Since the beginning of the 21st century, with the globalization of manufacturing technology, the competition in the manufacturing industry has become increasingly fierce. In the cutting process system consisting of machine tools, tools, fixtures and workpieces, the tool is the most active factor. Therefore, as high-speed machining technology is widely used in production today, high-performance tools are getting more and more attention and largely replace traditional tools. Although high-performance tools are more expensive than traditional tools, even 10 times that of traditional tools, the use of high-performance tools can still effectively reduce production costs.

Tool materials, geometric parameters and their structure are the most important key technologies for the design and manufacture of high-performance tools, as shown in Figure 1. At present, advanced tools are developing rapidly, and various special high-performance tools are constantly being introduced. In terms of tool materials, ultra-fine-grain carbide tools and super-hard material tools have been widely used; in terms of coatings, multi-layer gradient composite coatings and high-strength heat-resistant nano-coatings have also been developed by leaps and bounds. It is applied in the fields of aerospace, automobile and ship; in terms of tool structure, it will develop towards indexable, multifunctional, special composite tool and modular direction.

The latest development of tool materials

In recent years, developed countries in the world have been devoting themselves to the development of advanced cutting tool materials that are compatible with high-speed, high-efficiency, and high-quality cutting. Tool material has a great influence on tool life, processing efficiency, processing quality and processing cost. When cutting tools, they have to withstand high pressure, high temperature, friction, impact and vibration. Therefore, the tool material must have the following basic properties: high hardness, that is, the hardness of the tool material must be higher than that of the material to be processed; high strength and toughness, the tool cutting Part of the material is subject to great cutting force and impact force during cutting, so the tool material must have sufficient strength and toughness; wear resistance and heat resistance are good. Generally speaking, the higher the hardness of the tool material, the higher the wear resistance. The better the performance, and at the same time the wear resistance and heat resistance of the tool are closely related; the better the thermal conductivity, the better the thermal conductivity, the temperature of the cutting part can be reduced, thereby reducing the tool wear; the manufacturability and economy are good.

(1) New high-speed steel.

High-speed steel (HSS) is a high-alloy tool steel added with alloying elements such as W, Mo, Cr, and V. Although there are many varieties of tool materials currently available, high-speed steel has excellent comprehensive properties in terms of strength, toughness, hot hardness, manufacturability, especially sharpness (the tool nose radius can reach 12~15μm), so It still occupies a large proportion in the cutting of some difficult-to-machine materials and the manufacture of complex tools (especially gear cutting tools, broaches and end mills, etc.).

(2) New type of fine-grained and ultra-fine-grained cemented carbide.

Cemented carbide is a powder metallurgical product made of high-hardness, refractory metal compounds (mainly WC, TiC, etc., also known as high-temperature carbides) micron-sized powder, which is sintered with metals such as cobalt or nickel as a binder. Cemented carbide is the most widely used cutting tool material in the current cutting field, and the cutting efficiency is about 5-10 times that of high-speed steel. The world's cemented carbide output has grown extremely fast, and new materials and new grades of cemented carbide tools continue to appear, and their proportion in all tools is increasing. However, its technology is poor, and its use in complex tools is still greatly restricted.

The development of fine grain (1～0.5μm) and ultrafine grain (less than 0.5μm) cemented carbide materials and solid cemented carbide tools greatly improves the bending strength of cemented carbide, which can replace high-speed steel for manufacturing The cutting speed and tool life of small-scale drills, end mills, taps and other large-scale general-purpose tools far exceed high-speed steel. The use of solid carbide tools can significantly improve the cutting efficiency of most applications where high-speed steel was originally used. In order to improve the toughness of the cemented carbide, the method of increasing the Co content is usually adopted. The resulting decrease in hardness can now be compensated by refining the grains, and the bending strength of the cemented carbide can be increased to 4.3GPa, which has reached the same level. Exceeds the bending strength of ordinary high-speed steel. Another advantage of fine-grained cemented carbide is that the cutting edge of the tool is sharp, which is especially suitable for high-speed cutting of sticky and tough materials.

(3) Super hard tool.

The so-called superhard tool materials refer to synthetic diamond and cubic boron nitride, as well as polydiamond and polycrystalline cubic boron nitride sintered with these powders and binders. Because superhard tools have better wear resistance than cemented carbide and can adapt to higher cutting speeds, they have become the main tool material for high-speed cutting. More importantly, they can meet the cutting needs of difficult-to-machine materials. Therefore, superhard tool materials have played an increasingly important role in the entire cutting process.

Diamond is an allotrope of carbon, divided into two types: natural diamond and synthetic diamond (PCD). PCD is converted from graphite under the action of high temperature, high pressure and catalyst. Diamond tools have extremely high hardness and wear resistance, sharp cutting edges and good thermal conductivity. At the same time, the affinity between PCD tools and non-ferrous and non-metallic materials is very small, and it is not easy to produce accumulation on the tip of the tool during processing. Crumbs. At present, PCD tools are mainly used in the following two aspects: a. Difficult to process non-ferrous metals and their alloys. For example, when using PCD tools to process silicon and aluminum alloys, the tool life can reach 50 to 200 times that of cemented carbide; b. Difficult to process non-ferrous metals For metal materials, PCD tools are very suitable for the processing of difficult-to-process non-metallic materials such as stone, hard carbon, carbon fiber reinforced plastics and man-made plates. Therefore, it can be said that diamond tools are the best tools for precision processing of non-ferrous metals and their alloys, ceramics, glass, wood and other non-metallic materials.

However, the thermal stability of diamond is low, and its hardness will be completely lost when the cutting temperature exceeds 700~800℃. In addition, carbon and iron in diamond have a strong affinity. Under high temperature and high pressure, iron atoms interact with carbon atoms, resulting in graphitization of diamonds, which makes the tool extremely prone to wear. Therefore, diamond tools are generally not used to process steel and other materials.

Following the first synthesis of cubic boron nitride in 1957 by GE in the United States, cubic boron nitride was polymerized on cemented carbide under high temperature and high pressure conditions to obtain cubic boron nitride (CBN) blades with a composite structure. The CBN knife has two types of polycrystalline sintered block and composite blades. It can process hardened steel and cast iron at higher cutting speeds. It can be used for grinding instead of turning. It can cut some high-temperature alloys at high speeds. It has high machining accuracy and relatively low surface roughness. Moreover, cubic boron nitride is also suitable for processing various hardened steel, Ni-based, Fe-based and other wear-resistant and corrosion-resistant thermal spraying (welding) materials, vanadium-titanium cast iron, chilled cast iron and other wear-resistant cast irons, titanium Alloy materials, etc.

(4) Ceramic materials.

Ceramic tools have high hardness, wear resistance and good high-temperature mechanical properties, have low affinity with metals, are not easy to bond with metals, and have good chemical stability. Therefore, ceramic tools can process superhard materials that are difficult or impossible to process with traditional tools. Ceramic knives have two types: Al2O3 base and Si3N4 base. Adding various carbides, nitrides, borides and oxides can improve its performance. It can also be toughened by particles, whiskers, phase changes, microcracks and several The synergy of the mechanism improves its fracture toughness.

At present, some domestic products such as whisker toughened ceramics and functionally gradient ceramics have reached the performance of similar foreign blades, and some are even better than foreign ones. The main raw materials used in ceramic tools, such as alumina and silicon oxide, are abundant in the earth's crust, which is also of great significance for saving precious metals. Ceramic tools are mainly used for high-speed machining of difficult-to-machine materials. Internationally, ceramic material tools have been regarded as one of the most promising tools to further improve productivity.

The latest development of tool coating

A coating tool formed by coating one or more layers of metal or non-metallic compound film (such as TiAlN, TiC, TiN, Al2O3, etc.) with high hardness and good wear resistance on a relatively soft tool substrate is a cutting tool A revolution in development. Compared with uncoated tools, coated tools have obvious advantages: significantly reduce the friction coefficient, improve the tribological performance and chip removal ability of the tool surface; significantly improve the wear resistance and impact toughness, and improve the cutting performance of the tool. Improve processing efficiency and tool life; improve the oxidation resistance of the tool surface, so that the tool can withstand higher cutting heat, which is beneficial to increase the cutting speed and processing efficiency, and expand the application range of dry cutting. In advanced manufacturing, more than 80% of cemented carbide tools and high-performance high-speed steel tools use surface coating technology, and more than 90% of cutting tools used on CNC machine tools are coated tools.

Since the advent of tool coating technology, it has played an increasingly important role in the improvement of tool technology and processing technology, and has become a symbol of modern tools. Coated tools are obtained by coating a thin layer of a refractory metal compound with high wear resistance on a hard alloy substrate or high-speed steel substrate with better toughness, which greatly changes the performance of the tool. Commonly used coating materials include TiC, TiN, Al2O3, etc. Among them, TiC has higher hardness than TiN and better wear resistance. For cemented carbide, chemical vapor deposition (CVD) is generally used, and the layering temperature is 1000°C; for high-speed steel tools, physical vapor deposition (PVD) is generally used, and the layering temperature is about 500°C.

With the increasing maturity and continuous development of the coating process, from the beginning of a single coating, it has entered a new stage of developing multiple, multi-layer, gradient, and nano-coatings. As far as the current development status of PVD technology is concerned, the coating film structure can be roughly divided into single coating, composite coating, gradient coating, multi-layer coating, nano-multi-layer coating, and nano-composite structure coating.

The composite coating is a structure composed of various coating films with different functions or characteristics. It is also called a composite coating structure film. The typical coating is the current hard coating plus soft coating. Each film has a different Features, so that the coating has a better overall performance.

Gradient coating refers to the gradual change of coating composition along the film growth direction. This change can be a change in the ratio of each element of the compound, such as the change in the content of Ti and Al in TiAlCN, or it can gradually transition from one compound to another Compounds such as CrN gradually transition to CBC carbon-based coatings.

The multi-layer coating is composed of multiple films with different properties, and the chemical composition of each film is basically constant. At present, there are two different film compositions in practical applications. Due to the differences in the processes used, the size of each film layer is not the same. It is usually composed of more than a dozen films, and the size of each film is larger than tens of nanometers. Representatives are AlN+TiN, TiAlN+TiN coatings and so on. Compared with single-layer coating, multi-layer coating can effectively improve the coating structure and inhibit the growth of coarse grain structure.

The structure of the nano multilayer coating is similar to that of the multilayer coating, except that the size of each layer of the film is on the order of nanometers, which can also be called an ultramicrostructure. Theoretical studies have confirmed that in the nano-modulation period (a few nanometers to tens of nanometers), compared with traditional single-layer films or ordinary multilayer films, such films have super-hardness and super-modulus effects, and their micro-hardness is expected to exceed 40GPa, and at a very high temperature, the film can still retain a very high hardness.

It is precisely because the coated tool has a surface layer with high hardness, good chemical stability and low friction coefficient, it is not easy to produce diffusion wear, and at the same time it has the toughness of the matrix, so the cutting force and cutting temperature are low, which can significantly improve the cutting of the tool. performance. Therefore, coated tools have become the mainstream of modern cutting tools. The proportion of indexable inserts used by western industrialized countries in coated tools has risen from 26% in the 1980s to 90% at present. About 80% of the tools are coated tools. The proportion of coated blades from Sandvik Coromant of Sweden and Kennametal of the United States has reached more than 85%; the proportion of carbide coated blades used on CNC machine tools in the United States is 80%; coatings for turning in Sweden and Germany The tools are on 70%. my country's coated tools started late, but progressed quickly, and their coating outlets spread all over the country. Many cities have their own coating centers and undertake external processing operations. my country has been conducting research on CVD coating technology since the early 1970s. In the mid-1980s, my country's CVD coating technology has entered the practical level, and its process level has reached the international level. In general, the level of domestic CVD coating technology is not much different from the international level. However, my country only began to study PVD coating technology in the early 1980s. At present, the PVD coating technology of foreign tools has developed to the fourth generation, while the domestic is still at the second generation level, and single-layer TiN coating is still the mainstay.

The latest development of tool structure

The current change in tool structure is developing towards indexable, multi-functional, special-purpose composite tool and modular direction, and the tool structure is constantly innovating [.

The end mill adopts the design of variable helix angle or the unequal design of the groove, which can reduce the vibration in precision cutting and improve the surface quality; the large rake angle design of the high-speed steel end mill significantly reduces the cutting force and improves Chip removal can improve the surface integrity in precision machining; the integration of cemented carbide tools significantly improves the rigidity of small-diameter tools, and even complex tools such as gears and threaded tools are made of solid cemented carbide; solid carbide The alloy end mill adopts the design of one edge in the end tooth over the center, which expands the function of the end mill, without pre-drilling, and can achieve direct downward cutting within a certain depth range.

The working condition of the drill bit is relatively poor, and the chip removal is the most concerned issue, so we have been trying to improve it. The group drill is more typical, but its sharpening is more complicated; the German Guehring company developed the RT drill bit, whose parabolic groove increases the core thickness and the groove area; the twist drill with the S-shaped drill point has a very Good centering can significantly reduce the axial force of drilling and improve chip removal and chip breaking.

The composite tool fades the boundaries of different cutting processes such as traditional turning, milling, boring, drilling and threading, and can perform multi-process centralized processing of complex parts in one clamping, so as to reduce the number of tool changes and save tool change time. It can also reduce the number of tools and inventory, which is conducive to management and reducing manufacturing costs. The more common compound tools include multi-function turning tools, milling cutters, boring and milling cutters, drilling-milling thread-chamfering and other multi-function tools. American Kennametal's multi-function turning tool can complete the work of the outer circle, end face and boring of the car. At CIMT2001, the car and drill exhibited by the German Gun-ther company can drill flat-bottomed holes, boring holes, car end faces and car outer circles in solid materials, which can shorten the working time by 40%. Emuge's thread milling cutter can complete the drilling, chamfering and thread milling processes in one pass. The Octacot multi-function milling cutter developed by Mitsubishi Corporation of Japan can be equipped with octagonal blades or round blades to complete various processing such as milling planes, grooves, steps, chamfers, contour processing and bevels on the machining center.

An important aspect of the development of indexable tools is the development of chip breaker geometry. Sandvik Coromant’s R, M and F new geometry series (steel roughing, semi-finishing and finishing use PR, PM and PF geometries; MR, MM and MF geometries when cutting stainless steel; KR, KM and KF geometries for cutting castings and non-ferrous metals) and Iscar's typical geometries design with the "Overlord Knife" are unique. . Most of these blades are three-dimensional curved geometries, with a wide chip breaking range and good adaptability.

Concluding remarks

With the globalization of manufacturing technology, competition in the manufacturing industry has become increasingly fierce. As a new common basic technology of advanced manufacturing technology, high-speed cutting has become an important development direction of modern cutting technology. Advanced cutting tools have played an increasingly important role in machining. The selection of reasonable cutting tool materials, coatings and geometric parameters will be the key to high-efficiency cutting. Therefore, the tool as the most active factor in the cutting process system has become a necessary condition for high-speed cutting.