Work Material Properties and Their Role in Finishing

Hardness is the resistance to indentation. Because every surface finishing method involves creation of new surfaces, higher hardness always implies greater machining forces (e.g., it is more difficult to machine harder steel than softer steel). Higher hardness also implies greater wear of the cutting tool or abrasive product. This may be due to the wear caused by direct contact between the tool and the work, or it may be due to the wear caused by the hard and abrasive chip after it is released from the work material.

Toughness is the resistance to fracture. It is easier to propagate cracks or fracture through material of lower toughness. Hence, it is easier to machine high-hardness, low-toughness alloy steels than high-toughness stainless steel of equal hardness. However, extremely low toughness is not desirable, as is readily evident from the difficulty in machining glass or ceramics. In these materials it is desirable to use finishing methods or finishing process conditions that prevent the generation of cracks rather than minimizing or controlling crack propagation.

Stiffness is the resistance to deformation. It is often measured as the elastic modulus, or the slope of the stress-strain curve. Because most finishing methods require application of force for surface generation, it is desirable to have minimum deflection or distortion under the applied force to achieve close dimensional tolerances. In this regard, it is easier to achieve complex contours, closer geometry, and closer surface finish in higher-stiffness material. For instance, extremely thin sections can be machined in ceramics, and closer tolerances and surface finishes can be achieved in ceramics than in metals, because ceramics have about twice the stiffness (elastic modulus) of most metals or alloys.

Ductility is the ability of the material to deform plastically before fracture is initiated or propagated. This is generally measured in terms of percentage elongation. Ductile materials are generally more difficult to machine; they require considerably more energy to generate the chips. For instance, the specific energy required to machine soft steel is generally higher than that required to machine cast iron of equal hardness. This energy requirement is minimized by close control of the geometry of the cutting tool. Hence, positive-rake tools of small nose radius are required for machining aluminum, brass, and so forth.

The long, stringy chips produced in ductile materials often lead to extensive sliding that results in erosion or wear of the cutting tool or bond matrix (in the case of abrasive finishing methods). Hence, it is essential to have a means to control the shape (or curl) of the chip, as well as a means to dispose of the chip away from the cutting zone (chip clearance) in the finishing of ductile materials such as aluminum and copper and their alloys, as well as thermoplastic materials.

Ductility is also used to advantage in ultraprecision processes such as lapping or buffing, where mirror finish surfaces are achieved by extensive deformation of surface layers without affecting bulk material structure or properties. This is the technique used in the preparation of hard disks for computers and the machining of printing rolls.

Thermal Properties. Because most finishing methods are mechanical processes in the presence of a thermal front, the thermal properties of work materials play a key role in their response to finishing. For example, the conduction of heat away from the machining zone and the rate of heat buildup determine the maximum temperature at the surface, which influences tool life as well as the quality of the work surface after finishing. As a rule, poor thermal conductors are difficult to machine. Typical examples are polymers, stainless steel, and titanium. Any external effort to reduce surface sliding and heat conduction are very beneficial in the finishing of these materials.

Thermal cracks are generated when the thermal stress exceeds the strength of the material. Materials with high thermal conductivity, a low thermal expansion coefficient, and low stiffness (elastic modulus) are less susceptible to thermal cracks.

Thermal stability is resistance to decomposition. For example, in the presence of high temperatures, titanium readily reacts with carbon to form carbides. This is an inherent limitation for tool life in the finishing of titanium using diamond or silicon carbide tools.

The mechanical properties described earlier (hardness, toughness, ductility, and so on) need to be considered at room temperature as well as at elevated temperatures corresponding to the maximum "cutting" temperature. The conventional wisdom is that the strength of a work material decreases with temperature and increases with strain rate, and that these two opposing effects have a minimal net effect on machining processes. However, this may not always be the case.

Thermal properties are also of interest to the nonmechanical finishing processes such as electrical discharge machining and laser machining. These processes depend primarily on thermal energy and its use for material removal and surface generation.

Thermal effects are inherently unstable and should be carefully controlled in any finishing process. For instance, excessive heat buildup at the grinding zone increases the maximum temperature, which accelerates tool wear. This results in inefficient cutting action, which in turn leads to further increase in heat input to the grinding zone. These effects can proceed until severe damage is caused to the part, the abrasive tool, or both. Thermal effects also depend on the geometry of the work material and the nature of the applied forces. Thin cross sections of work materials are more susceptible to thermal damage than large cross sections, and heat distribution that minimizes thermal inputs to he cross section (e.g., up grinding vs. down grinding) also improves part quality and minimizes thermal damage.

Electrical and Magnetic Properties. Electrical conduction and resistivity are properties of importance, particularly in finishing methods such as electrical discharge machining. Magnetic materials such as ferrous alloys are readily clamped by electromagnetic chucks during their finishing, whereas paramagnetic and nonmagnetic materials require mechanical clamping.

Microstructure Effects. As a general rule, a uniform and predictable microstructure of the work material is preferable for reliable finishing processes and their automation. Inclusions, surface defects, and hard surface layers always impair performance. This is true for castings, forged components, rolled steels, densified ceramics, and so on.

The size, shape, content, and uniformity of hard second-phase material and second-phase particles also influence finishing results. As a general rule, the smaller the hard second-phase particles, the more efficient the finishing.

Grain size of the work material plays a key role in finishing results. Finer grain size is usually associated with higher strength and toughness and greater machining difficulty. Grain size modifications due to processing, such as by heat treatment and shot peening, yield results such as residual stresses, anisotropic properties, and work hardening, all of which pose difficulty in the finishing processes and hence should be carefully controlled or monitored. Changes in the morphology in one finishing process may influence the next finishing step. For example, brass is frequently work hardened during drawing or forming, which may contribute to a nonuniform finish. Polishing prior to buffing is desirable to reduce this nonuniformity.

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