On the Influence of the Cutter Insertion Angle on the Efficiency of Vibration-Rotary Drilling
Abstract Generated abstract
This paper examines how the insertion angle of hard-alloy cutters affects bit durability and drilling performance in vibration-rotary drilling. It argues that maximum cutter life should occur when the cutter insertion angle corresponds to the average angle at which the cutter penetrates the borehole bottom, then tests three 65 mm core bit designs in marble with insertion angles of 90 degrees, 82 degrees, and 110 degrees under specified vibration-rotary operating conditions. The experiments indicate that, unlike conventional rotary drilling, vibration-rotary drilling is strongly influenced by cutter insertion angle: the bit with the positive 82 degree insertion angle, close to the penetration angle of 81.5 degrees, gave the best penetration and cutter durability, while the negative-angle bit failed fastest.
Full Text
On the Influence of Cutter Insertion Angle on the Efficiency of Vibration-Rotary Drilling
V. M. Matrosov
(Presented by the scientific seminar of the Department of Exploration Technology)
In connection with the development of new methods for drilling exploratory boreholes, such as vibration-rotary drilling, percussion-rotary drilling using submersible hydraulic hammers, etc., the task has arisen of creating a more advanced rock-breaking tool with increased durability (service life). In recent years various organizations (TsKB MG and ON, VITR, and others) have created a whole series of core bits for the indicated drilling methods. Structurally, these core bits differ mainly in the shape and grade of the hard-alloy cutters and in their mutual arrangement on the face of the bit. Depending on the conditions under which the core bits are used, the hard-alloy cutters have different values of the front and rear cutting angles; the angle at the tip also changes. The insertion angle of these cutters in the bit, however—that is, the angle between the longitudinal axis of the cutter and the plane of the bit face—is, as a rule, \(90^\circ\).
Meanwhile, in vibration-rotary drilling, as well as in percussion-rotary drilling with continuous rotation of the bit, the cutters penetrate into the bottom of the borehole during impact at a certain angle, the average value of which depends on the vertical displacement velocity of the bit, its peripheral velocity, and the magnitude of the rotating masses of the drilling tool \([1, 2]\). In this case the resultant force of the rock reaction acts at the same angle on the cutters of the bit, tending to tear them out of their seats or break them. It may be assumed that maximum cutter durability can be achieved only when the cutter insertion angle \(\alpha\) is equal to the average value of the angle of penetration of the cutter into the surface of the borehole bottom \(\rho\) (Fig. 1). Under otherwise equal conditions such a drill bit will have the greatest service life.
As previously conducted studies have shown \([1]\), the maximum penetration rate in vibration-rotary drilling is observed when the angle of penetration of the cutters into the borehole bottom is equal to the optimal angle of encounter for the given “bit—rock” pair. This condition is ensured by an appropriate selection of the operating parameters of the vibration-rotary installation.
The values of the optimal encounter angles for different bit—rock pairs vary over fairly wide limits—from \(60^\circ\) to \(85^\circ\). Consequently, the cutter insertion angles, depending on the specific conditions, must vary within this range, provided that the combination
...of the operating parameters of the vibration unit will correspond to operation at the optimum angles of attack.
In order to determine the service life of core bits as a function of the cutter insertion angle, experimental studies were carried out on a vibration-rotary installation. It should be noted that these studies did not pursue the aim of establishing a rational geometric shape for the cutters or for the bit as a whole, but were directed only toward identifying the bit design with the greatest service life as a function of the cutter insertion angle.
Experimental drilling was carried out in marble with three types of core bits, differing from one another in the cutter insertion angle: type I—with vertical insertion (the angle between the cutter axis and the end face of the bit is \(90^\circ\)); type II—with a positive insertion angle of \(82^\circ\); type III—with a negative insertion angle of \(110^\circ\).
Fig. 1.
The cutters used were all of standard shapes G-4 and G-53, and the bit diameter was \(65\) mm. The operating parameters during drilling were characterized by the following values:
1) axial load — \(120\) kg;
2) bit rotation speed — \(75\) rpm;
3) moment of the vibrator eccentrics — \(25\) kg/cm;
4) vibrator oscillation frequency — \(1200\) oscillations/min.
The indicated parameters of the vibration-rotary drilling mode ensured penetration of the bit cutters into the bottom of the borehole at an angle of \(81.5^\circ\), which was close to the optimum angle of attack for these conditions. The data from the tests carried out are summarized in Table 1. On the basis of these data a graph was constructed showing the dependence of the penetration rate and the advance per run for different types of bits (Fig. 2).
In drilling without vibrations, both the advance and the penetration rate proved to depend only slightly on the cutter insertion angle: only for the bit with a negative cutter insertion angle were these indicators somewhat lower. The penetration rate during a run gradually decreased as the cutters wore.
When vibrations were imposed on the drill bit, the cutter insertion angle had a substantial effect on the magnitude of the advance per run, as well as on the penetration rate. As can be seen from the graph presented, the best indicators were achieved when drilling with a bit having a positive cutter insertion angle. In this case the durability of the cutters was maximal. The worst indicators were given by the bit with a negative cutter insertion angle; its cutters quickly failed.
Table 1
Change in penetration rate during drilling with cutters of various types
| Time interval, min | I: Drilling without vibration, by 4 cutters | I: Drilling with vibration, by 3 cutters | II: Drilling without vibration, by 3 cutters | II: Drilling with vibration, by 3 cutters | III: Drilling without vibration, by 3 cutters | III: Drilling with vibration, by 2 cutters |
|---|---|---|---|---|---|---|
| 0–10 | 0,67 | 1,00 | 0,65 | 1,03 | 0,61 | 0,91 |
| 10–20 | 0,58 | 0,95 | 0,59 | 0,93 | 0,47 | 0,86 |
| 20–30 | 0,52 | 0,83 | 0,48 | 0,87 | 0,45 | 0,73 |
| 30–40 | 0,46 | 0,78 | 0,43 | 0,88 | 0,41 | 0,68 |
| 40–50 | 0,38 | 0,76 | 0,42 | 0,95 | 0,36 | 0,62 |
| 50–60 | 0,40 | 0,71 | 0,42 | 0,74 | 0,34 | 0,47 |
| 60–70 | 0,37 | 0,62 | 0,36 | 0,73 | 0,27 | 0,38 |
| 70–80 | 0,29 | 0,52 | 0,21 | 0,69 | — | — |
| Total: | 3,67 | 6,17 | 3,56 | 6,82 | 2,91 | 4,65 |
Fig. 2. Graph of the dependence of the rate of penetration and penetration during a run for different types of bits. I, II, III — types of bits;
a — drilling without vibrations; b — drilling with vibrations.
The rate of penetration in vibration-rotary drilling with carbide bits proved to be 2–3 times higher than in drilling without vibrations. The footage per bit also increased, although it was not possible to establish a sufficiently reliable quantitative indicator, since even when all cutters failed, deepening of the borehole under vibrations did not cease.
Thus, the cutter insertion angle in vibration-rotary drilling has a major influence on the service life of the bit and, ultimately, on the efficiency of this drilling method as a whole. This circumstance should be taken into account when designing new types of bits for vibration-rotary and percussion-rotary methods of drilling boreholes.
REFERENCES
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V. M. Matrosov. Experimental studies on vibration-rotary drilling with an annular face. Proceedings of Higher Educational Institutions, series “Geology and Exploration,” No. 2, 1960.
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Yu. M. Pariiskii. Toward the development of the theoretical foundations of percussion-rotary drilling. Transactions of the Leningrad Mining Institute, Vol. XLI, Issue 2, 1961.