Comparison of Drilling Performance of Chisel and Button Bits on the Electro Hydraulic Driller

Transcription

1 Rock Mech Rock Eng (2013) 46: DOI /s ORIGINAL PAPER Comparison of Drilling Performance of Chisel and Button Bits on the Electro Hydraulic Driller Okan Su Olgay Yarali Nuri Ali Akcin Received: 12 August 2012 / Accepted: 13 December 2012 / Published online: 27 December 2012 Ó Springer-Verlag Wien 2012 Abstract Electro hydraulic drillers have been widely used in mining for drilling and roof-bolting. In the drilling process, the performance of the machine is predicted by selecting an appropriate bit type prior to drilling operations. In this paper, a series of field drilling studies were conducted in order to examine and compare the performance of chisel and button bits including wear on the bits. The effects of taper angle on chisel bits, which are at angles of 105, 110 and 120, were investigated in terms of rate of penetration, instantaneous drilling rate and specific energy. The results of drilling and abrasivity tests performed in the laboratory supported the outcome of the field studies. Based on laboratory studies and field observations, it was proven that the conglomerate encountered, though it is very abrasive, is easy to drill. The cutter life in the encountered series is also longer in sandstone formation compared to the conglomerate. Additionally, button bits resulted in lower specific energy and higher penetration rates relative to chisel bits, regardless of their taper angles. The results were also supported with statistical analyses. Keywords Drillability Specific energy Penetration rate Wear Drilling performance O. Su Department of Mining and Mine Extraction, Bulent Ecevit University, Zonguldak, Turkey O. Su (&) O. Yarali N. A. Akcin Department of Mining Engineering, Bulent Ecevit University, Zonguldak, Turkey okansu@karaelmas.edu.tr 1 Introduction The drilling-blasting method is widely applied in both mining and civil engineering when the conditions are not suitable for mechanized excavation systems. Drilling is the first step of blasting operations in surface and underground mining. There are several drilling techniques, but the most applied methods are rotary and percussive drilling. Rotary drilling is usually applied in soft rock formations, while percussive drilling is applied in hard rock formations. Penetration rate of the bit is the main difference between the two techniques. In rotary drilling, the penetration rate averages a few millimeters per second, versus few centimeters per second in percussive drilling (Wijk 1991). In order to determine a suitable drilling method, the physical, mechanical, drillability and abrasivity properties of the rock, as well as cost estimates, should be investigated prudently prior to performing drilling operations. When the limitations of both methods are taken into account, a combined rotary and percussive drilling technique called percussive-rotary drilling can be used to increase the drill rate with a lower required level of weight on a bit (Franca 2011). This combined method has been successfully applied so far. A general view of a bit connected to the shank via driller rod is shown in Fig. 1 (Thuro 1997). There are many kinds of drill bits used in the mining industry. By taking into account the drilling purpose and working conditions, the type of drill rig and the bit type can be selected carefully. Essentially, the geometry, dimension, length, and slope of the blasthole are taken into consideration. Tri-cone or roller bits are the most common bits used in rotary drilling (Fig. 2a). Tri-cone bits cut the rock with a crushing and chipping action. The first roller cone bit was introduced into the oil field in 1909 and it was primarily used in hard rock formations. It had two cones on the bit

2 1578 O. Su et al. Fig. 1 A general view of a driller (Thuro 1997) Fig. 2 The bits used in drilling operations (Mincon 2012) head, causing some issues in soft rock drilling. For this reason, in the early 1930s, the bit was redesigned for both hard and soft formations. A third cone was added later as well as water nozzles. The primary drilling mechanism of the roller cone bits is intrusion, which means that the teeth are forced into the rock by the weight-on-bit, and pulled through the rock by the rotary action. The journal angle of the bit is modified according to hardness of the formation (BHITD 1995). Button and cross drill bits shown in Fig. 2b are also widely used in percussive drilling. The wing angles and button configurations can be modified according to formation (Mincon 2012). Although button bits give higher penetration rates, they are more prone to deviation in long holes than cross bits. A successful drilling process can be performed relying on some factors such as bit selection, optimizing the drilling parameters, monitoring the drill rig, operator s experience, and so on. Huang and Wang (1997) reported that the drilling rate and energy requirement of a drill depends on bit properties (e.g. bit type, bit diameter, bit geometry), and operational properties (e.g. weight-on-bit, rotational speed, and circulating fluid). They also pointed out that formation properties, including rock properties and geological conditions, are the uncontrollable factors in drilling. However, it is recommended that detailed site investigations and laboratory studies should be performed to insure proper factors since drilling and blasting operations account for around 20 % of total excavation cost (Sharma et al. 1990). Drilling performance of a bit can be evaluated by means of specific energy, penetration rate, and instantaneous drilling rate of an excavation system. Specific energy, that is the work required to cut a unit volume of rock, is based upon the performance of the machine. Low specific energy values result in high efficiency from the driller. Bit wear, cutting forces on the bit, thrust force, and average torque of the driller also affect the performance and hence the specific energy. Reddish and Yasar (1996) stated that a number of parameters such as rock strength, rock stiffness, structural discontinues, abrasivity, hardness of the mineral constituents, rock matrix, and nature of mineral grain influence the specific energy. Protodyakonov (1962) indicated the variation of specific energy with axial pressure in rotary drilling, and also its variation with impact energy in percussive drilling. The rock types he selected were in different hardness classifications and he proposed that the specific energy decreases as the axial pressure and impact energy increase. Tandanand and Unger (1975) carried out drilling tests on two different drifts and calculated specific energy by using the laboratory and field tests results. Then, they compared the results and found out that there is a strong relationship between the laboratory and field tests equal to For this reason, Rabia (1982) later pointed out that specific energy is not a fundamental intrinsic property for the evaluation of drilling performance, and suggested empirical equations for the prediction of specific energy. Uniaxial compressive strength was also widely used for predicting the performance of tunneling machines and drill rigs (Evans and Pomeroy 1966; Nishimatsu 1972; Howarth et al. 1986; Reddish and Yasar 1996; Thuro and Spaun 1996). However, it is emphasized that knowledge of compressive strength in itself is insufficient datum to assess bit consumption and production capabilities of excavation machines (Johnson and Fowell 1986). McFeat and Fowell (1977, 1979) found the specific energy calculated in the laboratory decreases as the instantaneous cutting rate of roadheader increases. Kahraman et al. (2003) demonstrated an inverse relationship between penetration rate and specific energy. Another inverse relationship between penetration rate and uniaxial compressive strength was also presented in the same study. They concluded that the mechanical properties are closely related to penetration rate of percussive drills.

3 Comparison of Drilling Performance of Chisel and Button Bits Drillability and Abrasivity Properties of Rock Bit selection is one of the areas that has become very critical to overall drilling performance. Daily advance rates, rate of penetration, bit consumption, and strength of the rock are some of the parameters taken into consideration for bit selection. However, drillability and abrasivity are the dominant properties in a drilling and excavation process. Their effect should always to be studied before beginning to drill or excavate a formation in order to improve drilling tool efficiency. Bit wear in rock drilling is the dominant factor in determining the bit life and the cost of drilling. Significant savings can be achieved by effectively controlling the bit wear. Wear decreases penetration rates and increases required drilling forces (Ersoy and Waller 1995). Hence, minimization of wear on the bit and maximization of penetration rate will lead to the best performance for the driller. SINTEF (Norwegian University of Science and Technology) has developed new test methods in the last decades such as drillability rate index, brittleness test, Sievers miniature drill test, and bit wear index. The test methods were proposed by performing more than 2000 tests on different rock types in order to predict the drillability and abrasivity properties of bits. A considerable amount of literature has been published on the two parameters so far (Bruland 1998; Dahl 2003; Dahl et al. 2007; Plinninger 2008; Hoseinie et al. 2009; Kim and Bruland 2009; Gong and Zhao 2009; Yarali and Kahraman 2011; Dahl et al. 2012). In addition to drillability properties, wear on the bits is also assessed by considering some abrasivity tests such as bit wear index (BWI) and Cerchar abrasivity index (CAI). Bit wear index is usually used for the estimation of bit life and cost of drilling. It is associated with abrasion value (AV) test and DRI test results. Abrasion value of steel (AVS) test method, which is similar to AV test procedure, allows to calculate the cutter life index (CLI) by using Eq. (1). The entire test results are evaluated according to Table 1. SJ 0:3847 CLI ¼ 13:84 ð1þ AVS where CLI is cutter life index, SJ is Sievers J value, AVS is abrasion value steel. Furthermore, it is always suggested to perform other abrasivity tests in order to verify the results and better understand the cause of the wear on the bit. So far, there have been many studies describing the factors of the wear problem. The basic factors can be placed in the following groups (Deketh 1995): Rock material properties (texture, strength, composition, hardness, etc.) Rock mass properties (structure, inhomogeneities, etc.) Type of machinery (type of tools, machine cutting principle, etc.) Choice of machine setting (thrust, tare of penetration, power, etc.) Environment (submerged or dry, weather conditions, operator skills, etc.) Cerchar abrasivity index (CAI) test also gives preliminary knowledge about the cutter life used on excavation machines. The test has been widely used for the estimation of tool wear and is a suggested method by ISRM (1979) and ASTM (2010). The test results are evaluated according to Table 2. In addition to drilling and abrasivity properties of rock, the penetration rate and specific energy are also important parameters for the evaluation of the machine performance. The penetration rate and specific energy are calculated by Eqs. (2, 3). PR ¼ L ð2þ t P SE ¼ g ð3þ ICR where PR is penetration rate (m), L is length of the hole (m), t is drilling time (h), SE is specific energy (kwh/m 3 ), g is energy transfer ratio, P is power of the machine (kw), ICR is instantaneous drilling rate (m 3 /h). Table 1 Classification of drilling index tests (Bruland 1998) Category DRI BWI CLI Extremely low B25 B10 B5 Very low Low Medium High Very high Extremely high C83 C70 C75 DRI drillability rate index, BWI bit wear index, CLI cutter life index Table 2 Classification of Cerchar abrasivity index test (ASTM 2010) Cerchar abrasivity index Classification Very low abrasiveness Low abrasiveness Medium abrasiveness High abrasiveness Extreme abrasiveness Quartztic

4 1580 O. Su et al. In Eq. (3), the energy transfer ratio is a constant which varies depending on the type of excavation or drilling machine. By taking into consideration the diameter of the cutterhead on the mechanical excavator, the installed power on the machine and the rock quality index, reported energy transfer ratio ranges of some mechanical excavators as summarized in Table 3. By utilizing the current (I) and the voltage (V) on the machine, the power of the machine can also be determined by Eq. (4). P ¼ V:I ð4þ Instantaneous drilling rate is calculated according to volume of the hole and drilling time. Hence, it can be obtained by Eq. (5). ICR ¼ Vl ð5þ t where Vl is volume of the hole (m 3 ), t is drilling time of the hole (h). 3 Drilling Performance of Bits The Turkish Hardcoal Enterprises (TTK) located in Zonguldak is one of the largest basins in Turkey. It consists of five underground coal mines with the deepest reaching approximately 1000 m. Since the coal seams have more or less than of inclination and 4 5 m of thickness, the mines have been operated by a semi-mechanized system. At these mines, the jack hammers are used for coal extraction. The drifts are excavated by drill-blast techniques with electro hydraulic drillers, in which the percussive method is applied. A roadheader was also driven in the drifts few years ago, but it did not work efficiently due to high bit consumption rates. Essentially, either the bit geometry or the bit type on the driller influences the penetration rates since incorrect bit selection may cause an increase in tool wear, and hence, lower advance rates within the formations. Therefore, an optimum bit type depending on the rock should be determined firstly in drilling operations. The effect of different bit types on the drilling performance was conducted in this study. Two different bit types, chisel and button were taken into consideration and their drilling performances were investigated. 3.1 Chisel and Button Bits The chisel bits have been extensively used on jack hammers for coal production at the Turkish Hardcoal Enterprise. A taper angle of 110 is usually applied on those bits. In order to investigate the effect of taper angles on drilling performance, two other bits having 105 and 120 of taper angles were modified from the original ones by re-sharpening at the Eregli Iron & Steel Plant (Fig. 3). Later, the bits were welded to monoblock steel rods, which were in mm diameter and m lengths as shown in Fig. 4. In the meantime, eight specially designed shanks were fabricated in the TTK laboratories. Those shanks were used to mount the monoblock rods onto the electro hydraulic driller. In addition to chisel bits, button bits of 42 mm diameter were used in the course of the drilling process. Their performance was also monitored and compared with the chisel bits. 3.2 Monitoring of Drilling Performance Drilling operations were basically carried out in the development drifts of the Turkish Hardcoal Enterprises Kozlu mine at the level of -560 m. The drifts exist in Westphalia and Namurian formations with a number of faults, particularly, in the north-south direction and are in the Mesozoic age. An important feature of the Kozlu mine is that the drifts are currently advanced under the Black Sea. The driller was operated on three different rock types in sedimentary series including sandstone, conglomerate, and an intermediate layer which consists of sandstone and conglomerate. Before beginning the drilling process, in situ hardness of the locations was measured by using an N type Schmidt hammer (Fig. 5). Twenty readings in each location were conducted and the average values were Table 3 Energy transfer ratio of some common mechanized techniques (Rostami et al. 1994) Machine type Energy transfer ratio Tunnel boring machine Roadheader Raise borer Shaft drill Continuous miner-drum miner Fig. 3 The chisel bits in different taper angles

5 Comparison of Drilling Performance of Chisel and Button Bits 1581 calculated as Schmidt hardness value. The results are given in Table 4. In particular, drilling rates and wear on the bits were examined during drilling operations. The percussion pressure on the monoblock steel rod was selected as 50 bar and Fig. 4 Monoblock steel rods welded with chisel bits the water pressure was set up at 7 8 bar. At the beginning of drilling, the diameters of bits were measured. The chisel bit having an angle of 105 was firstly mounted to the shank in the sandstone formation which has 22 m 2 of cross section area (Fig. 5a). The telescopic boom was positioned perpendicular to the face. The machine was started and the current on the machine was measured by using a clamp meter. As soon as the machine began drilling, the current was measured one more time and was recorded. The voltage on the machine was also defined as 550 V. Hence, the power of the machine was calculated by Eq. (4). When the drilling was completed with a bit, the length of the hole was measured by a tapeline and the volume of the hole was calculated. An averaged distance of cm was drilled at each time. The time of drilling was recorded by a chronometer while the machine worked and an instantaneous drilling rate was estimated by Eq. (5). In addition, the diameter and the taper angle of the bits, i.e. wear on the bit, were monitored in every five holes and quantified. Approximately 160 holes were drilled by employing four types of bits in the sandstone formation. (a) Schmidt hardness measurement points in sandstone (22m 2 ). (b) Schmidt hardness measurement points in conglomerate (18 m 2 ). Fig. 5 The measurement points of Schmidt hardness at the drifts (c) Schmidt hardness measurement points at intermediate layer (conglomerate + sandstone) (22 m 2 ).

6 1582 O. Su et al. Table 4 Mechanical properties of rocks (Su and Yarali 2010) Test type Sandstone Sandstone? conglomerate Conglomerate r c ± ± ± 6.07 r t 6.58 ± ± ± 0.50 Is //(50) 2.87 ± ± ± 0.40 Is \(50) 1.59 ± ± ± 0.40 ScH ± ± ± 3.82 r c uniaxial compressive strength (MPa), r t indirect tensile strength, Is //(50) corrected point load index in diametral direction (MPa), Is \ (50) corrected point load index in axial direction (MPa), ScH Schmidt hammer Fig. 7 Wear on the bits to mechanical, drillability, and abrasivity testing according to the ISRM standards. 3.3 Laboratory Studies Fig. 6 The broken bits for the period of drilling operation However, some issues were encountered in the course of drilling close to the discontinuity plane in the middle of the face. The bit stuck in the hole a few times, resulting in lower advance rates. The drill machine was moved to another location two months later. There was an intermediate layer, which consists of conglomerate and sandstone at the face as seen in Fig. 5c. Although the upper part of the face was conglomerate and the downside was sandstone, drilling was performed in the middle part. Thirty-seven holes were drilled in 2 days. The low number of drilled holes was due to frequently broken bits as shown in Fig. 6. Some issues in the shank during installation and adaption also had to be resolved. For this reason, the ratio of machine utilization was very low in the second drilling trial. The drill machine was driven to conglomerate formations two months later (Fig. 5b). Two bits from each type were employed in this formation. The numbers of drilled holes are 65, 7, 46 and 4 for the button and the chisel bits at 105, 110, and 120 of taper angles, respectively. It was noticed that the bits were excessively worn during drilling as shown in Fig. 7. Minimum drilling rates were obtained by the bit having 120 of taper angle, whereas maximum drilling rates and number of drills were obtained using the button bit. Large block samples were also collected from the face following drilling operations. The samples were subjected In order to determine mechanical properties of the rock samples, uniaxial compressive strength, indirect tension strength, and point load index tests were performed in the laboratory. For this purpose, core samples were taken from large blocks. For the uniaxial testing, the samples were prepared in 54 mm mm dimensions and the surfaces of the samples were grinded in order to reduce surface roughness. The same procedure without grinding was also followed for the core samples prepared for indirect tension and point load index testing. However, the samples in 54 mm by 27 mm dimensions were employed for those tests. The point load tests were performed both in axial and diametral directions. The tests were repeated on seven or eight samples and the average values were calculated. The results are summarized in Table 4. As presented in Table 4, the strength properties of the rocks can be classified as medium hard rocks. However, it is clear that sandstone is harder than other formations. Moreover, drillability and abrasivity tests were performed in the laboratory. Drilling rate index, bit wear index, and cutter life index were determined. In addition, Cerchar abrasivity index test was carried out. The results are presented in Table 5. In accordance with Table 5, it is evident that the intermediate layer (sandstone? conglomerate) is the most difficult formation to drill since DRI value is in the high category according to Table 1. However, conglomerate is the most abrasive formation as shown from BWI and CAI values. It is extremely abrasive according to CAI values in Table 2. For this reason, abrasive mineral content of conglomerate might be in higher percentage than other rock types. This finding can also be supported with petrographic analyses. The results of AV and AVS also prove the same evidence. Nevertheless, there is a significant difference between abrasivity properties and cutter life index. Although conglomerate is more abrasive, the bit life will be longer when used in other formations due to low

7 Comparison of Drilling Performance of Chisel and Button Bits 1583 CLI values. Another reason is that it is easy to drill in terms of DRI and SJ values. It is apparent that any driller having appropriate bit type will lead to the highest advance rates although high bit consumption is expected. However, more research on this topic needs to be undertaken before the association between abrasivity and drillability studies. 4 Evaluation of Drilling Performances of the Bits Drilling performances of four different bits were evaluated by performing field measurements. For this purpose, m of length in total was drilled by employing the button bit. On the other hand, 95.34, , and 61.6 m of length were bored by using the chisel bits in the taper angles of 105, 110, and 120 respectively. A length of m was advanced throughout the entire drilling Table 5 Drillability and abrasivity index test results (Su and Yarali 2010) Test type Sandstone Sandstone? conglomerate AV S DRI AVS SJ BWI CLI CAI Conglomerate AV abrasion value, S 20 brittleness value, DRI drilling rate index, AVS abrasion value steel, SJ Siever s J value, BWI bit wear index, CLI cutter life index, CAI Cerchar abrasivity index operation in 319 blastholes. During drilling operation, drill time and the length were recorded. At the same time, the current charged by the machine was also measured in order to determine the power of the machine. At the end, average penetration rate, average instantaneous drilling rate, and average specific energy were calculated by Eqs. (2, 3) and (5). All results were obtained by in-situ measurements. The performance of the bits was assessed according to bit type and formation type. The results of average drilling length, average penetration rate, average instantaneous drilling rate, and specific energy are presented in Tables 6 and 7. In the course of specific energy calculation, the energy transfer ratio of the drilling machine was assumed as 0.3. The averages of all variables were also based on total number of drills. However, machine utilization was neglected due to maintenance shutdowns for replacing the bits and shank. Rock drilling studies of various researchers performed at the field showed that the results were included in the range MJ/m 3 (Tandanand and Unger 1975; Rabia 1982; Rabia and Brook 1981). However, the results we collected range from 278 to 655 MJ/m 3. In light of the results, the aspects of drilling performances of the bits have been discussed below, and the graphs between the variables are illustrated in Figs. 8, 9, and 10. Moreover, the drilling performance of the bits depending on the formation type is shown in Fig. 11. As seen in Fig. 8, the maximum penetration rate of m/h was obtained by button bit. In other words, the button bit has the best drilling performance in comparison to other bits. However, the bit having 110 of taper angle, still in use for coal production on jack hammers, has the best performance after button bit. So it is apparent that Table 6 The drilling performance of bits depending on the bit type ND number of drills, TDL total drilling length (m), L average hole length (m), t average drilling distance (s), t average drilling time (s), PR average penetration rate (m/h), ICR average instantaneous drilling rate (m 3 /h), P power of the machine (kwh/m 3 ), SE average specific energy (MJ/m 3 ) Bit type Rock type ND TDL L T PR ICR P SE Button Sandstone Conglomerate Conglomerate Sandstone? Conglomerate Sandstone Conglomerate Conglomerate Sandstone? Conglomerate Sandstone Conglomerate Conglomerate Sandstone? Conglomerate Sandstone Conglomerate Conglomerate Sandstone? Conglomerate

8 1584 O. Su et al. Table 7 The drilling performance of bits depending on rock type The notations are as given in Table 6 Rock type Bit type ND TDL HL t PR ICR P SE Sandstone Button Sandstone? Conglomerate Button Button Conglomerate Button PR (m/h) SE (MJ/m 3 ) Button Bit Type 0 Button Bit Type Fig. 8 Comparison of penetration rates versus bit type Fig. 10 Comparison of specific energy versus bit type Sandstone Conglomerate Sandstone+Conglomerate ICR (m 3 /h) Button Bit Type PR (m/h) SE (MJ/m3) Fig. 9 Comparison of instantaneous drilling rates versus bit type those bits can be kept working on the jack hammers with minimal performance loss. On the other hand, there is a good consistency between penetration rate and instantaneous drilling rates. As Fig. 11 Comparison of penetration rate and specific energy depending on formation types demonstrated in Fig. 9, ICR values of button bits are higher than other bit types. Maximum drilling length was also obtained with the button bits.

9 Comparison of Drilling Performance of Chisel and Button Bits 1585 Another important implication was also provided in specific energy. As presented in Fig. 10, minimum specific energy was obtained by button bit type. The results of this comparison can essentially be taken into consideration for the prediction of a machine s performance and guidance in the bit selection. Meanwhile, although the penetration rates are more or less close to each other as shown in Fig. 11, the best drilling performance was achieved in conglomerate formations because of faster penetration rates and less energy consumption. This evidence was also supported by laboratory drilling tests since minimum SJ and DRI results were obtained in the same formation. The comparison of the specific energy in terms of formation type in Fig. 11 also indicates that minimum energy was consumed in conglomerate. However, there is a significant difference between abrasivity and drillability studies as given in Table 5. The abrasivity of conglomerate is higher than other rock types in view of BWI and CAI values. It is also clearly noticed in Table 7 that the number of drills in conglomerate are less than in sandstone. This is also an indication of abrasivity effect even though minimum numbers of drills were derived from intermediate layer due to the smallest cross section area. Thus, it is inevitable that the bit consumption in conglomerate will be high in the course of drilling although lower specific energy and higher rates of penetration are exhibited. The effect of abrasivity can precisely be better observed in long drilling length. The cutter life will also be very long in sandstone since CLI obtained in the laboratory is higher than other formations. This finding was also supported with the declaration of the operator of the electro hydraulic driller. He emphasized that approximately m of length per bit is drilled in conglomerate despite the fact that it is around m/bit in sandstone. Rabia (1985) also pointed out that button bits have longer life compared to chisel bits. On the other hand, by utilizing the data provided in Tables 6 and 7, specific energy was correlated with PR and ICR. The relationship between the specific energy and instantaneous drilling rate is plotted in Fig. 12. In addition, the relationship between specific energy and penetration rate is drawn in Fig. 13. As indicated in Fig. 12, a good correlation, with a coefficient of 0.86, between the specific energy and penetration rate was obtained. Higher penetration rates were achieved in lower specific energy data, which belong to button bits. Reddish and Yasar (1996) created a rotary drill rig by using a masonry bit sharpened to 118. They performed drilling tests on 13 rock types and drilled 20 holes on each type. They recorded the SE values varying between 318 and 2928 MJ/m 3 and depicted the graph between SE and PR. The same downtrend was derived as in data shown in Fig. 12. Furthermore, another significant correlation was found between specific energy and instantaneous drilling rate where the correlation coefficient was determined to be 0.99 (Fig. 13). As the specific energy decreases, instantaneous drilling rates increase. McFeat and Fowell (1979) also demonstrated a similar curve for the roadheaders even though the specific energy was obtained from laboratory studies. Both of these graphs clearly reveal that the specific energy decreases, as both of penetration rate and instantaneous drilling rate increase. The variables, which are penetration rate, instantaneous drilling rate, and specific energy, were also statistically analyzed in order to examine whether the values are significant or not. In this context, one sample t test and oneway ANOVA test were performed at a confidence level of 95 % in SPSS. In these tests, specific energy was defined as the dependent variable while instantaneous drilling rate and penetration rates are defined as independent variables. The results are presented in Tables 8 and 9. Table 8 reports the results of one sample t test. The critical t value, which is obtained from t test table, with 15 degrees of freedom is Since the t values given in the Table 8 are much higher than and the P values are less than 0.05, the results are significant. For this reason, SE (MJ/m 3 ) y = x R² = PR (m/h) SE (MJ/m 3 ) y = 51313x x R² = ICR (m 3 /h) Fig. 12 The relationship between SE and PR Fig. 13 The relationship between SE and ICR

10 1586 O. Su et al. Table 8 One sample t test results PR penetration rate, ICR instantaneous drilling rate, SE specific energy Variable t value df P value Mean difference 95 % confidence interval of the difference Lower Upper PR ICR SE Table 9 One-way Anova test results Variables Source Sum of squares df Mean square F value P value SE ICR Between groups Within groups Total SE PR Between groups Within groups Total we reject the null hypothesis. In this manner, the results clearly reveal that the penetration rate and the instantaneous drilling rate can be associated with specific energy in order to evaluate the performance of a drill machine. On the other hand, Table 9 shows the output of the one-way ANOVA analysis. Since the P values are below 0.05, it is apparently said that the results of drilling tests performed underground are reliable and statistically valid. 5 Conclusion This study presents and compares drilling performance of chisel and button bits. The bits were fabricated in 105, 110 and 120 of taper angles. Drilling measurements were implemented at the -560 m level of an underground coal mine. As soon as the site was selected, three visits were made at regular intervals to monitor the performance of the electro hydraulic driller. In this context, the penetration rate, instantaneous drilling rate and specific energy of the bits were examined. As a result of the field studies, maximum penetration rate and minimum specific energy were achieved by using button bits. However, the machine has encountered some issues in conglomerate owing to the higher abrasivity of this stratum. It might be due to the larger grain size and higher amount of abrasive minerals such as quartz in the rock structure. Therefore, it is suggested to analyze the petrographic structure of the samples to have an idea about the composition of rocks. It would be better when both abrasion properties and petrographic analyses are combined and evaluated together in the course of performance prediction. Because of excessive wear or broken chips of the bit, it was necessary to re-sharpen the chisel bits at different intervals of the drilling. It is indeed a time consuming and expensive process. However, minimum wear was detected while drilling with button bits and they do not require any re-sharpening. It will save a lot of time in drilling. Moreover, strong correlations among penetration rate, instantaneous and specific energy were established. The results were also statistically analyzed and it was seen that the drilling measurements are valid. Those findings suggest that higher penetration and instantaneous drilling rates are obtained at lower specific energy, and that specific energy is directly related to penetration rate. Drillability and abrasivity index properties also have a significant effect on the bit selection. The consistency between laboratory and field studies verifies the importance of laboratory studies. However, further tests will lead to a better understanding of the rock behavior for the purpose of drillability. Hence, minimum costs and maximum advance rates will be achieved via accurate bit selection. As a result, we can clearly say from drill monitoring that button bits are more efficient than the chisel bits in terms of drilling rate, specific energy and bit life. They enable higher rates of penetration, lower mechanical wear and lengthen service life of machine. Moreover, chisel bits having 110 of taper angle can be kept in use on jack hammers since they represent the second best performance after button bits. Acknowledgments This study was supported by an industrial project between Bulent Ecevit University and Turkish Hardcoal Enterprises (TTK). The authors would like to acknowledge TTK for their support, funding, and the permission to perform the field studies. References ASTM D7625 (2010) Standard test method for laboratory determination of abrasiveness of rock using the Cerchar method, 6 p

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