Chapter 21: Machining of Metals

Machining of Metals from Mechanical Metallurgy presents a comprehensive analysis of the machining of metals, distinguishing it from deformation processes by defining it as a material removal operation where the shape is produced by straining a local region of the workpiece to fracture. The text begins by categorizing machining operations based on the relative motion between the tool and workpiece, describing primary motions and feed motions in fundamental processes such as turning, boring, shaping, and milling. A significant portion of the chapter is dedicated to the mechanics of machining through the model of orthogonal cutting, which simplifies the complex stress state to two dimensions to analyze the relationships between the rake angle, clearance angle, and the shear plane where plastic deformation occurs. The summary details the formation of different chip types—continuous, built-up edge (BUE), and discontinuous—and explains how the shear strain and strain rate in cutting are calculated, noting they are magnitudes higher than in standard metalworking. It further explores the force relationships in cutting using dynamometers to resolve cutting and thrust forces, allowing for the calculation of the coefficient of friction and specific cutting energy, which is primarily consumed by shear deformation and friction at the tool-chip interface. The discussion expands to three-dimensional machining, introducing geometric parameters like the angle of inclination and chip flow angle to determine the effective rake angle. Thermodynamics plays a crucial role, as the high strain rates create near-adiabatic conditions, necessitating an analysis of heat distribution and the thermal number to predict temperature rises that influence tool life. Consequently, the functions of cutting fluids are examined, highlighting their roles in cooling, lubrication, and chip removal. The chapter also provides an in-depth review of tool materials, ranging from carbon steels and high-speed steels to cemented carbides and ceramics, and analyzes tool wear mechanisms such as crater wear and flank wear. Taylor's tool life equation is introduced to mathematically relate cutting speed to tool longevity, serving as a basis for defining machinability. Furthermore, the text distinguishes grinding from cutting, characterizing it as a process with random abrasive cutting edges and high negative rake angles that result in high specific energy consumption and significant heat generation. Nontraditional machining processes, including electrical discharge machining (EDM), electrochemical machining (ECM), and ultrasonic machining (USM), are described for applications involving hard materials or complex geometries. Finally, the chapter concludes with an economic analysis of machining, deriving cost equations to determine the optimum cutting speed that balances machining time costs against tool changing and replacement costs.