Tuesday, April 19, 2011

Gem Cutting

In laser sawing, the stone is positioned on the X-Y table and moved across the laser beam that is fixed in position. Stepping motors control the movement, while the operator watches the stone through a stereomicroscope and protective glass. Sophisticated installations use a closed circuit TV monitor and a programmed microprocessor controlled movement of the platform. The laser beam converges asymmetrically at an angle of six degrees. As the stone is cut, the computer focuses the laser beam at a lower point in place. In order to saw the diamond efficiently, the laser has to be pulsed at widths of around excessive re-condensation and cooling before the carbon escapes from the hole, while longer pulses heat the mentire stone by conduction or radiation. A ten carat octahedral diamond can be laser sawn in one hour, while mechanical saws would take ten to thirty hours for the sam job. Breakages with laser sawing are greatly reduced and the weight yield is better, particularly for knaats. The disadvantages are the high capital and maintenance costs and the safety precautions that are to be observed. Internal stresses in the stones can cause breakage of the stone but this can also happen during mechanical sawing.

Depending on the size of the stone, the platform is moved from one to a hundred millimetre, in steps of 0.02 mm. As the stone is observed on the TV monitor, it is possible to move it in different directions and so cut circles and other fancy shapes. As a working rule, the width of the cut is about 0.05 times the depth. The V-shape of the cut is inevitable to prevent shadowing of the laser beam. After cutting and de-graphitising of the stone, polishing is done by the usual mechanical methods.

The laser is also used to drill inclusions cut of a stone. The stone is then boiled in acid to dissolve the spot and then filled with clear material. Only careful inspection through a microscope can detect this repair of the stone. Another interesting application of the laser drilling apparatus is scribing, to identify selected stones. Commercial systems and sevices offer to scribe characters in sizes ranging from 0.1 mm to 2 mm in height. A laser -drilling machine operated at low power is employed to scribe coded numbers on diamonds. The power level for this purpose is around 40 watts at ten kilocyles and the scribing rate is ten characters per second. The entire process is done by suitably programming the microprocessor that controls the X-Y movement of the table carrying the stone. Another installation has a microprocessor character generating system where a reflecting mirror is moved rapidly and generates characters on the diamond, which are monitored on the TV screen.

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