Wear of cutting tools occurs due to the high thermal and mechanical stress on the cutting wedge . Wear causes increasing cutting forces and geometrical deviations on the workpiece to be manufactured. This can concern the roughness , form errors or dimensional errors .
During machining, wear is caused by several wear mechanisms. These include mechanical abrasion ( abrasion ) due to friction between the chip and tool, adhesion (adhesive attachment of parts of the chip as pressure welding ), oxidation and diffusion . With the latter, desired alloying elements of the tool migrate into the chip and undesirable components of the chip into the tool at high temperatures.
The tool itself shows signs of wear and tear in various shapes and forms. The most important two are flank wear and crater wear . The first occurs on the flank of the tool, the second on the rake face . The wear mark width for the former and the scour depth for the latter is often used as a measure. They are used as service life criteria. If these criteria are exceeded, the tools must then be exchanged or reground.
In the literature, a distinction is often made between causes, forms and mechanisms of tool wear, but the terms are not always clear.
The causes of wear are the high thermal and mechanical loads on the tool. In some cases, chemical stresses are also included. They are each dependent on the material of the workpiece, the material of the tool ( cutting material ) and other parameters such as cutting speed , cutting force or the intervention parameters . (For the thermal loads, see energy conversion and heat during machining )
The mechanical stress results from the cutting force that acts on the tool and the resulting friction on the rake and flank face. Together with the high cutting speeds, this results in thermal loads. Heat is also generated inside the chips by shearing off as a result of internal friction. A large part of the heat is dissipated via the chip, but only about 5% to 20% via the tool. However, since the tool is constantly in contact with the chip, a large amount of heat is generated. The temperatures lead to thermal expansions and thermal stresses , which are superimposed on the mechanical stresses due to the cutting force. In processes in which individual cutting edges are not constantly in contact, such as milling , there is a changing thermal load. The temperatures can change by 600 ° C within a few milliseconds. The absolute values depend on the cutting values and the cutting materials: While high-speed steel loses its hardness at 600 ° C, other cutting materials can also be used at over 1000 ° C at high cutting speeds.
The wear is caused by various physico-chemical mechanisms. They are therefore sometimes also referred to as causes or processes. These include abrasion, adhesion, diffusion, oxidation ( scaling ) and various mechanical phenomena such as plastic deformation , surface disruption or cracks.
When adhesion is generally referred to sticking of particles due to the atomic bonds. Since the tendency to form bonds depends on the elements involved, the adhesive wear depends on the cutting material-material pairing used. The high temperatures and pressures result in pressure welds in the micro range between the rake face and the underside of the chip. If more and more material collects on the rake face, this is referred to as a built- up edge , as these accumulations now function as the actual cutting edge. If the built-up cutting edge or stuck particles are carried along by the running chip, small parts of the tool surface are torn out with it. The size of the adhesive wear and the built-up edge depend on the cutting speed. At first they grow with increasing speed and then they fall again. They rarely occur at high speeds. The surfaces of many bodies consist of a very thin oxide layer that forms as a result of what is known as passivation . However, these protective layers cannot form during machining. During the process, the surfaces of the chip and the tools correspond to the properties that normally occur in the interior of workpieces. They are therefore very reactive chemically, which promotes adhesion. Particularly soft and tough steel tends to adhere, especially if the grid of the tool corresponds to that of the material.
Under Abrasion is understood to mechanical abrasion by microscopic hard particles. It usually occurs together with other wear mechanisms. Three variants can be distinguished: micro plowing, micro chipping and micro breaking . The first two occur with ductile (soft, tough) cutting materials, the last with hard ones. In all variants, a hard particle slides on and at least partially in the surface of the tool.
- When micro plowing, the particle creates a furrow and pushes the material of the tool to the edges of the furrow through plastic deformation. With pure micro-plowing, there is no material removal; the stress can result in removal of subsequent particles.
- With microchipping, material from the tool surface is removed in the form of a chip. In the case of pure micro-chips, the volume of the chip corresponds to the volume of the separated material.
- When micro-breaking, the hard particle causes cracks on the surface of the tool, which spread and thus separate parts of the surface. When breaking, the wear particles are usually significantly larger than the wear groove.
Abrasion occurs at all cutting speeds. The hard particles can come from the material of the workpiece, such as oxides, carbides and nitrides. However, they can also be particles that have been separated from the tool surface by adhesion. Oxidation can also cause hard particles to form in the tool, which become detached during machining and lead to abrasion. In this sense, one also speaks of "self-wear", which occurs in particular on the open space.
When diffusion is a thermally activated mixing of the ingredients of the tool and workpiece. At high temperatures, individual atoms can leave their lattice position and penetrate into the respective partner. A distinction is made between the diffusion of atoms from the tool into the chip and the diffusion of atoms from the chip into the tool. The out-diffusion usually leads to only a small loss of material, but what is more important is that the tool changes its composition when it diffuses in and out, thereby losing its hardness and wear resistance. Diffusion occurs particularly with hard metals . High speed steel loses its hardness at temperatures of around 600 ° C, at which diffusion does not yet occur. Cutting ceramics, on the other hand, are only subject to very little diffusion wear. In the case of uncoated carbide tools, however, ideal conditions for diffusion exist at the pressures and temperatures typical for machining steel. Cobalt and tungsten, which give it its hardness, diffuse from the hard metal into the chip. Iron from the chip, on the other hand, diffuses into the cobalt binding phase of the hard metal. There the tungsten carbide also dissolves and forms mixed and double carbides in the form of Fe 3 W 3 C, (FeW) 6 and (FeW) 23 C 6 . To avoid diffusion wear, hard metal tools can be coated. Diffusion wear is particularly noticeable on the rake face as crater wear, since this is where the greatest pressures and temperatures prevail.
When using diamond tools to cut steel, the carbon of the diamond diffuses very quickly into the steel. A similar effect occurs with the use of silicon nitride - grinding wheels on, so that both materials are not suitable for the machining of steel.
Under oxidation refers to the chemical modification of the tool. Since the process is activated by friction, it is also called tribo-oxidation . It usually manifests itself in the form of scaling . Oxidation wear can increase or decrease wear. The latter is especially the case when the oxide layer is harder than the actual cutting material or when the layer protects against adhesion. Oxidation occurs in connection with the ambient air or the workpiece material. It does not occur with high-speed steel because it is already too soft at the temperatures required for machining. Ceramics, on the other hand, hardly oxidize even at high cutting speeds. Hard metals oxidize at temperatures between 700 ° C and 800 ° C. The scaling is particularly noticeable on the secondary flank and can lead to chipping of the cutting edges.
Surface distress, plastic deformation and cracks
Surface distress is a consequence of thermal and mechanical alternating loads. After a longer period of time, during which no wear can be measured, the surface distress becomes noticeable in the form of cracks that spread, as well as structural changes and fatigue that can lead to the separation of particles. The cutting materials lose their hardness at high temperatures, which can lead to plastic deformation. The cutting edge is plastically deformed, especially with new tools made of high-speed steel and hard metal and those that have been re-sharpened.
The main forms of wear (also phenomena) are the crater wear on the rake face and the flank wear . This also includes scaling, various breakouts and cracks.
The so-called scour forms on the rake face behind the main cutting edge. It is a depression that is created by the chip running off. The scour depth is also used as a wear criterion.
Breakouts are macroscopic defects in the tool. They can occur on the main or secondary cutting edge or on the rake face and flank face.
Cross cracks result from the changing mechanical stress, comb cracks, however, from the changing thermal stress.
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- Eberhard Paucksch: Zerspantechnik. 12th edition. Vieweg, 2008, ISBN 978-3-8348-0279-8 , p. 46.
- B. Denkena, HK Tönshoff: Machining - Basics. 3. Edition. Springer, 2011, p. 146.
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- Fritz Klocke, Wilfried König: Manufacturing process 1 - turning, milling, drilling. 8th edition. Springer, 2008, pp. 76, 80.
- B. Denkena, HK Tönshoff: Machining - Basics. 3. Edition. Springer, 2011, p. 145.
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- Eberhard Paucksch: Machining technology. 12th edition. Vieweg, 2008, ISBN 978-3-8348-0279-8 , p. 47.
- Fritz Klocke, Wilfried König: Manufacturing process 1 - turning, milling, drilling. 8th edition. Springer, 2008, p. 78.
- Fritz Klocke, Wilfried König: Manufacturing process 1 - turning, milling, drilling. 8th edition. Springer, 2008, pp. 76, 89-91.
- Fritz Klocke, Wilfried König: Manufacturing process 1 - turning, milling, drilling. 8th edition. Springer, 2008, pp. 78f., 85.
- Fritz Klocke, Wilfried König: Manufacturing process 1 - turning, milling, drilling. 8th edition. Springer, 2008, p. 91f.
- Fritz Klocke, Wilfried König: Manufacturing process 1 - turning, milling, drilling. 8th edition. Springer, 2008, p. 83.