Paul K. Wright

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.

Cutting tool materials III

Publisher Summary nThis 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 nMetal 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 nThere 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 nThis 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 nNo 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.

Cutting tool materials I

Publisher Summary nMetal 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.

Cutting tool materials II

Publisher Summary nDiamond, corundum, and quartzite, among many others, are natural materials used to grind metals. These are used in the form of loose abrasive or as grinding wheels, but are unsuitable as metal-cutting tools because of inadequate toughness. The introduction of the electric furnace led to the production of new hard substances at very high temperatures, the material was silicon carbide. This can be used loose as an abrasive and when bonded with porcelain, is very important as a grinding wheel material, but is not tough enough for cutting tools. The raw materials used in carbide cutting tools are made by melting and subsequent particle-size reduction by ball milling to a fine powder. Newer, chemical routes, such as the sol–gel technique can produce ultra fine materials in nanocrystalline size. These materials are also relatively free from defects. Cost permitting, they will find a niche for high-speed mining. Tungsten carbides are exceptionally hard and are of two types: WC and W2C. Cobalt is the most efficient metal for bonding WC. The carbides of tungsten and molybdenum have hexagonal structures, while the others of major importance are cubic. These rigid and strongly bonded compounds undergo no major structural changes up to their melting points, and their properties are therefore stable and unaltered by heat treatment, unlike steels that can be softened by annealing and hardened by rapid cooling.

Cutting tool materials II: Cemented carbides

Publisher Summary nDiamond, corundum, and quartzite, among many others, are natural materials used to grind metals. These are used in the form of loose abrasive or as grinding wheels, but are unsuitable as metal-cutting tools because of inadequate toughness. The introduction of the electric furnace led to the production of new hard substances at very high temperatures, the material was silicon carbide. This can be used loose as an abrasive and when bonded with porcelain, is very important as a grinding wheel material, but is not tough enough for cutting tools. The raw materials used in carbide cutting tools are made by melting and subsequent particle-size reduction by ball milling to a fine powder. Newer, chemical routes, such as the sol–gel technique can produce ultra fine materials in nanocrystalline size. These materials are also relatively free from defects. Cost permitting, they will find a niche for high-speed mining. Tungsten carbides are exceptionally hard and are of two types: WC and W2C. Cobalt is the most efficient metal for bonding WC. The carbides of tungsten and molybdenum have hexagonal structures, while the others of major importance are cubic. These rigid and strongly bonded compounds undergo no major structural changes up to their melting points, and their properties are therefore stable and unaltered by heat treatment, unlike steels that can be softened by annealing and hardened by rapid cooling.

Chapter 12 – Modeling of metal cutting

Publisher Summary nThe goals of any kind of modeling method are to predict physical behavior from known a priori conditions. Metal cutting poses great obstacles in comparison with other metal-processing operations. Armarego and colleagues, created one of the most comprehensive data, based on practical machining operations from which average forces and torque trends were curve fitted,using multivariable regression analysis. These provide empirical-type equations involving all the relevant operation variables. Armarego has used these empirical-type equations in the development of constrained optimization analyses and software for selecting machining conditions (e.g. feed and speed) for optimum economic performance. The processes considered include: high-speed steel, point-thinned general-purpose drills, high-speed steel, end-milling cutters, and flat-faced turning tools. From a pure research viewpoint, the empirical approach is sometimes criticized for relying on the original laboratory testing data. The underlying assumption behind the mechanistic methods is that the cutting forces are proportional to the uncut chip area. The constant of proportionality depends on the cutting conditions, cutting geometry, and material properties. In the finite element analysis model, four different approaches to formulating and calculating the properties of individual elements include: direct approach, variational approach, weighted residuals approach, and energy balance approach.