Edward M. Trent

Chapter 5 – Heat in metal cutting

Publisher Summary This chapter focuses on the role of heat in limiting the rate of metal removal when cutting the higher melting point metals. The power consumed in metal cutting is largely converted into heat near the cutting edge of the tool, and many of the economic and technical problems of machining are caused directly or indirectly by this heating action. The bulk of cutting, however, is carried out on steel and cast iron, and it is in the cutting of these, together with the nickel-based alloys, that the most serious technical and economic problems occur. With these higher melting point metals and alloys, the tools are heated to high temperatures as metal removal rate increases and, above certain critical speeds, the tools tend to collapse after a very short cutting time under the influence of stress and temperature. It is, therefore, important to understand the factors, which influence the generation of heat. The most important heat source responsible for raising the temperature of the tool is the flow-zone where the chip is seized to the rake face of the tool. The amount of heat required to raise the temperature of the very thin flow-zone is a small fraction of the total energy expended in cutting, and the volume of metal heated in the flow-zone may vary considerably. Therefore, there is no direct relationship between cutting forces or power consumption and the temperature near the cutting edge.

Chapter 4 – Forces and stresses in metal cutting

The forces acting on the tool are an important aspect of machining. For those concerned with the manufacture of machine tools, knowledge of the forces is needed for estimation of power requirements and for the design of machine tool elements. The cutting forces vary with the tool angles, and accurate measurement of forces is helpful in optimizing tool design. Scientific analysis of metal cutting also requires knowledge of the forces. Many force measurement devices, known as dynamometers, have been developed that are capable of measuring tool forces with increasing accuracy. There are three force components: cutting force ( F c ), feed force ( F t ), and the third component, acting in the direction OZ . F c is the largest of all the three and acts in the direction of the cutting velocity. F t acts on the tool in the direction OX, parallel with the direction of feed. The very high normal stress levels account for the conditions of seizure on the rake face. The existing knowledge of stress and stress distribution at the tool-work interface is far too scanty to enable tools to be designed on the basis of the localized stresses encountered. Even for the simplest type of tooling only a few estimates are available, using a two-dimensional model. However, the present level of knowledge is useful in relation to analyses of tool wear and failure, the properties required of tool materials and the influence of tool geometry on performance.

Chapter 10 – Coolants and lubricants

Publisher Summary Many machine tools are fitted with a system for handling the cutting fluids. Such systems include circulating pumps, piping and jets for directing the fluids to the tool, and filters for clearing the used fluid. A very large number of cutting fluids are available commercially, from which the production planners select the one most suitable for a particular application. There are two major groups of cutting fluid: water-based or water-miscible fluids, and neat cutting oils. Understanding of the action of coolants and lubricants in metal cutting is still at a rather primitive level. There are many applications where cutting is carried out dry, in air, with no advantage being found in the use of a cutting fluid. Lubricating requirements are most exacting with difficult operations, such as broaching, lapping, thread cutting, reaming, trepanning of deep holes, and the bobbing of gears. For such operations, workshop trials must be the criterion for the optimum cutting fluid. It is in connection with the machining of steel and other high melting-point metals that the use of coolants becomes essential. Their use is most important when cutting with high-speed steel tools, but they are often employed also with carbide tooling. Coolants cannot prevent the heat being generated since they cannot act directly on the thin zone, which is the heat source. However, the coolant can remove heat from those surfaces of the chip, the work piece, and the tool, which are accessible to the coolant.

Cutting tool materials III

Publisher Summary This chapter reviews the properties and performance of ceramic tools, aluminum-based composites, sialons, cubic boron nitride, and diamond. The basic raw material, alumina, is cheap and plentiful, but the processing is expensive and the tool tips are therefore not cheap compared with cemented carbides. The room temperature hardness of alumina tools is in the same range as that of the cemented carbides. The potential speeds using alumina tools, which are three to four times higher than those normally used with carbide tools, represent an increase in metal removal rate as great as that achieved by high-speed steel and by cemented carbides at their inception. There seems to be less potential advantage to be gained by increasing cutting speeds beyond those achieved by cemented carbides. Alumina-based composites are used mainly in high speed machining of cast iron and can be applied in a wider range of applications than pure AI2O3 because of the increased toughness that results from the addition of the TiC. Sialon has low coefficient of thermal expansion and high thermal conductivity, which provide increased resistance to thermal shock and thermal fatigue compared with alumina-based ceramics. Like diamond, CBN is a very rigid structure, but in this case not all the bonds between neighboring atoms are covalent. Synthetic diamonds are not made in sizes large enough to make single point tools; rather they are made by consolidating fine diamond-powder into blocks of useful size.This chapter reviews the properties and performance of ceramic tools, aluminum-based composites, sialons, cubic boron nitride, and diamond. The basic raw material, alumina, is cheap and plentiful, but the processing is expensive and the tool tips are therefore not cheap compared with cemented carbides. The room temperature hardness of alumina tools is in the same range as that of the cemented carbides. The potential speeds using alumina tools, which are three to four times higher than those normally used with carbide tools, represent an increase in metal removal rate as great as that achieved by high-speed steel and by cemented carbides at their inception. There seems to be less potential advantage to be gained by increasing cutting speeds beyond those achieved by cemented carbides. Alumina-based composites are used mainly in high speed machining of cast iron and can be applied in a wider range of applications than pure AI 2 O 3 because of the increased toughness that results from the addition of the TiC. Sialon has low coefficient of thermal expansion and high thermal conductivity, which provide increased resistance to thermal shock and thermal fatigue compared with alumina-based ceramics. Like diamond, CBN is a very rigid structure, but in this case not all the bonds between neighboring atoms are covalent. Synthetic diamonds are not made in sizes large enough to make single point tools; rather they are made by consolidating fine diamond-powder into blocks of useful size.

Cutting tool materials I: High speed steels

Publisher Summary Metal cutting tools are subjected to a tremendous range of environments. The pressures of technological change and economic competition have imposed demands of increasing severity. To meet these requirements, new tool materials are sought and a very large number of different materials should be tried. The novel tool materials, which have passed the trials, are the products of the persistent effort of thousands of crafts-people, inventors, technologists and scientists, blacksmiths, engineers, metallurgists, and chemists. The tool materials, which have survived and are commercially available are those that have proved fittest to satisfy the demands put upon them. The agents of this “natural selection” are the machinists, tool-room supervisors, tooling specialists, and buyers in the engineering factories, who effectively decide which of all the potential tool materials shall survive. The only tool material for metal cutting from the beginning of the Industrial Revolution until the 1860s was carbon tool steel. Prolonged industrial experience was the guide to the selection of optimum carbon content for particular cutting operations. High speed steels give a modest improvement in the speed at which steel could be cut compared with carbon steel tools. The properties of high-speed tools are the result of precipitation hardening within the martensitic structure of the chromium, tungsten, molybdenum, and vanadium tool steels after a very high temperature heat treatment. Coating of tools with thin layers of TiN by a PVD process, which prolongs tool life in most situations and in others gives a smoother surface finish on the machined part.

Chapter 1 – Introduction: Historical and economic context

Publisher Summary There is a great similarity between the operations of cutting and grinding procedure that are followed today and the one used by the ancestors. The grinding wheel does much of the same job as the file, which can be classified as a cutting tool, but has a much larger number of cutting edges, randomly shaped and oriented. In the engineering industry, the term machining is used to cover chip-forming operations, and this definition appears in many dictionaries. Most machining today is carried out to shape metals and alloys, but the lathe was first used to turn wood and bone. To summarize the economic importance, the cost of machining amounts to more than 15% of the value of all manufactured products in all industrialized countries. Progress in machining is achieved by the ingenuity, logical thought, and dogged worrying of many thousands of practitioners engaged in the many-sided arts of metal cutting. The machinist operating the machine, the tool designer, the lubrication engineer, and the metallurgist are all constantly probing for solutions to the challenges presented by novel materials, high costs, and the needs for faster metal removal, greater precision, and smoother surface finish. During cutting, the interface between tool and work material is largely inaccessible to observation, but many researchers have contributed indirect evidence concerning stresses, temperatures, metal flow, and other interactions.

Cutting tool materials III: Ceramics, CBN diamond

Publisher Summary This chapter reviews the properties and performance of ceramic tools, aluminum-based composites, sialons, cubic boron nitride, and diamond. The basic raw material, alumina, is cheap and plentiful, but the processing is expensive and the tool tips are therefore not cheap compared with cemented carbides. The room temperature hardness of alumina tools is in the same range as that of the cemented carbides. The potential speeds using alumina tools, which are three to four times higher than those normally used with carbide tools, represent an increase in metal removal rate as great as that achieved by high-speed steel and by cemented carbides at their inception. There seems to be less potential advantage to be gained by increasing cutting speeds beyond those achieved by cemented carbides. Alumina-based composites are used mainly in high speed machining of cast iron and can be applied in a wider range of applications than pure AI2O3 because of the increased toughness that results from the addition of the TiC. Sialon has low coefficient of thermal expansion and high thermal conductivity, which provide increased resistance to thermal shock and thermal fatigue compared with alumina-based ceramics. Like diamond, CBN is a very rigid structure, but in this case not all the bonds between neighboring atoms are covalent. Synthetic diamonds are not made in sizes large enough to make single point tools; rather they are made by consolidating fine diamond-powder into blocks of useful size.This chapter reviews the properties and performance of ceramic tools, aluminum-based composites, sialons, cubic boron nitride, and diamond. The basic raw material, alumina, is cheap and plentiful, but the processing is expensive and the tool tips are therefore not cheap compared with cemented carbides. The room temperature hardness of alumina tools is in the same range as that of the cemented carbides. The potential speeds using alumina tools, which are three to four times higher than those normally used with carbide tools, represent an increase in metal removal rate as great as that achieved by high-speed steel and by cemented carbides at their inception. There seems to be less potential advantage to be gained by increasing cutting speeds beyond those achieved by cemented carbides. Alumina-based composites are used mainly in high speed machining of cast iron and can be applied in a wider range of applications than pure AI 2 O 3 because of the increased toughness that results from the addition of the TiC. Sialon has low coefficient of thermal expansion and high thermal conductivity, which provide increased resistance to thermal shock and thermal fatigue compared with alumina-based ceramics. Like diamond, CBN is a very rigid structure, but in this case not all the bonds between neighboring atoms are covalent. Synthetic diamonds are not made in sizes large enough to make single point tools; rather they are made by consolidating fine diamond-powder into blocks of useful size.

Chapter 11 – High speed machining

Publisher Summary No single component or manufacturing process should be analyzed and optimized in isolation. There will always be something that can be improved, simplified, or made cheaper if design and manufacturing are viewed from a slightly wider system perspective. During the late 1960s and throughout the 1970s, experiments with projectiles and ultra high-speed cutting were conducted by several research teams. Experiments have reported that there was no sudden reduction in tool wears at the very high cutting speeds and that the tool life was extremely short. Such findings meant that interest in high speed machining dropped-off after the mid-1980s, and it is only recently that interest has been revived. Even now the interest is focused on the higher production rates for aluminum and cast iron—not particularly the reduction of tool wear. All the evidence points to the fact that most materials—even soft aluminum alloys—begin to machine with a segmental or serrated chip at some particular threshold value of the cutting speed. It is demonstrated that such chip forms accelerate attrition wear especially in carbide tools. An ultimate objective is thus to increase speed and yet minimize the degree of segmentation and reduce the fatigue type loading on the tool edge.

Chapter 3 – The essential features of metal cutting

Publisher Summary Metals and alloys are very hard, so that no known tool materials are strong enough to withstand the stresses, which they impose on narrow, knife-like cutting edges. In spite of the diversity of the geometries of machining operations, the restrictions on tool geometry are features of all metal cutting operations and they provide a common ground from where an analysis of machining commences. The formation of new surfaces requires energy, but in metal cutting, the theoretical minimum energy required to form the new surfaces is an insignificant proportion of that required to deform plastically the whole of the metal removed. The chip is enormously variable in shape and size in industrial machining operations and is of two types: continuous and discontinuous. The study of the formation of chips is difficult, because of the high speed at which it takes place under industrial machining conditions, and the small scale of the phenomena, which are to be observed. High-speed cine-photography at relatively low magnification is generally used. The simplified conditions used in the first stages of laboratory investigations of formation of chip are known as orthogonal cutting. Even with orthogonal cutting, the cross section of the chip is not strictly rectangular. In many cases, the work material is not available in a tube form, and what is sometimes called semi-orthogonal cutting conditions are used, in which the tool cuts a solid bar with a constant depth of cut. Detailed knowledge of the chip formation process is required for the understanding of the accuracy and condition of the machined surface of the desired component. Machined surfaces are inevitably damaged to some degree, since the chip is formed by the shear fracture at high strain.

Chapter 2 – Metal cutting operations and terminology

Publisher Summary Almost all metals and alloys are machined. Machining produces most shapes used in the engineering world. Many different machining operations are used, such as: turning, boring operations, drilling, facing, forming and parting off, milling, and shaping and planning. These operations are some of the more important ones employed in shaping engineering components. Each of these operations has its special characteristics and problems, as well as the features that it has in common with the others. It is difficult to appreciate the action of many types of tools without actually observing or, preferably, using them. Essentially the conditions of boring of internal surfaces differ little from those of turning, but this operation illustrates the importance of rigidity in machining. An essential feature of drilling is the variation in cutting speed along the cutting edge. The performance of cutting tools is very dependent on their precise shape. Turning, boring, and drilling generate cylindrical or more complex surfaces of rotation. Cutting is often carried out in air, but in many operations the use of a fluid to cool the tool or the work piece, and/or act as a lubricant, is essential to efficiency. This constitutes the environment of the tool.