In a workshop, metal parts are manufactured by performing different operations in lathe, shaping machine, milling machine, drilling machine or grinding machine. In order to enhance the quality of surfaces of these parts, several surface finishing processes are performed on them. If better finish is desired for looks, for accuracy, for wearing qualities or for better fits, one of the following processes is employed.
- Super finishing
Lapping is the abrading process that is used to produce geometrically true surfaces, correct minor surface imperfections, improve dimensional accuracy to provide a very close fit between two surfaces in contact. Very thin layers of metal (0.005 to 0.01 mm) are removed in lapping. Machining can be done to the accuracy of less than 1micron.
To perform lapping operation, lapping shoes and lapping mixture are needed. Laps may be made of almost any material soft enough to receive and retain the abrasive grains. They are made of soft cast iron, brass, copper or lead. It is made in different shapes. Abrasive powders such as emery, corundum, iron oxide and chromium oxide are mixed with oil or grease to make lapping mixture.
The face of the lap becomes charged with abrasive particles. Laps may be operated by hands or by machine. Cylindrical work may be lapped by rotating the work in a lathe and reciprocating the lap over the work. Flat surfaces may be lapped by holding the work against a rotating disc. Special lapping machines like vertical lapping machine, centreless lapping machine and abrasive belt lapping machines are also widely used.
Honing is the abrading process done mostly for finishing round holed produced by drilling, reaming or boring by means of bonded abrasive stones called ‘hones’. Honing is a machining process and is used to remove metal upto 0.25 mm. The surface roughness value can be maintained between 0.025 and 0.4 microns. So honing is used to correct some out of roundness, tapers, tool marks and axial distortion.
Honing stones are used for performing honing. Honing toolhead fitted with honing stones is fitted on spindles and rotated. The parts having holes to be honed are mounted on vises or suitable fixtures. The spindle is moved vertically to abrade the walls of the holes. A good quantity of coolant should be applied while honing.
Honing can be done on materials like plastic, silver, brass, aluminium, cast iron, steel and cemented carbide. Journal bearings supporting the crank shafts and long holes found in the barrels of guns are generally honed. The honing machines are of two types – Vertical & Horizontal.
The process of superfinishing is an operation intended to produce an extremely high quality of surface finish. The surface roughness value can be maintained between 0.015 and
0.32 microns. A very thin layer of metal (0.005 mm to 0.02 mm) is removed by using very fine size of abrasives (Size of 400 to 600)in superfinishing. It can be done on both external and internal surfaces.
The grinding stones are made to reciprocate and the workpiece is made to rotate or reciprocate. A fine surface is obtained by admitting coolant mixed with kerosene. Using some special machines, superfinishing is performed on crankshaft, journal bearings and cam shafts.
Polishing is a surface finishing operation performed by a polishing wheel for the purpose of removing metal to take out scratches, tool marks and other defects from rough surfaces. Polishing is performed only to provide better looks. Polishing wheels are made of leather, paper, canvas, felt or wool. The abrasive grains are setup sometimes on the faces of the wheel and work is held against it and rotated to give the desired finish.
Buffing is used to give a much higher, reflective finish that cannot be obtained by polishing. Buffing wheels are made of felt, leather and pressed & glued layers of a variety of cloth. The abrasive used are iron oxide, chromium oixde, emery, etc. The abrasive is mixed with a binder. The binder is a paste consisting of wax mixed with grease, paraffin and turpentire. It is applied either on the buffing wheel or on the work. Buffing wheels are rotated against the work to get a superior finish.
There will always be some minor imperfections on the machined surfaces. They are removed by an hand tool called scraper. There are three types of scrapers – flat, half-round and triangular. The part to be scraped is fitted in a vise and a thin layer of Persian blue is applied on the surface. A suitable scraper is selected and circular movements are made on the surface with it. Thin flakes of metal are removed. Persian blue is once again applied to check the flatness.
Electroplating is the process of applying metallic coatings on the surfaces of metal parts. It can be done on parts of non-metals also.
This is done for protection against corrosion or against wear and tear and for better appearance. It is also done to slightly increase the size of worn out parts and to make parts easy to solder. It may also be used to keep off selected areas on steel parts from being carburized during heat treatment.
Common plating materials are chromium, nickel, copper, zinc, cadmium, etc. The more precious metals like silver, gold, platinum, and radium are also applied for plating. Door handles and automobile parts are chromium plated for appearance. The method of plating a layer of zinc is known as galvanising.
Surfaces to be plated must be buffed smooth to eliminate scratches. The surface is cleaned by suitable cleaning solutions to remove all grease and dirt.
The four essential elements of plating process are the part to be plated (cathode), plating material (anode), electrolyte and direct current. The current leaves anode which is a bar of plating metal and migrates through the electrolyte to the cathode which is the part to be plated.