High-speed cutting machining is a high technology that faces 21st century. It is characterized by high efficiency, high precision, and high surface quality. It has been gaining more and more attention in the automotive industry, aerospace, mold manufacturing and instrumentation industries. Widely used, and has achieved significant technical and economic benefits, is an important part of contemporary advanced manufacturing technology.
High-speed cutting technology features
High-speed cutting is the core technology for achieving high-efficiency manufacturing. The intensification of processes and the generalization of equipment make it highly productive. It can be said that high-speed machining is an indispensable technology that can significantly increase machining efficiency without increasing the number of equipment. Its technical characteristics are mainly manifested in the following aspects:
1) The cutting speed is high and it is generally considered that the speed is more than 5 to 10 times that of ordinary cutting:
2) The spindle speed of the machine tool is very high, and the spindle speed is generally 10,000 to 20000 r/min.
3) The feed rate is very high, usually 15 ~ 50m/min, up to 90m/min
4) The meaning of high-speed cutting is not the same for different cutting materials and tool materials used;
5) During the cutting process, the passing frequency of the cutting edge (Tooth Passing Freqnency) is close to the Dominant Natural Frequency of the “Machine Tool-Workpiece†system.
In 1992, Professor H. Schulz of Darmstadt Institute of Technology in Germany put forward the concept of High Speed ​​Manufacturing (HSM) and its scope, as shown in Figure 1. It is considered that the transition zone shown in the figure is a so-called high-speed cutting range for different cutting targets. This is also a cutting speed that is expected or expected by a technician related to the metal cutting process.
Compared with conventional machining, high-speed cutting significantly increases the cutting speed, resulting in increased friction between the workpiece and the rake face and an increase in the temperature of the chip and tool contact surface. At this point of contact, the high temperature brought by the friction can reach the melting point of the workpiece material, making the chip soft and even liquefaction, thus greatly reducing the resistance to the cutting tool, that is, reducing the cutting force, making the cutting lighter and the chips The production is more smooth. At the same time, 70% to 80% of the heat generated by the processing is concentrated on the chips, and the removal rate of the chips is very fast, so the heat transferred to the workpiece is greatly reduced, and the machining accuracy is improved. The advantages of high-speed cutting technology mainly lie in: improving production efficiency; improving machining accuracy and surface quality; and reducing cutting resistance.
Application of High Speed ​​Machining in Automotive Industry
For workpieces with a large amount of material to be removed, workpieces with complex structures or ultra-thin structures (such as engine blocks, cylinder heads, car cover molds, etc.) traditionally require long working hours and rapid design changes. The workpieces with short product cycles can show the advantages of high-speed cutting. The following describes the high-speed machining of automotive engine parts and automotive parts and covers.
1. High-speed machining of automotive engines and their accessories
FTL: The high-efficiency flexible production line (FTL) is composed of a high-speed machining center, which is characterized by its miniaturization, outstanding flexibility, and easy change of processing contents. Fig. 2 shows an example in which an engine company of the SAIC Group uses the production line to process workpieces such as engine blocks, cylinder heads, and filter seats.
In order to utilize the advantages of high-speed cutting technology and automatic tool change functions represented by turning machining centers and boring and milling machining centers, and to improve machining efficiency, the principle of a concentrated process should be applied to the processing of complex parts as much as possible. The multi-channel centralized processing is realized in the clip, which reduces the limits of different cutting technologies such as traditional car, milling, boring, and thread processing, and gives full play to the high-speed cutting capabilities of the equipment and tools. At the same time, a new multi-functional requirement has been put forward for the tool, which requires that a tool can complete the processing of different parts of the workpiece, reduce the number of tool changes, save time for tool change, reduce the number of tools and inventory, and facilitate management and reduce manufacturing. cost. More commonly used are multi-function turning tools, milling cutters, boring and milling cutters, milling cutters, drilling - milling thread - chamfering and other tools. The use of some targeted process strategies in mass production lines also requires the development of special tools, compound tools, or smart tools to increase machining efficiency and accuracy and reduce investment. In the high-speed cutting conditions, some special tools can reduce the processing time of the parts to less than 1/10 of the original, and the effect is very significant.
Figure 3 shows the high-speed cutting process designed by the author for the machining of the joint between the top end of the engine block and the main bearing block. The body material is gray cast iron, and the tool is a CBN non-reground composite tool. The spindle speed is 12000r/min, and the cutting allowance is 0.02mm. The two key positions in the figure are milled in one position. The important dimension A is guaranteed by the compound milling cutter itself. The process also effectively avoids chattering caused by the longer milling cutter bar of the main bearing block on the single milling, which greatly improves the cutting accuracy, cutting efficiency and surface quality.
FMS: Since the life cycle of products is continuously being shortened, the number of breeds will continue to increase. Under this circumstance, how to shorten the time for changing the product becomes a key issue; due to the change of product design, how flexible the processing equipment is to adapt to it (ie, flexibility) is a major issue. Then came the FMS, which was made of a highly flexible universal machining center. The “high-flex general-purpose machining centers†mentioned here are different from the machining centers under the general concept. They are developed specifically for mass production, and fully satisfy the conditions for integrating FMS into mass production, ie, high productivity. The province's area, easy evacuation, ease of installation and displacement, and superior continuous running performance are all high-speed, compact machining centers.
Mitsubishi Heavy Industries of Japan, in order to adapt to the mass production of the urgent need to use this high-speed machining center as the host, developed the so-called "shuttle FMS" (see Figure 4). The FMS consists of eight M-H5A CMMs and an unmanned overhead vehicle (AGV) with two pallets in front of the machine. The trucks are used to exchange pallets and reciprocate to pallet loading and unloading stations. Between machine tools. The operator only needs to perform the loading and unloading of the workpiece by operating the button in one position, and it is not necessary to go between the machine tools. The carrier truck waits in turn in front of the machine tool that is about to finish machining. After the standby bed is processed, the pallet is exchanged between the machine tool and the carrier, and then the finished product is returned to the loading and unloading station.
2. High-speed cutting of automotive parts and parts
High-speed milling technology has significant advantages in machining three-dimensional free-form surfaces and superhard materials.
Automotive parts mold: A type of car often requires thousands of molds. In order to meet the needs of new models as quickly as possible, the exterior and interior cover molds and the resin impact-proof molding molds must shorten the production cycle and reduce production costs. Therefore, high-speed machining is the preferred process for manufacturing such molds. After high-speed, low-cutting precision machining of the die, the work of the fitter to trim the die is greatly reduced, and the die manufacturing cycle can be shortened by 40%. For example, the safety door lock injection mold using high-speed milling, material hardness HRC54 die steel, the smallest tool diameter can be used (φ0.6mm, the maximum depth of cut up to 4.8mm, processing time is 3h, surface roughness Ra0 .4μm, no need for fitter process, greatly reducing the processing time.
Automotive covers: Most of automotive interior and exterior covers, instrument panels, etc. are manufactured in batches using injection molds. These injection molds are generally complex cavity and thin shell structures, and ordinary cutting processes often cannot satisfy the surface roughness and bending at the same time. The degree of precision required, for this purpose also need to apply appropriate manual finishing process. The use of high-speed cutting can bring many benefits: 1) Compared with traditional finishing, further high precision is achieved; 2) High-speed cutting can be carried out at a high speed with a relatively small amount of resection. Machining reduces the amount of work done manually and greatly improves the machining efficiency of the cover mold. 3) Due to the greatly increased cutting speed, the machining cycle is significantly shorter than in the past.
For example, car cover molds are mostly composed of various free-form surfaces. High-speed milling uses high speed, small depth of cut, and large feed methods, which can increase the precision of mold manufacturing, extend the service life of molds, and improve the quality of injection molded parts. At high feed rates, high-speed milling machines have high-precision positioning and high-precision interpolation, especially arc-accurate interpolation.
For another example, for the finishing of the punching die of the front enclosure, the quality of the profile should be ensured. Under normal circumstances, large area finish milling requires that milling cutters or blades are not changed during the milling process, and milling of the entire profile is completed at one time, which requires a relatively high accuracy and durability of the milling cutter. Using high-speed cutting technology features, using high cutting speed, micro-feeds and finishing processes of multiple cuttings, it is possible to achieve long-term avoidance of Yin short, to meet the requirements of free-form surface machining of the mold. In order to ensure easy feeding during the stamping process, the cutting knife pattern and the punching material should be fed in the same direction. Therefore, it is advisable to adopt a radial knife. In order to ensure surface quality, the cutting depth is set to 0.2mm, the cutting feed is 0.2mm/r, the cutting speed is 200m/min, the initial cutter radius is 20mm, and the cutter radius is 8mm. Milling cutter. As this kind of process that can only be carried out by artificial (advanced fitter) can be completely completed by high-speed cutting, and the overall machining efficiency of the mold is significantly improved.
High-speed cutting machine tools
If the tool diameter is determined, increasing the cutting line speed will undoubtedly require a higher spindle speed; in order to obtain a proper feed rate for each tooth of the tool through high speeds, high feed rates and accelerations are also necessary. All this requires the application of a new machine tool manufacturing technology, such as high-speed motor spindles, high-pitch ball screws and other high-pitch ball screws. At the same time, in order to remove the heat-producing chips as quickly as possible and to better lubricate the front and rear flank, the application of oil mist lubrication and cooling devices is also particularly important. Obviously, in order to meet the requirements of high-speed cutting technology, the first requirement is very high for the machine itself: High-speed cutting machine tools must have sufficiently high spindle speed, high dynamic performance, and high stiffness and vibration absorption performance.
For example, the HSMU series of 5-axis simultaneous high-speed machining tools developed by the Swiss Mikron Company adopts a new type of artificial marble bed material, and the integrally formed closed O-shaped structure lays a good foundation for the vibration absorption and rigidity of the machine tool. The large pitch ball screw is driven by a water-cooled servo motor, which ensures the stability of the machine at high feed rates. The on-board laser tool setting device of the standard configuration ensures the measurement accuracy of the tool size at high rotation speed, thus ensuring the machining accuracy. China's Zhejiang Wanxiang Group, Yantai Huofu Automobile Lock Co., Ltd. and Changchun FAW Fuao Automobile Manufacturing and auto parts manufacturing companies have introduced Mikron's high-speed machining centers and high-speed 5-axis machining centers.
In the past decade, the rapid development of the automotive industry in Shanghai and East China has provided a huge market demand for high-speed machining. SAIC's Volkswagen, General Motors, Huizhong, Shanghai Diesel, Toneo, Ebara and other companies are setting up high-speed cutting flexible production lines or high-speed milling, boring, and drilling machining units for the machining of automotive interior and exterior cover molds and automotive engines. The more commonly used cutting parameters are: spindle speed 8000 ~ 12000r/min, feed speed 40 ~ 80m/min, depth of cut 0.01 ~ 0.05mm: blank material for cast steel, forging material, gray cast iron or aluminum alloy; tool used There are high speed tool steels, hard alloys, CBN, PCD, titanium coated tools or ceramic tools. Therefore, the machine tool equipment and tool materials within the above parameters have a large market demand.