Optimum cutting speed of block-cutting machines in natural stones for energy saving

Transcription

1 J. Cent. South Univ. (2012) 19: DOI: /s y Optimum cutting speed of block-cutting machines in natural stones for energy saving N. Bilim Department of Mining Engineering, Selçuk University, Konya, Turkey Central South University Press and Springer-Verlag Berlin Heidelberg 2012 Abstract: Energy consumption of block-cutting machines represents a major cost item in the processing of travertines and other natural stones. Therefore, determining the optimum sawing conditions for a particular stone is of major importance in the natural stone-processing industry. An experimental study was carried out utilizing a fully instrumented block-cutter to investigate the sawing performances of five different types of travertine blocks during cutting with a circular diamond saw. The sawing tests were performed in the down-cutting mode. Performance measurements were determined by measuring the cutting speed and energy consumption. Then, specific energy was determined. The one main cutting parameter, cutting speed, was varied in the investigation of optimum cutting performance. Furthermore, some physico-mechanical properties of the travertine blocks were determined in the laboratory. As a result, it is found that the energy consumption (specific energy) of block cutting machines is highly affected by cutting speed. It is determined that specific energy value usually decreases when cutting speed increases. When the cutting speed is higher than the determined value, the diamond saw can become stuck in the travertine block; this situation can be a problem for the block-cutting machine. As a result, the optimum cutting speed obtained for the travertine mines examined is approximately m/min. Key words: cutting speed; travertine; block cutting machine; specific energy; natural stone; energy saving 1 Introduction is preferred as a construction and decoration material for several reasons such as it displays attractive colors and designs and is a convenient and natural material. The objective of the natural stone processing facilities where travertine blocks are processed in order to obtain end product is to maintain production of the amount of products demanded by the market at the lowest possible cost. The degree of success of cutting can be reflected in the cutting performance of the saw used. Therefore, monitoring cutting machines to produce performance parameters can be considered an effective way to obtain a reliable and quantitative measure of the cuttability and/or effectiveness of production over a range of stone types [1]. Cutting mechanism, cutting speed, dimension of the blocks, and physical-mechanical properties of the natural stone all affect energy consumption in cutting. Block-cutting machines can generally be classified into two groups: horizontal saw block-cutting machines (frame saws) and circular diamond saw block-cutting machines. Block-cutting machines with circular diamond saws are machines used for cutting natural stone products with relatively smaller width, which are called plates. In the natural stone industry, circular diamond saw block-cutting machines are widely used for cutting natural stone blocks and rubbles and turning them into plates. The operating values that are appropriate to the type of the rock being cut are required to be continuously regulated and kept under control during the process of cutting natural stones by using these machines. For this reason, it is very important to appropriately select the peripheral speed of the vertical saw, cutting (advance) speed and cutting depth [2]. The sawing performance of the circular diamond saw in stone processing has previously been investigated in stone processing [3 7]. The most important subject in sawing processes at natural stone processing plants is the selection of suitable diamond segments and sawing parameters. Although classification systems are used in other areas of the mining sector, the classification system for sawability in natural stone processing has not been developed yet [8]. The performance and life of a circular diamond saw are affected by many factors. Parameters that affect the efficiency of block-cutting machines are given in Table 1. Received date: ; Accepted date: Corresponding author: N. Bilim; Tel: ; Fax: ; bilim@selcuk.edu.tr

2 J. Cent. South Univ. (2012) 19: Table 1 Parameters affecting efficiency of block-cutting machines Constant parameter of related rocks Related block cutting machine variable parameters Environmental condition Physico-mechanical properties Chemical properties Mineralogical properties Petrographic properties Discontinuities Textural properties Structural properties Distortion characteristics Cutting speed and depth Sawing mode Water flow and pressure Chemical properties of water Cutting block size Structure of machine and motor power Saw size and properties Wear patterns on saw Peripheral speed Technical staff Pressure distribution on saw Forces between diamonds pieces and marble Vibration TIRYAKI and DIKMEN [9] reported that some parameters related to rock texture and composition, such as texture coefficient and feldspar, mafic, and felsic mineral contents, which had been ignored in previous studies, were considered and the researches investigated the applicability of the texture coefficient concept to linear cutting of sandstones with picks. GÜNEY [10] reported that the influence of mineral grain size is significant on the performance of circular saws, and it must be taken into consideration in the estimation of hourly slab productions. The influence of processing parameters on sawing performance varies from one rock type to another. Consequently, it is very hard to establish optimum process parameters that are valid for all rock types. Therefore, considering this fact, the main purpose of this work was to focus on a particular group of rocks. This work consists of the plant and laboratory studies carried out to determine the relationships between energy consumption values and cutting speed of full-scale block-cutter machines (circular diamond saws) for travertine blocks of weak hardness. In this work, the results of circular diamond saw block cutting machine performance measurements for five different travertine stone units will be presented and discussed. In addition, suggestions to predict optimum cutting speed and energy consumption for practical purposes are given. The objective of the present work is to examine the influence of the cutting variables of the circular diamond saws. Relations between energy consumptions and cutting speed in travertine blocks are also investigated. As a result, optimum cutting speed was determined in consideration of energy consumption values. 2 Experimental procedures The studies conducted within the scope of this work were performed in three stages: plant stage, laboratory stage and evaluation stage. Studies intended for examining the parameters during the cutting process of the travertine with block-cutting machines were performed at the plant stage. The in-plant experimental studies were conducted in a travertine mine located in the Konya Region of South Central Turkey (Fig. 1). The blocks were cut using a block cutting machine (circular diamond saw) that was well-configured considering the technological conditions available at this establishment. Average cutting speeds applied during the cutting process, particularly while cutting blocks, were also determined at the in-situ stage. Cutting speed was obtained by dividing the dimension of the block by the cutting time. Furthermore, the current and voltage values measured during the cutting process were instantly recorded and the instantaneous power consumption values of the machine were calculated. The cutting performances of block cutting machines for five different travertine stone units were measured. All experiments were carried out using a mm diamond disc. The operation parameters of the block-cutting machine (engine power, saw size, socket design, amount of water used, operator, etc.) were always kept constant during the cutting experiments in order to obtain more reliable results. The physical and mechanical properties of the travertine blocks were determined at the laboratory stage. Laboratory experiments were performed on the samples according to ISRM [11] standards. The physico-mechanical properties of travertine obtained as the result of these studies are presented in Table 2. At the evaluation stage, which was the last stage, the goal was to evaluate the findings obtained from plant and laboratory results. The relationship between the specific energy values consumed during the cutting process and average cutting speeds were examined with the physical and mechanical properties of travertine. 3 Performance measurements of blockcutting machine Sawing tests in plant were performed on a block-cutting machine with a maximum spindle motor power of 110 kw and spindle speed of r/min. The

3 1236 J. Cent. South Univ. (2012) 19: Fig. 1 Location of travertine mine Table 2 Some physico-mechanical properties of travertines Sample type A B C D E UCS/ MPa PLT (Is50) Schmidt hardness Porosity/ % Density/ (g cm 3 ) cutting unit was placed on columns of the machine with sledge. Saw movements up and down in the vertical plane are shown in Fig. 2. In this work, a circular diamond saw is used for travertine stones, which are non-abrasive and low in hardness, generally medium- to coarse-grained. Water was used as flushing and cooling medium, and constant flow rate was maintained. Cutting parameters such as cutting speed, depth of cut, peripheral (rotational) speed and power consumed were measured. The same block-cutting machine was used at all the stages of the Fig. 2 Circular sawing experiments conducted with travertine mine on block-cutting machine work and the diameter of the circular diamond saw, socket structure, rotation speed, peripheral speed, amount of water supply and water pressure were kept constant, because these parameters affected the cutting speed. Unlike direct current circuits, it is essential to take into consideration the power factor in addition to current and voltage in alternative current circuits. The power factor is equal to the cosine of the phase angle between the current and the voltage. This angle is denoted by the symbol & and the power coefficient is denoted by cos&. The following equation was used for calculating the power consumption of the three-phase engine of the block-cutting machine: W= 3 V I cos& where W is the power of electrical motor, W; V is the operating voltage, V; I is the current, A; cos& is the power coefficient. 3.1 Specific cutting energy Specific energy can be calculated using the mean cutting force measured during rock-cutting tests. Specific energy is an important performance parameter as it relates cutting force to the amount of rock excavated. The specific cutting energy is a very significant measure used for the determination of the cutting performance, because it indicates the amount of energy required to cut the rock [12]. Specific cutting energy can also be used to quantify the efficiency of rock working processes (cutting, drilling, excavation, breaking, etc.) and to indicate sawblade conditions and rock characteristics such as strength, hardness, abrasiveness and texture [13]. The specific cutting energy is derived from the amount of energy required to remove a given volume of rock and has been successfully used in the diamond tool industry [14]. There have been many studies on the

4 J. Cent. South Univ. (2012) 19: sawing performance and the mechanism of the circular diamond saw [15 17]. Sawing or cutting drilling is an action process that can be assessed by cutting specific energy and drilling specific energy [18]. 3.2 Properties regarding cutting speed When the rock is cutting at a high speed, it causes excessive wear of the diamond sockets; on the other hand, when the cutting process is performed slower than required, the energy used for cutting increases and the machine wears out as a result of operating overmuch due to the increase of cutting time. Both conditions negatively affect the cutting cost and, consequently, the production cost. Selecting the optimum cutting speed that is appropriate for the rock to be cut is highly important for using block-cutting machines efficiently. It was found out that there was a significant relationship between the average cutting speeds and specific energy values (Figs. 3 7) by using the results obtained through the cutting processes performed at the travertine processing facility. Fig. 5 Relationship between cutting speed and specific energy for C Fig. 6 Relationship between cutting speed and specific energy for D Fig. 3 Relationship between cutting speed and specific energy for A Fig. 7 Relationship between cutting speed and specific energy for E Fig. 4 Relationship between cutting speed and specific energy for B Cutting performance measurement results are presented in Table 3. Performance monitoring of the block-cutting machine involved not only cutting speed but also energy consumption. The energy consumption for each case was calculated by utilizing the current and voltage data.

5 1238 Table 3 Specific energy values obtained by travertine cutting process using circular diamond saw block-cutting machine Average Average Saw Block Rock production specific diameter/ dimension/ type speed/ cm cm (m 2 h energy/ 1 ) (kw h m 2 ) A B C D E Relationship between cutting speed and specific energy The cutting procedure was performed at five different cutting speeds of travertine blocks in order to investigate the effect of cutting speed on energy consumption. The current voltage values consumed by the machine during the cutting process were recorded. The graphs based on the data obtained through the cutting processes performed on different sizes and types of travertine blocks can be seen in Figs Figures 3 7 were composed based on the data obtained through the measurements conducted of energy consumption at different cutting speeds from the beginning to the end of the block-cutting process. There will be a significant amount of energy saving if travertine blocks are cut in optimum cutting speed suggested in this work. More energy would be consumed according to the optimum cutting speed suggested in this work if block cutting machine cut the travertine blocks with the minimum cutting speed. Energy saving values got from graphic analyses are given in Table 4. Table 4 Energy saving values when cutting in optimum cutting speed Rock type Energy saved/% A 16.9 B 23.6 C 24.1 D 33.4 E Results and discussion What makes the present work different from other similar studies is that it was conducted on travertine materials. Marble samples (uniaxial compressive strength varying between 50 and 100 MPa) were used as the research material in various studies conducted in J. Cent. South Univ. (2012) 19: order to examine the cutting performances of block cutting machines. In the present work, however, travertine samples, whose uniaxial compressive strength is lower (<50 MPa) compared to that of marble, were selected. The formation processes and strength values of travertine samples are different compared to marble samples; for this reason, the cutting mechanisms of travertine blocks are also different. The importance of the study directly on travertine can be understood more clearly if we consider that the amount of travertine production is almost the same as marble production at the natural stone-processing facilities existing in studying day. The present work was conducted in order to bridge the gap that exists in the literature. material characteristics for each type were determined in the laboratory in order to investigate the relation between cutting speed and performance parameters. The effects of the physical and mechanical properties of the rock, which are among the constant parameters affecting the cutting efficiency of block cutting machines, the effects of cutting speed, which is listed among the semi-variable/variable parameters, on the energy consumption and strain of the machine, and the effects of the mechanical properties of the cut rock on cutting speed were examined in the present work. As a result, it was found out that the energy consumption (specific energy) of block cutting machines was highly affected by cutting speed. In this work, it was determined that specific energy value decreases when cutting speed increases. In other words, the consumption of energy decreases in high-speed cutting. This situation is very important for a mine. If optimum cutting speed value is known in a travertine mine, the less electric energy consumption will take place. Thus, the unit price of natural stone production will decrease for this travertine mine. 5 Conclusions 1) Cutting speed should be determined by taking the constant parameters related to the rock to be cut (physical and mechanical properties, and mineralogical properties, etc.) into consideration. 2) It is necessary that the cutting speed be adjusted to the optimum level in order to achieve the highest production capacity with the lowest cost. 3) It can be seen that the cutting speed for the travertine mines examined is approximately m/min. 4) High speeds cause some problems although the specific energy value decreases by a negligible quantity when the cutting speed is larger than the optimum cutting speed. When the cutting speed is higher than the determined value, the diamond saw can become stuck in

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