PROCEEDINGS
OF THE TOMSK ORDER OF THE RED BANNER OF LABOR POLYTECHNIC
INSTITUTE named after S. M. KIROV
Vol. 75　　　　　　　　　　　　　　　　　　　　　　　　　　　1954

EFFECT OF FEED FORCE AND DRILL BIT ROTATION ANGLE BETWEEN BLOWS ON THE RATE OF PERCUSSIVE-ROTARY BLAST-HOLE DRILLING

P. M. ALABUZHEV and O. D. ALIMOV

Drilling blast holes and boreholes in the conduct of drilling-and-blasting operations is one of the most labor-intensive processes. Therefore, for the further intensification of drilling-and-blasting operations it is necessary to study optimal regimes and methods of drilling and, on this basis, to create more advanced and productive machines. All methods currently used for drilling blast holes and boreholes are distinguished according to the nature of the action of the tool on the rock. Most of them may be divided into two principal groups:

1) methods of rotary drilling;
2) methods of percussive-rotary or percussive-turning drilling.

In rotary drilling (excluding drilling with roller bits and shot drilling), the tool penetrates into the rock under the action of a constant axial force. The principal destruction of the rock occurs through shearing of the rock in front of the leading face of the tool blade during its rotation [1; 2; 3]. Since the resistance of rocks to destruction by shear is considerably less than to destruction by penetration, the energy expenditure for destroying rock in rotary drilling is less than in percussive drilling. However, in practice this advantage is observed only when drilling soft rocks and, partly, rocks of medium hardness, and is lost when drilling rocks of high hardness.

The extension of the rotary method to drilling blast holes in rocks of high hardness, and also an increase in the productivity of rotary drilling when drilling blast holes in rocks of low and medium hardness, require the creation of considerable axial forces (feed forces) of the bit against the face. But increasing the feed force also greatly increases the friction of the tool blade against the face, as a result of which the power required for rotation increases, and tool wear increases greatly. Successful resolution of this contradiction when drilling rocks of high hardness is possible either by increasing the wear resistance of the tool blade or by reducing the frictional forces. Increasing the wear resistance of the tool is achieved by the use of special bits tipped with hard alloys, and, in the case of drilling rocks of high hardness, bits set with diamond (diamond drilling). The scarcity of diamond and the still considerable wear of hard alloys in rotary drilling limit the application of the latter to rocks of low and partly medium hardness.

The endeavor to reduce frictional forces in rotary drilling requires the search for new drilling regimes and methods. One such method is rotary drilling with roller bits and shot drilling [3]. In these methods the sliding friction of the bit along the face is partly replaced by rolling friction. In addition, owing to the nonuniformity of the rolling speed of the rollers and shot over the face, to some extent there is carried out...

percussive destruction of the bottom-hole surface. Drilling with shot bits, as well as shot drilling, is now widely used in drilling deep exploratory boreholes. The extension of these methods to the drilling of blast holes 40–45 mm in diameter is difficult because of design difficulties. In percussive-rotary drilling, or percussive-turned drilling, the main destruction of the rock is accomplished by penetration of the tool blade into it by means of impact energy. After each blow the tool is turned through a certain angle, after which a new blow follows. The rotation of the tool may be intermittent or continuous. Since turning of the tool is carried out, as a rule, with insignificant axial force, the losses to friction during turning, and the wear of the tool, are small in comparison with rotary drilling. This has led to the widespread use of percussive-rotary drilling for drilling rocks of medium and high strength having great abrasive capacity. Thus, the principal advantage of the percussive-rotary method of drilling over the rotary method is the relatively smaller wear of the tool when drilling rocks of high strength. In addition, penetration of the tool into the rock by impact does not require the creation of large feed forces of the machine. This makes it possible to use, along with powerful machines for drilling deep boreholes (cable-tool percussion drilling rigs), light and hand machines (drilling hammers) for drilling blast holes in rocks of high strength.

Despite the considerable use of drilling hammers in practice, the technical literature contains no data on the most expedient feed forces of drilling hammers on the bottom hole during drilling, on the most expedient angles of rotation of the drill between blows, and on methods of drill rotation. Meanwhile, the absence of data on these questions creates considerable difficulties in improving existing drilling hammers, in designing new drilling hammers and feed devices. Thus, for example, the opinion has become established that intermittent turning of the drill is most expedient in percussive-rotary drilling. On this basis, designers of new pneumatic and electric drilling hammers are in a number of cases compelled to create more complicated devices for intermittent turning of the drill instead of simpler devices that permit continuous rotation of the drill. In a number of cases, without sufficient grounds, it is proposed to increase considerably the feed forces of drilling hammers on the bottom hole, which supposedly will make it possible to combine the advantages of rotary and percussive-rotary drilling.

In this connection, the results of investigations carried out recently at the Tomsk Polytechnic Institute, on instructions from the Tomsk Electromechanical Plant named after V. V. Vakhrushev, are of considerable interest.

To determine the influence of various factors on the rate of percussive-rotary drilling of blast holes in rocks of medium and high strength, experiments were carried out on a special installation with independent units for impact, drill rotation, and feeding of the machine to the bottom hole.

The experimental installation, Fig. 1, created on the electropneumatic principle [4], made it possible to vary the number of blows per minute and the impact energy. The impact energy was varied within the limits of 0.5–12 kgm. The drive of the impact unit was effected from an asynchronous electric motor—1 through a V-belt transmission. When the piston—2 moved to the right, a rarefaction of air was created between it and the striker—3, and by excess pressure the striker moved to the right; on the return stroke of the piston, compression of the air enclosed between the striker and the piston occurred; the striker changed its direction of motion and, toward the end of the working stroke, delivered a blow to the drill—4, which destroyed the rock—6. Continuous turning of the drill was effected from an electro-

the direct-current motor—9 through the gear transmission—10; the number of revolutions of the drill, and consequently also the angle \(\alpha\) of rotation of the drill between blows, was varied by changing the generator voltage and changing the excitation current of the motor. The region of stable operation of the motor made it possible to vary the angle of rotation of the drill between blows from 18 to 180°. The design of the installation also made it possible to carry out intermittent rotation of the drill. Variation of the feed forces of the stand on the face was accomplished by means of different loads suspended through a pulley from the stand.

Fig. 1

Drilling was carried out in granite, with hardness according to Prof. Protodyakonov’s table \(f = 14\text{–}16\). During the tests the drilling rate, the energy and number of blows, the number of revolutions of the drill, the power expended on blows and on rotation of the drill were measured; vibrations of the body of the impact unit and longitudinal oscillations of the drill were also recorded on a paper tape mounted on drum 5, rigidly connected with drill 4, by pencils 7 (relative to the foundation) and 8 (connected with the hammer body).

In carrying out the experiments it was established that, for one and the same blow energy, the drilling rate depends to a considerable extent on the angle of rotation of the drill between blows and on the feed force of the machine on the face. Thus, for example, when drilling granite with blow energy at the striker equal to \(6\ \text{kgm}\), at one and the same feed force, the drilling rate varied by 70% depending on the angle of rotation of the drill between blows.

In preliminary experiments on copper, in studying the mechanism of destruction [5], it was found that, for one and the same blow energy, the minimum volumetric work of destruction when drilling with a single-bit crown is attained in the case when a series of blows produces indentations in the surface of the face with symmetrical chipping of the rock being destroyed, and the following blows are delivered on the projections that have formed. Such a combination of blows can be achieved if the circumference is divided into an odd number of equal angles so that the lines of impact of the drill blade during the second half-revolution would lie between the indentations already made earlier in the rock. It is not difficult to show that this condition is satisfied by the relation

\[ \alpha = \frac{2\pi}{2i - 1}, \]

where \(i = 1, 2, 3, 4, 5\)—integers of the natural series.

Proceeding from two-sided (symmetric) chipping of the rock relative to the drill blade, on the basis of the formula given above it is not difficult to specify the values of the optimal angles of rotation of the drill between two blows, as is seen from the appended table.

In specially conducted investigations [5; 8] by Engineer O. D. Alimov, when drilling blastholes in various rocks (diabase, granite, marble) with a single-bit crown \(40\text{–}45\ \text{mm}\) in diameter and with different blow energies, it was experimentally established that the optimal angles of rotation of the drill between blows of the striker are the angles: 40°; 32.7°; 27.8°; 24°; 21.2°; 19° and those close to them.

Table

The specific choice of the optimum value of the angle $\alpha$ through which the bit is rotated between two blows is determined by the purpose of the machine, i.e., it depends on the hardness of the rock being broken and on the blow energy. The value of the optimum angle of bit rotation increases with increasing blow energy $A_\delta$ and with decreasing rock hardness. By way of illustration, Fig. 2 shows the optimum values of the angle $\alpha$, obtained when drilling in granite (hardness $f = 14—16$), with the corresponding values of the blow energy $A_\delta$.

Fig. 2

The feed force has a substantial effect both on the drilling speed and on the stability of operation of the impact unit of the drilling machine, Fig. 3. At small feed forces of $10 \div 15\ \text{kg}$, the impact unit of the hammer, during drilling, moved very far back from the limiting collar of the bit. As a result, stable operation of the impact unit was disrupted and drilling became impossible. With an increase in feed force to $40—50\ \text{kg}$, the drilling speed reached a maximum—Fig. 3; in this case the displacement of the body of the impact unit from the limiting collar of the bit decreased to $2—6\ \text{mm}$, depending on the blow energy, while the torque resisting bit rotation remained almost constant. Moreover, the torque resisting bit rotation, when drilling with blows, was in all cases $1.5—2$ times less than the resistance to simple rotation of the bit (without blows) with the same feed force. A further increase in feed force considerably increased the torque resisting bit rotation, led to considerable wear of the tool, and had no noticeable effect on increasing the drilling speed. On the contrary, in some cases the drilling speed decreased. In carrying out the experiments it was observed that the drilling speed, the torque resisting bit rotation, and tool wear, when the feed force is varied, are connected with the amplitude of oscillation of the impact unit of the drilling machine and with its displacement from the limiting collar during drilling.

A study of the motion of the bit and the body of the drilling hammer, Fig. 4, during the drilling of horizontal blastholes showed that, as a result of the interaction of all forces in the process of percussive-rotary drilling, the impact unit

Fig. 3

MOVEMENT OF THE DRILL DURING BOUNDARY DRILLING ON A STAND

Weight of striker \(Q_b = 1.35\) kg.
\(R = 4.5\) cm.
\(n = 1400\) rev/min.

Weight of drill \(Q = 7.40\) kg.
\(\rho_e = 4.0\) cm.
\(n_{\text{drill}} = 226\) rev/min — in the established operating regime.

Drill length \(= 82\) cm.

Feed \(P = 25\) kg.

Feed \(P = 50\) kg.

Feed \(P = 75\) kg.

Vertical labels: drilling; direction

Labels in the enlarged detail:

• impact of striker on the drill
• impact of drill on the hammer body
• impact of drill on the rock
• angle between the drill blade and the rock
• angle of drill rotation between two impacts

CONVENTIONAL DESIGNATIONS:

— relative to the foundation
- - - relative to the body
← direction of drill rotation

Fig. 4

together with the entire moving part of the machine, in the presence of an elastic element in the feeder design, moves away from the limiting collar of the drill. Therefore, during drilling, the distance between the body of the drilling hammer and the bottom of the blasthole is in most cases greater than the length of the drill (from the bit blade to the limiting collar). The average magnitude of this gap varies depending on the feed force. The drill bit, in the drilling process, is not pressed against the bottom of the blasthole at all times. After each blow on the bottom, the drill rebounds from the bottom and strikes the body of the drilling hammer with its limiting collar. Therefore, when drilling horizontal blastholes, the blow of the striker on the drill usually occurs when the blade of the drill bit is at some distance from the surface of the blasthole bottom. When the striker strikes the drill, the latter moves toward the bottom, strikes the bottom with the bit, destroys the rock, and rebounds again.

Drilling with optimum feed forces, i.e., those at which the maximum drilling rate was ensured, corresponded to cases in which, before the striker and drill collided, there was a gap of \(1 \div 4\) mm between the blade of the drill bit and the surface of the bottom; moreover, the magnitude of this optimally necessary gap was the greater, the greater the impact energy. Owing to the presence of a gap between the bottom of the blasthole and the drill blade at the moment the latter is turned, the resistance to rotation of the drill in percussive-rotary drilling is less than in simple rotary drilling—without impacts—at the same feed force. The presence of a gap between the drill blades and the rock at the moment of collision of the striker and drill can also explain the fact that, in the process of drilling horizontal blastholes with drilling hammers, no rebound of the striker from the drill is observed after collision. In this case the collision of the striker and the drill may be regarded as the collision of two free rods [6]. As the experiments carried out showed, when the optimally necessary withdrawal of the body from the limiting collar of the drill is provided, in the presence of an elastic element in the feeder design, percussive-rotary drilling with continuous rotation of the drill is entirely possible. At the same time, the intensity of tool wear did not exceed the intensity of wear in percussive-rotary drilling with intermittent turning of the drill. The average moment of resistance to rotation of the drill in the blasthole when drilling holes 40–45 mm in diameter in diabase and granite, at an impact energy of 5–8 kgm and optimum feed force, did not exceed 45–60 kgcm. When the feed force is increased above the optimum, the gap between the drill blade and the rock decreases to zero; in this case the moment of resistance to rotation of the drill increases considerably, and tool wear proceeds more rapidly. As a result, at considerable feed forces of drilling hammers on the bottom, the main advantage of percussive-rotary drilling over simple rotary drilling is lost—less tool wear and lower resistance to rotation of the drill when drilling rocks of high strength.

As additional experiments [5], conducted on the penetration of a drill bit into rock under a copra at various impact energies, have shown, preliminary pressing of the drill against the rock before it is struck by the striker does not lead to an increase in the destroyed volume of rock in percussive-rotary drilling; on the contrary, in most cases it increases the volumetric work of destruction (the work expended on destroying by drilling \(1\ \mathrm{cm}^3\) of rock). On this basis it may be assumed that, when the striker strikes a drill pressed against the rock, unproductive losses in the collision are greater than in the case of a free collision of the striker and the drill, and then of the drill and the rock. This is also shown in work [7].

On the basis of the investigations carried out, the following conclusions may be drawn:

1. An increase in feed in percussive-rotary drilling does not exert

does not directly affect the process of penetration of the tool into the rock and the destruction of the latter, but only ensures operation of the drilling hammer in one or another regime, with one or another efficiency of conversion of impact energy.

1. The most expedient operating regime of the machine in percussive-rotary drilling of blast holes is one in which, after each blow of the bit against the rock, the bit moves away from the bottom by 1–4 mm. This ensures stable operation of the impact unit of the machine, the most effective action of the tool on the rock, and comparatively low resistance to rotation of the bit.

Such an operating regime of the drilling hammer can be achieved by a definite feed force, provided that the design of the feeder includes an elastic element ensuring a certain longitudinal displacement and oscillation of the machine body during drilling.

1. It is not expedient to choose the feed force of the drilling hammer from the condition of pressing the drill bit tightly against the bottom of the blast hole throughout the entire drilling time, since this leads to a considerable increase in the friction moment of the drill bit against the bottom and does not increase the drilling rate. In this case, the principal advantage of percussive-rotary drilling over simple rotary drilling of rocks of high strength is in fact lost, since tool wear is greatly increased.

2. Percussive-rotary drilling of blast holes with continuous rotation of the bit is entirely possible at bit rotation speeds from 90 to 230 rev/min and impact energies up to 10 kgm, if the feed force provides the optimally necessary withdrawal of the drilling hammer from the limiting collar of the bit,

\[ H_{\mathrm{av}} = 1 \div 7 \text{ mm}. \]

1. The conclusions drawn above: (a) concerning the dependence of the optimum angle \(\alpha\) of bit rotation between two blows on rock strength and on impact energy, and (b) concerning the influence of feed force on the rate of percussive-rotary drilling of blast holes, also apply to pneumatic drilling hammers.

REFERENCES

1. Fedorov V. S. Scientific Foundations of Drilling Regimes. State Scientific-Technical Publishing House of Petroleum and Mining Literature. Moscow–Leningrad, 1951.

2. Epshtein E. F. Theory of Drilling-Cutting of Rocks with Hard Alloys. GONTI, 1939.

3. Ostroushko I. A. Destruction of Rocks during Drilling. Gosgeolizdat, Moscow, 1952.

4. Alabuzhev P. M. Investigation of the working process of an electropneumatic hammer with an elastic (air) coupling. Tomsk, Izvestiya TPI, vol. 61, issue 1, 1947.

5. Alimov O. D. Investigation of the mechanism of destruction of rocks in percussive-rotary drilling and of the initial parameters of drilling hammers. Author’s abstract. Tomsk, 1953.

6. Dinnik A. N. Selected Works, vol. I, Publishing House of the Academy of Sciences of the USSR, Kiev, 1952.

7. Alabuzhev P. M., Alimov O. D., Tsukanov A. G. On the efficiency in percussive-rotary drilling. Izvestiya TPI, vol. 75, 1954.

8. Alimov O. D. Influence of feed force on the drilling rate with pneumatic drilling hammers. Izvestiya TPI, vol. 75, 1954.