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Down-the-hole around the world

Since the down-the-hole (DTH) method was originally developed to drill large-diameter holes downwards in surface-drilling applications, its name originated from the fact that the percussion mechanism followed the bit down into the hole.
Applications were later found for the DTH method underground, where the direction of drilling is generally upwards instead of downwards.
Major manufacturers began producing DTH drills in the 1950s, with Ingersoll Rand, for example, introducing its version in 1955.
DTH drills are developed in a variety of configurations, powered by compressed air or hydraulics. Hydraulic DTH drills are said to be faster and more accurate in precision-drilling. The drills can be mounted on either crawler tracks or a
rubber-tyred vehicle, depending on the terrain.
Today, DTH drills are developed to drill faster and deeper. As these types of drills are often used in quarry mines, it is within this industry that they are becoming more advanced. It is in mining for gold where most DTH drills are making huge developments as this metal requires more precision and tighter blast patterns.
A DTH drill consists of a drill bit that has three cones with teeth that are made out of tungsten carbide steel, but can also be made of industrial diamonds. The cones rotate, providing the drilling and cutting function.
The drill piston is powered by torque and typically rotates at a speed of 0-50rpm. DTH drills contain a shell that houses the piston. Normally, the drills do not consist of a valve system and operate with the use of ports to control the oscillation of the pistons. The exhaust air also makes its way through the port and flushes out air to clean the bit of the drill and to clear the hole of rock debris.
Bits come in different types: flat-faced, convex, concave, kavex, double-domed, ballistic and dome:
Flat-faced bits are used in heavy-duty drilling applications where the drilling is deep. The face of the bit is flat as it drills.
The convex bit is used in applications where the rock is somewhat worn away and drills with two buttons as opposed to one.
The concave bit is one of the most common. It is practiced on medium to hard-rock applications and drills a straight hole.
The kavex is a combination of both the concave and the convex, with a bit that works best on abrasive materials and drilling straight holes.
Dome bits with spherical faces are the bits mostly utilised for DTH drills. Well-suited for a variety of applications, from medium to hard rock, as well as broken formations, the dome-shaped face is less likely to break than the other types.
The double-domed face is similar to the dome shape, but with an additional dome positioned on top of the other. The additional bit provides extra life to the bit while providing sharper penetration.
Ballistic bits are capable of drilling in more rapid motions, but are used less because they tend to break when used in certain formations that they were not developed for.

Atlas Copco

Atlas Copco Secoroc has launched a series of general-purpose DTH hammers and an oil-free waterwell version. The new Secoroc Fusion series are general-purpose DTH hammers for drill holes down to a depth of 250m, and for those applications where acquisition economics are the priority.
This series is based on a combination of the DHD hammer and technology from the Quantum Leap series. This means Secoroc Fusion features a reversible hammer case for longer service life and a valveless air cycle for high reliability in demanding applications. The 12-spline QL shank, now standard on all new Secoroc hammers, facilitates better bit grip, less spline wear and a better service life for the chuck, according to Secoroc.
The Secoroc Fusion series is available in six models: Fusion 50, Fusion 55, Fusion 60, Fusion 65, Fusion 80 and Fusion 120, covering hole sizes from 130-560 mm.
The environmentally-friendly Aqua 60 is an oil-free waterwell DTH hammer, which uses water instead of hammer oil as a lubricant.
Leif Larsson, product line manager for DTH products, explains: “At Atlas Copco we are very proactive in our efforts to minimise any adverse effects our products may have on the environment. For example, it only takes a few drops of hammer oil to contaminate 1m3 of fresh water. As a standard DTH hammer consumes up to 800 litres of lubricating oil during its service life, the risk of contamination is high. This means that newly drilled waterwells need to be flushed for a considerable period of time before they can be put to work.”


Bulroc UK Ltd is the only British-owned company making DTH drilling equipment in the UK and has been engaged in this business for almost 40 years.During this time, Bulroc has built up an enviable reputation for producing and supplying a quite extensive range of DTH hammers, button bits and other DTH accessories to all corners of the world.
Presently, Bulroc produce a range of DTH hammers and button bits, capable of producing holes from 64mm through to 1,200mm. Bulroc produces the smallest DTH hammer on the market and recently introduced its largest hammer, the Hyper 301S.
All but the smallest hammers have available a range of overburden drilling systems for the continuous installation of casings through unstable ground. A new range of deep-hole hammers is in development and expected to be available later this year, aimed at the water and energy-well markets.
Bulroc has invested heavily in machinery to produce a comprehensive range of equipment for large-diameter drilling with its Hyper 181, 241 and 301S hammer range for hole sizes from 18in through to 43in, with corresponding, concentric overburden drilling systems – the Bulroc CDS ‘R’ for standard applications and CDS ‘RS’ for special drilling projects – and including sleeved, hexagon drive drill pipes to eliminate the need for threads and spanners.
Large-diameter DTH drilling equipment also has an important application in the field of horizontal drilling for installing pipes, and other services under highways and railways, with minimum disturbance and minimal cost compared to other methods.
For smaller-diameter, horizontal drilling, Bulroc produces a steerable range of equipment with a unique location system that eliminates the need for sonds and sophisticated sond housings. Generally, the Bulroc Steermole operates in a dry and clean environment with no need for mud or high-pressure water systems to contend with.
Bulroc was the first company in the world to design and produce a centre-sampling hammer or reverse-circulation hammer for the mining industry, and whilst the original hammers are still available, Bulroc is developing a new range of RC hammers more suitable to today’s operating conditions.


Over the past 12 years Numa has introduced more than 50 new hammer models, two overburden systems and Numa Lube, an environmentally-friendly alternative to rock drill oil. Today, over 45% of its sales are overseas, with Numa hammers and bits at work on land and marine drilling sites in more than 60 countries.
Numa offers one of the largest ranges of hammers and bits available in the drilling industry today, with a capability of drilling holes of 3½-43in (89-1,092mm) in diameter: the Patriot, Champion and Challenger hammer models.
Numa has engineered a new reverse-circulation bit design that improves hole cleaning and overall drilling speed. By deepening the channel ways and side exhausting, the new design is able to efficiently flush more air over the bit face, resulting in a greater recovery and accurate down-hole sample at the surface.
Currently, Numa’s reverse-circulation hammer and bit line has the capability to drill holes of 4¾-36in (121-914 mm) in diameter, with more designs in the works. Numa’s reverse-circulation hammers utilise an orifice ‘O’ ring as a choke. Removal of the orifice ‘O’ ring allows additional air to pass through the hammer for hole cleaning/flushing purposes.
The Super Jaws under-reaming bit has a design with wings that extend out wider than the casing while in the drilling position. The extended wings provide a large bit face striking area for transferring blow energy to the formation, yielding faster penetration rates. When drilling is complete, the bit is pulled off the bottom, and the wings extract back into the bit to allow the hammer and bit to be pulled back up through the casing to the surface – no parts are left down the hole. It is available for drilling holes of 150-1,245mm in diameter.
Numa’s pneumatic lubricators are positive displacement, air-operated, lubrication systems. They are designed to inject rock drill oil into the air line under positive pressure to ensure the proper amount of oil is supplied to the DTH hammer while drilling. Numa offers lubricators with a 15 gallon (57 litre) reservoir for 12in hammers and a 45 gallon (170 litre) reservoir for 18in hammers and larger. Both models offer a positive displacement pump that is capable of pumping up to 16 quarts/h (15.2 litres/h) at 1,500psi (103 bar). The required supply air pressure is a maximum of 85psi (6 bar), while the supply to output pressure ratio is 17.6:1. The pumps are adjustable for stroke length and number of strokes per minute.
Numa Lube is an environmentally-safe alternative to rock-drilling oil that has proven to be a major advancement in the quest for long-lasting hammer protection, according to the manufacturer. Numa says one 8oz bottle is required in a 6in (152mm) hammer every 300ft (91m) or six hours of drilling, while three or four gallons of rock drill oil is required for the same depth. Also, Numa Lube does not need to be continuously fed like rock-drill oil. The contents of one bottle are poured down the drill string and water is continuously injected while drilling.


Rockmore International, a global manufacturer of rock-drilling tools, has developed a new carbide insert design for its button bits called the MultiPoint.
In percussion drilling, penetration rates depend on efficient crack propagation. With a conventional, hemispherical insert, Rockmore says there is a single tangent point striking the rock, resulting in the rapid development of wear flats. Rockmore’s new carbide insert is designed to provide multiple strike points for more efficient rock fracture and longer insert life.
According to Rockmore, rock-drilling trials showed that button bits with MultiPoint inserts last up to 20% longer than the standard, hemispherical design and penetration rates increased by 10%. Wear flats appeared slower and later in the insert life than with hemispherical inserts. Testing also revealed that the MultiPoint inserts provide smoother bit rotation and advancement.
The MultiPoint design is currently available in ½-¾in (12-19mm)-diameter carbide inserts, and in Rockmore’s DTH and top-hammer bits of 3.5-10in (89-254mm)-diameter.
Rockmore International’s roots began in 1948, with the founding of ThrowAway Bit Corporation, aptly named for its flagship single-use drill-bit products. Back then these bits included tapered, cross, button and down-the-hole bits in all international thread sizes and diameters. The company’s product line was augmented to include the production of extension rods, couplings and shank adapters. These changes prompted the firm to realise it had outgrown the ThrowAway name because the rock-drilling tools they now manufactured
were no longer meant for single use. In 1996, ThrowAway became Rockmore International.


Longer life, higher penetration rates and cleaner holes are among the benefits of a new generation of Sandvik GT60 button bits called RT300, according to the manufacturer.
Thanks to a new, super grade of cemented carbide called XT48 and a new universal face design, Sandvik says its buttons are stronger and more wear resistant than ever, while energy distribution and cuttings evacuation are enhanced. Superseding current versions, the new RT300 bits drill holes of up to 33m deep with top-hammer rigs.
Sandvik says the RT300/GT60 bits drill holes of
92-152mm in diameter using only about half the fuel consumed by the DTH method. The new button bits have the suffix S48 (for spherical buttons) or R48 (for ballistic buttons) to indicate that they contain grade XT48 (extra tough) cemented carbide.
Spherical buttons are best in hard rocks while ballistic buttons drill faster in less- to medium-hard rocks. If ballistic buttons wear too fast, changing to spherical buttons will increase service life. With regard to skirt design, a regular one is usually more economical in solid and homogenous rocks. In variable and fractured-rock formations, however, from which it can sometimes be difficult to remove the drill string, a Retrac skirt is the answer.


A new water-powered DTH hammer, the W120, in combination with system-based thinking, is the latest offering from Swedish company Wassara.
The company’s water-powered down-the-hole technology has been attracting interest, and Wassara has now added the W120 5in hammer to its range to drill hole diameters of 130-152mm; commonly used in construction and geothermal applications.
One advantage of the Wassara W120 is its ability to drill straight, clean holes. This is made possible by the high operating frequency of the hammer and low up-hole velocity of the return water, allowing closer tolerances between the bit and hammer. Also with low velocity returns, much less wear on the hammer casing ensues, allowing guide ribs to be fitted on the standard hammer.
“When it comes to drilling operations in sensitive environments, such as city centres, dams and difficult ground conditions with backfill, the system is hard to beat,” says Johan Jeansson, Wassara’s marketing manager.
“That’s why the technology is often used for drilling grouting holes and deep, geothermal wells.”
Mr Jeansson adds: “By offering the entire system, we can guarantee that users get the right equipment for the right hammer every single time. The system includes a high-pressure pump, hammers, bits and drill tubes connected to an existing drill rig. There is also a sedimentation plant, which processes returns before discharge, so they do not contain any residues that might pollute the environment.”
Water-powered, down-the-hole drilling technology is claimed to be better for the environment than conventional pneumatic drilling. Energy consumption is lower, the working environment is dust-free and there is no oil mist.
Low vibration levels and the minimal effect on the surrounding formation are further advantages.

Courtsey GDI
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