The wide range of applications and drilling conditions suitable to down-the-hole pneumatic percussion technique requires consideration of more than one type of tool to drill efficiently and profitably.
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The selection of hammer size is largely determined by the hole diameter and type of rock formation. The optimum blasthole range for DTH drilling is 3½ to 10 inches. Smaller holes are typically drilled with a top hammer rather than DTH hammer, and larger holes generally use the rotary drilling technique, mainly with the focus on hole straightness.
As a general rule, the smallest hole diameter a DTH hammer can drill is its nominal size. A 4-inch hammer is optimally designed to drill a 4-inch hole. The closer the hole diameter is to the hammer's diameter, the more restricted the hole's evacuating airflow is. Drilling holes at the nominal size leaves adequate space for cuttings to evacuate the hole up the annulus between the hammer and drill pipe diameters and the internal diameter of the hole's wall.
Maximum bit size for production drilling is the nominal hammer diameter plus 1 inch. For instance, a 4-inch hammer's maximum bit size is regarded to be 5 inches in diameter. Pipe diameter should be close to the hammer diameter to provide optimum flushing, reducing the chances of getting stuck in the hole.
Pipe (tubes) made from cold-drawn tubing provide a superior surface finish and tolerance compared to tubes made from hot-rolled tubing. A better finish reduces the risk from metal chips from the tubes, called scaling. Scaling that flows through the hammer is a major cause of premature hammer failure.
Friction-welded joints add strength. Heat treating the threads of end-pieces ensures optimum durability and strength of the thread profile. Preserving the thread profile keeps coupling and uncoupling smooth, without adding time to the average rate of penetration.
The bodies of quality bits are precision machined from alloy steel, heat treated to a specified hardness, providing surface compression for fatigue resistance, and then fitted with precision carbide buttons.
Convex-faced, ballistic-button designs are preferable for fastest cuttings removal. This bit design cuts clean with minimal re-crushing, making it ideal for soft to medium, non-abrasive formations.
In hard and abrasive formations, flat-front designs offer best bit life, especially those that feature strong gauge rows with large spherical buttons that are easy to regrind and maintain. An alternative in these conditions is a concave design with spherical buttons.
Concave bit faces work well in medium to hard, fractured formations. In this type of application, a concave bit minimizes hole deviation.
A fundamental question is whether one needs a reliable, low-cost hammer or a premium hammer. Buying more than one less-expensive hammer to achieve the same drill feet of a single premium hammer is false economy.
Manufacturers design DTH hammers to address the challenges presented by the full array of rock types and applications. For deeper hole applications, hammers are designed to work with different air requirements as bailing velocity requirements change.
Quarrying projects are long-term jobs that will easily outlast the hammer. Frequently replacing hammers would be cost-prohibitive. Dimensional stone quarrying demands consistently straight holes. DTH hammers offer more precision to hole straightness than top hammers.
Mineral exploration generally requires robust hammers capable of running high pressures, often in dirty, remote environments. Operators tend to prefer simple, reliable designs in this application.
Reverse circulation with specialized DTH hammers is less expensive than diamond coring technique. The RC hammers use the same engineered technology and components as standard DTH hammers, but use RC tubes with inner and outer walls for direct airflow to allow the cuttings to pass up through the pipe to a cyclone for collection bags, rather than the typical hole cleaning up the hole's annulus.
Like quarries, open pit mines typically have high equipment utilization, drilling most of the working day.
Some premium hammers for these applications can now be rebuilt twice before replacement.
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Properly training drillers to recognize and maintain optimum drilling parameters prevents premature failure of hammers.
Hammer value involves more than component material and design. Additional factors must be taken into consideration. The market offers a range of quality in both economy and premium hammer selections. The hammer must not only be durable enough to be reliable throughout its anticipated lifespan, but its cost of operation figures in. The greater the hammer efficiency, the better the savings in fuel and energy, resulting in lower cost per drilled foot.
Additionally, drillers should consider the manufacturer's product support. A consultation from the manufacturer provides advice on DTH requirements. A knowledgeable product support specialist will also help determine which hammer provides the highest profitability for a project.
Down the hole (DTH) drilling rig is the equipment that the impactor dives into the hole and directly impacts the drill bit, while the rotary machine drives the drilling hole to rotate and drill to the ore rock. In addition to drilling main blast holes in open-pit mines, down-the-hole (DTH) drilling rigs are also used to drill pre-split holes, anchor holes, slope treatment holes and groundwater drain holes, as well as vent holes, filling holes, pipe cable hole, etc. Down the hole (DTH) drilling rig is mainly composed of impactor, drill bit, lifting mechanism, pressure device, walking mechanism, slag discharging system, drilling rig and drill pipe. When choosing a down-the-hole (DTH) drilling rig'you need to consider various factors. This article will guide you on how to choose the right down-the-hole (DTH) drill from three important parts: drill bit, drill pipe, and impactor.
In order to achieve higher drilling speed and lower perforation cost, it is necessary to select a suitable down-the-hole drill bit to drill in a specific rock.
(1) When drilling in hard rock, due to its large rock drilling power, each column tooth and bit body bears a larger load, so the bit body and column tooth are required to have greater strength. When choosing the drill bit, the number of chipway should not be too many, generally choose the double-airfoil drill bit. The size of chipway should not be too large, so as not to reduce the strength of bit body. At the same time, the alloy tooth of the drill bit is recommended to choose the ball tooth, and the height of the ball tooth exposure should not be too large.
(2) When drilling in the soft rock with good drillability, the drilling speed is fast and the relative slag discharge quantity is large, which requires the drill bit to have a strong slag discharge capacity. It is recommended to choose the tri-airfoil or quad-airfoil drill bit. Slagging chute can be appropriately larger and deeper. Alloy teeth can choose spring teeth or chipped teeth, and tooth height can be relatively higher.
(3) When drilling in crushed zones with relatively developed joints, in order to reduce the deflection, it is recommended to use the middle concave or middle protruding drill bit with better steering.
(4) When drilling in the rock with better toughness, it is recommended to use the wedge-shaped tooth bit.
The shape of the drill pipe will affect the drilling efficiency. According to the hydrodynamic theory, only when the air velocity in the annular channel formed by the drill pipe and the hole wall is greater than the suspension velocity of the slag, the slag can be smoothly discharged out of the hole. The airflow velocity in the channel is mainly determined by the channel cross-sectional area, the channel length and the impactor discharge volume.
It can be seen that the larger the drill pipe diameter, the higher the air velocity, and the better the slag discharge effect. Generally the diameter of the channel is between 10-25mm. Lower limit for deep holes and upper limit for high pressure.
The choice of drill pipe should not only consider the slag discharge effect, but also its bending resistance, torsion strength and weight, which are mainly determined by the wall thickness of the drill pipe. Under the premise of ensuring strength and rigidity, the selected drill pipe wall should be as thin as possible to reduce its weight, and the wall thickness is generally 4-7mm.
Geometric parameters of drilling hole, working pressure and rock consolidating coefficient are the original parameters for the design of drill impactor. Thus the appropriate size of the drill impactor can be determined and the ideal impact energy and impact frequency can be obtained. The drill impactor can only play a better rock drilling effect in the specific working pressure, specific process parameters and specific lithology.
The higher the working pressure of the down-the-hole drilling impactor, the faster the rock drilling speed. Since the drill impactor is designed according to a specific pressure, it performs best only within a given design pressure range. If the impactor is not used within the designed pressure range, not only will it not be able to exert its due efficiency, but it will cause the impactor to fail to work or prematurely damage. Therefore, the corresponding drill impactor must be selected according to the pressure grade.
The impact energy of the down-the-hole drilling impactor must ensure the specific energy of the drill bit, so as to effectively break the rock, and at the same time obtain a more economical specific energy and a higher drilling speed. If the impact energy is too large, it will not only cause energy waste, but also shorten the life of the drill bit. If the impact energy is too small, the rock cannot be broken effectively and the drilling speed will be reduced. Different rocks require different specific energies for chipping, so drill impactors with different impact energy need to be selected.
Generally, when the impact energy is constant, the higher the impact frequency, the greater the impact power. However, the outer diameter of the impactor is constrained by the diameter of the borehole. The higher the impact frequency, the smaller its impact energy. Therefore, the increase of impact frequency is limited by the specific work of rock chipping.
The above describes how to choose the right down-the-hole drill equipment from the perspective of the drill bit, drill pipe and impactor. In addition, the specific conditions of the mine, mining technology, blasting parameters, equipment prices and other factors must be considered comprehensively. Or you can directly consult a professional mining equipment manufacturer to choose a down-the-hole drilling rig that is really suitable for your mine.
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