The polishing process and mirror finishing

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* There is no mystique in producing a fine mirror finish, just a little common sense and the desire to achieve a special satisfying outcome. Aluminium is a very dirty material to work with, producing a lot of black dust when it is being polished,although the final outcome and sense of pride gained far outways the amount of times you have to wash your hands .Patiance and Commitment thats all you need. * What is a mirror finish? If you look at a polished surface magnified a couple of thousand times it looks like a view of the Rocky Mountains. Because the peaks and valleys are so small and close together and the tops have been rounded off from polishing, they reflect the light and to our eyes the surface appears as smooth as silk, although it is not. * Absolute flatness is actually impossible to achieve but we can get very close. Therefore all we are doing when we polish to a mirror finish is making a series of finer and finer scratches and eventually rounding off the peaks. This is achieved by stepping down through the abrasive grades until we achieve the finish we require. Various methods are employed to achieve a fine polished surface and can be placed into three main categories: Abrasive finishing polishing, Buffing or bright finishing and Colouring. * The first stage is often described as \'emery bobbing\' or \'scurfing\' and involves the use of abrasives which cut away the surface irregularities and imperfections. This can also be achieved by the use of abrasive belts, flap wheels, hand rubbing blocks or abrasive cloths and papers. * The second stage, polishing or buffing, requires the use of mops and polishing compositions and during this process a bright, smooth finish is achieved. The term \'Colouring\' is applied to the final polishing of any metal part when again use is made of mops and polishing compositions. After all the scratches have been polished out the metal is finally \'Coloured\' to make it clean and brilliant. For this purpose a lime-based or rouge composition is used on a finishing mop of soft calico or cotton. ABRASIVES * Three main abrasives are now in widespread use within the industry for metal finishing and these are the naturally occurring emery and two man-made abrasives aluminium oxide and silicon carbide, aluminium oxide being the most common. * These abrasives can be obtained in a wide range of grit sizes, from the finest \'flour\' grades of 1200 and 1000 through to the most common grades of 320, 240, 220, 180, 150, 120, 80, 60 and even coarser. Up to 500 grit the grain is sifted through a series of meshes, the grit size taking its number from the last mesh it passes through, i.e. the number of holes per linear square inch. Over 500 grit the grain is measured by the time it takes to settle in water. * For the purpose of abrasive finishing we only require grit sizes from 320 to 60. * To obtain maximum life from an abrasive dressed mop, belt or flap wheel it is essential to use a high grade abrasive such as aluminium oxide. The cost of dressing mops, making belts and flap wheels is many times the price of the material used. It is therefore false economy to use inferior grades. * Fused aluminium oxide and silicon carbide have short angular grains and are many times sharper and faster cutting than emery, therefore when making use of these man-made abrasives it is usual to select a finer grit size than one would use when using emery. * They are suitable when an extremely fast cut is required and also for polishing abrasive materials such as aluminium and are absolutely essential when polishing stainless steel. * Abrasives are used in metal finishing in the following ways: On abrasive belts, flap wheels, cloths and papers, etc.: On dressed wheels, mops, felt bobs, etc.: In grease bonded and lime-based polishing compositions: Loose grains for grit blasting, vapour blasting and lapping, in flexible rubber polishing blocks, sticks, polishing wheels and mounted point. Abrasive belts and flap wheels are now extensively used for preliminary abrasive operations prior to the polishing of components.is made up of strips of aluminium oxide coated belt material and bonded circumfrentially around a central resin core which is mounted on a steel shaft. This can be used on any D.I.Y. electric drill and hey presto you have a portable abrasive finishing machine which can be taken anywhere there is a power outlet. The abrasive flap wheel has all the advantages of the abrasive belt machine but with none of the disadvantages of high cost, non-portability, size and inability to work in small internal recesses. ABRASIVE DRESSED WHEELS * The wheels or bobs are dressed with abrasive grain (emery, aluminium oxide or silicon carbide) using glue or cement as adhesive. * Felt bobs are the most popular wheel used commercially, for their long life, although they are quite expensive, they are made of solid felt, blended to give uniform texture and resiliency. Felt wheels can be turned to any shape to suit particular trades or duty, such as grooved bobs for tubular work, small cones and fingers for jewellery and knife edged. FINISHING AND BUFFING * The primary function of the polishing process is to produce a smooth, reflective surface with only a minimum amount of material being removed. * To do this it is still necessary to use abrasives within a polishing composition, sharp enough to abrade the surface to remove marks left by the previous abrasive operations and impart a bright finish. * The polishing operation generally takes place in three stages, the first being to remove the marks left from the abrasive operation.. This is achieved with the use of a stiff sisal stitched mop and a fast cutting composition. grey or brown compound. This is then followed by a middle cutting operation using a white stiched mop with brown or grey polishing compound ,Finally a much softer unstitched calico mop is used and a white blue or rouge polishing compound. POLISHING MOPS * A wide range of fabrics are used in the manufacture of polishing mops and these, in order of hardness, include: * Cotton - in various qualities from soft, unbleached cloths to fast cutting, white fabrics incorporating special dressings, generally known as calico mops.and white stiched mops * Sisal - an extremely hard fibre generally used in conjunction with an abrasive grease based composition. The type of mop using 100% woven sisal with cloth interleaves to protect and separate each section is called a stapol mop. * Scotchbright* or Fibral** - a low density abrasive material provides a simple and effective method of cleaning materials and for producing a satin finish. This is a tough, chemically resistant material composed of tough fibres to which evenly distributed aluminium oxide abrasive grains are resin bonded. These mops can he used dry with relatively light pressure. They are available in a number of grades but generally come in coarse, medium and fine. The use of too high a mop pressure can result in overheating of the mop face and cause fusing of the synthetic fibres. * Leather - these mops are occasionally used in the traditional trades for the final finishing of non-ferrous metals. Generally chamois leather mops are used in the jewellery trade with a rouge composition for the polishing of gold and silver.