Polishing

Twenty times on the trade give your work: polish it unceasingly and repolish it.
Boileau

The polishing is the action to polish, to make plain and shining by:

  • abrasion: polishing with discs of various materials turning at high speed, with or without paste to be polished, polishing by vibration with abrasives.

  • work hardening: Use of a Burnisher, polishing by vibration with browning media (steel balls and needles).

Prehistory

Polishing is a technique of work of the stone which spreads with the appearance of agriculture to the Neolithic . This technique makes it possible to obtain Hache S and Herminette S with the regular and very resistant edges. It is important to stress that polishing is only the last stage of the manufacture of the blade of axe and that it generally intervenes after a Façonnage bifacial. The Outil S of polished stone are carried out starting from hard stones (Silex) or of tough green rocks, eruptive (Basalte S, Dolérite S…) or metamorphic (Amphibolite S, eclogite S, jadeite S…). The tough rocks are sometimes worked by sawing or roughening before being polished. Polishing is carried out by friction on a dormant or mobile polishing machine (sandstone, Granite, flint…).

Mechanical and Metallurgy

Polishing is a component of the Finition mechanical metal parts or of decoration by which one not only endeavors to respect the imposed dimensions, but in to more obtain a beautiful finished aspect, one or a state of Surface of high-quality.

This kind of work is attached to the Rectification, in the sense that the work tools are a wheel furnished with Abrasif and similar to a Meule punt. But it remotely however, because it is simply a question of obtaining one finished of worked surface and not of carrying out a given precision. Consequently, one will use machines less sophisticated than the grinding machines:

  • Lathes to be polished: the part is presented manually on the surface of grinding stones or discs of various materials (Coton, sisal, tampico, cord, synthetic fibers composite…), turning at high speed (about 3000 m/min) and generally coated paste to polish.
  • automated Machines: idem but the automat takes care of handling.
  • Polishing by vibration: A vibrator is a vibrating tank containing of the media abrasives (balls of steel, ceramics, wood, sand, abrasives synthetic,…) and various liquids with chemical action and mechanical. The parts are left in this tank until the abrasives act.
  • Polishing or fettling by satellite centrifugal machine: The process is also based on the use of media containing ceramics, porcelain, plastics or metal, of chemical additives composed of surface-active. The last innovations are due to the characteristic of the movements applied at piece-rates to polish, in particular with the " satellite centrifuges for axes obliques" who generate movement very complex (composition of a centrifugal movement and a " movement in huit"). Thanks to these very complex movements and with high-speeds of rotation, the cycle times of polishing can be reduced in a significant way; but the most important interest is due to the possibility of using media of very reduced size to reach zones of the very difficult parts of access (micromechanical, clock industry, micro-electronics…)
  • manual Polishing: Artisanal or of laboratory (see following paragraph: metallography)
  • Browning: polishing with a hard stone fixed by crushing of the surface of metal.

Metallography

Polishing is the principal stage of the Métallographie (method of observation of the structure of metal). Indeed, to be able to observe the microscopic details, even nanoscopic, of the structure, it is necessary to eliminate the stripes.

Polishing is also used when one wants to make a mechanical Essai:

  • in compression, to reduce the friction of the sample on the heap of compression to avoid the deformation “out of barrel” and the Buckling;
  • in a general way (in traction, compression and Inflection) to avoid the Concentrations stresses;
  • it is also essential to observe the traces in the tests of Microdureté.

Lastly, polishing can be used to remove the surface layer and to have access in the middle of the material, which does not have necessarily the same properties.

One aims the “polish mirror in general”, i.e. no défut, no stripe is visible with the optical Microscope, the residual defects thus have a size lower than the Micromètre; one content suvent oneself of one impression to the naked eye or the binocular Loupe.

One is satisfied sometimes with a coarser polishing, according to the applications.

Polishing is initially mechanical; sometimes for metals, one finishes by an electrolytic polishing (Dissolution controlled surface layer).

For mechanical polishing, one uses papers with abrasive particles, left Abrasive cloth, Sandpaper, but with a finer and controlled size of particles, in general in Carbure of silicon (Sic) or in Alumine (Hello). If surface is not plane, one starts with a paper “with coarse grain”, then passes successively to papers to finer grain. Between each paper, it is necessary:

  • to clean the sample well, in order to get rid of the particles of preceding paper which could have been encrusted; one in general uses a Bain with ultrasound;
  • to cross the direction of polishing: for a prapier to give, one always polishes in the same direction, and one crosses the directions from one paper to another, which makes it possible to emphasize the residual stripes of the preceding stage.

Then, one uses a fabric on which one deposits artificial Diamant S of controlled size (6 µm, 3 µm and 1 µm), either by pulverizing an aerosol, or by spreading out a paste.

The hard materials very are long to polish and use much of consumable (set with diamonds papers, paste or aerosol). The very soft material (as the Copper) are difficult to polish because one creates easily facets (faceting), and the abrasive particles are encrusted more easily; one cannot in fact not to use polishing diamond.

Electrolytic polishing is done in general with a Acide, or rather a mixture of acid (called “sauce”) depend on metal considered, and by applying a electric Tension about some Volt S. the dangerosity of “sauce” imposes precautions: not to handle (“maniper” in jargon of research) while being only in the laboratory, to carry protective gears individual (blouse, glasses, gloves), to respect the instructions according to the products (to work under hood or Sorbonne).

See also: Safety at the laboratory

Other fields of use of techniques of polishing

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