Tuesday, April 19, 2011

Gem Faceting

With use, plastic flow closed up the pores of the cast iron and the cutting power of the Scarface was greatly reduced unless the surface was machined again. Diamond impregnated scaifes, though expensive, are much more productive. However, this investment is made up as the consumption of the diamond power is minimal and far less time is spent on maintenance. Resin bonded diamond impregnated scaifes and those made by electroplating an aluminum or steel wheel with nickel in a suspension of diamond powder are useless for cutting diamonds, though softer coloured stones are polished on them. Metal bonded scaifes are much harder and now used for diamond processing. They are made by splintering a mixture of diamond powder and cobalt, molybdenum, tungsten and tungsten carbide on to a steel base. This process is done by cold pressing followed by heat treatment. Cobalt is ideal as the bonding material in the metal matrix as it wets and sticks to diamond. Unfortunately, toxic cobalt dust is generated during grinding; newer models of scaifes replace cobalt by iron and tin as the matrix that bonds the diamond powder. The matrix is usually an alloy containing tungsten and other metals, the composition tailored by each manufacturer according to the requirment of the user. The diamond grit is uniformly distributed throughout the bond, so that the diamond and the matrix wear out together. Ideally the matrix should be hard enough to wear away just fast enough to expose new diamond points, Grit size, proportion of diamond and the grade of matrix should ensure a working balance; this is determined by practical experience.

Grinding a diamond with coarse powder leaves parallel scratch marks that are obvious to the eye. The final polish with fine mesh diamond powder gets rid of these lines so that examination through a lens of 10x magnification cannot see them. However, even the finest polish leaves lines whose sides slpe at about one degree or so, when the stone is observed through a microscope of very high magnification. Several new techniques to obtain an extra fine finish on the surface of a diamond have now been developed. The most straightforward way is to use finer an very uniform diamond powder. Such fine powders are made by putting the suspension of the diamond dust into an ultra-centrifuge to obtain the required uniformly. The scaife must also be ground very flat to great accuracy and must be mounted on high precision and large bearings to reduce vibrations. This ensures stability at high speeds, even when the downward pressure of the drop increases friction. Some installations are fixed on shock absorber mountings and are located far from machinery that might cause vibrations.

Softer powders give a good polish, but the polishing rate drops so fast with decreasing hardness that this method becomes expensive. Chemical etching can also reduce surface roughness; for example, an iron scaife charged with potassium nitrate or potassium chloride removes polish lines, but the scaife has to be cleaned continuously. If the diamond is allowed to char at low temperatures on the scaife and the graphiote removed, a smoother polish results.A simpler technique is to put the diamond into an acid etch solution that removes only the outstanding points.

When facets are cut on a scaife a part of the diamond is ground to powder. A narrow laser beam on the other hand, can slice facets rather like a knife on the rough stone. The application of high power lasers to the processing of diamonds has been termed as the greatest advance in diamond cutting and sawing since the high speed saw was invented in 1930. The use of a laser beam saves a great deal of time and material and so is more economical than conventional techniques. Consequently, lasers are now widely used for drilling, kerning, sawing and bruiting not only single stones, but a set of stones of similar size in one operation. Improvements in technology have made laser processing more cost effective than manual cutting and polishing, particularly for bruiting and when fancy cuts have to be shaped.

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