Machining or machining is the collective term for a group of manufacturing processes that give workpieces a certain geometric shape by mechanically separating excess material from raw parts in the form of chips . The most important machining processes are turning , drilling , milling and grinding . According to DIN 8580, machining belongs to the main group cutting and is the most important group in terms of industrial importance.
During machining, a cutting edge of the machining tool penetrates the workpiece and separates chips. The parts of the tool that are used during machining are called cutting wedges . There are two subgroups in machining:
- When machining with a geometrically defined cutting edge , the number and geometry of the cutting edges are known. This mainly includes turning, drilling and milling.
- When machining with a geometrically undefined cutting edge, however, neither the number nor the geometry of the cutting edges are known. Above all, this includes grinding, in which the tools consist of numerous grains, the edges of which form the cutting edges and cut off chips in the micrometer range.
Most machining processes have been in use since ancient times at the latest. In the course of industrialization , many processes were machined in order to be able to use them on machine tools . In the middle of the 20th century, the individual machining processes were jointly defined by science and industry in DIN 8589. In addition, general terms of machining technology were defined in further standards in order to ensure uniform and unambiguous use of the language. Today machining is mainly used in metalworking , but it is also suitable for most other materials. The machinability of most metals is good; Composite materials or high-strength metals such as titanium pose particular problems . The machining processes are fairly precise and flexible, but tend to be less productive and resource-saving than primary forming and reshaping processes such as casting or forging.
The machining movement is the relative movement between tool and workpiece. It is divided into a cutting movement of the tool and a feed movement of the workpiece. The chips produced are classified according to the type of formation and chip shape that has an impact on the process. The tool materials are referred to as cutting material, among other things high-speed steel , cutting ceramics or diamond are used . The service life during machining , i.e. the service life of the tools, for a given cutting material and a given material depends above all on the cutting speed , which determines the wear during machining . Cooling lubricants are often used because of the high friction and high temperature . Special versions are the minimum quantity lubrication , the dry processing , the high-speed , micro- and Hartzerspanen .
The cutting force acting on the tool is divided into several components, including the cutting force and the feed force. Process parameters that are set on the machine are the so-called cutting or intervention variables such as the cutting depth or the working intervention . You determine the chip sizes that are decisive for the machining process, such as the chip width and thickness.
Definition and classification
DIN 8580 defines cutting as “ cutting , in which layers of material are mechanically separated from a workpiece with the help of the cutting edge of a tool in the form of chips to change its shape and / or workpiece surface”.
The DIN 8589 standard, which deals with the cutting processes, divides them into two groups, both of which are directly assigned to the main group of cutting: cutting with a geometrically defined cutting edge , where the geometry and number of cutting wedges are known, and that Machining with a geometrically undefined cutting edge , where neither the number nor the geometry of the cutting wedges need to be known. Statistical information is sufficient. The individual procedures are assigned to these two groups without further subdivision. In the specialist literature, on the other hand, the procedures are subdivided according to further criteria. This includes the type of cutting movement (rotating, linear) and the feed direction angle (constant 90 ° or variable). Further classification options are:
- Temperature (cold and hot machining),
- Degree of automation (manual, mechanical, conventional machines, CNC machines ),
- Control type ( cam , electrical, numerical ) and
- Amount of cooling lubricant (wet, dry).
In the standard, all procedures are classified according to a uniform scheme based on a numbering system. All machining processes begin with sequence 3.2 (machining with a geometrically defined cutting edge) or 3.3 (machining with a geometrically undefined cutting edge). The first digit refers to the main group of cutting, the third digit to the process (turning, grinding, ...). In the fourth place, the surface is divided into plane, round, screw, profile and shaped chips. In the fifth place, a distinction is made according to the location of the surface created (external and internal chip). The sixth digit provides information about the tool (e.g. hob or face milling cutter), the seventh about the kinematics (longitudinal and transverse cutting). The order number 220.127.116.11.1.1.1 therefore means the plan-circumference-longitudinal grinding.
Machining with a geometrically defined cutting edge
This group is divided into a total of nine subgroups in the relevant standards. Turning, drilling and milling are of particular importance.
Note: The following descriptions are kept as generally understandable as possible and are therefore in part very simplified. For the exact definitions of the individual processes, the associated tools, machines and achievable accuracies, see the list of machining production processes .
- Turning : Here the workpiece usually rotates around its own axis and the tool follows the contour to be generated. It is used to manufacture rotationally symmetrical components such as axles, shafts, bolts, screws, spindles or wheel hubs.
- Drilling , countersinking and reaming : These processes are grouped together because of the similar kinematics. Drilling is a roughing process, countersinking is a more precise variant of it. The most precise is rubbing, with which only surface and dimensional errors can be corrected, but not position errors.
- Milling : Here a mostly multi-edged tool rotates around its own axis. In contrast to drilling, the feed movement is perpendicular to the axis of rotation. It is mainly used to produce flat surfaces such as grooves and guides for moving machine parts. With modern CNC machines , three-dimensional geometries of any shape can also be created.
- Planing and slotting are also grouped together because of their identical kinematics. Both have a straight cutting movement and a step-by-step feed movement. They have been replaced by milling in almost all areas. Exceptions are woodworking, gear shaping for the manufacture of gears and the planing of guides for machine tools.
- Spaces : It is a broaching tool (usually a broach ) is performed, wherein the cutting edges are located one behind the other and are increasingly larger. The feed is therefore integrated in the tool.
- Sawing : It is used for slitting or cutting off workpieces with a multi-toothed tool. The cutting width is as small as possible.
- Filing , rasping : These processes are mostly used manually, sometimes also by machine. The tool ( file or rasp ) has numerous cutting edges that are arranged close together.
- Brush chipping : Chipping with brushes mostly serves to change the surface.
- Scraping , chiselling : This involves cutting with a single-edged tool. When scraping, the tool is pushed or pulled; when chiseling, a hammer hits it.
Cutting with a geometrically undefined cutting edge
When machining with a geometrically undefined cutting edge, the tools consist of small, hard grains. You can with a binder to abrasive wheels , ™ stones or tapes together or what is referred to as machining with bonded grain are mounted on a flexible support. They can also remain loose and in the form of powders or pastes (dispersed in a suspension as a carrier medium), as in lapping, which is known as cutting with loose grain. The achievable accuracies and surface qualities are better than when machining with a geometrically defined cutting edge, but the processes are less productive, but they are also suitable for very hard materials. Most workpieces are finished by grinding; only in rare cases are other procedures necessary. The grinding processes and honing belong to the machining with bonded grain, the rest to the machining with loose grain.
Note: The following descriptions are kept as generally understandable as possible and are therefore in part very simplified. For the exact definitions of the individual processes, the associated tools, machines and achievable accuracies, see the list of machining production processes .
- Rotating Tool Grinding : It requires rotating grinding wheels and is also known simply as grinding .
- Belt loops : Belts that run around two rollers are used for this.
- Stroke grinding: This requires grindstones with a reciprocating motion.
- Honing : Here a reciprocating cutting movement is superimposed with a rotating feed movement, so that crossed grooves are created on the surface . Honing serves to improve the surface, in particular of cylinder bores in piston engines.
- Lapping : This involves working with loose grain that is pressed onto the workpiece surface. The grains roll over the surface and separate microscopic chips. A non-standardized precision variant is called polishing .
- Jet cutting : Here the grains are thrown onto the workpiece at high speed. A variant is water jet cutting , provided that grains are added to the water jet.
- Sliding chips : The workpieces and the grinding bodies are located here as tools in a drum which, through rotation, causes a relative movement between them.
Significance and classification in process chains
Range of materials
The machining processes are mainly used in metalworking and follow casting and forging in the process chain . The most important metallic materials are steel , including free- cutting steel , which is specially designed for machining, cast iron and aluminum . Otherwise copper alloys and titanium are machined to a significant extent, but the latter causes problems due to its high strength.
In woodworking , machining processes are very important. This ranges from felling trees and sawing the trunks to grinding the end products. In principle, machining is also suitable for plastics and for more brittle materials such as glass , graphite , ceramics and other mineral substances as well as for composite materials such as carbon fiber reinforced plastic , which have been more popular as construction materials since the turn of the millennium, but are considered difficult to machine due to their structure.
Machines, cost shares and industries
About two thirds of all machine tools produced are cutting machines. The cost share of components from mechanical engineering or the vehicle industry for machining is between 40% and 70%. The rest is divided between forging and material costs. Machining is also used in aerospace technology , biomedical technology , device and drive technology and other industries.
Productivity and flexibility
The metal-cutting manufacturing processes - as the most important group in the main group cutting - are often compared with casting from the main group primary forming and forging from the main group forming , as they compete with each other. In terms of productivity (workpieces produced per time), machining is usually inferior to these two groups, which is due to the underlying mapping principle. During casting, the shape to be created is stored in the casting mold (for permanent molding ) or in the model (for lost molds ) and can therefore be transferred to the workpiece frequently and quickly. In die forging , the shape is contained in the die . When machining, on the other hand, the desired contour must be followed with the tool, which is less productive, but much more flexible . In order to create a different workpiece geometry, it is usually sufficient for machining to steer the tool onto a different path; for casting and drop forging, on the other hand, new and expensive tools must first be manufactured. When it comes to machining, the range of shapes that can be produced is hardly limited, and its high flexibility makes it suitable for small batches , and in extreme cases even for individual pieces. However, machining is also used for large batches, for example in the automotive industry on transfer lines for series production . For casting and forging, on the other hand, certain minimum batch sizes are necessary to ensure economical production. In some machining processes, too, the shape of the workpiece is partly contained in the tool: When drilling, the diameter of the hole roughly corresponds to that of the drill; only the depth of the hole is controlled by the movement.
Material utilization and energy expenditure
The material utilization in casting and forging is 75% to 90%. In the case of machining, on the other hand, between 50% and 60% of the material is removed in the form of chips. The situation is similar with the energy balance: For a workpiece weighing one kilogram, between 30 MJ and 50 MJ are required for casting and forging , whereas between 60 and 80 MJ are required for machining.
The machining processes usually achieve high levels of accuracy and surface quality. The achievable ISO tolerances (small numbers mean greater accuracy) are between IT16 and IT10 for casting and between IT16 and IT12 for forging (with special measures also up to IT8). The machining processes, on the other hand, achieve accuracies between IT10 and IT7, with grinding also up to IT5. The situation is similar for the surface qualities, measured as the mean roughness depth R z : For casting and forging, the values are between 1000 µm and 10 µm. When machining, on the other hand, usually between 250 µm and 10 µm, and when grinding even between 16 µm and 0.25 µm.
Because of the specific advantages and disadvantages of the process groups, they are often combined into process chains. In the case of metals, the shape to be produced is generally roughly worked out by casting and subsequent forging, in order to then obtain the final contour by machining. In the case of plastics or glass, too, primary forming and forming precede machining. Depending on the material, individual steps can or must be left out: Cast iron cannot be forged and is machined directly after casting. Ceramic cannot be shaped either. Not all wood-based materials can be shaped, solid wood can only be shaped to a certain extent like bentwood , which is why machining is the focus of wood processing . Workpieces made of natural stone are given their shape directly by machining.
The machining of metallic materials can be divided into several steps: It starts with rough machining ( roughing ). Since the workpieces warp by the high machining forces and solidify, sometimes a is normalizing necessary to make the structure to normalize and the subsequent finishing ( sizing ) to allow. For roughing and finishing, methods are used that count for machining with geometrically defined cutting edges, in particular turning, milling and drilling. The workpiece is then hardened, for example by quenching and tempering . The finishing is then mostly done by grinding. There are several reasons for this division: on the one hand, grinding is not very productive and is not very suitable for soft (unhardened) materials; on the other hand, turning, milling and drilling does not allow machining of hardened materials under normal circumstances, not even in the desired way Accuracy. In addition, hardening changes the shape of the workpieces slightly, so that finishing can only take place afterwards.
In industrial ceramics and powder metallurgy products, machining often takes place before sintering , i.e. before the workpiece has reached its final hardness. Because the green bodies are machined, they are called green machining . This is not possible with composite materials with particularly hard components. Special efforts have been made to solve such problems since around the year 2000.
Towards the end of the 20th century, the boundaries between the individual process steps began to blur: casting and forging became more and more precise, which made subsequent steps, and thus above all machining, partly superfluous. This mainly applies to die casting and thixocasting as well as precision forging , thixo forging and extrusion . On the other hand, special variants of machining processes were developed that significantly improved productivity. This includes, for example, high-speed machining . There were also changes within the machining process: grinding became more and more productive and could replace turning or milling. These processes for their part became more and more precise, so that subsequent grinding could be dispensed with. In addition, it was possible to machine hardened materials by turning and milling with super-hard cutting materials ( hard machining ).
The manufacture of everyday objects, vehicles, tools and weapons required heavy and time-consuming routine work, which was gradually made easier by new equipment and processes. The development went in the direction of driving the tools with a uniform movement, an increase in their driving force, their cutting ability and their more precise guidance.
Machining is one of the oldest machining methods known to man. In the Paleolithic Age , you could scrape, drill and scratch with simple stone tools such as a hand ax and chisel with a hammer . The flint was because of its hardness and cleavage is an important raw material for weapons and tools, to be specific stroke techniques developed. The Levallois technique of the Neanderthals has been examined more closely .
Since the Mesolithic there have been stone tools which, according to their shape , were used as saws . Since the Neolithic are Steinaxtklingen with clean drilled holes for shafts of wood known. It has been suspected since the 19th century that fiddle bits were used back then , which are set in a reciprocating rotation with a bow. In the absence of archaeological evidence, however, this is speculation.
In ancient times the arrived files , rasps , planing and turning add what was mainly used for the processing of wood, about the house or shipbuilding . In the handicraft sector, horn , ivory and amber were also machined. The artisanal chiselling of stone reached high points in construction and art. Filing was also used by blacksmiths in metalworking. The use of simple machines is assumed for grinding, turning and drilling . Many finds suggest that the Egyptians used a lathe very early on . The first pictorial representation of a pull cord lathe comes from the grave of Petosiris from the 4th century BC. The workpiece is wrapped around by a cord, the ends of which were pulled back and forth by an assistant so that it rotated and could be worked on by the craftsman with a tool. The tools were often made of obsidian and, since the beginning of the Bronze Age, increasingly made of bronze . Drilling tools were also used in conjunction with sand so that the hole was created through a combined drilling and grinding process.
In ancient times , which makes up the first half of the Iron Age , tools were made from iron. This includes the Greek auger bit with a square, conical tip, but blunt cutting edges that only cut off sawdust and only drilled in one direction of rotation. A further development was the spoon bit , which was easier to manufacture and drilled in both directions with its two cutting edges. In Roman times the center drill was added. These tools were only replaced by the twist drill in the 19th century . For the processing of glass and gemstones , drills whose tips were set with diamond chips were already used . In addition, were grinding stones made of pumice stone or emery known.
Middle Ages and Early Modern Times
In the Middle Ages, one used for boring the Bohrleier that produced a continuous rotation, instead of reciprocating the race spindle or fiddle drill . The Benedictine monk Theophilus Presbyter described the hardening of files in the 11th century : First ox horn was burned, mixed with salt and sprinkled over the files, which were then annealed in the oven . They were then quenched in water and tempered in the oven .
The antique cord pull lathe was further developed in two different ways: The rocker lathe , which was used in particular in woodworking, could be operated by only one person, as one end of the cord was attached to a rocker that was operated with the foot and that other end above the lathe on a swinging bar that worked as a spring. The English term " lathe " for lathe comes from this lath. This left both hands free to guide the tool. For metalworking, lathes were used that were equipped with wheel or crank drives and therefore enabled continuous rotary motion as well as higher forces and cutting speeds. The disadvantage was that an assistant was needed for the operation. Since 1528, vices have been used in the locksmith's shop , which allowed more precise work by allowing the worker to have both hands free to guide the tool.
Beginning of industrial metal cutting (1500–1900)
In the 16th century there was an increasing trend towards machining metals. Before that, only grinding and filing was common. For boring cannon barrels made of cast iron caused large boring mills that were driven by muscle power. Some engineers wrote machine books in which such boring mills are illustrated and described. These authors include the artillery captain Vannoccio Biringuccio with his ten-volume work De la pirotechnia (“From the art of fireworks”, Venice 1540). Practice outside of the craft of war was not necessarily appreciated. Biringuccio did not only describe weapons, but also founded metallurgy . Jacques Besson's book Theatrum instrumentorum et machinarum (“Theater of Instruments and Machines”), written in the scholarly language of Latin, was more highly regarded, and lathes are also shown on the display boards. As the title suggests, these complicated machines were meant to be viewed and amazed rather than constructed. The monk and botanist Charles Plumier put some of these ideas into practice a hundred years later and in 1701 published a frequently published book L'Art de Tourner ("The Art of Turning"), in which, among other things, he described copy turning and oval turning with mechanical controls (e.g. cams ) and templates.
In the Encyclopédie ou Dictionnaire raisonné des sciences, des arts et des métiers (approximately: "Encyclopedia or critical dictionary of the sciences, techniques and professions") published by Denis Diderot from 1751 , all machining techniques of that time were documented. In order to produce more powerful tools and machines, however, you needed those tools and machines that you did not yet have. This paradox can be seen most clearly in the development of the steam engine : In the 18th century, boring mills were required to manufacture cast iron cylinders. At the beginning of the century there were still major problems in producing the cylinders with the required accuracy. John Smeaton improved the details of both the steam engines and the boring mills. James Watt had problems with the production of the cylinders after his decisive improvement of the steam engine (patent from 1769). It was not until 1775 that John Wilkinson succeeded in reducing the vibrations of the drill with a double-bearing shaft and thus significantly improving the accuracy. The steam engine itself enabled and accelerated industrialization , which meant an increased use of ferrous materials. Since the machining of iron parts also resulted in higher cutting forces, the decision was made to manufacture the frames of the machine tools also from cast iron or steel.
In the course of industrialization, a process chain for metalworking was established: First, the shape to be produced was roughly worked out by casting and forging. Then the shape was worked out more precisely by turning, drilling and planing. Finally, the workpieces were finished by grinding.
Most of the known processes were machined in order to achieve large numbers of items: lathe , planing and drilling machines were created . The milling was in the 19th century a new process that simultaneously with the milling machine was built. It replaced a lot of routine work with hand tools such as carving , scraping or engraving .
Henry Maudslay , an English engineer and manufacturer, is of particular importance to machine tools . His influence on the development of machine tools corresponds roughly to that of Watt on the development of steam engines. Turning was particularly important for industrialization, as it was used to manufacture bolts, screws, spindles, axles and shafts for steam and textile machines. To ensure precise machining, tool holders were integrated into the machines, which were moved using cranks. Usually a central steam engine served as the drive for an entire factory hall.
The tools, which were made of carburized steel as in the Middle Ages, caused problems . During the machining of ferrous materials, due to their low temperature resistance, they lost their hardness at cutting speeds of a few meters per minute. Boring Watts cylinders therefore took almost a month. The first remedy came with an alloyed tool steel with a proportion of tungsten from Robert Forester Mushet , which is also known as Mushet steel. This enabled cutting speeds of around 10 m / min. Around 1870 the first automatic lathes appeared in America , which had to be set up by a skilled worker, but could be operated by a semi-skilled unskilled worker. The machine carried out all work independently, including changing tools for the various work steps, only the work piece was changed by the worker.
Machine tools were more accurate than handwork for the first time around 1900. Up until now, parts with high accuracy requirements , such as fits , could only be roughly machined and then adjusted by hand. The machining of components was also a prerequisite for the large-scale production of sewing machines and bicycles at the turn of the century and finally for the assembly line of cars at Ford from the 1920s. This means that sufficiently precise machines were available for all processes.
The major advances in machining technology in the 20th century, however, lay in productivity and flexibility: New cutting materials enabled ever higher cutting speeds and thus shorter machining times, mechanically and electrically controlled machines were also significantly more productive. Over the entire century, increasingly harder cutting materials were developed, which permanently increased the permissible cutting speeds. In 2000, a job that had taken 100 minutes in 1900 only took one minute. The numerical control that came up later also increased flexibility and enabled the production of workpieces with very complex geometry.
High speed steel
The most important development at the beginning of the century was that of high-speed steel (HSS). With it three times higher cutting speeds and thus significantly shorter machining times were possible. At the World Exhibition in Paris in 1900 , Frederick Winslow Taylor presented the high-speed steel developed together with Maunsel White and demonstrated its efficiency. With cutting speeds of up to 40 m / min, which was unimaginable at the time, the tools began to glow red and yet did not blunt, while the chips turned blue . However, the machines at that time were not designed for the higher forces and performance required for this. Experiments by Ludwig Loewe & Co. showed that such an increase in cutting speeds would make the machines completely unusable in a few weeks. Since machines were generally quite expensive and, if handled properly, they were also long-lasting, high-speed steel was initially used to increase the service life of the tools, and only shortly before the machines were decommissioned they switched to using the possible cutting speeds.
Electric drives and controls
The transition to the new generation of machines was also facilitated by the fact that the electric motor , which had been known since the 1860s, had become so robust around 1920 that it could withstand the high loads in industry. Now every machine was equipped with its own electric motor. The electrification in industry at the beginning of the century is also known as the second industrial revolution . The electric motors created new ways of controlling the machines. Using a button , the shape of a master model could now be transferred to a workpiece, with sensors transmitting the corresponding movements to the machines' motors. This made copy turning and milling economically feasible even for medium batch sizes.
In the 1930s there was a new cutting material in the form of hard metal , which could again increase the cutting speed three to four times. The first types of hard metal consisted of tungsten carbide and were initially used for machining aluminum . When machining steel, on the other hand, the carbon contained volatilized because the hard metal diffused when it came into contact with the steel . It was not until the mid-30s that hard metal types based on titanium carbide , tantalum carbide and niobium carbide had been developed that the basis for economic use in steel workpieces was created. The material, which was still quite new at the time, was used more and more frequently in the automotive industry. The tools mostly consisted of a shaft made of high-speed steel with a soldered-in hard metal plate that was ground into the desired shape by the workers.
Since the machines did not have sufficient stability and performance as with the development of the HSS tools and the hard metals were comparatively expensive, the introduction of the hard metal tools took place only gradually until the beginning of the Second World War. Moreover carbide was during the Great Depression seen in the 30's as strange and exotic cutting material. The hard metals only became more widespread after the war.
Toolholders, indexable inserts and coatings
In the mid-1950s, tool holders for the cutting inserts were created, which had several important advantages: the cutting edges could be changed more quickly and the blunt cutting edges could be reground in a device that was separate from the tool holders. Since the tools could remain in the machines, the achievable accuracy also improved. In addition, the cutting materials no longer had to be solderable, so that their exact composition could be better aligned with the cutting properties.
The decisive step on the way to the modern tool was the development of the indexable insert . Similar to a razor blade, the cutting edges were no longer sharpened after use, but first turned to use other edges as cutting edges, and finally thrown away. The tool manufacturers could now neglect the grinding suitability of the cutting inserts in their improvement and limit themselves to hard material coatings. This initially aroused astonishment among the skilled workers and engineers, as tools were considered valuable and were usually not thrown away, but rather reground. In addition, the knowledge of the required tool geometries was the responsibility of the experienced skilled workers, who ground them in hand. Since the elimination of regrinding was also associated with cost savings, indexable inserts soon became popular.
The sometimes long chips were problematic in automated production as they could get tangled up in the machine. Therefore chip breakers were placed on the cutting edges and clamped. Depending on the feed, their distance from the cutting edge could be adjusted in several stages. In the 1960s, indexable inserts with sintered-in chip breakers were created, sometimes as multiple chip guides for different feed rates. The previously sharp-edged cutting edges have been rounded off by drums and are therefore much less susceptible to fluctuations in the material composition and the workpiece dimensions. At the end of the 1960s, the indexable inserts were standardized internationally so that the various manufacturers now used the same abbreviations.
The first coated tools also appeared in the late 1960s. A tool or an indexable insert made of high-speed steel or hard metal was coated with hard materials, which further improved the tool life.
CNC machines, changed function of grinding
With the development of CNC machines from the 1970s onwards, it became possible to transfer the designers' CAD data directly to machines that move the tools independently and follow the required contour. However, the origins of CNC machines go back to the early 1950s. The component geometry in aircraft construction became increasingly complex, which is why the American engineer John T. Parsons came up with the idea of having the movements of the tools controlled by a computer. With the help of MIT and financial support from the US Air Force, a machine controlled with punch cards was finally built . Programming, however, was complex and the hardware was more expensive than the actual machine. It was only with the development of the microprocessor as part of the third industrial revolution that CNC machines brought economic advantages, so that they quickly became established.
In the 1970s, new tool geometries were created, which took into account research results regarding strength, wear, forces and temperatures. For example, wave-shaped chip breakers and rake faces were created.
There have been two different developments in the field of materials: On the one hand, stronger and harder materials were developed, which were often used in automotive or aerospace technology and were increasingly difficult to machine. For this reason, the cutting speeds in these industries fell, despite better cutting materials. On the other hand, types of material have been developed that are particularly easy to machine, such as free- cutting steel .
Through further developments such as improved CNC controllers and the use of modern tool geometries, higher feed rates and cutting depths could be achieved. This made it possible to significantly increase the metal removal rate of existing machine tools. These modern developments are known under the term high-performance cutting or high-performance cutting.
At the turn of the 21st century, some boundaries began to blur in machining technology: grinding, which had long been a fine machining process for hard materials, became more and more efficient and could now replace other machining processes. On the other hand, the super- hard cutting materials diamond and boron nitride made it possible to machine hard materials by turning, milling and drilling ( hard cutting ), which in turn could replace grinding. With high-speed machining , these processes also became more and more efficient.
In the 18th and 19th centuries, Johann Beckmann began to establish a new field that described the well-known manufacturing processes: technology . From this the field of mechanical technology developed at the technical universities . In the middle of the 19th century, Karl Karmarsch wrote a three-volume work in which he described, organized and systematized the processes. This can be seen as the beginning of the scientific penetration of manufacturing technology .
Machining research received an important impetus from the American Frederick Winslow Taylor . He carried out various experiments around 1900 and wrote a work On the Art of Cutting Metals (literally: "On the art of cutting metal ", New York 1906/07). Among other things, the Taylor straight line is named after him and provides a relationship between tool life and cutting speed. In the early 20th century, the basics of machining were researched at numerous technical universities and other research institutes . In countless experiments, for example, the v 60 values were determined for the most important materials, i.e. those cutting speeds at which the tool life is 60 minutes. At the beginning of the 1930s there were tables for all cutting materials and materials common at the time with guide values for the cutting speed. In the first two decades of the 20th century, theoretical and practical research into cutting forces was in the foreground, but the cutting resistance of various materials was also determined. The principles of cutting force decomposition were transferred from turning to milling and drilling. From the 1920s the influence of the cutting edge geometry on the forces and tool life was increasingly investigated. The new chrome-nickel steels , which were often machined in the automotive industry, caused problems there. Therefore, the machinability of these materials has been researched with extensive support from the steel and automotive industries. In addition, the first research approaches arose on chip formation and generation, surface testing and temperature measurement during machining. At about the same time, the term machinability, which was initially only measured by tool life, was expanded and now also took into account the forces, temperatures and the surface quality that can be achieved .
In Germany, a number of researchers dealt with machining, including Adolf Wallichs , Heinrich Schallbroch , Max Kronenberg (1894–1972), Franz Koenigsberger (1907–1979), Karl Gottwein , Ewald Sachsenberg , Georg Schlesinger and Friedrich Schwerd . The Association of German Engineers published the special issue Zerspanen as part of the Maschinenbau magazine in 1926 , with articles by well-known authors on current problems in research. In other European countries there was only sporadic research in this area. In the USA, on the other hand, there were numerous specialists. The term “machinability” developed in a similar way to “machinability” from the exclusive consideration of the service life to the inclusion of forces and the surface quality that can be achieved. In the 1930s, extensive tables were created on the machinability of the most important materials, which took all of these criteria into account. The research now focused on milling. The temperature distributions on the tool and the timing of the chip formation have now also been determined by means of cinematography .
In the second half of the century, Otto Kienzle found a simple method for determining the cutting force . In the last years of the 20th century the finite element method was used more and more frequently for complex calculations.
The machining of wood was no less practiced than that of metals, but it has only been systematically researched since the middle of the 20th century. This may be due to the fact that wood “is generally good and easy to work with, on the other hand, as an organic 'inhomogeneous' structure, in contrast to metals, it makes it difficult to establish general guidelines for machining”. Metals were the greater challenge for manufacturing technology and promised greater economic success.
The machining movement consists of two components: the cutting movement and the feed movement. Their nature and their direction to each other are used to delimit the various procedures. The relative movement between tool and workpiece is decisive for the result. Whether the tool performs the movement and the workpiece is stationary or vice versa is only important for the construction of the machines.
The cutting movement leads to the removal of chips during one revolution or one stroke. In processes with a rotating movement, i.e. turning, drilling, milling and sawing with circular saws, the cutting movement is a rotary movement. Planing, slotting, filing and sawing with hacksaws is a back and forth movement. In the direction of the cutting movement, the acting cutting speed and the cutting force (c; English cut = average).
The feed movement is the component that allows continuous chip removal. When drilling, this is, for example, the penetration of the drill into the hole, with the sawing process, penetration into the groove. It can be done step by step, between strokes as when planing, slotting and filing, or continuously as when drilling, milling, turning and sawing. In the direction of the feed movement, the acting feed rate and the feed force (f; English feed = feed rate).
The active movement is the resultant of the cutting and feed movement. Accordingly, the effective speed (e; English effective = effective) is the resultant of the two components. The resulting force is active force called; it is part of the cutting force .
The angle between the cut and feed directions is the feed direction angle ( phi ). When using a rotating tool, such as milling, it changes during one revolution. With the other methods it is a constant 90 °. The angle between the cutting direction and the effective direction is called the effective direction angle .
Chip formation is understood to mean the various types of chip formation . They differ in detail depending on the process, hardness of the material and many other influences. The material is first upset on the tool, which increases the shear stresses until the yield point is reached. A chip forms that runs off the rake face of the cutting part.
Chip types and shapes
Even with the same material to be machined, changing the process parameters can result in different types of chip:
- Continuous chip: A uniform chip. This type of chip is usually the one required because the tool is loaded evenly.
- Shear chip : In the shear zone separated chip parts, some of which weld together again. Scaly chip.
- Chipboard: Also friable chipboard ; a chip that is torn off and not cut off, resulting in poor surface quality.
The chip shapes, on the other hand, describe the shape of the chip after it has left the tool. They range from long ribbon and tangled chips to spiral chips to short broken chips and depend on the geometry of the cutting edge, feed rate and cutting speed. Long chips ensure an even load on the cutting edge, but they can get tangled in the machine and thus scratch the workpiece or endanger the operator. Short chips can be easily removed, but they cause increased tool wear due to the uneven tool load (relief in the event of chip breakage, load in the event of renewed chip formation).
Dependence on the materials
Chip formation and the cutting process differ depending on the material: When cutting flowable and isotropic materials such as (unhardened) metals and plastics, the material is plastically and elastically deformed by the cutting edge, and flow and separation processes occur along a shear zone. Usually a geometrically defined chip is created. The separation process is not direction-dependent.
When anisotropic , non-flowable wood chip formation also depends on the fiber cutting angle on what the precision of the cuts and the tool life significantly affected: Distinguish between the machining with the fiber by machining against the grain . As far as the rotation of the tools in relation to the fiber is concerned, one speaks of counter-rotating and synchronous operation. The cutting directions are roughly divided into brain section, longitudinal section and cross-section. Because machining can hardly be restricted to a single cutting direction, a mixture of chips is created . The pre-splitting of the wood, especially when chipping with the fiber, which increases with the rake angle, must always be taken into account: With the cutting edge as fast as possible, you try to use the inertia of the workpiece to break off the chips in time. Fundamental studies on the machining of wood have been around since the 1980s.
In the case of crystalline materials such as martensitic steel and mineral substances, the plastic deformation is low to very low, so that the chips are caused by brittle fracture . The tool must absorb the elastic deformation energy .
The tools consist of several parts: a shaft, a handle for manual production or a machine interface for machine tools and the cutting part. As solid tools, they can consist of a single, continuous material. If the cutting edges become blunt, they are reground. In the industry, however, indexable inserts are mostly used, which are inserted into the tools. Your edges then act as cutting edges. When they become dull, they are rotated and eventually exchanged.
Grinding tools, on the other hand, consist of a large number of grains that are joined together to form a tool. In the case of very expensive tool materials ( abrasives ), they consist of a cheap carrier material that is coated on the outside with the abrasive.
The tool material when machining with a geometrically defined cutting edge is referred to as cutting material, while that with a geometrically undefined cutting edge is usually called grinding material, and occasionally also cutting material.
Various requirements are placed on cutting materials, and not all of them can be realized equally. The most important are:
In order to make machining economical, high cutting speeds are sought on the one hand, which keeps the machining time as short as possible. At high speeds, however, the temperature load is high, which leads to increased wear and lower hardness of the tool. On the other hand, the feed rate can be increased, which leads to a larger chip cross-section and thus to greater forces.
Important cutting materials, roughly sorted according to increasing hardness and high temperature strength and decreasing toughness and flexural strength, are:
When machining with a geometrically undefined cutting edge, the tool material can either be bound (grinding wheels and honing stones) or loose (pastes for lapping and polishing).
The durability describes the ability of an active pair (workpiece and tool) to withstand certain machining processes. The longer it takes until the tool is worn, the better it is.
The service life is the time until the tool has to be replaced or reground. However, this only includes the time in which the tool is actually used for machining. Workpiece change times, for example, are not included. In industrial metalworking, downtimes of 15 to 30 minutes are common, on transfer lines, due to the long tool change times, several hours. It depends on numerous influencing factors; for a given tool and workpiece pairing, however, it is primarily dependent on the cutting speed . The Taylor line provides a mathematical relationship .
The end of the standing time is determined by so-called standing criteria. It can be about the cutting force that occurs and the surface quality achieved. However, a wear size is often chosen. The service life applies to certain conditions that are also specified. for example, a tool life of 30 minutes for a certain material-tool pairing at a cutting speed of 200 m / min until a wear mark width VB of 0.2 mm is reached on the tool.
Wear is the cause of the limited service life. It is caused by the high thermal and mechanical loads to which the tools are subjected. The forces can amount to several thousand Newtons and the temperatures at high cutting speeds can exceed 1000 ° C. Several mechanisms are responsible for wear: mechanical abrasion, microscopic pressure welds between chip and tool, oxidation , diffusion and surface disruption . The wear and tear can be felt in various forms on the tools. Most often, the are flank wear and crater wear , which manifests itself as a recess on the rake face.
Cooling lubricants should avoid the generation of heat through lubrication , remove hot chips and cool hot tools / pieces in order to avoid excessive thermal expansion . This enables a high level of performance in numerous manufacturing processes. Since cooling lubricants are expensive and hazardous to health, attempts are made to avoid them. One possibility is minimum quantity lubrication . Dry machining, on the other hand, works completely without cooling lubricants.
Machinability is the property of a workpiece or material that it can be machined under given conditions. It depends on the achievable surface quality, tool wear, the shape of the chips and the magnitude of the cutting force.
Edge retention is the ability of a tool to maintain its cutting ability during machining. Cutting ability is the ability of a tool to machine a workpiece or a material under given conditions. The stability is the ability of an active pair (tool and workpiece) to withstand a certain machining process. It is influenced by the quality of the tool and the machinability of the material.
Cutting force and performance
The force acting on the tool is called the cutting force. It is made up of three components: the cutting force in the cutting direction, the feed force in the feed direction and the passive force , which forms a right angle with the other two forces. In most procedures, the cutting force is significantly greater, so that often only this is considered. The amount of cutting force can be determined using various methods; Otto Kienzle's method is established in practice , which determines it from the cutting surface and the specific cutting force . The latter is the cutting force related to the cutting surface.
However, it is not a material constant, but depends on several influences, in particular the chip thickness .
The power required for machining , the effective power , is the product of the effective speed and the machining force.
The machine's drive must at least be able to deliver this power. Often you limit yourself to determining the cutting power, which roughly corresponds to the real power. It results from the cutting force and the cutting speed .
Energy conversion, heat and temperatures
The mechanical energy is almost completely converted into heat. This happens on the one hand through the friction between the tool and the workpiece, and on the other hand through the deformation of the chip. Most of the heat (approx. 95%) remains in the chip itself, so that the heating of the tool and the workpiece is comparatively low. Since the various frictions and deformations are extremely complex, it has not yet been possible to develop a theoretical model with which the heat generated can be calculated in advance. The previous findings are based on measurements.
The energy is converted in different places and through different mechanisms. The energy required for the deformation can be divided into the shearing work to shear the chip in the shear zone and the cutting work to separate the chip from the workpiece. Friction work is necessary in order to overcome the friction between the workpiece and the flank of the cutting wedge and that between the chip and the rake face. Their proportions depend on the chip thickness. With very small thicknesses, the free surface friction and the separation work are predominant. For larger thicknesses it is the shear work.
Geometries on the tool
The idealized cutting wedge consists of two areas - the rake face and the open space - that meet at the cutting edge. The angle between the two is the wedge angle . A distinction is also made between the main and secondary cutting edges. The rake and secondary flank are on the secondary cutting edge. The tool reference plane is perpendicular to the assumed cutting direction and in the considered point of the cutting edge. Together with the tool cutting edge plane and the tool orthogonal plane, it forms a Cartesian coordinate system . The tool cutting edge plane contains the cutting edge and is perpendicular to the tool reference plane. The tool orthogonal plane cuts the other two at a right angle and also runs through the point of the cutting edge under consideration. Further angles are defined in these planes, including the tool setting angle and the rake angle .
Engagement and stress quantities
Intervention variables are variables that are set on the machine. This includes the cutting depth and the cutting width, also called infeed and feed . They influence the cutting cross-section and, together with the cutting speed, the volume removed per time, the material removal rate , which is an important productivity indicator. Span ungs sizes are variables which influence the formation of chips. This includes the chip width and chip thickness , describing the thickness and width of the material layer to be separated. This must be distinguished from the chip thickness and width, which affect the geometry of the separated chip and differ from the chip sizes due to the chip compression. With the tool setting angle, the following relationships exist between the engagement and cutting parameters :
Surface and edge zone properties
Especially when turning, the traces of the tool can be seen as grooves or grooves in the surface of the workpiece, which also influence the roughness . The theoretical maximum roughness results from the feed and the cutting edge radius to
For this reason, a smaller feed rate is selected for finishing (fine machining) than for roughing. The roughness that can be achieved in practice is, however, always worse because of grooves on the tool that are caused by wear. The structure of the workpiece also changes near the surface. Due to the high machining forces, some layers of grains are flattened and stretched in the machining direction. This introduces internal stresses into the workpieces. In addition, plastic deformations , changes in hardness and cracks occur.
Choice and optimization of cutting values
If the blank, the tool and the machine for machining are already fixed for a certain workpiece, numerous process parameters can still be freely selected. The most important are the feed rate , the cutting depth and the cutting speed . There are numerous tables with guide values that can be used for smaller quantities and serve as a starting point for experiments for optimization. However, there are certain limits to be observed. The cutting depth is usually chosen as large as possible in order to machine the workpiece with as few cuts as possible if the allowance is greater than the maximum possible cutting depth. Otherwise, it is limited by the stability of the tool, as chattering can occur at great depths of cut, i.e. vibrations that affect the surface quality.
There are further limitations to the feed rate. In principle, it should also be selected as large as possible, as it increases the metal removal rate and thus reduces the machining time. An upper limit results from the influence on the roughness of the workpiece and from the increasing cutting force. If the torque of the drive is not sufficient to generate the appropriate force at the point of action, high feed rates cannot be achieved. The chip shape plays a role, especially for automated production, and becomes cheaper as the feed rate increases. Another limitation is the minimum chip thickness that is necessary to ensure chip formation at all.
The choice of cutting speed is primarily related to wear. At high speeds it often increases disproportionately, but the processing times decrease.
The optimal cutting values result from the total manufacturing costs. They are made up of machine costs, labor costs and tool costs.
Newer process variants
At the turn of the 21st century, variants of the established processes emerged that opened up new possibilities. They place special demands on machines and tools. In some cases, effects are of importance that can otherwise be neglected.
High speed machining
Machining at high speeds (HSC machining, from high-speed cutting) requires lower forces and enables better surfaces and shape accuracy as well as lower machining times with the same cross-section . The boundary between conventional cutting and high-speed cutting is not precisely defined. For turning, it is at cutting speeds between 500 m / min and 1500 m / min. However, the high speeds required for these cutting speeds also result in high centrifugal forces. After the technical prerequisites for machine technology were in place at the turn of the millennium, it became increasingly widespread in industry. With HSC machining, chip formation takes place according to different principles than with conventional speeds. See chip formation in high-speed machining .
High-performance cutting, also known as “High Performance Cutting” (HPC), is a process that is optimized for the production of large metal removal rates.
Powerful and high-torque tool spindles, high cutting widths, cutting depths and high cutting speeds enable high material removal per unit of time. HPC is usually a pure roughing process, so that more precise machining processes such as HSC (“High Speed Cutting”) are usually after the HPC.
Mold making , in particular , can benefit from the more economical machining of matrices implemented by HPC, due to the high metal removal rate and significantly longer tool life.
Micro machining is the processing of workpieces with tools that are in the micrometer range. The diameters of milling cutters or drills are between 10 and 50 micrometers. The size of the structures produced is between 10 and 1000 µm. Micro-machining has significantly higher removal rates than conventional micro-machining processes such as etching .
Hard machining is used when machining materials with a hardness of more than 47 HRC using processes that are part of machining with a geometrically defined cutting edge. Until the development of the super-hard cutting materials boron nitride and cutting ceramics , the machining of such materials was only possible with grinding and honing. Hard machining is mainly used when the subsequent fine machining by grinding can be omitted, which shortens the process chains. It also offers advantages in terms of energy efficiency and environmental friendliness. The latter is due to the fact that hard machining is usually carried out as dry machining (i.e. without cooling lubricants), so that the chips can be recycled without special measures to separate the cooling lubricant and chips. When grinding, on the other hand, the chip-lubricant mixture often has to be disposed of as hazardous waste.
With hard machining, the tools are subjected to particularly high mechanical loads and special chip formation processes, in which the material behaves plastically despite its hardness due to the high deformation speeds and mechanical stresses , i.e. like a soft material. Due to the high tool loads, the cutting speeds are limited to around 200 m / min.
Simulation and modeling
Simulations of the machining processes are based on models that can be more or less accurate and detailed. The standard software contains a program for work preparation in industrial companies that provides a virtual image of the machine as well as the tools and workpieces. Simulations are used to check the planned machining and to calculate simple process parameters such as forces and temperatures. Either the geometric CAD data or the CNC code of the machine tool serve as the basis for these simulations .
With the finite element method, there is a possibility to model the machining process much more precisely. It is used to calculate the distribution of mechanical stress or the temperature field on the tool or to simulate chip formation processes . For this purpose, the tools or workpieces are broken down into a finite number of (finite) elements. Relationships exist between them in the form of equations that express mechanical stresses, speeds, friction or thermal convection and are combined to form systems of equations with a large number of equations and unknowns. In general, efforts are made to simplify such relationships and express them using linear equations ; in the case of machining, however, this would lead to unusable results. For example, Hooke's law , which creates a linear relationship between stress and strain , no longer applies because the deformations that occur are too large. The FEM simulation of machining leads to systems of equations with numerous non-linear equations and constraints that can only be solved numerically . This can be done with standard FEM software. However, there are also several commercial solutions that are specifically tailored to machining technology.
As early as 1954, Kronenberg suggested describing the process of chip formation analytically using the laws of similarity mechanics. The laws derived from this approach are now available for the machining of steel with geometrically defined and indefinite cutting edges. For machining with a geometrically defined cutting edge, these equations provide a relationship between the cutting data and the tool life of a cutting edge. The link here are the dimensionless parameters kinematic and thermal speed ratio and the Fourier number of chip formation. With appropriate design programs, cutting data can be calculated which exclude machining in the area of built-up edge formation and thus reliably lead to economical machining results. The dimensionless parameters active and wear index are decisive for the design of machining with an indefinite cutting edge. The programs for the design of grinding processes are structured in such a way that the occurrence of grinding burn is avoided through the selection of the cutting data.
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