of the important reasons that affect the popularity of high-speed machining in China. Among them, coated cemented carbide is most widely used in high-speed machining, which can be used for high-speed cutting of heat-resistant alloy, titanium alloy, high-temperature alloy, cast iron, pure steel, aluminum alloy and composite materials

Table 2 cutting adaptability of common high-speed tool materials

among the cutting tools for machining cast iron and alloy steel, cemented carbide is the most commonly used tool material. Cemented carbide tools have good wear resistance, but their hardness is lower than that of cubic boron nitride and ceramics. In order to improve the hardness and surface finish, cemented carbide tools are coated with hard coating materials, such as titanium nitride, titanium nitride aluminum and titanium carbonitride. The cemented carbide blade with diameter in the range of 10 ~ 40mm and titanium carbonitride coating can process materials with Rockwell hardness less than 42; The tool coated with titanium aluminum nitride can process materials with Rockwell hardness of 42 or higher. Different cutting tool materials and coating materials can be selected according to the use requirements. Table 3 shows the cutting parameters of aluminum alloy materials processed by cemented carbide tools

the cutting tools and coating materials used in high-speed cutting can be divided into: cubic machining of cast ironrdquo; Boron nitride and silicon nitride cutting tools with Rockwell hardness of 42

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