KR102404517B1 - Rock drilling apparatus and method of crushing rock using the same - Google Patents

Abstract

The present invention relates to a rock drilling device, comprising: a main housing having a through hole, a first receiving part provided on an upper portion of the through hole, and a second receiving part provided on a lower portion of the through hole; a motor provided with a rotating pin and provided on an upper portion of the main housing; a rod connection part rotated by the motor; a bit rod coupled to a lower portion of the rod connection part; a drill bit coupled to a lower portion of the bit rod; A first spherical roller bearing and a second spherical roller bearing are provided between the rod connection part and the main housing, wherein, in the first spherical roller bearing, an inner surface of an inner ring is coupled to an upper portion of the rod connection portion, and the outer ring An outer surface is coupled to the first receiving part, and a pair of rollers are vertically disposed between the inner surface of the outer ring and the outer surface of the inner ring on a vertical cross-section of the first spherical roller bearing, and the inner surface of the outer ring and A radial gap is provided between the rollers so that the axis of rotation of the rod connection part can be inclined, and in the second spherical roller bearing, the inner surface of the inner ring is coupled to the lower part of the rod connection portion, and the outer surface of the outer ring is the A pair of rollers coupled to the second accommodating portion are vertically disposed between the inner surface of the outer ring and the outer surface of the inner ring on the vertical cross-section of the second spherical roller bearing, and between the inner surface of the outer ring and the roller It is possible to improve the efficiency of the rock crushing process, shorten the construction period, and reduce labor costs because a radial gap is provided so that the rotation shaft of the rod connection part can be inclined.

Description

Rock drilling device and rock crushing method using the same

The present invention relates to a rock drilling device and a rock crushing method.

In general, when performing rock crushing work at various construction sites, a vibrating hammer drill is often used. When rock crushing with a vibrating hammer drill is used, severe vibration and noise are induced, causing a lot of damage to people around the workplace. However, the working speed is slowed down, and the working efficiency is reduced.

Accordingly, a method has been proposed in which a hole of a certain size is drilled in the rock, and then a rock crusher is put into the hole to crack the rock, or explosives such as dynamite are put into the hole to explode. to be.

On the other hand, in the prior art, the rock crushing method was performed by forming a hole in the rock by the so-called core method of cutting the rock in a cylindrical shape and then extracting the rock inside the cylinder and inserting a crusher into the hole. This core method had to proceed in the following order: rock cutting, extraction of the cylindrical core rock, inserting a crusher, and crushing the rock. As a result, there were limitations in reducing construction time, reducing labor costs, and improving construction efficiency.

Korean Patent Publication No. 10-2222785 (Registered on Feb. 25, 2021)

According to one aspect of the present invention, it is possible to provide a rock drilling device capable of at least one of improving the efficiency of the rock crushing process, shortening the construction period, reducing labor costs, and extending the life of the equipment.

A rock drilling apparatus according to an embodiment of the present invention includes: a main housing provided with a through hole, a first accommodating portion provided on an upper portion of the through hole, and a second accommodating portion provided on a lower portion of the through hole; a motor provided with a rotating pin and provided on an upper portion of the main housing; a rod connection part rotated by the motor; a bit rod coupled to a lower portion of the rod connection part; a drill bit coupled to a lower portion of the bit rod; A first spherical roller bearing and a second spherical roller bearing are provided between the rod connection part and the main housing, wherein, in the first spherical roller bearing, an inner surface of an inner ring is coupled to an upper portion of the rod connection portion, and the outer ring An outer surface is coupled to the first receiving part, and a pair of rollers are vertically disposed between the inner surface of the outer ring and the outer surface of the inner ring on a vertical cross-section of the first spherical roller bearing, and the inner surface of the outer ring and A radial gap is provided between the rollers so that the axis of rotation of the rod connection part can be inclined, and in the second spherical roller bearing, the inner surface of the inner ring is coupled to the lower part of the rod connection portion, and the outer surface of the outer ring is the A pair of rollers coupled to the second accommodating portion are vertically disposed between the inner surface of the outer ring and the outer surface of the inner ring on the vertical cross-section of the second spherical roller bearing, and between the inner surface of the outer ring and the roller A radial gap may be provided so that the axis of rotation of the rod connection unit may be inclined.

At this time, the height difference between the vertical center line of the first spherical roller bearing and the second spherical roller bearing is H1, the inner diameter and the outer diameter of the first spherical roller bearing are D1 and D2, respectively, and D2 is 1.75~ of D1 It is preferable that 1.85 times, H1 is 1.85 to 1.95 times D1, and the radial clearance of the first spherical roller bearing is 0.0004 to 0.00075 times D1.

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In the rock crushing method according to an embodiment of the present invention, a bit including a polycrystalline diamond compact (PDC) is applied to the drill bit, and the drill bit is rotated at 1000 to 2000 RPM to perforate the rock. , it proceeds in such a way that a blasting process or a granite process is performed.

At this time, in a rock with a uniaxial compressive strength of 125 kgf/cm2 or less, drilling is performed at a descending speed of 1.0 to 2.0 m/min and a downward pressure of 2ton/m2, and 0.5 to 1.0 m in a rock with a uniaxial compressive strength of 126 to 600 kgf/cm2. After drilling at a descending rate of /min and a downward pressure of 1 ton/m2, in a rock with a uniaxial compressive strength of 601 to 1200 kgf/㎠, drilling at a descending rate of 0.3 to 0.7 m/min and a downward pressure of 0.6 ton/m2 It is preferable to proceed with the blasting process or the granite process.

In addition, the halam process is performed by inserting the lower part of the ripper blade of the vibrating ripper into the drilling hole, and the lower part of the ripper blade has a conical shape with a minimum diameter of R2 and a maximum diameter of R1, and R1 is the It is preferable that the diameter of the drilling hole is greater than that of R2, and that R2 is smaller than the diameter of the drilling hole.

In addition, it is preferable that R1 is 1.5 to 1.8 times of R2, and an acute angle formed by the side surface of the lower portion of the reper blade with the ground is 84 to 86°.

According to an embodiment of the present invention, it is possible to reduce the rotational vibration and noise of the bit rod by stably supporting the bit rod that rotates the punching bit when performing the rock drilling operation, and in particular, by the irregular external force applied to the bit rod It can solve the problem of motor damage.

In addition, since the rock drilling apparatus according to an embodiment of the present invention does not require a separate process for removing the cylindrical core, the efficiency of the rock crushing process can be improved, and effects such as shortening of construction period and reduction of labor costs can be realized.

1 is a view schematically illustrating an excavator equipped with a rock drilling device according to an embodiment of the present invention;
Figure 2 is a diagram schematically illustrating a rock drilling apparatus according to an embodiment of the present invention,
3 is a diagram schematically illustrating a cross-section of a major part of a rock drilling apparatus according to an embodiment of the present invention;
4 is a view for explaining a rock drilling device according to an embodiment of the present invention,
5 is a view for explaining the first spherical roller bearing of the rock drilling device according to an embodiment of the present invention,
6 is a diagram schematically illustrating a vibration ripper used in a rock crushing method according to an embodiment of the present invention;
7 is a view for explaining a rock crushing method according to an embodiment of the present invention.

Hereinafter, specific embodiments of the present invention will be described in detail with reference to the drawings.

In addition, in describing the present invention, if it is determined that a detailed description of a related known configuration or function may obscure the gist of the present invention, the detailed description thereof will be omitted.

1 is a diagram schematically illustrating an excavator equipped with a rock drilling device according to an embodiment of the present invention, FIG. 2 is a diagram schematically illustrating a rock drilling device according to an embodiment of the present invention, FIG. 3 is a diagram schematically illustrating a cross-section of a major part of a rock drilling apparatus according to an embodiment of the present invention, FIG. 4 is a view for explaining a rock drilling apparatus according to an embodiment of the present invention, and FIG. 5 is a view of the present invention It is a view for explaining a first spherical roller bearing of a rock drilling device according to an embodiment, and FIG. 6 is a view schematically illustrating a vibration ripper used in a rock crushing method according to an embodiment of the present invention, FIG. 7 is a view for explaining a rock crushing method according to an embodiment of the present invention.

Referring to the drawings, the rock drilling apparatus 1000 according to an embodiment of the present invention may perform a function of creating a drilling hole 2 in the rock 1 . In one embodiment, the rock drilling apparatus 1000 may include a housing 110 , a drilling module 200 , a bit rod 300 , a vibration reduction unit 400 , and a drill bit 500 . In this case, the perforation module 200 includes a main housing 230 , a motor 210 , a rod connection part 220 , and first and second spherical roller bearings 260 and 270 .

In one embodiment, the housing 110 constitutes the overall appearance of the rock drilling apparatus 1000 , and may have a predetermined accommodation space therein so that other components may be accommodated in the housing 110 .

In one embodiment, the housing 110 has a vehicle connection unit 140, 150 for connecting the housing 110 to the lifting rod 120 for elevating the drilling module 200, the operation rod 30 of the vehicle 31, and the like. ) may be provided. In addition, a fixing pin 130 may be further provided below the housing 110, and the fixing pin 130 contacts the rock when the housing is placed on the rock to perform a function of fixing the housing. have.

In one embodiment, a control unit (not shown) for controlling the operation of the rock drilling device 1000 may be provided on one side of the rock drilling apparatus 1000 . In addition, the rotation speed and torque of the motor 210 to be described later may be controlled by the controller.

In one embodiment, the driver's seat of the vehicle 31 is provided with a manipulation device capable of controlling the operation of the rock drilling device 1000, and the manipulation device is connected to the control unit, so that the driver can operate the vehicle 31 and control the rock bed The manipulation of the drilling device 1000 may be simultaneously performed.

On the other hand, in order to reduce the vibration of the bit rod 300, the vibration reducing unit 400 may be provided. In one embodiment, the vibration reducing unit 400 may include a support bearing that contacts the outer surface of the bit rod 300 to support the bit rod 300 and a holder for fixing the support bearing. In one embodiment, the support bearing and the holder may be coupled to the lower plate of the housing 110 . At this time, the lower plate of the housing 110 is provided with an inlet and outlet so that the bit rod 300 can pass through the inlet and outlet. In addition, a receiving part to which the holder is fastened may be provided on the lower plate. Vibration and noise generated during the drilling process of the drill bit 500 may be reduced by the vibration reducing unit 400 .

In one embodiment, a PDC lock bit including a polycrystalline diamond compact (PDC) may be utilized as the drill bit 500 . Accordingly, the drill bit 500 can be rotated at high speed to form the drilling hole 2 in the rock mass 1, and in particular, after cutting the rock in a cylindrical shape, drilling without proceeding with the process of extracting the rock inside the cylinder A perforated hole can be formed with just work.

On the other hand, when the rotation speed of the PDC lock bit is less than 1000 RPM, the time required to drill the rock increases rapidly, and when the rotation speed of the PDC lock bit exceeds 2000 RPM, the motor 210 is overloaded, or the drill bit 500 The lifespan of the rock drilling apparatus 1000 may be rapidly shortened due to the vibration generated in the , or the drill bit 500 may not be stably supported, resulting in a sharp decrease in drilling precision or safety. Therefore, in order to secure the efficiency of the work of drilling the rock using the PDC lock bit, it is preferable that the rotation speed of the PDC lock bit be 1000 to 2000 RPM.

On the other hand, the rock drilling apparatus 1000 according to an embodiment of the present invention has a main housing in order to minimize a problem due to vibration generated in the PDC lock bit even if the PDC lock bit is rotated at a high speed in the range of 2000 RPM or less. 230 , the rod connection part 220 , the first spherical roller bearing 260 and the second spherical roller bearing 270 are included.

In one embodiment, the main housing 230 performs a function of firmly and stably supporting the motor 210 and the rod connection unit 220 . In addition, a fastener is provided on one side of the main housing 230 to allow the perforation module 200 to be raised and lowered along the lifting rod 120 of the housing 110 .

In one embodiment, the rod connection part 220 is formed in a cylindrical shape as a whole, is connected to the motor 210 and rotates according to the driving of the motor 210 , and the bit rod 300 is located below the rod connection part 220 . As it is coupled, the rotational force generated by the motor 210 is transmitted to the drill bit 500 through the rod connection part 220 and the bit rod 300 sequentially, and the drilling process can be performed by the rotation of the drill bit 500 . have. In this case, the rod connection part 220 may be coupled to the main housing 230 via the first spherical roller bearing 260 and the second spherical roller bearing 270 .

In one embodiment, the first spherical roller bearing 260 includes an inner ring 261 , an outer ring 262 , and rollers 263 , and the second spherical roller bearing 270 is the first spherical roller bearing 260 . It is provided at positions spaced apart vertically downward.

In one embodiment, inside the main housing 230 , the first receiving part 231 for accommodating the first spherical roller bearing 260 and the second receiving part 232 for accommodating the second spherical roller bearing 270 . is provided The first accommodating part 231 and the second accommodating part 232 are vertically spaced apart from each other, and a through hole 233 is provided in a region therebetween. In addition, the outer ring 262 of the first spherical roller bearing 260 is supported in contact with the surface of the first receiving portion 231, and the outer ring 262 of the second spherical roller bearing 270 is supported by the second receiving portion ( 232) is supported in contact with the surface.

In one embodiment, the first area 221 is inserted into the inner ring 261 of the first spherical roller bearing 260 on the outside of the rod connection part 220 and the inner ring 261 of the second spherical roller bearing 270 . A second region 222 inserted into the The first region 221 and the second region 222 are vertically spaced apart from each other, and a third region 223 is provided therebetween.

In one embodiment, inner diameters of the first and second accommodation units 231 and 232 may be larger than those of the through hole 233 . Accordingly, the first spherical roller bearing 260 is supported by the inner surface of the first receiving portion 231 and the upper surface of the through hole 233 portion, and the second spherical roller bearing is the inner surface of the second receiving portion 232 . And it may be supported by the lower surface of the through-hole 233 portion. And, the outer surface of the first region 221 of the rod connection part 220 is fitted inside the inner ring 261 of the first spherical roller bearing 260, and the inner ring 261 of the second spherical roller bearing is inside the rod. The outer surface of the second region 222 of the connecting portion 220 is fitted. Accordingly, external forces to move the rotation shaft of the rod connection part 220 in the horizontal direction at the coupling site between the upper part of the rod connection part 220 and the motor 210 are caused by the main housing 230 and the first spherical roller bearing 260. can be reduced. In addition, external forces to move the rotation shaft of the rod connection part 220 in the horizontal direction in the lower part (eg, the second region 222) of the rod connection part 220 are caused by the main housing 230 and the second spherical roller bearing 270. can be reduced. In addition, external forces to incline the rotation shaft of the rod connection part 220 may be reduced by the main housing 230 , the first spherical roller bearing 260 , and the second spherical roller bearing 270 .

In one embodiment, the height difference between the vertical center line of the first spherical roller bearing 260 and the second spherical roller bearing 270 is defined as H1, and the inner diameter and outer diameter of the first spherical roller bearing 260 are It is defined as D1 and D2, respectively, and the radial internal clearance of the first spherical roller bearing 260 may be defined as C1. Referring to FIGS. 3 and 5 , a pair of rollers 263 are vertically disposed between the inner surface of the outer ring 262 and the outer surface of the inner ring 261 on the vertical cross-section of the first spherical roller bearing 260 . and a radial gap C1 is provided between the inner surface of the outer ring 262 and the roller 263 . And, according to the basic principle of the spherical roller bearing, the pair of rollers 263 and the inner ring 261 rotate on the inner surface of the outer ring 262 and the inclination angle may be changed at the same time. Accordingly, even in a state in which the main housing 230 is fixed, the rotation shaft of the rod connection unit 220 may be inclined. Here, as H1 decreases or C1 increases, the inclination range of the rod connection part 220 increases, and as H1 increases or C1 decreases, the inclination range of the rod connection part 220 decreases. In addition, when C1 becomes excessively large, the rate at which lateral stress or vibration force is absorbed in the spherical roller bearing is reduced. Therefore, compared to D1, when H1 is less than the standard, bending direction vibration or stress is transmitted to the motor 210, so that the fatigue of the motor 210 increases, and compared to D1, when H1 exceeds the reference range, the bit load 300 The rate at which the provided bending direction vibration or stress is absorbed by the first and second spherical roller bearings 260 and 270 is reduced, and as the amount transferred to the housing 110 through the main housing 230 increases, the rock drilling device (1000) There is a problem in that the overall bending direction vibration increases rapidly. Therefore, it is preferable that H1 is 1.85 to 1.95 times D1, and the radial clearance C1 of the first spherical roller bearing 260 is 0.0004 to 0.00075 times D1. Also, the range of D2 may be determined according to the size of D1, and it is preferable that D2 be 1.75 to 1.85 times that of D1. For example, when D1 is 100 mm, H1 is preferably 185 to 195 mm, while C2 is 0.04 to 0.075 μm. And, in this case, it is preferable that D2 be 175 to 185 mm.

Accordingly, the vibration of the bit rod 300 is reduced by the first, spherical roller bearings and the second spherical roller bearing 270, so that bending vibration and bending stress transmitted to the motor 210 can be minimized, and as a result, the motor ( 210) and the overall lifespan of the parts of the rock drilling apparatus 1000 may be extended, noise may be reduced, the construction period may be shortened, and the efficiency of the drilling process may be improved.

The rock drilling apparatus 1000 according to an embodiment of the present invention may further include an upper cover 240 covering an upper portion of the first spherical roller bearing 260 and an upper portion of the main housing 230 . In this case, the motor 210 may be fixedly coupled to at least one of the main housing 230 and the upper cover 240 .

The rock drilling apparatus 1000 according to an embodiment of the present invention may further include a lower cover 250 covering a lower portion of the second spherical roller bearing and a lower portion of the main housing 230 . At this time, the lower portion of the rod connection part 220 may protrude downward of the lower cover 250 , and a mounting part to which the bit rod 300 is coupled may be provided at the lower portion of the rod connection part 220 .

<rock crushing method>

The rock crushing method according to an embodiment of the present invention may be performed by rotating the PDC lock bit at 1000 to 2000 RPM to perforate the rock, and then performing the blasting process or the rock breaking process. In one embodiment, it is preferable that the rock drilling apparatus 1000 according to the above-described embodiment is used in the rock drilling process.

In the rock crushing method according to an embodiment of the present invention, the falling speed and the falling pressure of the PDC lock bit are adjusted differently according to the hardness of the crushing target rock.

If the descent speed is excessively fast compared to the hardness of the rock to be constructed, or if the downward pressure is excessively strong, the lifespan of the rock drilling apparatus 1000 and work safety may be drastically reduced.

In addition, if the descent speed is excessively slow compared to the hardness of the rock to be constructed, or if the downward pressure is excessively weakened, the time required for the drilling of the rock increases unnecessarily, resulting in a sharp decrease in construction efficiency.

Table 1 below is a table showing the optimal range of the descending speed and downward pressure according to the hardness of the rock when the PDC lock bit is rotated at 1000~2000RPM.

In the rock crushing method according to an embodiment of the present invention, as shown in Table 1, in the rock with a uniaxial compressive strength of 125 kgf/cm 2 or less, the drilling is performed at a downward pressure of 1.0 to 2.0 m/min and a downward pressure of 2 ton/m it is preferable

In addition, in a bedrock having a uniaxial compressive strength of 126 to 600 kgf/cm2, it is preferable to drill at a downward pressure of 0.5 to 1.0 m/min and a downward pressure of 1 ton/m2.

In addition, in a bedrock having a uniaxial compressive strength of 601 to 1200 kgf/cm 2 , it is preferable to perform a blasting process or a granite process after drilling at a descending speed of 0.3 to 0.7 m/min and a downward pressure of 0.6 ton/m 2 .

In one embodiment, after the drilling process is performed as described above and the drilling hole 2 is formed in the bedrock 1, the granite process may be performed, and the arm that is the lower part of the ripper blade 2100 of the vibrating ripper 2000 After inserting the crushing unit 2110 into the drilling hole, vibration is generated to perform the granite process. 6 and 7, the dark crushing part 2110, which is the lower part of the reper blade 2100, has a conical shape with a minimum diameter of R1 and a maximum diameter of R2, and R2 is larger than the diameter of the drilling hole. , R1 may be made smaller than the diameter of the drilling hole. Accordingly, when a pressure to vertically lower the vibration ripper 2000 is applied, a pressure is generated in a direction in which the drilling hole is opened due to the tapered shape of the arm crushing unit 2110, and the stepping arm process can be performed efficiently by this pressure. have. As a result, while the noise and vibration generated when the sharp lower end of the vibration ripper 2000 collides with the rock is reduced, the rock formation process can be effectively performed.

In one embodiment, R2 is preferably 1.5 to 1.8 times that of R1. For example, when R1 is 80 mm, R2 is preferably 120 to 144 mm.

On the other hand, when the acute angle formed by the side surface of the female crushing unit 2110 with the ground exceeds 86°, the force to open the perforation hole by the descent of the female crushing unit 2110 is sharply reduced, and the side of the female crushing unit 2110 is When the acute angle with the ground is less than 84 degrees, the rate at which the force for lowering the rock crushing unit 2110 is converted into the force to open the drilling hole sharply decreases. Since the downward pressure of the arm crushing unit 2110 required to create a force to open the perforation hole increases rapidly, there may be a problem in that the efficiency of the work progress is sharply decreased. In order to solve this problem, in the rock crushing method according to an embodiment of the present invention, it is preferable that the acute angle formed by the side surface of the rock crushing unit 2110 with the ground is 84 to 86°.

As described above, although the embodiments have been described with reference to the limited drawings, those skilled in the art may apply various technical modifications and variations based on the above. For example, the described techniques are performed in an order different from the described method, and/or the described components of the system, structure, apparatus, circuit, etc. are combined or combined in a different form than the described method, or other components Or substituted or substituted by equivalents may achieve an appropriate result.

1: bedrock
2: perforated hole
30: working rod
31: vehicle
1000: rock drilling device
110: housing
120: lifting rod
130: fixing pin
140, 150: vehicle connection part
200: perforation module
210: motor
220: rod connection part
221: first area
222: second area
223: third area
230: main housing
231: first receiving unit
232: second accommodation unit
233: through hole
240: upper cover
250: lower cover
260: first spherical roller bearing
261: inner ring
262: outer ring
263: roller
270: second spherical roller bearing
300 : bit load
400: vibration reduction unit
500: drill bit
2000 : Vibration Ripper
2100: Reaper Blade
2110: dark crushing part

Claims (7)

a main housing having a through hole, a first accommodating portion provided on the upper portion of the through hole, and a second accommodating portion provided on a lower portion of the through hole;
a motor provided with a rotating pin and provided on an upper portion of the main housing;
a rod connection part rotated by the motor;
a bit rod coupled to a lower side of the rod connection part;
a drill bit coupled to a lower portion of the bit rod;
Including; a first spherical roller bearing and a second spherical roller bearing provided between the rod connection part and the main housing;
In the first spherical roller bearing, the inner surface of the inner ring is coupled to the upper portion of the rod connection part, and the outer surface of the outer ring is coupled to the first receiving portion, and the inner surface of the outer ring and the inner surface of the outer ring on the vertical cross-section of the first spherical roller bearing A pair of rollers are arranged in the vertical direction between the outer surface of the inner ring, and a radial gap is provided between the inner surface of the outer ring and the roller so that the axis of rotation of the rod connection part can be inclined;
In the second spherical roller bearing, the inner surface of the inner ring is coupled to the lower part of the rod connection part, and the outer surface of the outer ring is coupled to the second receiving part, and the inner surface of the outer ring and the inner surface of the outer ring on the vertical cross section of the second spherical roller bearing A pair of rollers are vertically disposed between the outer surface of the inner ring, and a radial gap is provided between the inner surface of the outer ring and the roller so that the rotation axis of the rod connection part can be inclined. perforation device.
According to claim 1,
The height difference between the vertical centerline of the first spherical roller bearing and the second spherical roller bearing is H1, the inner diameter and the outer diameter of the first spherical roller bearing are D1 and D2, respectively,
D2 is 1.75~1.85 times of D1,
H1 is 1.85 to 1.95 times that of D1,
The radial clearance of the first spherical roller bearing is 0.0004 to 0.00075 times of D1.
In the rock crushing method of crushing the rock using the rock drilling device according to claim 1 or 2,
A bit containing a polycrystalline diamond compact (PDC) is applied to the drill bit, and the drill bit is rotated at 1000 to 2000 RPM to drill the rock, and then a blasting process or a granite process is performed, characterized in that Rock crushing method.
4. The method of claim 3,
In rock with a uniaxial compressive strength of 125 kgf/cm2 or less, drilling is performed at a downward pressure of 1.0 ~ 2.0 m/min and a downward pressure of 2 ton/m2,
In rock with uniaxial compressive strength of 126~600 kgf/cm2, drilling is performed at a descending speed of 0.5 to 1.0 m/min and a downward pressure of 1 ton/m2,
A method of crushing rock, characterized in that in a rock with a uniaxial compressive strength of 601 to 1200 kgf/cm2, a blasting process or a granite process is performed after drilling at a descending speed of 0.3 to 0.7 m/min and a downward pressure of 0.6 ton/m2.
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