A good horse needs a good saddle, and advanced CNC processing equipment is used. If the tools used are wrong, it will be useless! The selection of suitable tool materials has a great influence on tool life, processing efficiency, processing quality and processing cost. This article provides dry goods about knife knowledge, bookmark and forward, let's learn together.
01
Tool materials should have basic properties
The choice of tool material has a great influence on tool life, processing efficiency, processing quality and processing cost. When the tool is cutting, it must bear the effects of high pressure, high temperature, friction, shock and vibration. Therefore, the tool material should have the following basic properties:
(1) Hardness and wear resistance. The hardness of the tool material must be higher than that of the workpiece material, generally above 60HRC. The harder the tool material, the better the wear resistance.
(2) Strength and toughness. Tool materials should have high strength and toughness to withstand cutting forces, shocks and vibrations, and prevent brittle fracture and chipping of tools.
(3) Heat resistance. The heat resistance of the tool material is better, it can withstand high cutting temperature, and it has good oxidation resistance.
(4) Process performance and economy. Tool materials should have good forging performance, heat treatment performance, welding performance, grinding performance, etc., and should pursue high performance-price ratio.
02
Types, properties, characteristics and applications of cutting tool materials
1. Diamond tool material
Diamond is an allotrope of carbon, the hardest material ever found in nature. Diamond tools have high hardness, high wear resistance and high thermal conductivity, and are widely used in the processing of non-ferrous metals and non-metallic materials. Especially in the high-speed cutting of aluminum and silicon-aluminum alloys, diamond tools are the main types of cutting tools that are difficult to replace. Diamond tools that can achieve high efficiency, high stability, and long-life machining are indispensable and important tools in modern CNC machining.
⑴ Types of diamond tools
① Natural diamond tool: Natural diamond has been used as a cutting tool for hundreds of years. The natural single crystal diamond tool has been finely ground, and the cutting edge can be ground extremely sharp. The cutting edge radius can reach 0.002μm, which can realize ultra-thin cutting and can It is a recognized, ideal and irreplaceable ultra-precision machining tool for processing extremely high workpiece precision and extremely low surface roughness.
② PCD diamond tool: Natural diamond is expensive, and polycrystalline diamond (PCD) is widely used in cutting. Since the early 1970s, polycrystalline diamond (Polycrystauine diamond, PCD for short) was developed After success, natural diamond tools have been replaced by artificial polycrystalline diamonds in many occasions. PCD raw materials are rich in sources, and its price is only a few tenths to one tenth of natural diamonds. PCD tools cannot be ground into extremely sharp The surface quality of the processed workpiece is not as good as that of natural diamond, and it is not convenient to manufacture PCD blades with chip breakers in the industry. Therefore, PCD can only be used for fine cutting of non-ferrous metals and non-metals, and it is difficult to achieve super Precision mirror cutting.
③ CVD diamond tools: From the late 1970s to the early 1980s, CVD diamond technology appeared in Japan. CVD diamond refers to the synthesis of diamond film on heterogeneous substrates (such as cemented carbide, ceramics, etc.) by chemical vapor deposition (CVD). CVD diamond has exactly the same structure and characteristics as natural diamond. The performance of CVD diamond is very close to that of natural diamond, and it has the advantages of natural single crystal diamond and polycrystalline diamond (PCD), and overcomes their shortcomings to a certain extent.
⑵ Performance characteristics of diamond tools
① Extremely high hardness and wear resistance: Natural diamond is the hardest substance found in nature. Diamond has extremely high wear resistance. When processing high-hardness materials, the life of diamond tools is 10 to 100 times that of cemented carbide tools, or even hundreds of times.
② Very low coefficient of friction: the coefficient of friction between diamond and some non-ferrous metals is lower than that of other tools, the coefficient of friction is low, the deformation during processing is small, and the cutting force can be reduced.
③ The cutting edge is very sharp: the cutting edge of the diamond tool can be sharpened, and the natural single crystal diamond tool can be as high as 0.002-0.008μm, which can perform ultra-thin cutting and ultra-precision machining.
④ High thermal conductivity: diamond has high thermal conductivity and thermal diffusivity, the cutting heat is easy to dissipate, and the temperature of the cutting part of the tool is low.
⑤ Low thermal expansion coefficient: The thermal expansion coefficient of diamond is several times smaller than that of cemented carbide, and the change in tool size caused by cutting heat is very small, which is especially important for precision and ultra-precision machining that requires high dimensional accuracy.
⑶ Application of diamond tools
Diamond tools are mostly used for fine cutting and boring of non-ferrous metals and non-metallic materials at high speed. It is suitable for processing various wear-resistant non-metals, such as FRP powder metallurgy blanks, ceramic materials, etc.; various wear-resistant non-ferrous metals, such as various silicon-aluminum alloys; various non-ferrous metal finishing processing.
The disadvantage of diamond tools is that they have poor thermal stability. When the cutting temperature exceeds 700°C to 800°C, it will completely lose its hardness; in addition, it is not suitable for cutting ferrous metals, because diamond (carbon) is easy to bond with iron at high temperatures. The atomic action converts the carbon atoms into a graphite structure, and the tool is easily damaged.
2. Cubic boron nitride tool material
Cubic boron nitride (CBN), the second superhard material synthesized by a method similar to that of diamond, is second only to diamond in terms of hardness and thermal conductivity. It has excellent thermal stability and can be heated to 10,000°C in the atmosphere. Oxidation does not occur. CBN has extremely stable chemical properties for ferrous metals and can be widely used in the processing of steel products.
⑴ Types of cubic boron nitride cutting tools
Cubic boron nitride (CBN) is a substance that does not exist in nature. It can be divided into single crystal and polycrystalline, namely CBN single crystal and polycrystalline cubic boron nitride (PCBN). CBN is one of the isomers of boron nitride (BN), and its structure is similar to that of diamond.
PCBN (polycrystalline cubic boron nitride) is a polycrystalline material that sinters fine CBN materials through a bonding phase (TiC, TiN, Al, Ti, etc.) under high temperature and high pressure. Diamond tool material, it and diamond collectively referred to as superhard tool material. PCBN is mainly used to make knives or other tools.
PCBN tools can be divided into integral PCBN blades and PCBN composite blades sintered with cemented carbide.
PCBN composite inserts are made by sintering a layer of PCBN with a thickness of 0.5 to 1.0mm on a cemented carbide with good strength and toughness. Its performance has both good toughness and high hardness and wear resistance. The problems of low bending strength and welding difficulties of CBN inserts are solved.
⑵ Main properties and characteristics of cubic boron nitride
Although the hardness of cubic boron nitride is slightly inferior to diamond, it is much higher than other high hardness materials. The outstanding advantage of CBN is that its thermal stability is much higher than that of diamond, which can reach above 1200°C (700-800°C for diamond). reaction. The main performance characteristics of cubic boron nitride are as follows.
① High hardness and wear resistance: CBN crystal structure is similar to diamond, and has similar hardness and strength to diamond. PCBN is especially suitable for processing high-hardness materials that could only be ground before, and can obtain better surface quality of workpieces.
② High thermal stability: The heat resistance of CBN can reach 1400-1500°C, which is almost 1 times higher than that of diamond (700-800°C). PCBN tools can cut high-temperature alloys and hardened steels at a speed 3 to 5 times higher than that of cemented carbide tools.
③Excellent chemical stability: It does not have chemical interaction with iron-based materials at 1200-1300°C, and it will not wear out as sharply as diamond, and it can still maintain the hardness of cemented carbide at this time; PCBN tools are suitable for cutting hardened steel Parts and chilled cast iron, can be widely used in high-speed cutting of cast iron.
④ Good thermal conductivity: Although the thermal conductivity of CBN is not as good as that of diamond, the thermal conductivity of PCBN is second only to diamond among various tool materials, and is much higher than that of high-speed steel and cemented carbide.
⑤ Has a low coefficient of friction: a low coefficient of friction can reduce the cutting force during cutting, reduce the cutting temperature, and improve the quality of the processed surface.
⑶ Cubic boron nitride tool application
Cubic boron nitride is suitable for finishing various difficult-to-cut materials such as hardened steel, hard cast iron, superalloy, hard alloy, and surface spraying materials. The machining accuracy can reach IT5 (the hole is IT6), and the surface roughness can be as small as Ra1.25～0.20μm.
The cubic boron nitride tool material has poor toughness and bending strength. Therefore, cubic boron nitride turning tools are not suitable for rough machining with low speed and high impact load; Severe built-up edge will occur in the case of metal, which will deteriorate the machined surface.
3. Ceramic Knife Material
Ceramic knives have the characteristics of high hardness, good wear resistance, excellent heat resistance and chemical stability, and are not easy to bond with metal. Ceramic cutting tools occupy a very important position in CNC machining. Ceramic cutting tools have become one of the main cutting tools for high-speed cutting and processing of difficult-to-machine materials. Ceramic cutting tools are widely used in high-speed cutting, dry cutting, hard cutting and cutting of difficult-to-machine materials. Ceramic knives can efficiently process high-hard materials that traditional knives cannot process at all, and realize "replacing grinding with a car"; the optimal cutting speed of ceramic knives can be 2 to 10 times higher than that of cemented carbide knives, thus greatly improving the production efficiency of cutting processing The main raw material used in ceramic tool materials is the most abundant element in the earth's crust. Therefore, the popularization and application of ceramic tools is of great significance to improve productivity, reduce processing costs, and save strategic precious metals, and will also greatly promote the development of cutting technology. progress.
⑴ Types of ceramic tool materials
The types of ceramic tool materials can generally be divided into three categories: alumina-based ceramics, silicon nitride-based ceramics, and composite silicon nitride-alumina-based ceramics. Among them, alumina-based and silicon nitride-based ceramic tool materials are the most widely used. The performance of silicon nitride-based ceramics is superior to that of alumina-based ceramics.
⑵ Performance and characteristics of ceramic cutting tools
① High hardness and good wear resistance: Although the hardness of ceramic tools is not as high as that of PCD and PCBN, it is much higher than that of cemented carbide and high-speed steel tools, reaching 93-95HRA. Ceramic tools can process high-hardness materials that are difficult to process with traditional tools, and are suitable for high-speed cutting and hard cutting.
② High temperature resistance and good heat resistance: Ceramic tools can still cut at high temperatures above 1200 °C. Ceramic knives have good high-temperature mechanical properties, and the oxidation resistance of A12O3 ceramic knives is particularly good. Even if the cutting edge is in a red-hot state, it can be used continuously. Therefore, ceramic tools can achieve dry cutting, which can save cutting fluid.
③ Good chemical stability: Ceramic cutting tools are not easy to bond with metal, and are corrosion-resistant and chemically stable, which can reduce the bonding wear of cutting tools.
④ Low coefficient of friction: The affinity between ceramic tools and metal is small, and the coefficient of friction is low, which can reduce cutting force and cutting temperature.
⑶ Application of ceramic knives
Ceramic is one of the tool materials mainly used for high-speed finishing and semi-finishing. Ceramic cutting tools are suitable for cutting all kinds of cast iron (gray cast iron, ductile iron, malleable cast iron, chilled cast iron, high alloy wear-resistant cast iron) and steel (carbon structural steel, alloy structural steel, high strength steel, high manganese steel, quenched steel etc.), can also be used to cut copper alloys, graphite, engineering plastics and composite materials.
There are problems of low bending strength and poor impact toughness in the performance of ceramic cutting tool materials, which are not suitable for cutting under low speed and impact load.
4. Coated Tool Material
Coating the tool is one of the important ways to improve the performance of the tool. The emergence of coated cutting tools has made a major breakthrough in the cutting performance of cutting tools. The coated tool is coated with one or more layers of refractory compound with good wear resistance on the tougher tool body, which combines the tool substrate with the hard coating, so that the performance of the tool is greatly improved. Coated cutting tools can improve processing efficiency, improve processing accuracy, prolong tool life and reduce processing costs.
About 80% of the cutting tools used in new CNC machine tools use coated tools. Coated cutting tools will be the most important tool varieties in the field of CNC machining in the future.
⑴ Types of coated tools
According to different coating methods, coated tools can be divided into chemical vapor deposition (CVD) coated tools and physical vapor deposition (PVD) coated tools. Coated carbide tools generally use chemical vapor deposition, and the deposition temperature is around 1000 °C. Coated high-speed steel tools generally use physical vapor deposition, and the deposition temperature is about 500 °C;
According to the different substrate materials of coated tools, coated tools can be divided into carbide coated tools, high-speed steel coated tools, and coated tools on ceramics and superhard materials (diamond and cubic boron nitride).
According to the nature of the coating material, coated tools can be divided into two categories, namely "hard" coated tools and 'soft' coated tools. The main goals pursued by "hard" coated tools are high hardness and wear resistance Its main advantages are high hardness and good wear resistance, typically TiC and TiN coatings. The goal pursued by "soft" coating tools is a low coefficient of friction, also known as self-lubricating tools, and its friction with the workpiece material The coefficient is very low, only about 0.1, which can reduce bonding, reduce friction, reduce cutting force and cutting temperature.
Nanoeating tools have recently been developed. This coated tool can use different combinations of various coating materials (such as metal/metal, metal/ceramic, ceramic/ceramic, etc.) to meet different functional and performance requirements. A properly designed nano-coating can make the tool material have excellent anti-friction and anti-wear functions and self-lubricating properties, which is suitable for high-speed dry cutting.
⑵ Characteristics of coated tools
① Good mechanical and cutting performance: The coated tool combines the excellent properties of the base material and the coating material, which not only maintains the good toughness and high strength of the base, but also has the high hardness, high wear resistance and low wear resistance of the coating. coefficient of friction. Therefore, the cutting speed of the coated tool can be increased by more than 2 times than that of the uncoated tool, and a higher feed rate is allowed. Coated tool life is also increased.
② Strong versatility: Coated tools have wide versatility, and the processing range is significantly expanded. One coated tool can replace several non-coated tools.
③ Coating thickness: With the increase of coating thickness, the tool life will also increase, but when the coating thickness reaches saturation, the tool life will no longer increase significantly. When the coating is too thick, it is easy to cause peeling; when the coating is too thin, the wear resistance is poor.
④ Regrindability: Coated blades have poor regrindability, complex coating equipment, high process requirements, and long coating time.
⑤ Coating material: tools with different coating materials have different cutting performance. For example: when cutting at low speed, TiC coating has an advantage; when cutting at high speed, TiN is more suitable.
⑶ Application of coated tools
Coated cutting tools have great potential in the field of CNC machining, and will be the most important tool variety in the field of CNC machining in the future. Coating technology has been applied to end mills, reamers, drills, compound hole processing
Cutting tools, gear hobs, gear shaper cutters, gear shaving cutters, forming broaches and various indexable inserts for machine clamps meet the needs of high-speed cutting and processing of various steels, cast irons, heat-resistant alloys and non-ferrous metals.
5. Carbide Tool Material
Carbide cutting tools, especially indexable carbide cutting tools, are the leading products of CNC machining tools. Since the 1980s, various integral and indexable carbide cutting tools or blades have been expanded to various In the field of various cutting tools, indexable carbide tools have expanded from simple turning tools and face milling cutters to various precision, complex and forming tool fields.
⑴ Types of cemented carbide tools
According to the main chemical composition, cemented carbide can be divided into tungsten carbide-based cemented carbide and carbon (nitride) titanium (TiC(N))-based cemented carbide.
Tungsten carbide-based cemented carbide includes three types: tungsten-cobalt (YG), tungsten-cobalt-titanium (YT), and rare carbides (YW), each of which has its own advantages and disadvantages. The main components are tungsten carbide (WC), titanium carbide (TiC), tantalum carbide (TaC), niobium carbide (NbC), etc., and the commonly used metal binder phase is Co.
Carbon (nitride) titanium-based cemented carbide is a cemented carbide with TiC as the main component (some other carbides or nitrides are added), and the commonly used metal binder phases are Mo and Ni.
ISO (International Organization for Standardization) divides cemented carbide for cutting into three categories:
K category, including Kl0 ~ K40, is equivalent to my country's YG category (the main component is WC.Co).
P category, including P01~P50, is equivalent to my country's YT category (mainly composed of WC.TiC.Co).
M category, including M10~M40, is equivalent to my country's YW category (the main component is WC-TiC-TaC(NbC)-Co).
Each grade represents a series of alloys from high hardness to maximum toughness with numbers between 01 and 50.
⑵ Performance characteristics of cemented carbide cutting tools
① High hardness: Cemented carbide cutting tools are made of carbide with high hardness and melting point (called hard phase) and metal binder (called bonding phase) by powder metallurgy method, and its hardness reaches 89-93HRA , much higher than high-speed steel, at 5400C, the hardness can still reach 82-87HRA, which is the same as that of high-speed steel at room temperature (83-86HRA). The hardness value of cemented carbide varies with the nature, quantity, particle size and content of the metal bonding phase of the carbide, and generally decreases with the increase of the bonding metal phase content. When the binder phase content is the same, the hardness of YT alloys is higher than that of YG alloys, and the alloys added with TaC (NbC) have higher high temperature hardness.
② Bending strength and toughness: The bending strength of commonly used cemented carbide is in the range of 900-1500MPa. The higher the metal binder phase content, the higher the flexural strength. When the binder content is the same, the strength of YG type (WC-Co) alloy is higher than that of YT type (WC-TiC-Co) alloy, and the strength decreases with the increase of TiC content. Cemented carbide is a brittle material, and its impact toughness at room temperature is only 1/30 to 1/8 of that of high-speed steel.
⑶ Application of commonly used carbide cutting tools
YG alloys are mainly used for processing cast iron, non-ferrous metals and non-metallic materials. Fine-grained hard alloys (such as YG3X, YG6X) have higher hardness and wear resistance than medium-grained hard alloys when the cobalt content is the same, and are suitable for processing some special hard cast iron, austenitic stainless steel, heat-resistant alloys, Titanium alloy, hard bronze and wear-resistant insulating materials, etc.
The outstanding advantages of YT type cemented carbide are high hardness, good heat resistance, higher hardness and compressive strength at high temperature than YG type, and good oxidation resistance. Therefore, when the knife is required to have higher heat resistance and wear resistance, the grade with higher TiC content should be selected. YT alloys are suitable for processing plastic materials such as steel, but not suitable for processing titanium alloys and silicon-aluminum alloys.
YW alloy has the properties of YG and YT alloys, and has good comprehensive performance. It can be used not only for processing steel materials, but also for processing cast iron and non-ferrous metals. If the cobalt content is appropriately increased, the strength of this type of alloy can be very high, and it can be used for rough machining and intermittent cutting of various difficult-to-machine materials.
6. High Speed Steel Knives
High Speed Steel (HSS for short) is a high-alloy tool steel with more alloying elements such as W, Mo, Cr, and V added. High-speed steel cutting tools have excellent comprehensive performance in terms of strength, toughness, and manufacturability. In complex cutting tools, especially in the manufacture of hole processing tools, milling cutters, threading tools, broaches, gear cutting tools and other complex cutting tools, high-speed steel is still occupy a dominant position. High-speed steel knives are easy to sharpen cutting edges.
According to different uses, high-speed steel can be divided into general-purpose high-speed steel and high-performance high-speed steel.
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⑴ General-purpose high-speed steel cutting tools
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General purpose high speed steel. Generally, it can be divided into two types: tungsten steel and tungsten molybdenum steel. This type of high-speed steel contains additive (C) of 0.7% to 0.9%. According to the different tungsten content in steel, it can be divided into tungsten steel with 12% or 18% W, tungsten-molybdenum steel with 6% or 8% W, and molybdenum steel with 2% or no W. . General-purpose high-speed steel has a certain hardness (63-66HRC) and wear resistance, high strength and toughness, good plasticity and processing technology, so it is widely used in the manufacture of various complex tools.
① Tungsten steel: The typical grade of general-purpose high-speed steel tungsten steel is W18Cr4V, (W18 for short), which has good comprehensive performance. The high-temperature hardness at 6000C is 48.5HRC, and can be used to manufacture various complex tools. It has the advantages of good grindability and low decarburization sensitivity, but due to the high content of carbides, the distribution is relatively uneven, the particles are large, and the strength and toughness are not high.
② Tungsten-molybdenum steel: refers to a high-speed steel obtained by replacing part of the tungsten in the tungsten steel with molybdenum. The typical grade of tungsten-molybdenum steel is W6Mo5Cr4V2, (M2 for short). The carbide particles of M2 are fine and uniform, and its strength, toughness and high temperature plasticity are better than those of W18Cr4V. Another tungsten-molybdenum steel is W9Mo3Cr4V (W9 for short), its thermal stability is slightly higher than that of M2 steel, its bending strength and toughness are better than W6M05Cr4V2, and it has good machinability.
⑵ High-performance high-speed steel cutting tools
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High-performance high-speed steel refers to a new type of steel that adds some carbon content, vanadium content, and alloying elements such as Co and Al to the general-purpose high-speed steel composition, thereby improving its heat resistance and wear resistance. There are mainly the following categories:
① High carbon high speed steel. High-carbon high-speed steel (such as 95W18Cr4V), with high hardness at room temperature and high temperature, is suitable for manufacturing and processing ordinary steel and cast iron, drills, reamers, taps and milling cutters with high wear resistance requirements, or tools for processing harder materials. It is not suitable to withstand large impacts.
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② High vanadium high speed steel. Typical grades, such as W12Cr4V4Mo, (referred to as EV4), containing V increased to 3% to 5%, good wear resistance, suitable for cutting materials with great tool wear, such as fiber, hard rubber, plastic, etc., can also be used for processing Materials such as stainless steel, high-strength steel, and high-temperature alloys.
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③ Cobalt high speed steel. It is a cobalt-containing superhard high-speed steel, a typical grade, such as W2Mo9Cr4VCo8, (M42 for short), has a high hardness, and its hardness can reach 69-70HRC. It is suitable for processing high-strength heat-resistant steel, high-temperature alloys, titanium alloys, etc. Machining material, M42 has good grindability and is suitable for making precision and complex tools, but it is not suitable for working under impact cutting conditions.
④ Aluminum high speed steel. It belongs to aluminum-containing superhard high-speed steel, typical grades, such as W6Mo5Cr4V2Al, (abbreviated as 501), the high-temperature hardness reaches 54HRC at 6000C, and the cutting performance is equivalent to M42. It is suitable for manufacturing milling cutters, drills, reamers, gear cutters, and broaches. etc., used to process materials such as alloy steel, stainless steel, high-strength steel and superalloy.
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⑤ Nitrogen superhard high-speed steel. Typical grades, such as W12M03Cr4V3N, referred to as (V3N), are nitrogen-containing superhard high-speed steels. The hardness, strength, and toughness are equivalent to M42. processing.
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(3) Melting high-speed steel and powder metallurgy high-speed steel
According to different manufacturing processes, high-speed steel can be divided into melting high-speed steel and powder metallurgy high-speed steel.
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① Smelting high-speed steel: Both ordinary high-speed steel and high-performance high-speed steel are manufactured by smelting. They are made into knives through processes such as smelting, ingot casting and plating and rolling. The serious problem that is likely to occur in smelting high-speed steel is carbide segregation. Hard and brittle carbides are unevenly distributed in high-speed steel, and the grains are coarse (up to tens of microns). and adverse effects on cutting performance.
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② Powder metallurgy high-speed steel (PM HSS): Powder metallurgy high-speed steel (PM HSS) is molten steel smelted in a high-frequency induction furnace, atomized with high-pressure argon or pure nitrogen, and then quenched to obtain fine and uniform crystals Microstructure (high-speed steel powder), and then press the obtained powder into a knife blank under high temperature and high pressure, or first make a steel billet and then forge and roll it into a knife shape. Compared with the high-speed steel produced by melting method, PM HSS has the following advantages: the carbide grains are fine and uniform, and the strength, toughness and wear resistance are much improved compared with the high-speed steel produced by melting. In the field of complex CNC tools, PM HSS tools will further develop and play an important role. Typical grades, such as F15, FR71, GFl, GF2, GF3, PT1, PVN, etc., can be used to manufacture large-sized, heavy-duty, high-impact knives, and can also be used to manufacture precision knives.
03
Selection principles of CNC tool materials
At present, the widely used CNC cutting tool materials mainly include diamond cutting tools, cubic boron nitride cutting tools, ceramic cutting tools, coated cutting tools, carbide cutting tools and high-speed steel cutting tools.
