Super Finishing Processes

Super Finishing Processes, Surface finishing, Surface roughness, Super finishing, Honing, Lapping, Burnishing, Buffing, Polishing

Surface finishing:

               Good surface finish on part is an essential requirement of a product quality and for its better life. Surface finishing process on the part, does not changes its dimension significantly. In surface finishing process, very negligible amount of material is removed or material is added to the surface of the job. Surface finishing processes should not be misunderstood as metal removing processes. Surface cleaning process is also accepted as a surface finishing process.

Some of the surface finishing processes are:

• Buffing

• Lapping

• Electroplating

• Polishing

• Super finishing

• Metal spraying

• Honing

• Belt grinding

• Anodizing.

Surface roughness:

               It refers to relatively finely spaced surface irregularities of small wavelength on the part, which results from the inherent action of the manufacturing process. It may involve transverse, feed marks and the irregularities on the machined surface.

Super finishing:

               Super finishing is an operation using bonded abrasive stones in a particular way to produce an extremely high quality of surface finish in conjunction with an almost complete absence of defects in the surface layer. Avery thin layer of metal (0.005 to 0.02 mm] is removed in super finishing. This operation may be applied for external and internal surfaces of parts made of steel, cast iron and non-ferrous alloys. In super finishing, a very fine grit [grain size 400 to 600] abrasive stick is retained in a suitable holder and applied to the surface of the work-piece with a light spring pressure. The stick is given a feeding and oscillating motion and the work-piece is rotated or reciprocated according to the requirements of the shape being super finished. In this process, the work rotational speed is low [2 to 20 m/min] the longitudinal feed ranges from 0.1 to 0.15 mm per work-piece revolution. The abrasive stick oscillates rapidly in short strokes [2 to 5 mm] with a frequency from 500 to 1,800 strokes per minute and the springs hold the stick against the work with a force from 2 to 10 kg. Special general-purpose machine tools are available for super finishing. Other types of ordinary machines, in particular, lathes, are sometimes employed for this purpose. Single purpose machine tools for example, for finishing crankshaft journals, camshafts, etc., are also used.

 Super Finishing Processes 

Honing:

               Honing is grinding or abrading process mostly for finishing round holes by means of bonded abrasive stones, called hones. Honing is cutting operation and has been used to remove as much as 3 mm of stock but is normally confined to amount less than 0.25 mm. Honing is primarily used to correct some out of roundness, taper, tool marks, and axial distortion. Honing stones are made from common abrasive and bonding materials, often impregnated with sulphur, resin, or wax. Materials honed range from plastics, silver, aluminium, brass, and cast iron to hard steel and cemented carbides. When honing is done manually the tool is rotated, and the work-piece is passed back and forth over the tool. For precision honing, the tool is given a slow reciprocating motion as it rotates. A typical honing tool head is shown in figure:

               The honing tool may be so made that a floating action between the work and tool prevails and any pressure exerted in the tool may be transmitted equally to all sides. Honing is done on general purpose machines, such as the lathe, drill press, and portable drills. There are two general types of honing machines:

1.Horizontal

2.Vertical.

               A honing machine rotates and reciprocates the hone inside holes being finished. The two motions produce round and straight holes that have a very fine surface finish free from scratches. Vertical honing machines are probably more common. Horizontal honing machines are often used for guns and large bores.

Lapping:

               Lapping is an abrading process that is used to produce geometrically true surfaces, correct minor surface imperfections, improve dimensional accuracy, or provide a very close fit between two contact surfaces. Very thin layers of metal (0.005 to 0.01 mm] are removed in lapping. Lapping is unable to correct substantial errors in the form and sizes of surfaces. It is low efficiency process and is used only when specified accuracy and surface finish cannot be obtained by other methods. Abrasive powders [flours] such as emery, corundum, iron oxide, chromium oxide, etc., mixed with oil or special pastes with some carrier are used in lapping. Most lapping is done by means of lapping shoes or quills, called laps, that are rubbed against the work. The face of a lap becomes "charged" with abrasive particles. Charging a lap means to embed the abrasive grains into its surface. Laps may be operated by hand or machine, the motion being rotary or reciprocating. Cylindrical work may be lapped by rotating the work in lathe or drill press and reciprocating the lap over the work in an ever changing path. Small flat surfaces may be lapped by holding the work against a rotating disc, or the work may be moved by hand in an irregular path over a stationary faceplate lap. In equalizing lapping the work and lap mutually improve each others surface as they slide on each other. There are three important types of lapping machines. The vertical axis lapping machine laps flat or round surfaces between two opposed laps on vertical spindles. The centreless lapping machine is designed for continuous production of round parts such as piston pins, bearing races and cups, valve tappets and shafts. The centreless lapping machine operates on the same principle as centreless grinding. The abrasive belt lapping machine laps bearings and cam surfaces by means of abrasive coated clothes.

Burnishing:

               Burnishing is a cold working process in which plastic deformation occurs by applying a pressure through a ball or roller on metallic surfaces. It is a finishing and strengthening process. Improvements in surface finish, surface hardness, wear resistance, fatigue resistance, yield and tensile strength and corrosion resistance can be achieved by the application of this process. Burnishing is one of the important finishing operations carried out generally to enhance the fatigue resistance characteristics of components. Burnishing tools are;

• Used to impart a gloss or fine surface finish, often in processes that involve the cold working of metal surfaces.

• Used for the sizing and finishing of surfaces.

               A burnishing tool develops a finished surface on turned or bored metal surface by performing a continuous planetary rotation of hardened rolls. The rotation of the rolls increases the yield point of the soft portion of the metal surface at the point of contact. The point of contact results in the deformation of the metal surface to generate a finished metal surface. The point of contact results in the deformation of the metal surface to generate a finished metal surface. There are many types of burnishing tools. Burnishing is not a metal cutting process. In burnishing process, chips are not produced. It is essentially a cold forming process, in which the metal near a machined surface is displaced from protrusions to fill the depressions. Due to the work hardening of the surface during burnishing, there will be a hardened layer on the surface and it is expected to increase the fatigue resistance of the component. Apart from improvement in surface finish and fatigue strength, burnishing process imparts improved wear and corrosion resistance. In today's manufacturing industry, special attention is given to dimensional accuracy and surface finish. Thus, measuring and characterizing the surface roughness can be considered as the predictor of the machining performance. Burnishing is a process that leads to an accurate change in the surface roughness of the work piece by a minor amount of plastic deformation. In burnishing process, the metal on the surface of the work piece is redistributed without material loss. Besides producing a good surface finish, the burnishing process has additional advantages such as securing increased hardness, corrosion resistance and fatigue life as result of the produced compressive residual stress.

Buffing:

               Buffing is used to give a much higher, lustrous, reflective finish that cannot be obtained by polishing. The buffing process consists in applying a very fine abrasive with a rotating wheel. Buffing wheels are made of felts pressed and glued layers of duck or other cloth, and also of leather. The abrasive is mixed with a binder and is applied either on the buffing wheel or on the work. The buffing wheel rotates with a high peripheral speed upto 40 m/sec. The abrasive may consist of iron oxide, chromium oxide, emery, etc.

Polishing:

               Polishing is a surface finishing operation performed by a polishing wheel for the purpose of removing appreciable metal to take out scratches, tool marks, pits and other defects from rough surfaces. In polishing, usually accuracy of size and shape of the finished surface is not important, but sometimes tolerances of 0.025 mm or less can be obtained in machine polishing. Polishing wheels are made of leather, papers, canvas, felt, or wool. Polishing may follow any of the machining methods except honing, lapping, or super finishing. Commonly several steps are necessary, to remove the defects and then to put the desired polish on the surface. The polishing method is very similar to grinding. The work may be pressed by hand to wheels mounted on floor stand grinders. They may be broadly classified in two groups: The endless-belt machines and the coated abrasive wheels.

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