Diamond wire is used in the cutting of ornamental stones, both in quarries. (loop configuration) and in block- processing plants (thanks to frames working in the ...
Stone
The influence of operating parameters on the total productivity of diamond wire in cutting granite D
iamond wire is used in the cutting of ornamental stones, both in quarries (loop configuration) and in blockprocessing plants (thanks to frames working in the ‘inverted catenary’ mode). The technology is also very widespread in the controlled demolition of reinforced concrete structures. The tool, widely used in limestone stone quarries since the late 1970s, with electroplated diamond beads, has seen a far slower introduction into cutting operations in granite quarries because of problems due to its rapid wear. Only since the beginning of the 1990s, thanks to improvements in technology, including the quarry machines, the sintering of the beads, the manufacture of industrial diamonds and the plasticization of the wire, have we arrived at a lower tool cost and higher tool life in the quarry. In addition, the need to apply precise, non-destructive cutting technology in ornamental stone quarries has become fundamental due to the pressure of competition on international markets which pushes for a greater recovery (utilisation) of the volume of stone, an improvement in block quality, lower cutting costs and above all a higher profit [1]. Particularly significant is the use of granite in claddings for high-rise buildings. In this case the technical requirements for the stone are such as to exclude materials quarried by ‘drill & blast’ technology. As well as this, strict environmental protection laws require the application of methods less harmful to health and safety in excavation, to replace the present ones based on drilling, which entail the production of noise, dust and vibrations often well above tolerable levels. It follows that, in quarries of silicaceous ornamental stone, low-cost rudimentary technologies are still favoured only in the case of low cost products, while the more INDUSTRIAL DIAMOND REVIEW
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This report by O. Cai, N. Careddu, M. Mereu and I. Mulas describes a research project carried out at the DiGITA laboratories of the University of Cagliari in Italy, which had the objective of improving knowledge of the methods for predicting the total productivity of diamond wire. The study of the tool performance considers the variations in operating parameters such as the peripheral wire speed and the cutting rate.
expensive advanced, precise technologies are becoming steadily more advantageous in the case of materials with a high market value, especially if these are quarried with a high recovery level [1]. Diamond wire for granite consists essentially of a high-strength steel cable, with diamond beads threaded on it. The cable, usually 4.8 mm in diameter, consists of 6 7-ply strands on a 19-ply steel core. This has a breaking strength of about 17 kN. There are also cables of 133 wires (7 19-ply strands). The bead consists of a cylindrical support surrounded by a ring formed from metal powder (mainly cobalt and, in a smaller quantity, bronze, although today there are low-cobalt alloys with mixtures of Fe + Cu e P) and diamond, which arrives from the pre-moulding line. Before mounting on the wire the beads (on average 40 per m with a thickness varying from 6.5 to 8 mm, with an outer diameter of 11 mm) are subjected to sintering. The bead sintering process is carried out according to a cycle in which the principal variables (temperature, pressure, time and sintering-chamber atmosphere) are combined in such a way as to obtain an excellent result in terms of wear and hardness in relation to the stone to be cut. There then follows the plasticization process, required for the protection of the cable from the abrasive material arising from the sawing process. The cutting action is due to dragging and to the force pressing the beads onto the stone surface.
The movement of the wire is given by the drive pulley attached to the electric motor shaft. The pressing force is linked to the pulling of the machine itself (in the quarry) and the downfeed rate of the mobile equipment (in the frame). The wear of the beads is due to: ◆ chipping and/or smoothing of the diamond ◆ diamond pullout ◆ abrasion of the embedding metal alloy Chipping is caused by an excessive normal force between the bead and the stone especially in the case of fine-grained heterogeneous materials (granite) with an abundance of hard minerals. Smoothing occurs when the wire speed is too low, in fact the chip thickness increases and the bond is worn but without the crystal being rounded. The latter happens at high peripheral speeds due to excessive development of heat at the point of contact between the diamond and the stone [2]. Wear of the diamond-impregnated elements must necessarily occur by chipping in such a way that new surfaces of contact with the stone are continually created in the diamond, keeping the cutting capacity unchanged [3]. Bead wear is irregular in the absence of rotation round the metal cable. Hence the need to give an initial torsion (usually one per m of wire) before closing the wire with a joint.
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Stone
Experimental equipment The frame: The experimental apparatus consists essentially of a frame, of small dimensions, for diamond wire (Fig 1). This is made up of two steel guide-pillars (fixed to the floor) down which the metal hull, housing the pulleys, the diamond wire and the spray cooling system, moves vertically. The structure is completed by an upper beam connected to the guide pillars. The system is balanced by a counterweight which makes for a symmetrical distribution of the weights in the frame. Tensioning of the wire is given by a pneumatic piston (the force of which can be adjusted) connected to the guide pulley. The downfeed is given by two engines (placed at the top of the guide pillars) which drive two worm-screws connected to the hull. The diamond wire is driven by a 5.5 kW engine which transmits the movement to the main pulley by a transmission belt. Below the hull is the block-carrying trolley, connected via a worm-screw to the motor which governs its forwardbackward movement on cylindrical rails. Complete control of the apparatus is enabled by a mobile electric panel connected to a portable PC with data acquisition software (Fig 2). The system is completed by a series of ducts for evacuating the sawing sludge. Table 1 gives the maximum operating parameters of the apparatus. The stone: The experiments were carried out on cubic granite blocks, with a side of about 500 mm, of the commercial variety known as ‘Rosa Ferula’. This stone, quarried near Orosei (Sardinia), is shown in Fig 3. The blocks were all taken from the same zone as a rocky massif whose homogeneity on a metric scale had been ascertained. In this way it was attempted to minimize the effect of the variability of the rock characteristics. In any case, by the time the experiments started some blocks had been chosen with practically identical macroscopic characteristics. It is well known that granites, by their nature, have preferential cutting planes; for this reason, in positioning the blocks on the trolley, extreme care was taken so that the cuts were made parallel to the preferential plane [4]. The chemical, physical and mechanical characteristics of the rock are shown in Tables 2 and 3.
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The diamond wire: The tool used, suitable for this stone and supplied by a well-known manufacturer, is a plasticized wire with a stranded steel cable core, holding 40 beads/metre. The beads are of the sintered type with an outer diameter of 11 mm and length 6.2 mm. The copper junction was pressed with pliers onto the ends of the wire after the latter had been subjected to seven twists (one twist per metre).
3.2
0.24
1.0 0.6
Wire tension
0.5 0.5
Fig 1 Frame layout and dimensions used for sawing the blocks
Experimental plan In order to obtain as much information as possible about the behaviour of the wire during cutting, the experimental tests were carried out with the following variation in operating parameters: ◆ wire speed [m.s-1]: 20, 25, 30 ◆ downfeed rate [cm.min-1]: 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0
Fig 2 Control panel and data acquisition
It was not possible to increase the downfeed rate to more than 6 cm/min as the engine consumed an excessive amount of power and the wire was under a tension high enough to break it at the junction. Throughout the experiment the same wire tensioning was maintained: ◆ wire tension [N]: 2000 ◆ wire length [m]: 6.70 For each test the following measurements were made: ◆ variation of current consumption [A] ◆ inlet and outlet angles of wire (w.r.t. horizontal) in steady cutting state [°]
Fig 3 Rosa Ferula granite
Maximum wire speed [m·s-1]
◆ test duration [min]
15.4
Maximum wire tension [N]
3080
◆ sawn area [m ] 2
◆ weight of wire [kg] SiO2 [%] Al2O3 [%] CaO [%] MgO [%] Na2O [%] K2O [%] F2O3 [%] MnO [%] PbO [ppm] CuO [ppm] Cr2O3 [%] Altro [%]
66. 13 11.87 0.15 0.05 4.83 10.83 1.04 0.03 0.99). On the basis of the exponent values obtained for the variables considered and taking into account the positive values obtained for coefficient c, the following considerations can be formulated: ◆ the tool yield is strongly influenced by the current consumption, as already pointed out at the beginning of this section. As power consumption increased, the tool life decreases ◆ the increase in peripheral speed favours the improvement of wire total productivity up to a maximum (as determined also from graph in Fig 7), after which the performance falls ◆ the tool life decreases as the cutting rate increases ◆ the increase in the inlet angle of the wire into the cut leads to a more rapid tool wear
100
200 300 vαp.vβat.Aγ.Φiδ
400
500
Fig 10 Correlation between wire total productivity and operating parameters according to the relationship Rf = cNiρ
35 Wire costs Other costs Total costs
30 25 Cost (€/m2)
12
20 15 10 5 0
vo Cutting rate (m2/h)
Fig 11 Total cost of cutting with diamond wire against cutting rate
to the excessive wire consumption, which is preponderant compared with the incremental saving on other costs (capital, power, water, labour, maintenance and spares) achieved with an increase in vat. Fig 11 shows the graph relating to unit cutting costs as a function of the area cutting rate. It reaches an optimum value (vo) in correspondence with the minimum of the total cost curve (obtained as the sum of the other two). Of course in general the position of the vo on the x-axis varies according to the rock type and the price and quality of the wire. For the granite tested in this research a vo of 2.2 m2/h was found with a wire yield of about 8.35 m2/m). In common static machines with two pulleys a lower vo value is observed (generally limited to an average of 1.5 - 1.9 m2/h) together with a great yield.
Economic considerations Conclusions The wire yield is an important technicaleconomic factor in sizing both extraction works in the quarry and sawing in the laboratory. The obstinate pursuit of high area cutting rates (leading to productivity both in the quarry and on static machines) can actually cause an economic loss due
The experiments made it possible to show that the peripheral speed of the wire is important for the purposes both of tool life and current consumption by the main motor of the frame. From the tests carried out it can be said that, for this granite, INDUSTRIAL DIAMOND REVIEW
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there surely exists a maximum tool life for vp values between 25 and 30 m.s-1 (which for class 4 granites goes down also to 22-23 m.s-1). The current consumption, a factor giving a measure of the ‘effort’ of the apparatus in sawing the stone, has an
The increase in area cutting rate leads to a faster wear of the wire, which must be estimated economically: an optimum vat will lead to a minimum of total cutting process costs. ◆
excellent correlation both with inlet angle of the wire into the cut and the area cutting rate (directly) in steady state phase and with the total productivity itself (by a 2nd degree function). The latter is also strongly dependent on the current consumption by the principal engine.
V. Tönshoff and Warnecke, undertaking research at the Polytechnic of Hanover, introduced a factor hc (already used in machining with formation of chips) which influences several parameters in diamond-tool cutting processes such as wear and the energy required to crush the stone. The average thickness is given by:
vv 1 hc = v · [ µm] C·λ·r p
vp
vv
In this relationship, originally applied to diamond blades, we have: vp [mm.s-1] peripheral speed;
hc
vv [mm.min-1] frame descent rate C [mm-2] number of grains working per unit area (for sintered wire this has values of 1.5 - 3.1) λ
ratio between pearl length (L) and spacing (P)
r
‘indentation’ coefficient of the diamond which for the wire can be taken as 10
L P
It can be deduced that the greater the chip thickness and the harder the bead wear, the higher the power needed to drag it through the cut.
References [1] N. Careddu: ‘Taglio in cava del granito con lancia waterjet’, Tesi di Laurea, Aprile 1995. [2] H. K. Tönshoff, G. Warnecke: Research on stone sawing. In Ultra hard materials application technology, vol. 1. 1982. pp36-49. P. A. Daniel, editor. [3] M. Agus, A. Bortolussi, A. Caranassios, R. Ciccu, P. P. Manca, G. Massacci: ‘Ricerche sperimentali sull’usura delle perle diamantate nel taglio dei graniti’, Eurocave ‘92, 1a Conferenza Europea sulle Cave, 6-8 ottobre 1992, pp221-224. [4] S. Giuliani, G. Loi, M. Louafi, G. Rossi, G. Siotto, P. Trois: La perforazione a rotopercussione: analisi delle prestazioni di una macchina elettrica commerciale su banco di prova, Resoconti della Associazione Mineraria Sarda - Iglesias, Anni IC-C, N.1, 1994-1995, pp97-113. [5] O. Cai: ‘Considerazioni sull’economicità del filo diamantato nel taglio del granito, in comparazione con altri sistemi di taglio (multidisco e grandi diametri)’, Rivista tecnica ‘Il Diamante - Applicazioni e Tecnologie’, Ottobre 1995, pp116-119. Ed. G & M Associated Sas. [6] O. Cai: ‘Comparison between diamond multi-blade and diamond wire cutting of granite’, Rivista tecnica ‘Il Diamante - Applicazioni e Tecnologie’, Dicembre 1998, pp99-112. Ed. G & M Associated Sas.
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[7] M. Agus, A. Bortolussi, N. Careddu, R. Ciccu, B. Grosso, G. Massacci: ‘Influence of stone properties on diamond wire performance’, Proc. Fourth International Conference on Computer Applications in the Minerals Industries (CAMI 2003). September 8 - 10, 2003. Calgary, Alberta, Canada. Ed. Singhal/Fytas/Chiwetelu.
Authors O. Cai is an independent diamond tooling consultant. N. Careddu, M. Mereu and I. Mulas work at the Department of Geoengineering and Environmental Technologies (DiGITA), University of Cagliari, Italy.
[8] M. Agus, A. Bortolussi, N. Careddu, R. Ciccu, B. Grosso, G. Massacci: ‘Influenza delle caratteristiche della roccia sulle prestazioni del filo diamantato’, Rivista tecnica ‘Diamante Applicazioni e Tecnologia’, Anno 11, n. 42, Settembre 2005, pp99-109. Ed. G & M Associated Sas. ISSN 1824-5765. [9] R. Ciccu, M. Agus, A. Bortolussi, G. Massacci, N. Careddu: ‘Diamond wire sawing of hard rocks’, Ultrahard Materials Technical Conference Proceedings of ‘Superabrasive & CVD Diamond - Theory & Application’ May 28-30, 1998 - Windsor, Ontario, Canada. Also printed in Finer Points, Vol 11, No. 4 1999, pp22-30. [10] R. Ciccu, M. Agus, A. Bortolussi, G. Massacci, N. Careddu: ‘Il Taglio delle rocce dure con filo diamantato’, Rivista tecnica ‘Il Diamante - Applicazioni e Tecnologie’, Dicembre 1998, pp78-95. Ed. G & M Associated Sas. [11] N. Careddu, I. Mulas: ‘Diamond wire equipment in granite quarries: safety and standards’, Rivista tecnica ‘Diamante - Applicazioni e Tecnologia’, Anno 9, n. 35, Dicembre 2003, pp97-109. Ed. G & M Associated Sas.
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