Machinability
The machinability is the property of a material by machining to have it processed. It represents one of the most important parameters of mechanical engineering materials. DIN 6583 defines machinability as "[...] the property of a workpiece or material that can be machined under given conditions". Correspondingly, castability , formability and weldability are understood to mean the properties of materials that can be processed by casting , forming or welding .
Machining, such as turning , milling and drilling , changes the shape of workpieces. Easily machinable materials show smooth surfaces after machining, produce chips that do not hinder the production process, the machining forces are low and the tool life is long . Often some, but not all of these criteria are easily achievable. Whether a material is easy to machine therefore also depends on the requirements. Because of the high use of steel and cast iron, the machinability of steel and the machinability of cast iron are also very important. For both, it depends on the exact type of material, the most common types (structural steel and cast iron with lamellar graphite) are considered to be easy to machine.
Influencing factors
The machinability of a material depends on many parameters. Usually strength and toughness are important factors. For example, materials with high strength are less easy to machine because higher cutting forces and thus higher energy are required for machining.
Other factors, such as the material's thermal conductivity, are also important. For this reason, materials with low thermal conductivity, such as plastics, are less easy to machine because the frictional heat generated cannot be dissipated quickly enough.
The quantitative assessment of machinability is difficult because it depends not only on the material but also on the machining conditions. This means the chip sizes and the intervention parameters cutting depth and width as well as the cutting speed and the cutting material (tool material). The use of cooling lubricants also has an influence. Furthermore, the different manufacturing processes also require different cutting conditions.
Quantification of machinability
Not only are the influencing variables of machinability complex, the quantifiability of machinability itself is not trivial either. There are several variables that can be used to describe the machinability quantitatively.
Tool life
The service life of a tool is the time that a tool can machine a material under given conditions before it has to be replaced. On a large scale, the tool life is important because it has an important meaning on the maintenance intervals of the machines and the consumption of tools and thus on the costs of machining.
However, the tool life is not an absolute measure of the machinability of a material because it depends not only on the material but also on the machining conditions (e.g. cutting speed) and the tool.
Tool wear
Tool wear is another assessment criterion for machinability. It has a direct influence on the service life. But the cutting force is also influenced by tool wear, since a worn, blunted tool requires a higher cutting force. The surface quality of the material also decreases with increasing tool wear. As a measure of tool wear which serves width of wear or scour depth .
Cutting force
The cutting force is important for the profitability of the cutting process, as it is directly related to the energy consumption. The drives of the machines can only provide a limited power, which is calculated as the product of the cutting force and the effective speed. The higher the forces that occur, the lower the cutting speed must be, which leads to longer machining times.
Surface finish
An important quality criterion of the finished workpiece is its surface quality . The usual roughness parameters are used as parameters for the surface quality.
Chip shape
The chip shape allows direct conclusions to be drawn about the machining process, which affects tool wear and the surface quality. A balance is desirable between short, compact chips that allow easy removal and long, even chips that allow a higher surface quality of the workpiece. If the chips are too long, there is a risk, for example when drilling, that the chips jam and block the chip evacuation, which leads to tool breakage or at least increased wear on the tool. Chips that spiral up are cheaper than those that fold in a leporello shape , as the latter involve a high risk of jamming.
Influence of the machining conditions
Cutting speed and feed
In principle, it is desirable to machine with the highest possible cutting speeds and large feeds. This enables cycle times to be minimized. However, poor machinability sometimes requires a drastic reduction in these speed parameters if excessively high speeds would result in unacceptably high tool wear and thus a short tool life and inadequate surface quality.
Cooling lubricant
All assessment criteria for machinability can be improved by using a cooling lubricant . The main tasks of the cooling lubricant are cooling and lubrication of the cutting process. The cooling means that the tool and workpiece do not overheat locally. This reduced cutting temperature leads to less wear. By enabling lower cutting forces, lubrication also leads to less wear and less energy consumption. In addition, the lubrication improves the surface roughness.
Machinability of certain materials
Ferrous materials
Iron materials are materials that mainly contain iron. A distinction is made between steel and cast iron.
Steels
Steel is the most frequently machined material. Together with cast iron , it is a ferrous material and is characterized by a carbon content of less than 2%, while cast iron contains more than 2%. Steels are very diverse materials. Their machinability depends primarily on the structure, which in turn depends on the exact carbon content and the condition of the heat treatment. Numerous alloying elements also play a role. Some are deliberately added to the alloy in order to improve machinability, others in order to increase properties such as strength, with a deterioration in machinability being accepted for better performance properties. Other elements such as phosphorus are actually undesirable, but improve machinability.
cast iron
Cast iron, together with steel, is one of the ferrous materials and is characterized by a carbon content of over 2%. Cast iron is used very often, can be cast very well but cannot be formed. The shaping is therefore mainly done by casting and subsequent fine machining by machining.
The machinability depends heavily on the structure and the precise formation of the graphite. Cast iron types with a high proportion of cementite are very difficult to work with. Other grades that contain ferrite or pearlite are considered to be easier to machine due to the embedded graphite, as the material is interrupted by the graphite and thus has a lower strength, which leads to lower cutting forces and easier chip breaking. Furthermore, the graphite develops a lubricating effect on the rake face and thus forms a protective layer so that the service life can be very long.
Non-ferrous metals
Aluminum and aluminum alloys
Aluminum and aluminum alloys are considered to be easy to machine. It is especially in the aviation and aerospace industry and in the automotive industry an important construction material, which is good for the lightweight suitable. Up to 90% of the raw parts are machined. However, low-strength grades can form long chips and tend to stick to the cutting edge. The cutting forces are generally low, the wear depends on the structure. Aluminum and its alloys are well suited for high speed machining . The temperatures that occur are only around 300 ° C, which is very little compared with temperatures that occur with steel, but relatively high compared with the melting point of aluminum alloys (580 ° C to 660 ° C). The cutting speed can vary within wide limits; downwards it is limited by the built- up edge formation and upwards by the melting temperature. Despite the low cutting forces, drives are required due to the high cutting speeds, which have to provide about five times the power than is necessary for machining steel. High-speed steels are used as cutting materials for simple machining such as drilling. Often the hard metal types are based on tungsten carbide. On the other hand, grades with titanium or tantalum are not suitable, as these elements enter into chemical reactions with aluminum. Coatings are therefore not suitable either. The cutting ceramics are also not chemically resistant and wear out very quickly. Diamond , on the other hand, is well suited for machining aluminum and is used because of its very long service life and high surface quality. This is particularly useful when working with mirrors. The wear is usually low, but some alloys contain hard, abrasive additives that increase wear but improve chip breaking. The wear is almost always on the open space; Crater wear only occurs with highly abrasive structural components at high cutting speeds.
The exact conditions depend heavily on the alloy.
- Soft materials such as the non-hardenable wrought alloys and those which can be hardened in the soft state tend to produce long chips and the formation of built-up edges. The surface quality is rather poor, it can be improved at high cutting speeds. If possible, such materials are machined after cold forming, as the work hardening leads to more favorable chip shapes and surfaces.
- Stronger materials such as hardened wrought alloys are usually easier to machine
- Cast alloys often contain silicon which has an abrasive effect. The higher the silicon content, the higher the tool wear. The chip shapes are good.
Since aluminum tends to stick, it is often machined with a large rake angle.
Titanium and titanium alloys
Titanium and titanium alloys are considered difficult to machine materials. Its strength is relatively high, the strength related to the mass is even higher than that of steel or aluminum, which is why it is well suited for lightweight construction . Applications is in aerospace and sports. It is biologically compatible and is therefore also suitable for implants.
The thermal conductivity of titanium is very low, which means that up to 80% of the heat has to be dissipated via the tool. With steel it is only about 20%. Titanium dust can be produced during dry machining. Since this is highly flammable (ignition temperature 33 ° C), titanium dust can explode. The machines are therefore equipped with carbon dioxide extinguishing systems. For wet cutting, cooling lubricant is used, which is based on oil at low cutting speeds. Lubricants containing phosphorus and chlorine are used, but the concentration of chlorine should only be 0.01% when machining highly stressed engine rotors, otherwise the surface is too poor. At higher cutting speeds, water-based cooling lubricants are used, which can dissipate the heat better. Titanium has a special chip formation, with sawtooth chips, which is similar to chip formation in high-speed machining . Hard metal is usually used as the cutting material. Ceramics react chemically with titanium and therefore wear out very quickly.
Magnesium and magnesium alloys
Magnesium and magnesium alloys are often used for lightweight construction because of their low density. They are mostly processed by die casting ; Sand casting or wrought alloys are therefore only of secondary importance. Magnesium is in a hexagonal lattice structure below 225 ° C, which has only two slip planes and is therefore brittle. Above this temperature there is a cubic lattice structure and the material becomes ductile.
Magnesium tends to form lamellar chips . The distance between the lamellas depends on the frequency with which the cutting force changes. It can be influenced by the material-cutting material pairing and the tribology of the interface, which is influenced by the feed rate and cutting speed. The dynamic loading of the tool can therefore be adapted to its load capacity by a suitable choice of the cutting values. The temperature is increased on the underside of the chip so that the chip lamellas are connected there by plasticization. This results in easy chip breaking and short chips.
The cutting edge geometry is similar to that used for machining aluminum. During fine machining, the stiffness of the workpiece can limit the cutting values.
Magnesium alloys contain only a few abrasive components, so the service life is long. This also applies to the edge zones of the workpieces, as they were mostly made by die casting. The adhesion, i.e. the tendency for the material to stick to the cutting edge, is low. Therefore built-up edges rarely occur. Since the melting temperature is around 420 ° C to 435 ° C, the temperature on the tool is only low.
High-speed steels, hard metals and diamond are used as cutting materials. Very fine grain carbides of group N10 / 20 or diamond coated carbides are often used. This enables high cutting speeds and feeds. In addition, these cutting materials are very wear-resistant, which leads to a high level of process reliability. Magnesium alloys tend to form pseudo chips . The cutting force is low and is roughly that of hypoeutectic aluminum alloys.
Copper and copper alloys
Copper and copper alloys are used in air conditioning, technical building equipment, food technology, chemical systems and apparatus as well as fittings . Copper alloys consist of at least 50% copper and are usually considered to be easy to machine. The most important alloying elements are tin ( bronze ), zinc ( brass ), aluminum ( aluminum bronze ), nickel and silicon. Special free-cutting alloys, like free- cutting steel, contain small amounts of lead, sulfur, selenium and tellurium, which above all promote chip breaking.
The copper alloys are usually divided into wrought alloys (for forming ) and cast alloys for casting. Within the two groups, subdivision is usually made according to the alloying elements. However, since groups with the same composition can differ greatly in terms of their machinability, this scheme is not suitable. Instead, a distinction is made between the following three groups:
- Pure copper and alloys with zinc, tin, nickel and aluminum as long as they only form a homogeneous mixed crystal. This mainly includes brass. These alloys are characterized by their high formability and can be cold formed. The machinability is considered to be rather poor.
- Alloys with zinc, tin, nickel, aluminum and silicon that form a second mixed crystal, but without chip-breaking additives. These alloys are harder and stronger, have less formability and are easier to machine. This group includes, in particular, German silver, which consists of copper-tin-zinc or copper-nickel-zinc.
- Free-cutting alloys containing additions of lead, sulfur, selenium and tellurium to improve chip breaking. They are very easy to machine.
Cast workpieces have a cast skin that is very difficult to machine. The core material, on the other hand, can usually be machined very well. Cold-formed wrought alloys have an increased strength which has a positive effect on chip breaking. Age-hardenable alloys are mostly machined in the soft state. Only the fine machining by grinding or polishing takes place in the hardened state.
At low temperatures and with continuous chip formation, built-up edges can occur which lead to increased wear. Because of the great hardness and the high formability of nickel silver, the service life is shorter than with brass, which also tends to adhesion and built-up edge formation. Carbides from group K10 / 20 are used as cutting materials for HSC milling. The types that tend to stick, such as pure copper, can be machined inexpensively with diamond as a cutting material, as this also enables high surface qualities to be achieved. Cutting ceramics, on the other hand, are unsuitable because they tend to stick.
The cutting force is well below that of steel and decreases with increasing cutting speed. With an increase from 5 m / min to 160 m / min, it drops to about 33%. A further increase only leads to a slight reduction in the cutting force, which asymptotically approaches a limit value. Since in practice the cutting speeds are over 160 m / min, the influence of the cutting speed is negligible.
The flank wear and the built-up edge formation lead to poor surfaces. Since copper materials only have a low modulus of elasticity , thin-walled workpieces can warp, which can also lead to dimensional errors and internal stresses. A low cutting force can lead to improvements here, as can the use of cooling lubricants.
The chip shapes of nickel silver vary greatly depending on the alloying elements and their content, but they are mostly usable. Pure copper tends to produce long ribbon chips. The free-cutting alloys, on the other hand, form short-breaking chips.
Wood
Most woods can be machined well. This is especially true when they are separated in the direction of the grain. If they are processed perpendicular to what is necessary for so-called end grain , breakouts can occur.
plastic
literature
- Eberhard Paucksch, Sven Holsten, Marco Linß, Franz Tikal: Machining technology. 12th edition. Vieweg + Teubner Verlag, Wiesbaden 2008, ISBN 978-3-8348-0279-8 .
- Fritz Klocke, Winfried König: Manufacturing process I. 8th edition. Springer Verlag, Berlin / Heidelberg / New York 2008, ISBN 978-3-540-23458-6 .
Individual evidence
- ↑ DIN 6583 , 1981, p. 1.
- ↑ Herbert Schönherr: Machining production. Oldenbourg, 2002, p. 60.
- ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, p. 307.
- ^ Aluminum headquarters in Düsseldorf (ed.): Aluminum paperback. Volume 3: Further processing and application. 16th edition. Aluminum-Verlag, 2003, pp. 13-27.
- ^ JR Davis: Aluminum and Aluminum Alloys. 4th edition. ASM International, 1998, pp. 328-332.
- ^ Friedrich Ostermann: Application technology aluminum. 3. Edition. Springer, year ?, pp. 567-581.
- ↑ Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur: Handbuch Spanen. 2nd Edition. Hanser, Munich 2014, pp. 1274–1276.
- ^ Edward M. Trent, Paul K. Wright: Metal cutting. 4th edition. Butterworth-Heinemann, 2000, pp. 303-306.
- ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, pp. 321-325.
- ^ Fritz Klocke, Wilfried König: Manufacturing process. Volume 1: turning, milling, drilling. 8th edition. Springer, 2008, pp. 341-345.
- ↑ Bernd Wittchen, Elmar Josten, Thomas Reiche: Holzfachkunde. 4th edition. Teubner, 2006, ISBN 3-519-35911-1 , p. 141.