Post 174 –by Gautam Shah
Material finishing processes are employed essentially for changing the surface textural quality, by alteration, removal or addition. The Finishing processes may improve sensorial aspect (touch-feel-texture), look-colour-pattern, adhesion-wettability, weldability-solderability, corrosion resistance, tarnish resistance, chemical resistance, wear resistance, hardness, electrical conductivity, etc. An unfinished surface is often called mill finish. The finishing processes include acid-alkali treatment, chemical reactions, electro chemical deposition-removal (plating), mechanical procedures, burnishing, singeing, washing, coating, plastering, cladding, etc.
Of these processes GRINDING, POLISHING, HONING, LAPPING AND BUFFING, are briefly described here.
Grinding removes material from the surface to roughen a normally glossy surface like Glass, or polishes a rough surface like Stone. Grinding requires material of higher hardness than the surface material. Grinding is usually a step up or down procedure, using graded series of coarse to fine or fine to course abrasives. The grinding media or abrasives are of many types and forms, such as Carborundum, sandstone, emery, pumice, sand, glass and diamond powders, leather, jute, etc.
Where a material constitution permits, very fine grinding may polish the surface. Grinding is also a cutting operation, where the material is cut or removed as minute chip, swarf or dust.
Grinding wheels usually consist of particles of a synthetic abrasive, such as silicon carbide or aluminum oxide, mixed with a vitrified or resinoid bonding material. Grinding can be coarse or fine, depending on the size of the grit used in the grinding wheel. Metal and glass can be ground to a mirror finish and an accuracy of 0.0000025 cm. Abrasive are used as grinding wheels, sandpapers, honing stones, polish, cut-off wheels, tumbling and vibratory mass-finishing media, sandblasting, pulp-stones, ball mills, and many other tools and products.
Stone surfaces are chiselled to split the material into thinner sections, to remove the weathered crust and also to level out the surface. Grinding and polishing is done to Hard materials such as building stones, marbles, granite, metals, glass; Precious and Semiprecious stones like gems, diamonds; Animal products like ivory, bones, horns, teeth, leather; Plant products like timbers, seeds; Ceramics like pottery products, bricks, cement concrete and other cement products.
POLISHING, HONING, LAPPING, BUFFING
Polishing uses extremely fine abrasive substances, such as jewellers rouge, Tripoli, whiting, putty powder and emery dust to rub or burnish an extremely smooth and brilliant finish on the surface of a material. The polishing materials are coated on the surface of cloth, felt, leather, rubber pr polymer wheels or as belts. Metal surfaces are levelled and finished by honing and lapping. Honing removes less than 0.0125 millimetres of material from the surface to eliminate micro scratches and machine marks from ground machine parts. It is done with bonded abrasive sticks or stones that are mounted in a honing head. Lapping is a process in which a soft cloth (wool, linen and chamois-leather) impregnated with abrasive pastes (rubbing compounds), is rubbed against the surface of a work-piece. Honing and lapping, are essentially metal finishing techniques. Buffing is a term used for polishing of metals. Buffing is done with polishing compounds and brushes of various shapes, and materials, likes: (animal hair, synthetic fibres, plant fibres -coir), flex, wool and leather. Barber polishing the razor on a leather stripe is a buffing process that levels out small nicks on the blade.
There are four types of precision grinding machines: Centre-type grinders used for tiny valve spools to steel mill roles. Centre-less grinders used for bowling balls, surgical sutures, and tapered roller bearings. Internal grinders are employed for inside diameters of gears, bearing races, and similar parts. Surface grinders are used for die tops, bench surfaces.
Lapping is used to produce a high-quality surface finish or to finish a work-piece within close size limits. Dimensional tolerances of 0.00005 millimetres can be achieved in the hand or machine lapping of precision parts such as gauges or gauge blocks.