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Meeting Rock Drillers’ Needs: DTH Percussive Hammers

Cary Cooper

The down-the-hole (DTH) percussive directional rock drilling market is a fast growing market, particularly in the Southeastern United States due to the extremely abrasive igneous type rock found in the region. A method to get through the rock was something that was lacking in the early days of directional drilling.

It used to be a common misconception that a percussive hammer was not a proven method for directional rock drilling. Well, those days are gone. There have been thousands of rock bores completed using this method, and a wave of companies are hitting the market every year with a steerable percussive system that can be attached to just about any directional drill.

What makes these systems unique is that the air conversion usually takes place in the field in about 30 minutes. A basic system consists of a carbide button bit, a down-the-hole hammer, bent sub, a special sonde housing designed to absorb vibration and a support station. The support station is set up between the air compressor and the drill and has three functions: 1.) Turn air on/off; 2.) Inject water into a high pressure air stream for sonde cooling and keeping dust down; and 3.) Oil injection system for lubricating the piston in the hammer.

To best understand how DTH evolved, it would be beneficial to have a basic knowledge of the other two methods of drilling rock.

Rotary Drilling

Whether it is a mechanical rotational motor turning the drill bit or fluid gallons per minute, rotary drills drilling vertical and horizontal rely on high rotational speed and high thrust/torque to penetrate the rock. The harder the rock the greater the thrust is required to get the same effect.

Drifter Percussive Drilling

These drills do not rely on high thrust or high torque for the cutting action. They operate from percussive energy that comes from a hammering action from the carriage and is transmitted through the drill string to a button carbide bit that is engaged into the rock. They pass low pressure air through the drill string to remove the cuttings. This method is good for short holes because the deeper they drill the farther the percussive energy must travel to engage the bit. These are the drills that helped build our great interstate system, and we’ve all seen their handy work in the pre split rock walls they leave behind.

DTH Percussive Hammer

Unlike the drifter drill, the DTH hammering action only takes place at the end of the drill string, with little loss of energy. As the high-pressure air passes through the hammer with the button bit pressed against the face of the rock, the piston will hit the back of the bit forcing the bit into the rock. The air then passes through the end of the bit removing the rock cuttings back through the bore hole to atmospheric air pressure. DTH is unique in that the more air pressure the hammer can hold, the faster the penetration rate. Most DTH hammers on the market today will hit anywhere from 1,600 to 2,300 beats per minute (bpm) at 350 psi 900 CFM, and most can steer in rock or dirt. They do not require much thrust or rotational torque for the hammer to work, and you do not have to anchor the drill. You need just enough thrust to keep up with the production rate of the bit, and they only use one to four gallons of water per minute.

We think of the air compressor like a fire hydrant — a continuous supply of water that you never have to go get or clean up.

Cary Cooper is owner of Hardrock HDDP Inc., which is based in Atlanta.

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