Diamond Tooling in the New Stone Age

  The recent slowdown in global economies affected all areas of industry, particularly those involving new building and construction. Despite this, the market for natural stone has actually increased over this same period. The world output of stone has increased by around 80 percent from 1994 to 2001, with nine-percent growth in the last year alone.
  One reason for this increase is the emergence of new major players in the market, particularly China, which is challenging the previous European domination in this area. In the early ‘90s, countries such as China, India and Iran would not even have featured in the top 10.
  It is also interesting to look at the breakdown of the end use of the quarried stone. The majority of this is in the form of stone furniture, i.e. stone slabs for use as work tops or sink tops for bathrooms and kitchens.
  Overall though, the increases in stone output from countries such as China and an increase in the use of stone in certain product areas such as kitchen and bathroom furniture would not have been possible without the considerable improvements that have been made in the diamond tooling, and the machines now used to quarry and process the stone.
  To examine the influence of diamond tooling in the stone industry, it is necessary to define the three main categories of stone processing – quarrying, primary sawing and finishing – and discuss the developments in each one separately.
 
QUARRYING
  Back in the 1960s, the traditional methods of quarrying involved very little use of diamond tooling. Stone was extracted by four basic techniques:
  • Cutting with wire and steel shot;
  • Blasting with explosives;
  • Cutting with thermal lances; or
  • Slot drilling and splitting with wedges.
  Each of these had many disadvantages in terms of both waste and adverse environmental effects that are less-tolerated today.
  The major change in block extraction has been the introduction of diamond wire to the quarries. The basic diamond-wire sawing technique employed has not altered since the first machine, based on a steel wire set with diamond beads spaced at regular intervals, was first introduced into a marble quarry more than 30 years ago.
  In the setup, the wire is threaded through holes pre-drilled in the quarry face, and passed over a flywheel driven by a track-mounted machine. Other secondary pulleys are placed to act as guides for the cut.  As the machine drives the wire and cuts the stone, the machine reverses along the tracks to maintain the correct tension in the wire. When it reaches the end of the track, the machine is returned to its starting point, a section of wire removed accordingly, and the cut resumed.
  The spread of quarrying machines into granite-block extraction was made possible by developments in both the machine itself and the specification of the diamond beads. To cut the much-harder and -abrasive granite, larger, more-powerful machines were needed and the wire beads were manufactured using diamond sintering technology rather than the initial electroplating used for marble extraction.
  The reasons for the success and overall acceptance of diamond-wire saws within the industry are the advantages of reduced noise and pollution, less waste, reduced transport costs and reduced factory processing after quarrying.
  Factors such as noise and pollution now weigh heavily against conventional methods such as thermal torches and blasting. This is likely to become even more important as governments around the world tighten up more and more in these areas. In many countries, there are also restrictions against blasting after certain hours which do not apply to wire sawing; this means the quarry can be worked, as far as the actual process itself, for up to 24 hours a day.
  Apart from these environmental considerations, there is considerably less waste when using diamond wire compared to any other method. This has two major effects. The first is that the precise, accurate cuts achievable with diamond wire ensure that the maximum of stone is usable, and maximum yield is obtained from the stone. This factor alone can be paramount when extracting some of the more-expensive stone types.
  Secondly, apart from this increase in yield, there are also significant economic benefits by the reduction in the amount of waste generated by other methods, leading to savings in waste disposal.
  A factor that is often forgotten is the cost of transporting the stone. This is usually a function of weight, and blocks extracted by conventional means are more likely to be of irregular shape and needing an initial block-squaring operation to facilitate further slabbing.
  The final factor in favor of wire sawing is that blocks cut by diamond wire often require less time for further processing, since the cut is more-accurate. This is particularly relevant on the first calibrating stages of a polishing line, where diamond tools are usually employed to remove the bulk of the surplus thickness. By maintaining a constant thickness of slab, the demands on this tooling are less and the slab processor can optimise the life of the polishing heads and make significant reductions in overall costs.
  An example of the success of introducing wire saws into a granite quarry is the experience of a South African company which turned to diamond-wire sawing for block extraction eight years ago. It now has seven Benetti Macchine wire saws for cutting the vertical faces of each bench.
  The average size of a bench is 65.6’/20m wide by 32.8’/10m high and the bench floor is still drilled and blasted conventionally. The wire achieves an average cutting rate of 37.6 ft²h /3.5 m²hr and a wire life of 172.2 ft²/1 ft  (or 16 m²/lm) of wire, far in excess of anything achievable with any other method. Just recently, a wire with an improved specification has been used to achieve even higher wire life at no reduction in cutting rate, around 322.7 ft²/1 ft (or 30 m2/lm of wire) – almost three times the life achieved eight years ago.
 
PRIMARY SAWING
  Primary sawing involves cutting the blocks into slightly oversize slabs, usually with frame saws and conventional abrasives. With the softer marbles, diamond-segmented frame saws are now used and bring the benefits of increased cutting rate and reduced overall tooling costs. With granites, however, it has as yet not been possible to use diamond frame saws because of the aggressive nature of the stone.
  With circular saws, the blade is always cutting in one direction and the diamond particles wear out in a controlled manner. With frame saws, because of their reciprocating action, the blade cuts in two opposite directions; this leads to early pullout of the diamond particles and premature failure of the blade.
  However, two major alternative techniques, both based on diamond tooling, are now extensively used in primary sawing of granite as a replacement for conventional steel shot frame saws.
  The first is the stationary diamond wire saw, an adaptation of the original quarry wire saw mentioned above. Unlike the quarry machines, a stationary saw – as its name suggests – does not move along a track, but cuts the blocks using downward pressure of the wire on the block. Stationary saws can be either single or multi-wire; multi-wire machines can have between five and 60 wires mounted in parallel on the one machine.
  Four years ago, the first 50-wire machine was installed in northern Spain. The machine can process blocks with a maximum length of 12.4’/3.8 m and a height of 6.8’/2.1 m, with a variable block width. Driven at 130 kW, the 50 diamond wires – when cutting a block into slabs of .78”/2cm – are accelerated to a cutting speed of 108.3 f/s (33 m/s). The feed rate on this block is 7.8” (20 cm/h); future development of the multi-wire saw should help to increase this value to 11.8”(30 cm/h).
  A block can be cut with a multi-wire in approximately seven hours; the same job took 72 hours with the frame saw. Even when cutting three blocks at a time, which is sometimes possible with frame sawing, the time increase is still more than 3:1.
  With a dimensional tolerance of ± 0.5 mm, the quality of the sawn slabs more than satisfies the requirements for subsequent polishing. The improved results, in terms of dimensional accuracy and surface quality, added to significant time savings in processing.
  The second development used solely for granite slabbing is the diamond multi-saw. This consists of a number of diamond saw blades parallel-mounted on a single spindle, which then cut the block into slabs or tiles of the required thickness. The main advantages these machines have over conventional frame sawing are accuracy of cut and increased production rate.
  An example of multi-blade sawing can be found at a stone yard in Brazil which processes around 193,750 ft²/18,000m² of granite slabs and 53,819 ft²/5,000m² of tiles. The company recently acquired a Breton multi-blade granite block cutter equipped with diamond saw blades; previously, all block cutting involved frame saws using conventional abrasives.
  The company found that one of the main advantages of using the multi-blade machine was a much-more-accurate cut, therefore increasing the yield from the granite block. This, in turn, means that the strips that enter the calibration lines have a more-consistent thickness, leading to less material removal during calibration and greater efficiency and longer life from the calibration tools. Both of these factors resulted in significant savings in costs.
  Another advantage of the multi-blade was its flexibility in use. It coped with large or small blocks with ease, and could also machine granite of different hardnesses without the need for blade changing. Frame saws, on the other hand, needed frequent blade changing to suit the variation in hardness found from different granite types.
 
FINISHING
  Finishing operations involve cutting to final size and calibration/polishing of the slab or tile. These operations are normally carried out on fully automatic conveyor lines, where the primary sawn slabs or tiles enter at one end and come out as finished products at the other. The normal sequence of events is cutting into longitudinal strips, thickness calibration, polishing, transverse cutting into final size, and finally edging and chamfering.
  Diamond tools play an important role at all these stages, and improvements to the basic machines used combined with advances in diamond-tool specifications – in terms of diamond-grit selection, diamond concentration and bond formulation – have led to significant savings and increased production.
  Also, diamond tools are being introduced at more stages of the operation, such as calibration and polishing. A typical polishing line will have 20 heads and, until recently, often only the first two or three heads would be fitted with diamond tools. Now, stone producers are fitting diamond polishing heads on the first nine or 10 stations, and polishing costs per square meter have been shown to be halved.
 
DIAMOND SYNTHESIS
  Perhaps the most-important influence on the increased use of diamond tooling in the stone industry is the increase in volume of synthetic diamond now produced worldwide. In the mid 1960s, total output of industrial diamond was around 30 million carats. split almost 50-50 between natural and synthetic. Today, nearly all diamond used for industry is synthetic – as much as 99 percent, perhaps – with the market estimated as high as 2 billion carats. The huge increase in the use of diamond tooling would not have been possible without the existence of enough volume of raw material to support it.
  But another – and equally important factor in this increase in synthetic diamond production – is the fact that diamond particles can now be ‘engineered’ to suit particular applications. I simple terms, diamond needed to cut the very hard granites should be blocky in shape and highly impact-resistant, whereas diamond used for free-cutting and less-demanding applications should be more-irregular in shape and more-friable.
  With the sophisticated diamond synthesis techniques available today, diamond can be manufactured to suit whatever is required. Diamond toolmakers now have an extensive choice of diamond available and this, coupled with advances they themselves have made in the bond formulations and sintering techniques used, ensures that the right tool is used for the right job.
  Thus even ‘standard’ diamond saw lades – such as the relatively smaller diameter ones which have always been required for finishing cuts on granite tiles and slabs – now perform much more efficiently, at higher cutting rates and lower tooling costs.
 
  Martin Jennings is the managing editor of Industrial Diamond Review in Wembley, England.

This article first appeared in the August 2004 print edition of Stone Business. ©2004 Western Business Media Inc.