Traditional Finishing Processes

Before continuing discussion on advanced ultra precision finishing processes, it is useful to understand the principle of working of commonly used traditional finishing processes; grinding, lapping and honing. All these processes use multipoint cutting edges in the form of abrasives, which may or may not be bonded, to perform cutting action. These processes have been in use from the earliest times because of their capability to produce smooth surface with close tolerances. Higher hardness of abrasive particles compared to workpiece hardness is an important prerequisite of these processes. If properly conducted, these abrasive machining processes can produce a surface of high quality with a controlled surface roughness combined with a designed residual stress distribution and freedom from surface and sub-surface damages (Schey 1987).

8.2.1 Grinding

Grinding is the most widely used abrasive finishing process among all traditional finishing processes used in production. In grinding, the material is removed from the workpiece surface by relative motion between the workpiece and the grinding wheel having abrasive particles embedded on its periphery. The abrasive particles are bonded together to form porous revolving body (Shaw 1996) which when come in contact with workpiece results in material removal. Grinding in a broad perspective is divided into two regimes - stock removal grinding (SRG), and form and finish grinding (FFG). In SRG, the main objective is to remove the superfluous material from the surface, while in FFG, the surface quality is a main concern. The abrasives on a grinding wheel are firmly bonded with an appropriate binder and at the same time also have possibility to allow grain fracture to generate new cutting edges. Abrasive grain wears rapidly on grinding harder materials so a less strongly bonded wheel is preferred for operation. Porosity on the grinding wheel is a controllable factor to provide room to accommodate chips. Wheels bonded with glass are strongest and hardest while organic bonds are of lower strength. The size and distribution of grits along with wheel structure play an important role in grinding performance. A proper selection of wheel according to finishing requirements is very important. Grinding is mainly employed for simple geometries like cylindrical or plane surfaces where size is limited by grinding wheel movement.

8.2.2 Lapping

Lapping uses loose abrasives to finish the surface. It works on three body abrasive wear principle in which finishing action takes place through abrasion by hard particles trapped between workpiece surface and a relatively soft counter formal surface called lap. After introducing abrasive slurry between workpiece and lap surface, the workpiece is held against lap and moved in random paths under pressure. Simple three-dimensional shapes and curved surfaces (concave, convex etc.) can be finished by designing a compliant lap. As this process is generally employed for improving surface finish and accuracy, the amount of material removed is insignificant.

8.2.3 Honing

Honing is another abrasive finishing process generally used to finish internal cylindrical surfaces. The abrasives in the form of stones or sticks carried in an expanding and oscillating mandrel are used to generate random cross-marked surface with good finish. The stick pressure on workpiece surface is comparatively more than lapping. The surface produced after honing has self-lubricating property due to oil retaining capability in crosshatched pattern.

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