Experimental Studies of New Types of Bits
Abstract Generated abstract
This paper reports laboratory experiments on carbide drill bit designs intended to improve the productivity of core drilling in geological exploration boreholes. Using a ZIF-75 drilling rig with dynamometric recording, the authors tested sandstone drilling with varying cutter numbers, cutter rake angles, wedge-shaped kerfing cutters, and stepped cutter arrangements. The experiments indicate that, under the tested conditions, circumferential force was nearly independent of cutter number, while mechanical drilling speed decreased approximately in inverse proportion to the square root of the number of cutters. Bits with a 30 degree rake angle, wedge-shaped kerfing cutters with small protrusion, and stepped cutter arrangements produced substantially higher drilling speeds than comparable ordinary bits, leading the authors to recommend these designs for improving core drilling productivity.
Full Text
EXPERIMENTAL STUDIES OF NEW TYPES OF DRILL BITS
S. S. SULAKSHIN, V. I. ZVARYGIN
(Presented by the scientific seminar of the Department of Exploration Technology)
A significant factor influencing the productivity of core drilling of geological-exploration boreholes is the type and design of the bit. The success of drilling boreholes depends to a considerable extent on the design of the drill bits and on the quality of the hard alloy.
The main design elements that influence drilling productivity to one degree or another are:
a) the protrusion of the cutters from the bit ring; b) the width of the cutters; c) the cross section of the cutters; d) the clearance angle of the cutters; e) the rake angle of the cutters; f) the shape of the cutters; g) the orientation of the cutters; h) the arrangement of the cutters in the bit ring across its section; i) the arrangement of the bit cutters relative to the bottomhole; k) the number of cutters in the bit.
In order to determine the optimal parameters of bits, experimental studies were carried out on a laboratory installation. A ZIF-75 drilling rig was used for the studies. The recording of circumferential forces and axial pressure was made on the paper of an N-700 oscillograph by means of a dynamometer specially designed for the studies. To record the mechanical drilling rate, the simplest device was made, consisting of a wooden board fastened to the vertical frame of the rig and a bracket fastened to the spindle.
Millimeter paper was pinned to the board. During drilling, over time, the spindle moved downward; together with the spindle the bracket with a transverse ruler also moved downward. Every 10 seconds, marks were made with a pencil on the millimeter paper along the ruler. From the magnitude of the advance and the corresponding time, the mechanical drilling rate was determined.
The main objective of the studies was:
a) to determine the influence of the number of cutters in the bit on the mechanical rate and circumferential forces;
b) to determine the operating efficiency of bits with cutters whose rake angle is \(\gamma = 30^\circ\);
c) to determine the operating efficiency of bits with triangular cutters with a flat rake face;
d) to determine the operating efficiency of stepped bits.
Determining the Effect of the Number of Cutters in a Bit on Drilling Efficiency
There are many works in the literature devoted to analyzing the influence of the number of cutters on drilling efficiency. S. B. Kocharov and N. F. Pravdyuk [4] believe that the number of cutters in a bit must necessarily be taken into account when drilling wells, since with a small number of cutters their breakage and rapid wear are possible. E. V. Borovskii and F. A. Shamshev [1] established that there is an optimum number of cutters at which the maximum drilling productivity is achieved. With a large number of cutters, the mechanical drilling rate decreases very appreciably; with a small number of cutters, the cutters wear out rapidly, as a result of which the mechanical rate also drops quickly.
N. I. Lyubimov and V. I. Morozov [4] believe that, when the diameter of bits is increased, the specific saturation per \(1\ \mathrm{cm}^2\) of the bit face should be constant. However, while giving a qualitative estimate of the dependence of the mechanical drilling rate on the number of cutters in the bit, previous authors do not establish a quantitative dependence \(v_{\text{mech}} = f(m)\), nor do they establish the influence of the number of cutters on the circumferential force.
The study to determine the influence of the number of cutters in a bit was carried out while drilling sandstones with bits fitted with carbide cutters No. 3903 \((3 \times 3 \times 20)\). The number of cutters in the bits was 3, 6, 9, and 12. For all bits, the axial pressure and rotational speed remained constant, equal to \(P = 140\ \mathrm{kg}\) and \(n = 405\ \mathrm{rpm}\). For each bit, 8 experiments were performed. The results of processing the oscillograms are entered in Table 1.
It follows from the table that the circumferential forces acting during drilling on bits with different numbers of cutters remain the same.
Table 1
| Rocks | Magnitude of circumferential force, kg, with number of cutters in the bit: 3 | Magnitude of circumferential force, kg, with number of cutters in the bit: 6 | Magnitude of circumferential force, kg, with number of cutters in the bit: 9 | Magnitude of circumferential force, kg, with number of cutters in the bit: 12 |
|---|---|---|---|---|
| Sandstone | 70 | 72 | 71 | 75 |
| Sandstone | 73 | 70 | 71 | 69 |
| Sandstone | 80 | 76 | 79 | 78 |
| Sandstone | 69 | 69 | 63 | 65 |
| Sandstone | 74 | 74 | 73 | 72 |
| Sandstone | 81 | 80 | 80 | 79 |
| Sandstone | 76 | 75 | 74 | 74 |
| Sandstone | 70 | 70 | 70 | 69 |
| Mean values | 74 | 73 | 72.5 | 72.5 |
Table 2
| Rock | Mechanical drilling rate, cm/min, with number of cutters in the bit: 3 | Mechanical drilling rate, cm/min, with number of cutters in the bit: 6 | Mechanical drilling rate, cm/min, with number of cutters in the bit: 9 | Mechanical drilling rate, cm/min, with number of cutters in the bit: 12 |
|---|---|---|---|---|
| Sandstone | 11.04 | 7.4 | 6.5 | 5.6 |
| Sandstone | 2.1 | 1.5 | 1.3 | 1.1 |
| Sandstone | 15.0 | 10.7 | 8.4 | 7.5 |
| Sandstone | 5.5 | 4.0 | 3.2 | 2.8 |
| Sandstone | 3.8 | 2.6 | 2.0 | 1.8 |
| Sandstone | 2.7 | 2.4 | 1.6 | 1.3 |
| Sandstone | 12.8 | 8.7 | 7.5 | 6.3 |
| Sandstone | 10.3 | 6.5 | 5.5 | 5.0 |
| Mean values | 7.75 | 5.9 | 4.5 | 3.9 |
Therefore, the circumferential forces acting on the bit during drilling do not depend on the number of cutters.
The results of the experimental data for determining the dependence of the mechanical drilling rate on the number of cutters in the bit are presented in Table 2 and in Graph 1.
From an analysis of the results obtained it is evident that, when the number of cutters in the bit is increased under identical drilling conditions, the mechanical drilling rate and, consequently, the depth of penetration of the cutters (per revolution) decrease. From Table 1 and Fig. 1 it is also evident that the mechanical drilling rate is inversely proportional to the square root of the number of cutters in the bit:
\[ v_{\mathrm{mech}}=\frac{K_m}{\sqrt{m}}, \tag{1} \]
where \(K_m\) is the coefficient of proportionality.
In the graph, the theoretical curve calculated from formula (1) is plotted with a dashed line. A comparison of the theoretical and actual curves shows that they are situated close to one another.
Fig. 1. Dependence of the mechanical drilling rate on the number of cutters in the bit
Study of bits with cutters having a rake angle equal to
\[ \gamma=+30^\circ \]
In the course of experimental studies of single cutters with different values of rake angles \(\gamma\), it was established that the most optimal rake angles of cutters are \(\gamma=+(30 \div 45^\circ)\). To verify the effectiveness of bits with cutters installed in the bit ring at an angle \(\gamma=30^\circ\) directly during the drilling process, corresponding bits of diameter \(46\) mm with three and six cutters were manufactured and tested. The front face of the cutters was exposed by means of a special cutout. For comparison, simultaneously with the study of the indicated bits, tests were also carried out on ordinary bits (bits with normally installed cutters), also with 3 and 6 cutters. The studies of the bits were conducted on the laboratory installation described above while drilling sandstones from the Anzhero deposit.
When comparing the results obtained after processing the experimental studies in cutting with bits having normally and obliquely positioned cutters (Table 3), it is evident that the mechanical drilling rate in the latter case, with the same number of cutters, is significantly higher.
Table 3
| Bit and number of cutters \(m\) | Bit rotation speed, rpm | Average mechanical drilling rate, cm/min, at axial pressure, kg | Average mechanical drilling rate, cm/min, at axial pressure, kg | Average mechanical drilling rate, cm/min, at axial pressure, kg |
|---|---|---|---|---|
| 140 | 240 | 310 | ||
| Bit with \(\gamma = 30^\circ\) and \(m = 3\) | 205 | 3.5 | 6.5 | 10.1 |
| Bit with \(\gamma = 30^\circ\) and \(m = 3\) | 405 | 5.5 | 10.5 | 14.2 |
| Bit with \(\gamma = 30^\circ\) and \(m = 3\) | 810 | 7.0 | 12.6 | 18.9 |
| Bit with \(\gamma = 0^\circ\), \(m = 3\) | 203 | 2.6 | 4.4 | 7.5 |
| Bit with \(\gamma = 0^\circ\), \(m = 3\) | 405 | 4.1 | 6.5 | 10.1 |
| Bit with \(\gamma = 0^\circ\), \(m = 3\) | 810 | 5.4 | 9.0 | 15.1 |
| Bit with \(\gamma = 30^\circ\), \(m = 6\) | 203 | 2.5 | 4.6 | 7.1 |
| Bit with \(\gamma = 30^\circ\), \(m = 6\) | 405 | 3.8 | 7.0 | 10.5 |
| Bit with \(\gamma = 30^\circ\), \(m = 6\) | 810 | 5.1 | 8.9 | 14.3 |
| Bit with \(\gamma = 0^\circ\), \(m = 6\) | 203 | 1.8 | 3.2 | 5.1 |
| Bit with \(\gamma = 0^\circ\), \(m = 6\) | 405 | 2.8 | 4.3 | 7.3 |
| Bit with \(\gamma = 0^\circ\), \(m = 6\) | 810 | 3.9 | 6.0 | 10.5 |
Study of bits with cutters having a wedge-shaped cutting edge with a flat rake face
According to experimental studies, the most optimal form of the cutter’s cutting edge is the wedge-shaped form. But when drilling with a bit in which all the cutters have a wedge-shaped form, the expected effect is not obtained. On the contrary, the mechanical drilling rate in this case is significantly lower than the drilling rate with ordinary bits having the same number of cutters. This is apparently explained by the fact that, as the depth of cutting increases, the length of contact between the cutter and the rock is considerably greater with a wedge-shaped cutting edge than with a straight one. Obviously, the greatest drilling efficiency of bits with a wedge-shaped cutting-edge form will occur in the case when the indicated cutters are able to penetrate into the rock from a leveled surface, with a sufficient amount of free space.
In order for the cutter to have such a possibility, it is necessary, in addition to the wedge-shaped cutters used as kerf-forming cutters, to install additional prismatic or plate-type bottom-hole cutters. Thus, the wedge-shaped (kerf-forming) cutter, projecting above the others by some amount, during the drilling process will constantly penetrate into the surface prepared by the bottom-hole cutters. At the same time, the cutter itself, as a result of its ability to penetrate to a greater depth under the action of relatively small loads, creates the possibility of more favorable operating conditions for the bottom-hole cutters. But in manufacturing bits, apparently, the amount of protrusion of the wedge-shaped cutters relative to the bottom-hole cutters should be taken into account. On the one hand, the greater the depth of the groove cut by the kerf-forming cutter, i.e., the greater its protrusion relative to the bottom-hole cutters, the more favorable the operating conditions of the bottom-hole cutters and the higher the productivity of the bit. But the greater the protrusion of the wedge-shaped cutters, the greater the amount of contact between the cutting edge and the rock, and the lower the efficiency of the cutters used (in comparison with ordinary ones). Obviously, the greatest efficiency of wedge-shaped cutters will occur in the case when
the total length of contact of the cutting edge with the rock will be less than the width of an ordinary cutter.
For the experimental studies, bits were manufactured with three cutters: a kerfing cutter with a wedge-shaped cutting edge and two plate-shaped cutters of form G-3. The kerfing cutter, for different bits, protruded above the others by a height \(\Delta h\) equal to 3 mm and 0.5 mm. When drilling with bits having kerfing cutters protruding above the others by 3 mm, the mechanical drilling speed, in comparison with \(v_{\text{mech}}\) for bits with all three prismatic cutters, is 1.1–1.2 times lower. When drilling with bits with \(\Delta h = 0.5\) mm, the mechanical drilling speed of the bits tested increases very sharply in comparison with ordinary bits (approximately by a factor of 1.7), as can be seen from Table 4.
Table 4
| Bit | Bit rotation speed, rpm | Average mechanical drilling speed, cm/min, at axial pressure, kg: 140 | Average mechanical drilling speed, cm/min, at axial pressure, kg: 240 | Average mechanical drilling speed, cm/min, at axial pressure, kg: 340 |
|---|---|---|---|---|
| With wedge-shaped kerfing cutters | 203 | 4.8 | 7.8 | 12.0 |
| With wedge-shaped kerfing cutters | 405 | 7.2 | 10.2 | 18.5 |
| With wedge-shaped kerfing cutters | 810 | 10.0 | 16.0 | 24.0 |
| Ordinary, with prismatic cutters G-3 | 203 | 2.8 | 4.3 | 6.7 |
| Ordinary, with prismatic cutters G-3 | 405 | 4.2 | 6.5 | 11.0 |
| Ordinary, with prismatic cutters G-3 | 810 | 5.5 | 7 | 13.2 |
Study of Bits with a Stepped Arrangement of Cutters
On the basis of theoretical studies it was established that the most favorable bottom-hole shape for cutting rocks is a stepped one.
The effectiveness of drilling with bits having a stepped arrangement of cutters has also been noted by a number of other authors. M. M. Tsukurov experimentally established that a stepped bit, when drilling sandy shales, increased the drilling speed by 55% in comparison with ordinary bits [3]. The influence of a stepped bottom-hole shape on drilling efficiency was also noted by V. N. Novichenko. When drilling by the percussion method with a solid bottom hole, F. I. Volkov [2] established that the drilling speed with stepped bits was higher than \(v_{\text{mech}}\) with ordinary bits by 30–40%, and wear resistance by a factor of 2.5–6.
F. I. Kucheryavyi [3] also carried out a series of experiments on drilling apatite rocks with bits having a stepped arrangement of cutters. In this case the kerfing cutters protruded 1.5 mm more than the breaking cutters. On the basis of the experiments he drew the following conclusions:
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At low feed forces, the stepped bit does not differ in operating efficiency from the ordinary one.
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The operating efficiency of the stepped bit, in comparison with the operation of the ordinary bit, can be increased by 30%.
But all the above-mentioned studies were carried out with bits in which the kerfing cutters protruded relative to the breaking cutters by an insignificant amount, as a result of which the height of the step was very small. The latter circumstance affects the distribution of stresses in the rock. It is quite obvious that the height of the step substantially
affects the nature of the stress distribution. In theoretical studies we established that, when cutting rock along the edge of a groove of sufficiently great depth (a depth permitting the full development of the stresses characteristic of cutting along the edge of a specimen), the forces acting on the relief cutters are 4 times smaller than those acting on the kerf cutters when their widths are the same. As a result, drilling efficiency should increase. Therefore, in order to create the possibility for the full development of stresses in the ledge, bits should be made with a sufficiently large distance between the cutting edges of the kerf and relief cutters. To determine the effectiveness of stepped bits with a large protrusion of the kerf cutters relative to the relief cutters, special studies were carried out.
For the experimental studies, bits were made with six cutters: three kerf cutters 3 mm wide and 20 mm long, reinforced along the face of the leading step (step width 2.5 mm), and three undercutting cutters 8 mm wide, located on the second step, 20 mm away from the first. The cutters of the first step were brazed into the bit without protruding beyond the face; then the front face of the cutters was exposed to a height of 5 mm by a special cutout, and the back face of the cutter was sharpened together with the face of the bit. Such sharpening reduces the protrusion of the cutter and, consequently, protects it from breakage.
For comparison, tests were carried out simultaneously with an ordinary bit reinforced with 6 prismatic cutters \(3 \times 3 \times 20\) mm.
The results of comparing the experimental data (Table 5) show that, although the total width of the ring increased considerably, the mechanical drilling speed of bits with a stepped arrangement of the cutters is approximately 1.9 times higher than that of the ordinary bit.
Table 5
| Bit, number of cutters | Bit rotation speed, rpm | Average mechanical drilling speed, cm/min, at axial pressure, kg | Average mechanical drilling speed, cm/min, at axial pressure, kg | Average mechanical drilling speed, cm/min, at axial pressure, kg |
|---|---|---|---|---|
| 140 | 240 | 340 | ||
| With stepped arrangement of cutters (6 cutters) | 203 | 4.2 | 7.3 | 11.3 |
| With stepped arrangement of cutters (6 cutters) | 405 | 6.1 | 10.5 | 16.2 |
| With stepped arrangement of cutters (6 cutters) | 810 | 8.4 | 14.4 | 21.8 |
| Ordinary, 6 cutters No. 1309 | 203 | 2.3 | 4.0 | 5.6 |
| Ordinary, 6 cutters No. 1309 | 405 | 3.2 | 5.5 | 8.0 |
| Ordinary, 6 cutters No. 1309 | 810 | 4.5 | 8.5 | 10.2 |
Conclusions
-
As a result of the experimental studies it has been established that, with an increase in the number of cutters in the bit under identical cutting conditions, the mechanical drilling speed is inversely proportional to the square root of the number of cutters.
-
The mechanical drilling speed for bits with cutters set at an angle \(\gamma = 30^\circ\) is considerably higher than when drilling with ordinary bits under identical conditions with a cutter rake angle \(\gamma = 0^\circ\).
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The mechanical drilling speed of bits with a stepped arrangement of cutters and wedge-shaped kerf cutters increases almost twofold in comparison with ordinary bits.
- The investigations carried out make it possible to recommend more modern designs of bits, the use of which in practical work will substantially increase the productivity of core drilling of boreholes.
REFERENCES
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E. V. Borovsky, F. A. Shamshev. Study of the technological process of drilling. Transactions of TsNIGRI, issue 95, 1937.
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F. I. Volkov, S. L. Iofin, M. L. Stebakov. Pneumatic drilling of deep boreholes in hard rocks. Mining Journal, No. 1, 1954.
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F. I. Kucheryavyi. Ways of increasing the efficiency of a bit with a stepped arrangement of cutters. Exploration and Protection of Mineral Resources, No. 3, 1957.
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N. I. Lyubimov, V. I. Morozov. The role of the specific saturation with cutters in hard-alloy bits. Exploration and Protection of Mineral Resources, No. 6, 1954.