Cutting material
When cutting materials those are materials designated, from which the cutting portion of a cutting tool with a geometrically defined cutting edge is. These include, in particular, turning tools , drills , milling tools , saws and broaching tools . Cutting materials have a significant impact on the profitability of machining processes. The development of cutting materials, which continues today, goes back to the middle of the 19th century and produced an abundance of different materials. There is a particularly suitable cutting material for almost every machining case.
The material responsible for chip removal on cutting tools with a geometrically undefined cutting edge (e.g. grinding wheels ) is called an abrasive .
Cutting materials are grouped together. Sorted according to increasing hardness, wear resistance, price and achievable cutting speed as well as decreasing breaking strength, these are:
- Tool steel (especially high speed steel (HSS))
- Hard metals
- Cutting ceramics
- Boron nitride
- diamond
history
Since the first carbon steels, cutting materials have undergone rapid development, which has led to a sharp increase in cutting speeds and feed rates , especially since 1970 with the introduction of coated hard metals . The values listed here always relate to the machining of steel under favorable conditions.
As early as 1850, the Englishman Robert Muchet developed a low-alloy steel with tungsten, manganese, silicon and chromium especially as a cutting material and thus found an alternative to the carbon steels used until then . While the average speed was with carbon steel in 1894 still at about 5 m / min, it was with that of Taylor and Maunsel White developed and the Bethlehem Steel Corporation at the World Exhibition in Paris in 1900 featured high-speed steel , HSS abbreviated, more than doubled. HSS spread very quickly and as early as 1901 a competition was held in Germany to determine the best German cutting steel. Another significant increase occurred in 1913 with improved high-speed steels to 30 m / min and in 1914 with cast hard alloys to 40 m / min. The new cutting materials very quickly pushed the limits of the machine tools available . Tests by the Ludwig Loewe company showed that their most stable machine tools, which were operated at the maximum cutting speed for HSS, failed after only four weeks with considerable damage. Another revolution was the hard metals introduced in 1926 and sintered with tungsten carbide in 1931 , which allowed speeds of around 200 m / min. A further doubling finally took place in 1955 with hard metals with a high titanium carbide content. Finally, in 1958, the first ceramics appeared on the broad market as a cutting material and made it possible to increase the speed to 500 m / min. Synthetic diamond was also used industrially as a cutting material for the first time at this time. With super-hard cutting materials based on boron nitride , hardened steel could now also be machined economically in 1965. Coated hard metals based on titanium carbide increased the service life of the tools considerably in 1970 , but shortly afterwards, in 1975, multi-coated hard metals came onto the market. The major disadvantage of cutting ceramics , brittleness, was counteracted in 1978 with improved cutting ceramics and silicon nitride. In 1979, super-hard composite and mixed cutting materials were also developed for hardened steels and the heaviest machining work. The last major milestone was the development of so-called fine-grain carbides, which have several advantages over conventional carbides, such as high toughness and high hardness at the same time.
Requirements and properties
Cutting materials are exposed to loads such as sudden cutting forces , high temperatures and temperature fluctuations as well as friction and wear . At the tool cutting edges of machine tools, almost all of the drive power introduced is converted into frictional heat ; only a negligibly small part is converted into solidification of the workpiece surface. That is why this heat must be dissipated well. Most of it is carried away with the chip , a small part remains in the workpiece or gets into the tool and must be kept at a permissible temperature level by cooling with a cooling lubricant or by dissipating the energy through the tool itself. Since the cutting speed is the decisive factor for the generation of heat, the desire of the industry to keep increasing it has brought the previously known cutting materials to their performance limits. One possibility of increasing the metal removal rate and also the surface quality is implemented with workpiece materials that have been optimized for machinability , such as free - cutting steel or leaded aluminum alloys .
So that the cutting materials can withstand the loads, they must have the following properties:
- Hardness / cutting ability : In order to be and remain good cutting ability (edge retention), a material must be much harder than the material to be cut.
- Wear resistance : In order to be wear-resistant, the cutting material must have sufficient resistance to the removal of cutting material particles when it comes into contact with the material. In addition to the hardness, the decisive influencing factor is the temperature at the cutting edge.
- Hot hardness: The cutting material must retain its hardness even under the high temperatures that occur during machining.
- High toughness and breaking strength : Cutting edge breaks and crack propagation under bending stress should be avoided as far as possible.
- Heat resistance : It is a measure of how well a cutting material retains its strength at high temperatures and thus withstands mechanical loads.
- Resistance to temperature changes: Serves to avoid cracking caused by material fatigue as a result of strong temperature fluctuations. This inevitably occurs when the cutting edges are only used briefly and alternately, as is the case with milling.
- Thermal shock resistance: This is understood to mean the property of withstanding sudden temperature changes without breaking edges. A low coefficient of thermal expansion and good thermal conductivity increase the resistance. This is important when milling or when there is insufficient cooling lubricant supply.
- Chemical stability: The cutting material should not form any connection with the surrounding materials. Especially compared to the chip, where, from a chemical point of view, diffusion or electrochemical wear can occur on the one hand due to the contact , but also the cooling lubricant and the air, which can cause oxidation and, due to the high temperatures, also scaling .
- Thermal conductivity : the resulting heat can be dissipated. This prevents high mechanical stresses in the tool due to thermal expansion and the associated cracks in the cutting material.
The requirements for the properties of the cutting materials are sometimes contradicting one another. For example, a cutting material with high toughness does not have high hardness. With increasing wear resistance, a cutting material is also more and more sensitive to impact loads. Therefore, the selection of the right cutting material always remains a compromise in which individual properties of materials have to be weighed against each other according to the specific machining conditions. This requires precise knowledge of the mode of operation of the respective type of machining.
Classification
Cutting materials are divided into three main groups: metallic cutting materials, composite cutting materials and ceramic cutting materials, each with several subgroups. With the exception of tool steels, the abbreviations used here are based on ISO 513.
Unalloyed and low-alloy tool steels
Unalloyed tool steels ( cold work steel ) are carbon steels with a carbon content between 0.45% and 1.5%. The C content influences the hardenability of the steel and is based on the various requirements placed on the tool, such as hardness or toughness. The working temperature for unalloyed tool steels is a maximum of 200 ° C. They are therefore only used for hand tools and wood saw blades.
Alloyed tool steels ( hot-work tool steel ) have a maximum working temperature of 400 ° C with a C content between 0.2% and 1.5%, depending on the content of alloy components . Their properties are also primarily adjusted via the carbon content, but the metallic additives also have a strong influence here. Due to their good edge retention and the low price, a wide variety of mostly hand-held cutting tools are made from them. The cutting speed for steel is around 15 m / min, so, like the unalloyed steels, they no longer play a role in industrial machining.
High speed steel
A high-speed steel (HS according to EN ISO 4957, workshop designation HSS) is a high-alloy tool steel that is very tough and insensitive to fluctuating forces. The working temperature can be up to 600 ° C. It is mainly used for tools that have to have high toughness, large rake angles , small wedge angles, high cutting edge strength and a sharp cutting edge, although the low possible cutting speed is insignificant. They are also suitable for individually adapted cutting edge geometries. Typical tools are drills , countersinks , broaching tools , reamers , profile and gear cutting tools or special milling cutters . In the meantime, they play a subordinate role in industrial production, but in some machining cases they will not be able to be replaced by other cutting materials in the foreseeable future.
In the 1980s and 1990s, due to the process reliability and the low price, coating in the PVD process with a hard material layer of 2 µm to 4 µm thickness made of titanium nitride or titanium carbide became popular , the process temperature being between 450 ° C and 500 ° C and thus a structural change remains small. The increased surface hardness and the lower surface roughness prevent the formation of built-up edges , i.e. the chip sticking, and thus contribute to the dimensional accuracy of the workpieces and the increase in the service life of the tool.
There is also the possibility of improving the properties of the steels through the powder metallurgical manufacturing process, sintering . Achievable grain sizes of less than 1 µm and a more uniform structure increase the edge strength and edge retention.
Cast hard alloys
Cast hard alloys are characterized by a base metal (cobalt, iron or nickel) and some carbide formers (chromium, molybdenum, vanadium or tungsten). They were launched in the USA in 1907 under the brand name Stellite . In contrast to high-speed steels, the proportion of carbide-forming alloying elements is significantly higher. Heat treatment of the cast and ground tools is usually not provided and often not possible. They have a high hot hardness, but are also very brittle. The spread is limited almost exclusively to the USA, because the machining work that can be carried out with stellite can be done just as well with high-speed steel or carbide.
hard metal
Hard metals are composite materials made by sintering . They consist of a soft metallic binder phase (mostly cobalt ) and the hard carbides titanium , tungsten , tantalum or titanium nitride embedded in it . The abbreviations are HW for mainly made of tungsten carbide, HT ( cermets ) for hard metals mainly made of titanium carbide and titanium nitride, and HC for coated variants. The hardness and toughness depend on the composition of the carbides with a size of 1–10 µm and the soft binder, which usually has a volume fraction of up to 20%, with more binder making the cutting edge softer and tougher. In addition to steel and cast iron , carbide cutting materials can also machine hard materials such as glass and porcelain . They are mostly used in the form of indexable inserts , but they are also available as tools made of solid carbide or as soldered carbide cutting inserts on tool bodies made of steel ( concrete drill bits ).
Solid carbide
Cutting tools are classified as solid carbide or VHM if they consist entirely of carbide, in contrast to coated tools or tools composed of different materials.
Tungsten carbide free and containing titanium carbide
Hard metals with tungsten carbide (DIN abbreviation 'HW', chemically WC-Co) have high wear resistance and are primarily characterized by their high toughness. They are divided into titanium carbide-free and titanium carbide-containing cutting materials. The former are based on tungsten carbide and are extremely tough, which means they can withstand high mechanical loads well. The hot hardness is somewhat lower than with the titanium carbide-containing variants, which contain up to 60% titanium carbide in addition to tungsten carbide. (DIN abbreviation also HW, chemical WC-TiC-Co ) This addition makes them suitable for machining steel. Furthermore, there is an increased resistance to oxidation at higher cutting temperatures.
Fine grain carbide
The grain size of the carbides also has a decisive influence on the properties and so fine grain carbide (HF) was developed on the basis of tungsten carbide and cobalt. Depending on the grain size, they are also called ultra-fine grain carbides. The grain size is in the range of 0.2–1 µm, which means that properties are achieved that are opposed to one another in normal hard metal. The hardness and flexural strength increase without changing the binding phase, which makes the cutting material particularly suitable for dynamically demanding machining, such as those that occur when cutting is interrupted. Tools made of fine grain carbide can also be used to machine materials that are difficult to machine, such as hardened steel.
Tungsten carbide-free hard metal (cermet)
Tungsten carbide-free hard metals are generally called cermets (HT), a made-up word from Cer amic and Met al. The mixed ceramics described below are also occasionally sold under the name cermet. The hard materials titanium carbide, titanium nitride with a volume fraction of more than 85% and, more rarely, niobium carbide and nickel or molybdenum and cobalt serve as the basis . Since rare raw materials such as tungsten, tantalum and cobalt are not used and instead titanium, which is available everywhere, can be used as a carbide former and nickel as a binder phase, cermets are expected to increase their market share. The advantages over hard metals based on tungsten carbide are furthermore, due to their higher hardness, less mechanical wear and less diffusion wear. They are therefore primarily suitable for the fine machining of all steel and cast steel materials.
Coated hard metal
By coating with hard materials , with which the majority of hard metal tools are now treated, the wear resistance can be increased with a tough base body at the same time. This is done by applying several hard material layers made of titanium carbide , titanium nitride , titanium carbonitride , aluminum oxide , titanium aluminum nitride , chromium nitride or zirconium carbonitride in the PVD , CVD or PACVD process. The CVD process, which is characterized by process temperatures between 850 ° C and 1000 ° C, is preferred. Almost exclusively multi-layer layers with a total thickness of up to 25 µm are applied. The advantage of coated as opposed to uncoated hard metal plates is the longer service life and the higher achievable cutting speed. Coatings also broaden the area of application of a carbide grade (grade adjustment). The coating has a disadvantageous effect on the cutting edge, the radius of which increases to 20–100 µm and thus loses its sharpness.
In addition, there are a number of new developments such as metal-containing molybdenum disulfide coatings, CVD diamond layers and amorphous carbon layers for machining superabrasive materials such as graphite , green ceramic and hard metal parts , fiber-reinforced plastics or metal matrix composites . The aim is to achieve either an extremely hard, slippery or a soft, lubricating surface or a combination of these.
Cutting ceramics
Werner Osenberg carried out his first cutting tests with ceramics as a cutting material in 1938 at the Technical University of Dresden , choosing aluminum oxide . The Second World War prevented further development and so ceramics did not gain acceptance until after 1950.
The production of the mostly aluminum oxide-based cutting ceramics (abbreviation CA, for c eramic, a luminum) is carried out in a similar way to hard metals. The economic advantages of not using heavy metals such as tungsten or cobalt as well as the high wear resistance and hot hardness helped cutting ceramics to displace hard metals from some areas. However, due to the higher hardness, the cutting ceramic can also machine materials and perform fine machining in which hard metals fail and expensive diamonds would have to be used. The majority of cutting ceramics are not coated, but there is now a wide selection of ceramics with multi-layer TiCN-TiN coating (CC, ceramic coated). The resulting lower friction of the chip on the cutting surface reduces the thermal load and enables a higher cutting speed. Cutting ceramics are essentially divided into three groups: oxide ceramics , non-oxide ceramics and mixtures of different ceramics (mixed ceramics).
Oxide cutting ceramic consists of aluminum oxide (Al 2 O 3 ) and up to 15% zirconium oxide and has a high wear resistance and hardness up to 2000 ° C. It is sensitive to changing cutting forces and temperature changes and is used in very even cutting conditions without cooling. It is characterized by lower wear than pure aluminum oxide, since the fracture energy of growing cracks is scattered on the dispersed zirconium dioxide and partially absorbed by phase transformation. Furthermore, the zirconium dioxide particles act as a particle reinforcement by deflecting and branching cracks and thus also slows down the propagation of cracks.
The non-oxidic cutting ceramics are silicon nitride ceramics (CN) consisting of rod-shaped, completely isotropic silicon nitride crystals (Si 3 N 4 ) with significantly better strength values than oxide and mixed ceramics. Similar to oxide ceramics with zirconium dioxide, fracture toughness is increased through crack deflection and crack branching. It is relatively insensitive to temperature fluctuations, but has a tendency to diffusion and pressure welding wear when machining steel. It is also used to make tools from one piece, such as drills or milling cutters.
Mixed ceramics (CM) are sintered from aluminum oxide and hard materials such as titanium carbide, tungsten carbide or titanium nitride, which make them appear black or gray. It has a higher resistance to temperature changes and edge resistance and is suitable from finishing to rough turning of many materials up to approx. 62 HRC .
Whisker-reinforced cutting ceramics (CR) are ceramic composite materials reinforced with silicon whiskers based on aluminum oxide. They have high strength values and high thermal shock resistance, which means that smaller wedge angles can be achieved. They are used in the machining of cast materials and heat-resistant nickel alloys as well as in high performance cutting , since large chip chambers are required with HPC .
Diamond and boron nitride
The two materials are also summarized under the term super-hard cutting materials . This primarily refers to cutting materials that have a Knoop hardness of more than 50,000 N / mm² (50 GPa).
Monocrystalline diamond (DM or workshop designation MKD) has the greatest hardness of all materials and is mostly used for precision work. The extremely sharp cutting edges with a radius smaller than 1 µm allow surface roughness of less than R Z 0.02 µm. Since single crystals have direction-dependent strength values, the diamonds must be installed in accordance with the maximum cutting force direction. Diamond-tipped tools are well suited for non-ferrous metals and their alloys , fiber and fill-reinforced plastics, rubber, pre-sintered hard metals, glass and ceramics.
Polycrystalline diamond (DP or workshop designation PKD) as a cutting material made from a hard metal base onto which a thin metal layer and then a 0.5 to 1.5 mm thick layer of synthetic diamond powder is sintered. Due to the polycrystalline structure, DP is isotropic and its strength properties are therefore independent of direction. The cutting speed is almost half that of single crystals, but the feed speed can be increased tenfold. The use of DP increases the service life by a factor of 100 compared to monocrystalline diamond.
Polycrystalline cubic boron nitride (BN or workshop designation CBN or PKB) is mainly used for machining hard and abrasive ferrous materials with a hardness of up to 68 HRC, as, unlike the super-hard cutting materials DM and DP, it does not react with iron and has a heat resistance of up to 2000 ° C. BN is applied as a layer up to 1.5 mm thick by the so-called high pressure liquid phase sintering on hard metal plates or manufactured as a solid body. As a rule, titanium nitride or titanium carbide is used as the binding phase.
The cutting material diamond is also used as a coating, see section Coated carbide .
Monocrystalline diamond (2 carats) soldered onto a steel body
Polycrystalline diamond with hard metal body
Polycrystalline cubic boron nitride sintered onto a hard metal body
standardization
| License plate |
Cutting material group |
|---|---|
| H *, HM * - hard metal | |
| HW | forward Tungsten carbide |
| HT | Cermet ( TiC / TaC ) |
| HF | Fine grain carbide |
| HC | Coated HM. |
| C - cutting ceramics | |
| CA | Oxide ceramics |
| CM | Mixed ceramics |
| CN | (Silicon) nitride ceramics |
| CC | Coated ceramic |
| CR | Whisker Reinforced C. |
| D - diamond | |
| DM | Monocrystalline D. |
| DP | Polycrystalline D. |
| B - boron nitride | |
| BN | Polycrystalline B. |
| * optional, mostly HM | |
To help the user choose the right cutting material, high-speed steels are generally divided into four groups and hard metals, cermets, cutting ceramics and polycrystalline cubic boron nitride are divided into main machining groups and further into application groups.
The high-speed steels are divided into four groups according to the alloy components molybdenum and predominantly tungsten. The first group contains about 18%, the second about 12%, the third about 6% and the fourth about 2% tungsten. The molybdenum content is between 0 and 10%. According to EN ISO 4957, which replaced DIN 17350 in Germany , high-speed steels are marked with a prefixed HS and then the percentages of the alloy components are specified in the order tungsten-molybdenum-vanadium-cobalt. For example HS6-5-2-5, a high-speed steel for highly stressed twist drills, milling cutters or roughing tools.
According to ISO 513, hard cutting materials are not classified according to their chemical composition like HSS, but according to their area of application. The identification consists of five elements. The code letter indicates the cutting material group. It is followed by one of the main machining groups with different colors: P for long-chipping, M for long and short-chipping and K for short-chipping materials. The machining application group is appended to the main group in the form of a number. Two letters for the suitable material and the preferred machining process can follow. For example, the HW-P20N-M label stands for an uncoated hard metal of medium hardness and toughness suitable for the machining of non-ferrous metals using the milling process. Every tool manufacturer has the task of dividing his cutting materials into a group. The last two letters are often omitted. Instead, the manufacturer either provides the customer with detailed chip parameters or does not specify the cutting materials in more detail.
According to ISO 513, the two overview tables in this article section represent, on the one hand, the abbreviations for hard cutting materials and, on the other hand, the main groups and extracts from the most important application groups. The materials and work processes are only intended as guidelines and are not listed in full for reasons of space.
| application | Main group, ISIN color |
short character |
Materials | Machining form | |
|---|---|---|---|---|---|
|
1) ↑
↑ ↑ |
2) ↓
↓ ↓ |
P (blue) |
P01 | Steel (S), cast steel (GS) | Finishing |
| P10 | S, GS, long-chipping malleable cast iron | Turning (D), milling (F), tapping | |||
| P20 | Turning, milling | ||||
| P30 | S, GS with blowholes | Roughing | |||
| P40 | S, GS, free cutting steel | unfavorable processing cases | |||
| P50 | |||||
| M (yellow) |
M10 | S, GS, cast iron (GJ), manganese steel | Turn, high v c | ||
| M20 | S, GS, G, austenite. -S | Turning, milling | |||
| M30 | S, G, high temperature resistant S. | Finish roughing | |||
| M40 | Free cutting steel, non-ferrous metals (NEM), light metals | Turning, parting off | |||
| K (red) |
K01 | hard GJ, thermosets, Al Si alloys | D, F, peel turning, scraping | ||
| K10 | GJ (HB ≥ 220), hard steel, stone, ceramics | D, F, drilling, broaching, scraping, internal turning | |||
| K20 | GJ (HB ≤ 220), NEM | D, F, internal turning | |||
| K30 | Steel, GJ (HB <220) | D, F, slot milling | |||
| K40 | NEM, wood | with a large rake angle | |||
Changes in properties (according to the respective arrow directions):
1) Increase in: hardness or wear resistance of the cutting material, cutting speed, chip length
2) Increase in: toughness of the cutting material, feed rate, cutting edge load
CVD diamond tools are not dealt with in ISO 513. They are described in detail in VDI guideline 2841 CVD diamond tools - systematics, manufacture and characterization , which was published in August 2012.
literature
- Hans Berns (Ed.): Hard alloys and hard composite materials. Structure, properties, processing, application. Springer-Verlag Berlin, Heidelberg 1998, ISBN 3-540-62925-4 .
- Friemuth, Thomas: Manufacture of cutting tools. Düsseldorf: VDI Verlag 2002.
- DIN EN ISO 4957: 2001-02 tool steels.
- DIN ISO 513: 2005-11 Classification and use of hard cutting materials for metal cutting with geometrically determined cutting edges - designation of the main groups and application groups.
- Draft VDI 2841 sheet 1: 2008-09 CVD diamond tools - systematics, production and characterization.
Individual evidence
- ^ A b c Günter Spur: On the change in the industrial world through machine tools, a cultural-historical consideration of manufacturing technology , Carl Hanser Verlag, Munich Vienna 1991, ISBN 3-446-16242-9 , p. 347
- ↑ a b c d e f g h Werner Degner, Hans Lutze and Erhard Smejkal: Spanende Formung , Carl Hanser Verlag, 2002, ISBN 3-446-22138-7 , pp. 60–79
- ↑ a b Ulrich Fischer (Ed.): Fachkunde Metall , Verlag Europa-Lehrmittel, 53rd edition 1999, ISBN 3-8085-1153-2 , pp. 97-99
- ^ A. Herbert Fritz, Günter Schulze (ed.): Manufacturing technology , VDI-Verlag, 3rd edition, Düsseldorf 1995, ISBN 3-18-401394-4 , pp. 242-249
- ↑ a b Wolfgang Beitz, Karl-Heinz Küttner (both ed.): Dubbel - Taschenbuch für den Maschinenbau , Springer Verlag Berlin, Heidelberg, 18th edition 1995, ISBN 3-540-57650-9 , pp. 50–51
- ↑ a b c d e f g h i j Herbert Schönherr: Spanende Produktion , Oldenbourg Verlag, 2002, ISBN 3-486-25045-0 , pp. 26–41
- ↑ Engelbert Westkämper, Hans-Jürgen Warnecke: Introduction to manufacturing technology. Stuttgart, Leipzig, Wiesbaden: BG Teubner April 2001, 4th edition, ISBN 3-519-36323-2 , p. 135
- ↑ a b Heinz Tschätsch: Praxis der Zerspantechnik , Vieweg Verlag, Braunschweig, Wiesbaden, 6th edition October 2002, ISBN 3-528-34986-7 , pp. 307-314
- ↑ Wilfried König , Fritz Klocke: Manufacturing process 1 turning, milling, drilling, Springer Verlag, 1997, ISBN 3-540-63202-6 , p. 161
- ↑ a b Johannes Schneider: Schneidkeramik , Verlag modern industry, Landsberg am Lech 1995, ISBN 3-478-93141-X , p. 10
