THE Diamond-SLITTING SAW

The progressive grinding away of much Diamond material with a scaife is so laborious and so time-consuming that it was inevitable that faster auxiliary machining methods would be invented. Two processes emerged which considerably facilitated shaping of Diamond. These are ‘bruting’ and ‘slit­ting’. It is difficult to pinpoint any date for the invention of bruting. Bruting in the early days simply meant that the work­man fixed two Diamonds on sticks and rubbed one against the other to wear them down to a more rounded desired shape, I prior to fabricating an octahedron into a brilliant-shaped gem-stone. Nowadays the Diamond to be bruted is rotated in a lathe chuck and another Diamond pressed against it. Both Diamonds chip off at the edges and become rounded.

This method is used only occasionally, but it does have its advo­cates. Of much greater importance is the Diamond-slitting saw. This enables a Diamond to be sawn across, in perhaps from minutes to hours (according to size). Only 'soft' direc­tions can be sawn. By cutting across the section of an octa­hedron, as in Fig. 10, the larger portion is already well on the way to the shape of a brilliant, whilst the smaller off-cut can be converted into a smaller brilliant by grinding. Thus all the material, except that lost in the slitting, is conserved and used, and the time consumed is much less than that of grinding away the smaller part, which was the early practice before the saw was invented.

1823) Diamonds were sawn by means of a fine brass wire or iron wire, held taut by a piece of bent cane. The wire was con­stantly impregnated with a mulch of Diamond dust and olive oil, and just as a fret saw is used today on plywood, so this

wire saw was rubbed back and forth over the Diamond. The J method was exceedingly slow and it is on record that the J sawing in half of the celebrated Regent Diamond (410 carats) took a whole year of continuous work, and incidentally used up a good deal of grit.

The Diamond slitting saw is a thin disc of bronze, of thick- * ness perhaps 1/200 inch, of diameter some 3 inches. It is held between thick strong disc flanges to stiffen the bronze disc, such that only perhaps half an inch of the thin bronze disc protrudes. This bronze disc is driven by a motor at about 4000 rev/min or a little faster.

The Diamond to be cut is held on an arm above the wheel and rests on the wheel, i.e. a ‘gravity feed’ is used. Sawing must be carried out on soft planes and in specified directions on such planes, for in both polishing and sawing a selected soft plane has also favoured directions which operatives call the ‘grain’. Sawing or polishing times are reduced if correct the grain directions are exploited and better ultimate finishes aril also secured.

Demands on the bearings and smoothness and trueness of rotation are high, for if the slit cut widens, there is excess loss of weight in the Diamond being cut and more and more time is needed for the work. The sawing is activated by charging the edge of the bronze disc with a roller coated with a mulch of fine Diamond grit and olive oil. Frequent recharging is necessary. Special precautions must be taken to avoid vibrations, and saws are usually bedded firmly on concrete. Of course, the whole outfit is quite small.

With modern machines such a saw takes two hours to cut| across a 1-Carat crystal. Large Diamonds may take several days of continuous operation. Great difficulties are experienced where the Diamond is twinned. When this happens, the saw, whilst traversing the soft plane, suddenly meets a fiercely hard twin edge and this is so very hard that sawing is brought to a standstill. These twinning conditions are dreaded in the industry, such a twin plane being called a ‘naat’. There is al tradition that Diamonds from Australia cannot be sawn. This is really because they have regions of repeated twinnings

(naats) in their construction. This has led to the mistaken belief that Australian Diamonds are harder than other dia­monds. Intrinsically this is not really the case at all. All twinned Diamonds are difficult to saw, whatever their country of origin. The rate of sawing is such that it takes about 10 minutes to cut through one square millimetre of Diamond surface. So efficient are Diamond saws that material lost in the cut is slight. As a rule, less than 5 % of the weight of a dia­mond is lost through the actual cutting. Yet in many modern factories both air, dust and sludges are treated for recovery, and much of the waste Diamond dust is recovered. Sawing is by far the most important industrial method for pre-shaping. In factories banks of saws (maybe a 100) are set up and an operative walks down the line continuously recharging with grit.

Diamond sawing has become so specialised an industry that factories often devote themselves to this and this only. The polishing factories send their Diamonds away to be separately sawn. Much skill is involved in selecting the particular planes for sawing, especially when dealing with initially irregular shapes. The objective is to cut a crystal with the minimum of cuts into the best pieces for ultimate fabrication, whether it be a gem or a tool.