Introduction

THE FINISH MACHINING of materials is commonly accomplished by surface finishing processes such as grinding, lapping, and polishing. All of these processes typically use abrasive particles to carry out material removal. In grinding, the abrasive particles are usually bonded to a wheel using a vitrified or resin bond, whereas in lapping and polishing, the particles are usually present in a slurry that is between the workpiece and a lapping block. The material removal action in grinding resembles two-body abrasion, whereas in lapping and polishing, this action is akin to three-body wear. The surface finish (roughness average, or Ra) on ground surfaces is typically in the range of 0.2 to 1 ^m, whereas lapped and polished surfaces can have Ra values of less than 0.05 ^m. The material removal rates in lapping and polishing are usually an order of magnitude less than in grinding. At the fundamental level, material removal takes place in all of these processes due to the localized action of an abrasive particle on the workpiece surface. The local pressures prevalent in the abrasive-workpiece contact region are of the order of the hardness of the workpiece material (Ref 1), and the relative sliding speeds between the particle and the workpiece are in the range of a few centimeters per second to 150 m/s, with the higher speeds occurring for modern high-speed grinders (Ref 2). At the local level, the action of the abrasive particle resembles that of a sliding indenter applied to the workpiece surface.

The localized abrasive-workpiece contact pressures and high sliding speed cause high temperatures to be generated at the interface between the abrasive particle and the work surface, as well as in the work subsurface, due to frictional heating (Ref 3, 4). This is especially so in grinding, where sliding speeds are high (30 to 150 m/s). The high temperatures are an important source of several forms of damage on the machined surface. First, the transient temperatures contribute to residual stresses and microcracking on ground surfaces (Ref 5, 6, 7). Second, the localized temperatures can cause warping of the component being finished, especially when it is of small size, having a relatively large surface-to-volume ratio. This is a serious problem in the finishing of small electronic devices such as recording heads (Ref 8). Third, they can induce phase transformations in the materials being finished. For example, during the grinding of hardened steels, if the surface temperature of the workpiece is sufficiently high, the surface reaustenitizes; a consequence of the transformation is the formation of brittle, untempered martensite. This type of thermal damage is commonly referred to as workpiece burn and is highly undesirable (Ref 9, 10, 11, 12). Another type of phase transformation occurs during the grinding of transformation-toughened zirconia. Here, the phase transformation is due to the transient mechanical and thermal stresses generated during grinding. These forms of thermal damage alter the mechanical, magnetic, and electrical properties of finished materials.

Thus far, thermally induced damage in work materials produced by finishing processes has been highlighted. The local temperatures also play an important role in the degradation of abrasive particles and their bonding material. The transient temperatures prevailing at the abrasive particle tip during grinding contribute significantly to wheel wear. For example, during grinding with diamond wheels, wheel wear can occur by thermally induced degradation of the bond holding the diamond abrasives together on the wheel, by graphitization of the diamond particles when heated above 1200 °C (2200 °F) in air, by microfracture of the abrasive grain as a consequence of repeated heating and cooling, or by diffusion wear of diamond at high temperatures when grinding ferrous metals (Ref 9).

Thermal phenomena thus play a key role in the mechanics of surface finishing processes. The analysis and measurement of temperatures and associated thermal damage generated by finishing processes are essential to the production of engineered components with controlled surface properties. A discussion of these topics constitutes the bulk of this article. Focus is placed on kinematically simple configurations of finishing processes such as surface grinding and flat surface polishing and lapping. The discussion will be applicable with some modifications to kinematically more complex finishing configurations such as form grinding and polishing as well as honing.

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