Aerospace parts cutting requirements for tool cutting
December 03, 2019
The aviation manufacturing field has always been one of the industries with high level of advanced technology, mainly because of the complex shape and structure of aerospace products, various materials and strict processing precision. The complexity of the manufacture of aerospace products is mainly reflected in: (1) usually with complex theoretical surface, crisscross rib structure, thin wall structure with small thickness, etc.; (2) high-strength aluminum alloy for parts and materials Titanium alloys, high-temperature alloys, stainless steels, composite materials, honeycomb structures, mostly difficult-to-machine materials; (3) Modern aircraft have long life and high reliability requirements, which makes the quality control requirements of parts surface more stringent; (4) Increasingly, the overall structural design is used, and the outer dimensions of the parts are also increasing. In order to meet the design performance and use requirements of aerospace products, parts manufacturing often uses a variety of processes, such as cutting, electro-physical processing, electrochemical processing, beam processing, precision casting and precision forging, among which the cutting process is still Currently the most widely used and widely used processing method in the field of aerospace manufacturing.
In the main bearing structure of modern aircraft and engines, the proportion of the overall structural components increases rapidly. These parts are usually cut with integral blanks (plates or forgings), and the weight of the finished parts is only 10% to 20% of the blank. The remaining 80% to 90% of the material becomes a chip. The beam, frame, ribs and siding of the aircraft body, as well as the compressor fan and the integral blade disc of the engine are the key parts of modern aircraft and aeroengine. The materials used are high-strength aluminum alloy, titanium alloy, superalloy, composite material, etc. Most of them are based on the overall structure, with complex structure, large material removal, high precision and surface quality requirements, and long processing cycle. The cutting process of these parts has an urgent need for efficient and accurate machining.
Machining has always been one of the main technical means of machining parts. Although with the advancement and development of science and technology, new processing methods have emerged and become more and more widely used, but cutting is still the most widely used and widely used. The processing method, the parts with higher requirements on the matching accuracy of size and shape, need to be completed by cutting processing methods, and there is no better processing method [1].
Cutting tools are a key factor in supporting and facilitating advances in cutting technology. In recent years, the wide application of high-speed and high-efficiency CNC machine tools has brought modern cutting technology to a new stage. The application of advanced and efficient tools is one of the basic preconditions for expensive CNC machine tools to fully utilize their high-efficiency machining capabilities.
The application basis of cutting tools Manufacturing technology has always been advancing along with the development of human civilization. In the late 18th century, the emergence of machine tools with moving tool holders and guide rails marked the beginning of the era of machining. With the invention and improvement of new smelting technology, internal combustion engine technology and electrical technology in the 19th century, as well as the emergence of H. Ford's large-scale production methods and Taylor's scientific management theory, the machinery manufacturing industry has entered the era of mass production. Machining is one of the most basic processing methods in modern manufacturing technology. It uses tools (or tools) to remove excess material from the object being processed, resulting in a surface with the desired shape, precision and surface quality. In a cutting process system consisting of machine tools, tools, workpieces and fixtures, the tool is an active factor that is subject to change and affects the machining state.
The selection and development of tool materials is the key to achieving the cutting process and promoting the advancement of cutting technology. The tools used by humans began with natural materials. After the development of the Stone Age, the Bronze Age, and the Iron Age, the cutting tools were prepared to be cut/cutting tools by stone, animal bone, and bronze, and developed into tool steel, high speed steel, and hard alloy. Advanced cutting tools made of ceramics and super-hard alloys have been developed from natural materials such as stone, wood and animal bones to different types of copper, iron, steel, various metal alloys and non-metal materials. A new situation in which metal materials, ceramic materials, and organic polymer materials are “three-legged” has been formed, and a plurality of solid materials obtained by combining two or more substances different in physical and chemical properties—a composite material is a modern material revolution. An important direction [2] also requires cutting technology in the component manufacturing process.
The quality of the tool depends on the material and structure of the tool. The cutting performance of the tool material must meet the following basic requirements:
(1) Hardness. The tool material must be higher than the hardness of the workpiece material. The normal temperature of the modern tool material is usually above HRC60.
(2) Strength and toughness. Higher strength can withstand greater cutting forces, and better toughness can withstand large impact loads and vibrations.
(3) Wear resistance. The tool material should have good resistance to wear and is a comprehensive reflection of the strength, hardness and microstructure of the tool material.
(4) Thermosetting. The tool maintains material hardness, strength, toughness and resistance to oxidation at elevated temperatures.
A variety of different tool materials can only maintain their cutting performance over a range of temperatures.
During the cutting process, the cutting edge of the tool must withstand high cutting temperature, high pressure and high strain rate. This requires the tool to have high high temperature hardness and wear resistance, as well as high strength and toughness. Layer technology and tool surface treatment techniques have been developed to meet these comprehensive performance requirements for tooling materials. The coated tool is physically or chemically coated with one or more layers of high temperature and wear resistant coating material on the tool base material, so that the tool has both a strong matrix and high hardness and wear resistance. s surface. The basic tool coating materials can be divided into single coating, multi-component coating, soft coating, etc. [4], the common coating material properties and application range are shown in Table 2. Single coatings have certain limitations in application, and multi-coating structures are widely used in modern coated tools. Multi-layer coatings can effectively improve the texture of the coating and improve the performance of the tool. Since the coating material has better resistance to sintering, abrasion and heat resistance than the substrate, it can be cut at a cutting speed exceeding the substrate. In addition, due to the small coefficient of friction of the coating, the life of the tool can be extended.
Application status of cutting tools in aerospace parts cutting The materials used in aerospace products mainly involve high-strength steel, aluminum alloy, titanium alloy, superalloy, composite materials and other types. The parts involved in the machining of the aircraft body mainly use aluminum alloy, titanium alloy, composite material, high-strength steel and other materials. The structure size is large and the size coordination parts are many; the parts involved in machining on the aero engine mainly use titanium alloy and high-temperature alloy. , stainless steel, composite materials, the processing accuracy requirements are higher.
From the processing form, the aircraft body structural parts are typically represented by wing beams, fuselage frames, ribs and siding. The parts are in the form of flat structures with large dimensions, with the fuselage and wing theoretical shape. These parts are mainly milling, and different types of milling tools are used in the gantry structure and five-axis CNC milling machine. The commonly used knives include disc milling cutters, end mills, ball end mills, etc. Commonly used are special milling cutters with rounded corners; aero-engine parts are typically represented by casings, integral blade discs, blades, shafts and discs, except for shafts and discs, which are suitable for turning, and other parts are swivel. Shaped structure, some parts need turning, most of the parts involved in the installation and airflow passages need to be milled on the CNC machine with five-coordinate linkage control and turntable structure. The machining process requires various forms and structures of tools, such as Round turning tools, internal turning tools, end mills, ball end milling cutters, etc.
In the process of machining parts, insert cutters, welded cutters and integral cutters have been widely used. In recent years, insert cutters and integral cutters have gradually become the main tool structures used in the field, and the application range of welded cutters has been gradually reduced. These tools are mainly from three sources: enterprise-made, domestic professional production plants and foreign tool suppliers. Among them, high-end tools are mainly based on the products of foreign tool manufacturers. The main problems of domestic tools are low manufacturing precision and surface treatment technology. There is a gap, the tool quality is not stable enough, and the accuracy and life of each batch of tools are sometimes inconsistent, which makes it difficult to consistently control the consistency of the machining accuracy of the parts at the production site.
High-speed machining has entered the practical stage, and aircraft structural parts are the main fields of application of high-speed machining, especially in the cutting of aluminum alloy structural parts and composite parts. At present, the cutting speed of aluminum alloy materials has reached 1500 ~ 5500m / min (the highest speed is 5000 ~ 7500m / min), cast iron finishing and semi-finishing speed is 500 ~ 1500m / min, fine milling gray cast iron up to 2000m / min The ordinary steel is 300-800 m/min, and the hardened steel (HRC 45-65) speed is 100-500 m/min [5]. The cutting speed of high-speed machining is 5 to 10 times that of conventional cutting. The safety, high temperature stability and dynamic balance of the tool have become the key to high-speed machining tools. At present, the high-speed milling equipment equipped with high-speed milling equipment on the aerospace parts cutting machine has reached 24000r/min. The high-speed cutting tools used are mainly foreign brands, and the cutting speed of aluminum alloy parts has reached more than 1300m/min. The tools used in high-speed milling are mainly cemented carbide cutters with two types of inserts and the whole structure. New super-hard materials (such as PCD, PCBN) are used less.
The machining of aerospace product parts usually requires the use of tools of different sizes, specifications and structures, and requires continuous, no-to-interference, and no interference between the surfaces of different tooling. In most cases, the application of cutting tools in the production site is mainly extensive, and the problems of the shape of the cutting edge on the machining accuracy, the error of the tooling of the rough finishing tool, and the accuracy of the tool installation adjustment are lack of detailed and in-depth analysis. And control, mainly through the tool setting, the tool setting on the machine and the nominal size value to obtain the diameter of the tool, the installation length, the size of the key parts (such as the corner of the tool tip). The critical dimensions between different tools have at least an error of more than 0.05 mm (especially for insert cutters), which causes residual, step and damage on the surface of the machined workpiece and has to be processed by subsequent manual dressing and polishing.
From the aspect of cutting parameters, due to the different technical level and supporting environment of each production site, the cutting parameters used in tool cutting are quite different. The selection and use of cutting parameters are mainly based on the experience of the craftsman or machine tool operator. The cutting speed of aluminum alloy material is between 200mm and 2000m/min, and the removal rate of material per unit time is 10~30kg/h; the cutting speed of titanium alloy material is between 9~50m/min, the removal rate of material per unit time Generally no more than 1 ~ 2kg / h. From the aspect of tool life, the production site is basically not strictly controlled, mainly because the operator determines the usable state of the tool by experience, which is one of the reasons for the unstable surface quality of the workpiece.
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