SELECTION OF RATIONAL CUTTER GEOMETRY FOR DRILLING BLAST HOLES IN ROCK

M. K. TSEKHIN

The effectiveness of rock-heading operations under the conditions of the Kuzbass is at present determined to a considerable extent by drilling operations. In the heading cycle, as the investigations carried out have shown [1], drilling takes up to 50% or more of the time, while expenditures for the principal cost items—wages, materials, and equipment—exceed half of the total cost of driving workings. In the mines of the Kuzbass, drilling operations also determine the rate and cost of driving horizontal workings in rock.

The unsatisfactory state of drilling operations in rock is explained primarily by the poor utilization of column-type electric drills. The operating conditions of drills often do not correspond to the mining-geological conditions under which the drilling process is carried out. As a result, low drilling rates are obtained, along with a high consumption of cutters, drill steel, and electric power.

Reserves have been identified for accelerating and reducing the cost of drilling operations. First of all, this involves setting drills to optimal drilling regimes, with replacement of the dry method of removing drill dust by flushing the holes with water.

The introduction of the proposed optimal regimes for rotary electric drilling of blast holes with flushing, as established in [1], makes it possible to increase significantly the technical-economic effectiveness and safety of drilling operations in various rocks composing the Prokopyevsk–Kiselevsk coal deposit. At the same time, in the course of the investigations it was established that a significant reserve in rotary electric drilling of blast holes in rock is the introduction of cutters of rational geometry.

Fig. 1. Elements and geometry of cutter No. 3 for drilling blast holes in rock

Figure 1 presents the basic geometry of a cutter that has been used for more than twenty years in coal mines for drilling blast holes in rock.

in rock and to date has not undergone serious changes that would make it possible to increase its durability and operating efficiency.

Depending on the strength and abrasiveness of the rock, as well as on the drilling regime adopted, the serviceability of the cutter before resharpening, for rocks of medium and above-medium strength, ranges from 20–30 cm to several drill meters. Thus, in dry rotary electric drilling of holes in the strong, highly abrasive quartz sandstone of the roof of the Kharakternyi seam (the Kalinin Mine of the Prokopyevskugol trust), with a feed force of 500 kg, a speed of 200 rpm, and a specific spindle feed of 0.9 mm/rev, the average serviceability of the cutter before resharpening was only 28 cm of hole. For fine-grained sandstone of medium strength (the interseam interval between the Kharakternyi and 1st Vnutrennii seams), under the same drilling regime, it was 4.5 drill meters. Increasing the axial force to 850–1100 kg made it possible, for the same rocks, to increase cutter serviceability respectively to 60–80 cm for the first rock and 7–8 linear meters of holes for the second rock.

An analysis of time observations of drilling operations during the driving of horizontal workings through rock in the mines of the Prokopyevsk ore district shows that, in workings driven through strong rocks, in many cases the cutter drills only 20–60 cm of hole before resharpening. Examples of such workings are crosscut No. 2 and the incline of the Yuzhnaya mine, the main crosscut of the Voroshilov Mine, and the field drift of the Kharakternyi seam at the Kalinin Mine. In most cases, when drilling rocks of above-medium strength, cutter serviceability does not exceed 1.0–1.5 drill meters. Such high wear of cutters when drilling holes in the rocks indicated above is explained by the physico-mechanical properties of the sandy rocks and, first of all, by their abrasiveness and hardness. At the same time, the magnitude and character of wear are determined by the drilling regime and by the cutter geometry adopted.

It has been established that, with an increase in axial pressure and in the specific spindle feed, other conditions being equal, the thickness of the “chip” removed by the cutter increases and the specific amount of wear of the cutter end face decreases.

It has also been established that the expediency of increasing these parameters, beginning from certain critical values for the given drilling conditions and the adopted type of cutter, decreases noticeably as a result of the occurrence of spalling and breakage of the cutting edges of the cutter, especially at its corner junctions on the periphery and at the tips near the center. The frequency and size of breakages and spalls increase with increasing strength of the rock being drilled, with increasing size and hardness of the mineral particles composing it, and with increasing spindle rotation speed.

The investigations carried out [1] showed that an increase in the efficiency of drilling operations can be achieved by introducing optimal regimes for drilling holes with flushing, by increasing axial pressure, and by increasing the thickness of the chip removed by the cutter.

A cutter of the geometry currently adopted, even under drilling regimes that are the best in terms of drilling speed and economic efficiency, is not very satisfactory. Frequent spalls, arising predominantly at the tips of the corner junctions, and high wear of the end face on the periphery of the cutting edge cause frequent cutter replacement and high costs for their depreciation.

The cause of the increased wear of the end face on the periphery, as is seen in Fig. 2, is the peripheral speed—higher than at the center—at the same penetration of the cutter into the rock and with the same specific resistance on the cutting edge along its entire length from the side-

of the zone of the rocks being drilled. Owing to the nonuniform loading of the cutting edge of such a cutter, its wear proves to be high not only during drilling, but also when it is reground on the carborundum wheel of the grinding machine. With such geometry, as has been established by experimental observations [2], only an insignificant part of the cutter wears during drilling, while the greater part is lost in the process of sharpening. Therefore, in order to reduce irrational wear of the cutter during sharpening, as early as 1941 [2] it was proposed to bring the actual wear of the cutting edge at the periphery up to the full thickness of the inserted plate (up to 4 mm), and to perform sharpening with a variable vertex angle. This proposal made it possible to solve the problem of saving hard-alloy plates during sharpening. However, it led to a reduction of the sharpening angle of the cutter at the vertex and, consequently, to more frequent breakage of it in the process of drilling holes in rock of above-average strength.

Fig. 2. Diagram of penetration and wear of the cutting edge of cutter No. 3

The standard cutters “RP,” produced since 1953 by the Kuznetsk Machine-Building Plant, the geometry of which does not differ in principle from cutters No. 3, do not give better performance. On the contrary, the number of possible regrindings of such cutters, owing to the reduced height of the plate at the periphery, has noticeably decreased, while the magnitude of the actually unused central part of the plate has noticeably increased.

Investigations carried out by the author in the mines of the Prokop’evsk ore mine made it possible to propose a rational cutter geometry for drilling holes in rock. A diagram of the cutter of the proposed geometry is shown in Fig. 3. The fundamental difference between the proposed cutter and cutters No. 3 and “RP,” now in use, consists above all in a different geometry of the hard-alloy inserted plate and in the subsequent sharpening of the cutter on the grinding wheel with preservation of the original geometry of the plate until its complete wear. As is seen from Fig. 3, the proposed cutter is sharpened along a circumference which at the periphery passes into a straight line parallel to the longitudinal axis of the cutter and coinciding, in the process of rotation, along its entire length with the cylindrical side surface of the hole. The center of the circumference is displaced from the axis by a distance equal to half the length of the notch.

Such geometry of the cutting edge makes it possible, as is seen from Fig. 4, with equal advance per revolution of all points of the cutting edge of the cutter along the feed, to remove a “chip” whose thickness decreases uniformly toward the periphery. For cutters No. 3 and “RP,” the maximum path of circular displacement of the peripheral point of the cutting edge corresponds (Fig. 2) to the same chip thickness as for the central points and to the same resistance to penetration. For a cutter of the proposed geometry (Fig. 4), the increase in the friction path for points farther from the periphery corresponds to an equally uniform decrease in the thickness of the “chip” and an equally uniform decrease in the resistance to penetration.

of the cutting face into the rock. Therefore, a cutter sharpened according to the proposed geometry wears uniformly along the entire cutting edge, from the center to the periphery. Wear at the periphery of the cutting face (Fig. 4) proves to be equal to the wear in its central part. With the same thickness of the hard-alloy insert, equal to 4 mm, the service life of the cutter before resharpening increases considerably. Thus, from 18 observations made when drilling blastholes with No. 3 cutters and cutters sharpened according to the proposed geometry, an increase in cutter service life within the range from 1.8 to 2.9 times was established. At the same time, from eight observations made when drilling blastholes in fine-grained sandstone of medium strength (the interbed between the Kharakternyi and First Inner seams), the increase

Fig. 3. Cutter geometry with an equal-strength cutting face

Fig. 4. Diagram of penetration and wear of a cutter with an equal-strength cutting face

in cutter service life ranged from 180 to 260%. From ten observations when drilling blastholes in the very strong coarse-grained quartz sandstone of the roof of the Kharakternyi seam, where along with wear breakages of cutter No. 3 are especially noticeable, it ranged from 225 to 290%.

Thus, even under ordinary drilling conditions, the service life of the modernized cutter, in comparison with cutter No. 3, increases by more than two times. Even for such difficult-to-drill rocks as the sandstones of the roof of the Kharakternyi seam, one cutter before resharpening proves sufficient to drill a blasthole to its full depth (1.6–1.8 linear m). All this makes it possible to switch to long-stroke continuous blasthole drilling, providing a considerable saving of time through the elimination of labor-intensive auxiliary operations for replacing cutters and repositioning rods.

The cutter of the proposed geometry has a number of other substantial advantages over standard No. 3 and “RP” cutters. It has been established [1] that the drilling speed increases sharply, while the energy consumption and specific wear of the cutters decrease noticeably with an increase in axial force and spindle feed per revolution. However, for No. 3 and “RP” cutters, which are mechanically weak sharp-angled joints of the cutting faces, a feed force exceeding 1000–1200 kg proves destructive. A specific feed exceeding 0.9–2.0 mm/rev is likewise destructive for strong rocks. The cutter of the proposed geometry has no sharp-angled joints of the cutting faces. Owing to its distinctive geometry, variable sharpening angle, gradual

but increasing from the center toward the periphery, of the variable thickness of the removed “chip,” gradually decreasing toward the periphery, the cutting edge over its entire length proves to be of equal strength and withstands, without failure, an axial pressure of up to 1200–1500 kg.

The proposed cutters therefore make it possible to drill blast holes in hard rocks with an increased specific feed, since the actual thickness of the removed “chip,” in comparison with cutters No. 3 and “RP,” increases noticeably only in the central part of the cutting edge of the modernized cutter, where the circumferential speed of motion is much less than at the periphery and the dynamic destructive loads are weaker. Thus, the introduction of a cutter of the proposed geometry provides the possibility of a significant increase in drilling speed, both in purely machine time and in total time.

An important feature of the proposed cutter is the configuration of the inserted plate, which, as can be seen from Fig. 3, makes it possible not to change the cutter geometry during drilling and resharpening until the plate is completely worn out. Thus, the useful wear of the pobedit for one drilling cycle before resharpening can be brought from 9–27 [2] to 40–50%. Its abrasion on the grinding wheel can be reduced by a factor of one and a half.

In the process of drilling and resharpening, the proposed cutter is scarcely reduced in diameter up to its complete wear. This is an extremely important factor in increasing the efficiency of the entire complex of drilling-and-blasting operations. It is known that, when drilling with an ordinary cutter of the No. 3 and “RP” type, the blast holes do not come out cylindrical. Their surface assumes a screw-like form, reducing the internal diameter of the blast hole. Owing to rapid wear of the peripheral tips, a noticeable reduction in cutter diameter, and repeated replacement of cutters during drilling, the diameter of the bottom part of the blast hole proves in most cases to be 3–4 mm smaller than the diameter of the mouth of the blast hole. The blast hole is drilled in the form of a truncated cone having a helical surface. Therefore, cartridges of explosive of 32 mm diameter are often inserted with difficulty into such a blast hole drilled with a cutter 38 mm in diameter. The loading density drops sharply and, along with it, the blasting result seriously deteriorates.

The use of the modernized cutter eliminates the noted shortcomings. The inner surface of the blast hole assumes a cylindrical form. In this connection, with cutters 42 mm in diameter, it becomes possible to bring the diameter of the cartridges up to 36–38 mm. This makes it possible to reduce the number of blast holes in the round, reduce the time spent on drilling operations, and increase the blasting effect owing to the increased loading density.

For the mines of the Prokop'evsk ore field, where more than 20 km of main horizontal workings in rock are driven annually and where, at present, the costs for cutter amortization per 1 linear meter of these workings average 15–20 rubles, and when drilling hard sandstones—100 rubles and more, the actual savings from the introduction of modernized cutters may amount to not less than 100 thousand rubles per year, apart from the savings that may be obtained as a result of accelerating drilling operations.

Thus, the introduction of cutters of the proposed geometry makes it possible to ensure a higher technical and economic effect of drilling operations.

REFERENCES

1. Tsekhin M. K. On the question of establishing optimal conditions for electric rotary drilling of blast holes in the rocks of the Prokop'evsk deposit, Izvestiya TPI, vol. 93, 1958.

2. Mikhailov V. G. Investigation of cutters for drilling blast holes. Collection Mining and Geology, published by Sovetskaya nauka, 1941.

Noted Misprints

Izv. TPI, vol. 103.