Boris Dimitrov

Tribology of Abrasive Machining Processes

This book draws upon the science of tribology to understand, predict and improve abrasive machining processes. Pulling together information on how abrasives work, the authors, who are renowned experts in abrasive technology, demonstrate how tribology can be applied as a tool to improve abrasive machining processes. Each of the main elements of the abrasive machining system are looked at, and the tribological factors that control the efficiency and quality of the processes are described. Since grinding is by far the most commonly employed abrasive machining process, it is dealt with in particular detail. Solutions are posed to many of the most commonly experienced industrial problems, such as poor accuracy, poor surface quality, rapid wheel wear, vibrations, work-piece burn and high process costs. This practical approach makes this book an essential tool for practicing engineers. Uses the science of tribology to improve understanding and of abrasive machining processes in order to increase performance, productivity and surface quality of final products. A comprehensive reference on how abrasives work, covering kinematics, heat transfer, thermal stresses, molecular dynamics, fluids and the tribology of lubricants. Authoritative and ground-breaking in its first edition, the 2nd edition includes 30 per cent new and updated material, including new topics such as CMP (Chemical Mechanical Polishing) and precision machining for micro-and nano-scale applications.

6 – Thermal design of processes

Well-designed abrasive machining processes usually enhance workpiece surface quality producing low roughness, compressive or neutral residual stresses, and improved fatigue life. Conversely, abusive machining leads to a range of forms of surface damage. Generally, the higher the machining temperature, the greater is the damage caused to the surface. Thermal effects are usually deleterious to surface integrity as demonstrated by diffusion leading to grain growth, precipitation, and softening; phase transformations leading to re-hardening; thermal expansion leading to expansion, contraction, possible cracking, and tensile residual stresses; and chemical reactions leading to increased oxidation. In this chapter, consideration is given to surface damage with examples of different types of thermal damage and explanations. Examples of the effects of temperature on grinding swarf and of transition temperatures for tensile residual stresses are given. Relationships are explored between process energy, selection of grinding conditions, and aspects of the process design to achieve high productivity and surface integrity. Examples of measured grinding temperatures showing the effects of different abrasives and grinding conditions are given. The latest Rowe thermal model is presented for prediction of grinding temperatures including data for workpiece conduction at different speeds and depths of cut. Application to deep and shallow grinding at high and low work speeds with case studies is demonstrated. A new model for fluid convection is presented.

15 – Tribochemistry of Abrasive Machining

Chapter 16 describes tribochemical reaction processes and wear processes involved in abrasive machining. Tribochemical wear is the most complex type of wear involving chemical processes, due to active agents in the process fluid and/or the surrounding atmosphere, and mechanical action due to the removal process under the action of the applied forces. Chemical action results from the active substances in the environment and in the process fluid. Corrosion wear is manifested by formation of reaction products as a result of chemical interactions between the elements of a tribosystem by tribological action. The prefix tribo is used to denote the special nature of a mechanical, physical, and chemical process when subject to intense surface deformation, friction, and micro-cutting in abrasive contact. For better understanding of tribochemical processes in the contact between abrasive and work-material, it is necessary to consider the nature of the components of an abrasive and the nature of the work-material. Other elements involved are the abrasive bond, the process fluid and the environment. Each of these is considered and their roles reviewed in terms of such characteristics as chemical composition and affinity. This is a substantial chapter analyzing complex effects and selection aspects of great importance. Simple conclusions are achieved for preferred corrosion types. References are provided to previous work on tribochemistry of abrasive machining processes.

Process fluids for abrasive machining

Chapter 15 reviews the main types of process fluids used in abrasive machining and their characteristics. In abrasive machining, liquid fluids are usually preferred to gaseous fluids. Besides cooling, a process fluid has the important function of lubricating. Lubrication means interposing a layer of low shear-strength at the interface between the elements of the friction pair. The main role of a lubricant is to prevent or, at least, to diminish solid contact between the elements and to reduce adhesion wear. This chapter develops from the principles of fluid delivery described in Chapter 8 to discuss regimes of lubrication and the tribochemistry implied in the selection of process fluids. Types of lubricants are discussed including mineral oils, organic oils, synthetic oils, emulsions, solutions, and additives for a range of essential functions. These are listed and reviewed. Physical properties of process fluids are described. Chemical properties, biological, and tribological properties are reviewed. This leads to degradation of fluids, and the need for analysis, monitoring, and correction of fluids. References are given to previous work on process fluids for abrasive machining.

13 – Loose abrasive processes

Chapter 13 describes a range of loose abrasive processes. Two main processes of lapping and polishing are mainly employed in high-precision finishing. Typical examples of various components used in aerospace, automotive, mechanical seals, fluid handling, and many other precision engineering industries are furnished. Advanced abrasive powders often of nanometer sizes are used. Because the abrasives are often of submicron size for ultraprecision processing, the term nanotechnology may be employed. These types of operations are capable of producing fine finishes on both ductile and brittle materials. The relative speeds in lapping and polishing are much lower than in grinding. Consequently, the concentration of energy in the contact area is much lower. The benefit is that average temperatures tend to be lower than in grinding and may be negligible; the disadvantage is that specific energy is higher, although the volumes of material to be removed are small. Types of abrasives and differences between two- and three-body abrasion are discussed. Also discussed are the nature of lapping and polishing tools and process fluids and fluid delivery. References are given to previous work on loose abrasive processes.

11 – Abrasives and Abrasive Tools

Materials used as abrasives include both natural minerals and synthetic products. Abrasive grains can be considered as randomly shaped cutting tools characterized by high hardness, sharp edges, and good cutting ability. This chapter discusses the nature of common abrasives and machining characteristics. Standard methods of specifying abrasive wheels are described. Conventional abrasives are described including silicon carbide, aluminium oxide, and garnet. Two superabrasives described include diamond and cubic boron nitride. Abrasives and new abrasive grain developments are discussed in some detail including grains produced by sol gel and related processes. The requirements of an abrasive are discussed in relation to application. Various selection criteria apply depending on workpiece material, work geometry, grinding fluid and removal conditions. The structure of abrasives is discussed and the nature of abrasive tools including types of wheel bonds. Bonded structures include vitrified bonds, organic bonds, and for super-abrasives single layer bonds and metal bonds. Essential calculations methods are provided for the design of grinding wheels. Coated abrasives and abrasive belts are also discussed. References are provided to previous work including a range of abrasive manufacturers.

10 – Grinding wheel and abrasive topography

Topography is concerned with defining and mapping the shape of a surface, in this case the grinding wheel or other abrasive tool. Both macro-topography and micro-topography are important, although in different ways. The basic wheel shape is part of the macro-topography and is important for the overall accuracy of the workpiece profile generated. Micro-topography is significant for maintaining the required workpiece profile, but additionally for workpiece roughness, energy requirements, downtime for redressing, wheel-life, surface integrity, and removal rates. This chapter discusses the importance of grinding wheel and abrasive topography with examples of machining inaccuracies that can arise. Parameters for definition of abrasive topography including cutting edge dullness are given. Measurement techniques are described for topography including two-dimensional and three-dimensional techniques. Techniques may be stylus or optical. Replication techniques may be employed to avoid removing a grinding wheel from the machine. Image processing techniques are also described. Different techniques each have their own advantages and disadvantages. Bond elasticity is shown to affect cutting edge density. Results are presented for wear characteristics. More porous wheels are shown to have different wear characteristics from wheels of conventional porosity. References are given to previous work on abrasive topography.

Electrolytic in-process dressing grinding and polishing

This chapter introduces and reviews abrasive processes assisted by electrolytic in-process dressing (ELID) techniques first introduced by Hitoshi Ohmori in 1990. This in situ dressing method is used for metal-bond wheels and is relatively new. This technique is highly successful for fine grain wheels efficiently used to obtain very low surface roughness, and when hard ceramics have to be machined using a very small grain cutting depth in order to avoid failure by cracking. The basic system, principles, and characteristics of ELID abrasion mechanisms are introduced first. Electrical, mechanical and chemical aspects of the process are described. The success and wide application of ELID principles to ceramic grinding are explained. Fourteen applications of the ELID principle to modern abrasive processes are documented to illustrate the scope of application. It is shown how super-finishes can be achieved and greatly increase removal rates through the application of ELID dressing, grinding and polishing processes.

7 – Molecular dynamics for nano-contact simulation

Many areas, such as optical systems, bearings, computer memory components, and microelectronic devices, rely on advances in fabrication techniques for the manufacture of cheaper, smaller, and more precise components. Work-materials of choice for most advanced technologies are ceramics because of excellent thermal, shock, chemical, and wear resistance. However, brittleness prevents ceramics from being machined using conventional machines in a way that ensures high precision in both form and surface finish. Generally in grinding, material removal proceeds through controlled localized surface fracture, leaving a rough surface unacceptable for most applications. The challenge is to avoid fracture and chipping while grinding ceramics. Material removal for ceramics has to be carried out in the ductile regime. Current research efforts are focused on achieving machining accuracy of less than 100 nm. The demand is for nanometer scale depth of the cut, so that mechanical interactions at the machining interface play a stronger role than macroscopic chemical and physical actions. Atomistic simulation of the workpiece material/machining tool interface coupled with an analysis of surface/interface mechanics provides a powerful approach to understanding factors that govern nanoscale precision. This chapter introduces basic concepts and challenges to be overcome in providing solutions to abrasive nano-processing. Application examples are presented to illustrate typical results.

2 – Tribosystems of abrasive machining processes

Chapter 2 builds on the nature of the system introduced in Chapter 1 . The aim is to develop the concept of a tribosystem and, in particular, the nature of an abrasive machining process tribosystem. The system concept and system approach are very helpful in setting up a process to investigate and optimize a process. Systems involve inputs, outputs, and relationships between inputs and outputs, and it is necessary to define the nature of these relationships. The relationships between inputs and outputs involve a complex range of factors. A systems analysis attempts to define these factors and seeks to build a description of the transfer functions that transform inputs into outputs. The basic elements that contribute to abrasive machining tribosystem analysis are described systematically. The inter-relationship processes between the main structural elements of the tribosystem are described including contact processes, friction processes, tool-wear processes, workpiece wear processes, and lubrication processes. The outputs of the system are also considered in terms of the factors that may be considered as describing the efficiency of the system. Efficiency factors include volume removed, removal rate, tool-wear volume, forces, machine stiffness, process energy, processing time, costs, surface roughness, and integrity. This chapter also defines basic parameters available for process optimization. References are provided to published works that underpin the study of the systems approach.