PROCEEDINGS
OF THE TOMSK ORDER OF THE RED BANNER OF LABOR
POLYTECHNIC INSTITUTE named after S. M. KIROV

Volume 108  STUDY OF DRILLING MACHINES  1959

RESULTS OF A STUDY OF A DRILLING TOOL FOR ROTARY-PERCUSSIVE DRILLING OF BLAST HOLES

Ya. A. Serov

The study of the operating conditions of the rotary-percussive method of drilling blast holes was carried out by us [1] with various types of removable drill bits, the choice of which was made on the basis of the following considerations. With some approximation, it may be considered that the method of rotary-percussive destruction of rock is derived from two known methods: rotary and percussive-rotary. On the basis of many years of experience, for these methods of destruction a tool has been developed that, in the main, satisfies operational requirements. The requirements for a tool for rotary drilling include the smallest sharpening angle of the cutter blades, provided that there is sufficient strength and resistance to abrasive wear. The geometry of such a cutter is shown in Fig. 1, б. The blade sharpening angle is \(\beta = 65^\circ\). To increase resistance to abrasive wear, the cutters are equipped with hard-alloy plates VK-6, VK-8.

For a tool used in percussive-rotary drilling, the foremost requirement is the strength of the blade under impact loading, as well as relatively high resistance to abrasive wear. For this purpose, the sharpening angle of the drill bit is increased to \(\beta = 100—110^\circ\) (Fig. 1, а). The bits are reinforced with hard-alloy plates with an increased cobalt content, of the VK-11, VK-15 types.

As regards the geometry and quality of the drilling tool for rotary-percussive drilling, there are as yet no sufficiently substantiated recommendations. Cutters for rotary-percussive drilling, evidently, must satisfy both of the above-mentioned requirements.

It may be assumed that the rational geometry of the tool for this method of drilling should be intermediate between the geometries of cutters for rotary and percussive-rotary drilling.

Within this range, a large number of varied bits can be proposed, with different sharpening and blade shapes and made of different materials.

Testing a large number of bits when drilling various rocks would be very laborious and time-consuming work without sufficient scientific justification and consistency. Therefore, in order to resolve the question posed concerning the advantages of rotary-percussive drilling, as a first approximation the number of drill bits selected for testing should be reduced to a minimum. In our opinion, in comparative tests of rotary, percussive-rotary, and rotary-percussive drilling, the first to be used should be bits widely employed for the first two drilling methods (Fig. 1, a and b).

When drilling with these bits under the corresponding regimes, the principal parameters will be obtained: drilling speed and cutter life, which characterize these drilling methods. When these same bits are tested under rotary-percussive drilling regimes, limiting values of speed and wear resistance under the new operating conditions will be determined. These data make it possible, as a first approximation, to decide the question of the advantages of one or another method of destruction, as well as the design of a drilling tool intended for rotary-percussive drilling.

The drilling tool created on the basis of these recommendations must undergo additional comparative tests, the results of which may serve as the basis for a final comparison of the technical and economic indicators of the various drilling methods.

Proceeding from these considerations, for comparative tests in rocks of varying strength we adopted five types of bits: a single-chisel bit, the one most widely used at present in percussive-rotary drilling (bit No. 1, Fig. 1, a); rotary-drilling bits (bits Nos. 2 and 3, Fig. 1, b, c), used in drilling with PEB-2m type drills. In addition, two types of bits were tested (Fig. 2, a, b, bits Nos. 4 and 5) when drilling hard rocks. Their geometry was developed with allowance for the possibility of producing chipping of the rock as a result of the rotary motion of the tool. For this purpose, the rake angle of these bits was reduced to \(\gamma = -25^\circ\), as compared with the single-chisel bit, which has \(\gamma = -55^\circ\).

When drilling rocks of medium strength, the average width of the wear flat was adopted as the criterion for tool dulling.

Fig. 1. Drilling tools used for various drilling methods:

a — single-bit crown No. 1 for percussive-rotary drilling;
b — crown No. 3 for rotary drilling;
c — crown No. 2 for rotary-percussive drilling of medium-hard rocks;
d — template for checking the sharpening shape of crown No. 2.

Visible labels in the figure:

• Insert G-32
• Section along AA
• Along BB
• \(\alpha = 25^\circ\)
• \(r = 0\)
• \(r = 10^\circ\)

Fig. 2. Drill bits for rotary-percussive drilling of hard rocks:

a—bit No. 4, recommended for rotary-percussive drilling of hard rocks; b—bit No. 5 for rotary-percussive drilling of hard fractured rocks.

Labels in the drawing: “View along B–B”; “Section A–A”; “taper 1:12”; “2 holes, ⌀5.”

the chamfer of blunting \(m\) of the hard-alloy insert along the rear face of the main cutting edge (Fig. 3). When drilling hard rocks, however, it is the width of the end wear land, measured at a distance of 3 mm from the edge of the insert (Fig. 4). The width of the wear land was measured at a distance of 3 mm from the edge, since wear at the edge is random in character, while the intensity of wear is almost the same for all drilling regimes. When measuring with a template, a 3-mm-long section of the hard-alloy insert from the edge was excluded from the field of view of the MIR-1m measuring microscope.

By wear of the tool we mean the width of the blunting land, and by wear resistance—the number of meters of blasthole drilled by one bit before sharpening.

Fig. 3. Wear of a bit during rotary-percussive drilling of hard rocks of medium hardness.

Figure 3 gives a photograph of bit No. 2 with blunted blades. Drilling was carried out in sandstone with \(f = 6—8\). As a result it was found that, in this case, the character of bit wear in rotary and rotary-percussive drilling is entirely the same. Wear of the bits is a land of approximately equal width, located along the entire length of the main cutting edge (Fig. 3). The surface of the wear land of the bit’s hard-alloy insert is matte and even. On the surface of the insert that has not been subjected to the abrasive action of the rock, transverse bands are clearly visible—traces of sharpening (Fig. 3). The character of bit wear changes somewhat with different methods of removing drilling fines from the blasthole. When drilling with air flushing, the wear surface of the hard-alloy insert is less smooth and the boundaries of the blunting land are more sharply outlined than when drilling with water flushing.

When drilling rocks of high hardness, wear of the bit blades is most intense at the edge. The character of wear in

in percussion-rotary (Fig. 4, a) and rotary-percussive (Fig. 4, b) methods of drilling rocks of high strength is the same. In this case the blunting surface is not flat but convex, without sharp transitions from the worn to the unworn part of the hard-alloy insert. In rotary drilling (Fig. 4, v) the wear land is even, with a sharp transition from the worn part of the insert to the surface not subjected to the abrasive action of the rock.

Wear resistance of the tool is one of the principal indices of rotary-percussive drilling. The wear resistance of drill bits was tested under the following regimes of rotary and rotary-percussive drilling of sandstone \(f = 6 \div 8\).

Fig. 4. Wear of bits in percussion-rotary, rotary, and rotary-percussive methods of drilling rocks of high strength

number of revolutions of the drill per minute, 196 and 408; feed forces, 500, 750 and 1000 kg; hammer of the impact unit operating at an air pressure of 4 atm; removal of drill cuttings was carried out by blowing or flushing.

The dependence of bit wear on the number of drilled borehole meters at various feed forces is shown in Fig. 5, according to which the wear of bits in rotary drilling is higher than the wear of bits in rotary-percussive drilling. This increase averages 150% at a drill rotation speed of 196 rpm and 200% when the number of revolutions of the drill is 408 per minute. It follows from this that the wear resistance of bits, expressed by the number of drilled borehole meters, in rotary-percussive drilling is 1.5–2 times higher than the wear resistance of bits of the rotary method.

With an increase in the feed force, in all cases the wear of bits increases both in rotary and in rotary-percussive methods of drilling. However, the rate of increase of wear with

with an increase in the feed force is significantly less in rotary-percussive drilling than in rotary drilling.

Increasing the drill rotation speed from 196 to 408 rpm leads to an increase in tool wear in rotary drilling by 150%, and in rotary-percussive drilling by 130%.

The use of flushing instead of blowing leads to an increase in bit wear by 10–16%. This situation can be explained as follows.

As was established in the study of drilling regimes [1], the use of flushing instead of blowing leads to a significant reduction in the drilling rate. The reduction in the drilling rate

Fig. 5. Wear of bits under different drilling regimes: rock—sandstone, \(f = 6 \div 8\); impact-unit hammer RH-754; bit No. 2; \(n_b = 408\) rpm; removal of drill cuttings by flushing; \(a\)—rotary drilling; \(b\)—rotary-percussive drilling, \(p = 4\) atm; 1—feed force 500 kg; 2—feed force 750 kg; 3—feed force 1000 kg.

occurs because flushing, compared with blowing, provides poorer removal of drill cuttings from the cutting edges of the drilling tool. For this reason the drill cuttings, being in the working zone of the cutting edges of the bit, lead to their more rapid wear. When the width of the wear land was measured with a microscope, it was observed that the boundaries of the wear surface are sharply defined with blowing and blurred with flushing. All this indicates that, with flushing, the bit blades move in a mixture formed from water and drill cuttings and are thereby subjected to more intense abrasive wear.

The wear resistance of the tool in rotary-percussive drilling of high-strength rocks—diabase, \(f = 14 \div 16\), and hornfels, \(f = 18\)—was studied under the following regimes: number of revolutions

rotations of the drill per minute of 51, 95, and 152; feed forces of 100, 500, 1000, and 1500 kg; impact frequency of 2700 and 3700 per minute; impact energy of 4 and 5 kgm, respectively, at air pressures in the hammer of the RH-754 percussion unit of 4 and 6 atm; drill cuttings were removed by blowing.

To compare tool wear in rotary-percussive drilling and in percussive-rotary drilling with large feed forces, the wear of bits in percussive-rotary drilling was taken as the wear of bits in rotary-percussive drilling with a feed force of 100 kg.

Fig. 6. Wear of bits under various regimes of rotary-percussive drilling:

rock—diabase, \(f = 14 \div 16\); bit No. 4; \(n_b = 95\) rpm;
\(a\)—\(p = 4\) atm; \(b\)—\(p = 6\) atm;
1—feed force 100 kg; 2—feed force 500 kg; 3—feed force 1000 kg; 4—feed force 1500 kg; 5—feed force 250 kg.

From Fig. 6 it is evident that bit wear increases with increasing feed force and the number of drilled shpur-meters. When the rotational speed of the drill is increased approximately twofold (from 51 to 95 rpm), bit wear when drilling hornfels increases by 30–40%. Increasing the number of drill rotations from 95 to 152 per minute leads to an increase in tool wear of 4–15% for hornfels and 6–14% for diabase.

The wear resistance of bits depends on their geometry. A reduction in the sharpening angle of the bits leads to an increase in their wear. Therefore, the wear of bits No. 4 is higher than the wear of bits No. 1 by 23–27% for hornfels and by 16–24% for diabase.

Increasing the air pressure in the hammer of the percussion unit somewhat increases tool wear.

From the standpoint of tool wear resistance and drilling speed, within the investigated range of numbers of blows, for drilling hard rocks it is possible to recommend a drill rotation rate of 95 rpm. Drilling speeds at higher drill rotation rates (for example, at 152 rpm) increase, but the wear resistance of the tool decreases sharply.

A distinctive feature of rotary-percussive drilling is the large feed force. The wear of bits in rotary-percussive drilling with feed forces of 1000–1500 kg is higher than the wear of bits at a feed force of 100 kg by 40–80% when drilling hornfels, and by 50–100% when drilling diabase. Consequently, tool wear in rotary-percussive drilling increases on average by a factor of 1.5–2 in comparison with percussive-rotary drilling, when the feed force is equal to 100 kg.

As a result of the investigations carried out on the regimes of rotary-percussive drilling of rocks of medium hardness [1], it was established that the principal motion of the tool, which provides the greatest work in breaking the face and the drilling speed, is the rotary motion. From this point of view, the design of the cutting part of the tool for rotary-percussive drilling must be very close to the design of the tool for rotary drilling. However, the presence of axial blows requires strengthening and, in connection with this, some change in the shape and sharpening angles of the cutting face of the tool. This leads to a convergence of the tool designs for the rotary-percussive and percussive-rotary methods of drilling. The shapes and sharpening angles of the cutting blade of the tool for rotary-percussive drilling must be intermediate between the characteristics of the tool for the rotary and percussive-rotary methods.

In rotary and rotary-percussive drilling with a feed force above 1000 kg, the strength of the blades of bit No. 3, fitted with hard-alloy plates of grade VK-6 with a rake angle $\gamma = 0^\circ$, proved insufficient. Frequent breakages of the hard-alloy plates were observed, mainly at the edge and in the center of the bit. After replacing the VK-6 hard alloy with the tougher alloy VK-8, cases of plate chipping decreased, but it was not possible to eliminate breakages completely.

Under analogous drilling conditions, bits No. 2 (with a blade shape as in bit No. 3), having a negative rake angle $\gamma = -10^\circ$, provided somewhat lower drilling speeds in comparison with bits No. 3. However, in bits No. 2 fitted with hard-alloy plates of grade VK-8, breakages and chipping of the plates were, as a rule, not observed. On the basis of the foregoing, we adopted bit No. 2 as the basis for developing the design of a tool for rotary-percussive drilling of rocks of medium hardness.

Rotary-drilling bits have insufficient blade strength and uneven wear of the hard-alloy plates. To strengthen the blade tips, the auxiliary faces can be eliminated and a web formed. However, this will lead to the need to increase the feed force, since the web has large negative angles. Therefore, for example, when drilling in high-strength steels, web-less drills have been developed with a small split between the blades; compared with ordinary drills without a split, these provide a 35% reduction in axial feed forces and a 2.5–3-fold increase in drill life at the same feed rate [2]. Consequently, the presence of auxiliary cutting edges forming an angular recess in drill bits may be considered necessary. To strengthen the blades and eliminate uneven wear of the cutting faces, and especially of their tips, it is necessary to round the apex at the junction of the auxiliary and main cutting edges and to provide sharpening of the cutter with two or more angles in plan. M. K. Tsekhin [3] arrived at similar conclusions for rotary drilling of rocks. In this case, the increase in blade life is achieved by increasing the length of the cutting edge and correspondingly reducing the specific load on the blade. With the same feed of the entire cutter, the chip thickness, measured normal to the cutting face, decreases as the distance from the center of the cutter increases and as the angle in plan decreases. A gradual, smooth decrease in the angle in plan leads to the elimination of rapidly wearing tips. In this case the cutting faces of the bit assume a rounded shape (Fig. 1, v).

On the basis of the foregoing, we developed the design of bit No. 2 (Fig. 1, v), which showed good results and can be recommended for rotary and rotary-percussive drilling of rocks of medium hardness, \(f = 6 \div 10\). The drilling rate with bit No. 2, with rounded blades and with the blade shape as in bit No. 3, was the same.

In rotary-percussive drilling of rocks of medium hardness with bits No. 2, at a feed force of 1500 kg, breakage of the hard-alloy plates was observed (Fig. 7), especially when drilling rocks with a nonuniform structure or with the presence of cracks. Therefore, to ensure reliable operation of the tool and to eliminate chipping of the hard-alloy plates, the feed forces when working with these bits should be taken within the range of 1000–1200 kg.

In rotary-percussive drilling of high-strength rocks (diabase, \(f = 14 \div 16\), and hornfels, \(f = 18\)), it was found that the principal destruction occurs through penetration of the bits under the action of impacts, and the tool design must satisfy the requirement of exceptional resistance to breakage. Therefore, in developing the design of a tool for rota-

the rotary-percussive drilling bit was taken as the basis (Fig. 1, a). On the basis of these premises the geometry of bits No. 4 and 5 was also developed (Fig. 2, a, b).

As a result of tests of the tool for rotary-percussive drilling of blast holes in solid hard rocks, we recommend drill bit No. 4 (Fig. 2, a), which, when drilling diabase and hornfels, provided good results in drilling rate and wear resistance. However, when drilling fractured rocks with this bit, breakage of the hard-alloy plates was observed. Under analogous conditions, plate breakage, as a rule, was not observed when drilling with bit No. 5 (Fig. 2, b). In drilling rate and wear resistance, bit No. 5 is not inferior

Fig. 7. Chipping of hard-alloy plates during rotary-percussive drilling of rocks of medium hardness.

to bit No. 4, but it is more difficult to manufacture and sharpen. Therefore bit No. 5 can be recommended for use in hard and fractured rocks.

In manufacturing bits No. 4 and 5, the hard alloy was brazed to the body of the bits with brass. Such fastening of the hard-alloy plates fully justified itself. Some foreign authors [4] recommend, in rotary-percussive drilling, in view of the high bending and impact loads, that the hard alloy not be brazed to the bit body but press-fitted. In our opinion, such fastening will be unreliable, since the considerable dynamic loads acting on the bit will very quickly loosen the hard-alloy plate and the bit will fail.

Bits for rotary-percussive drilling may be equipped with hard-alloy plates of grades VK-11 and VK-15 with a thickness of 8 mm. The use of plates 10 mm thick does not increase the strength-

the efficiency of the tool, and increases the consumption of hard alloy. For bits used in rotary-percussive drilling, in our opinion, it is advisable to produce special hard-alloy inserts with a front angle \(\gamma = -30^\circ\), since when ordinary inserts with \(\gamma = -55^\circ\) are reground, a large part of the hard alloy is lost.

The tool for rotary-percussive drilling, with an appropriate design, requires especially careful heat treatment. Insufficient hardness and thickness of the bit collar leads to a significant widening of the conical part, after which the bit is not secured on the cone of the drill rod. Excessive hardness of the collar leads to rupture of the bit body (Fig. 8). Only the collar (the conical part) of the bit should be subjected to heat treatment. Hardening the feathers of the bit makes them brittle, and during drilling, together with the feather, the hard-alloy insert breaks under the action of large bending loads (Fig. 8).

Fig. 8. Appearance of a broken bit during rotary-percussive drilling of high-strength rocks

It should be noted that when the air pressure in the hammer of the RN-754 impact unit is increased to 7.5 atm, and consequently when the impact energy is increased above 5 kgm, the resistance of the tool to breakage at feed forces of 1000–1500 kg decreases sharply. In addition, the character of bit wear changes. The wear surface of the hard-alloy insert becomes not smooth, but covered with numerous depressions; i.e., in this case the wear will be nonuniform. As the air pressure is increased, not only the energy but also the number of hammer blows increases. The increase in the number of blows, in our opinion, helps reduce the share of rotary destruction of the rock and, consequently, has a favorable effect on reducing wear. Therefore, the reduction in the wear resistance of bits occurs as a result of increasing the impact energy above 5 kgm.

In carrying out the investigations, blast-hole drilling was performed with drill rods made of hexagonal steel 25 mm in diameter. When drilling blast holes more than 1.5 m deep, the rigidity of the rods proved insufficient, as a result of which the blast holes became curved when drilling rocks of medium strength, and the rods twisted when drilling fractured hard rocks, when the rotational speed of the drill was low and the torque on the drill was greatest. Therefore, for drilling blast holes 2–3 m deep without changing the tool, it is necessary to use

use drill steel 32 mm in diameter with a central axial hole for supplying air or water into the blast hole.

For greater durability, it is advisable to manufacture drill rods and bits for rotary-percussive drilling from alloy steels.

REFERENCES

1. O. D. Alimov, I. G. Lyapichev, Ya. A. Serov. Study of rotary-percussive drilling. Proceedings of TPI, vol. 106, Metallurgizdat, 1958.

2. G. A. Krivoukhov, M. A. Myakishev. Physical substantiation of the machinability of high-alloy steels by drilling, Mashgiz, 1951.

3. M. K. Tsekhin. Study of drilling-and-blasting operations and improvement of their efficiency in driving horizontal workings through rock at the mines of the Prokopyevsk ore mine. Candidate’s dissertation, Tomsk, TPI, 1955.

4. K. H. Voss. Kritische Untersuchungen und Betrachtungen über das Drehschlagbohren. Bergfreiheit, No. 10—11, 1954.