One of the most common misconceptions about a percussive hammer is that it is not a proven method for rock drilling. Most people in directional rock drilling do not realize that the rock is in the asphalt, on the interstate system, and the state roads were drilled with a percussive hammer not a tricone. Percussive hammers have been around for decades in the vertical industry.

Remember the unfortunate mining accident in Pennsylvania when the miners got trapped in the flooded mine shaft? The rescue shaft was drilled with a percussive hammer.

Call your local water well drillers and the crusher stone quarries and ask what method they use to drill rock. If they say a hammer, then you should probably be drilling with one. There are three basic categories that all drills fall into: rotary, percussive drifter, and the DTH—down the hole—percussive hammer.

First Category
All Directional drills including: Ditch Witch™ ATs, Vermeer's™ RS 6, and all mud motors are classified under rotary drilling. Vertical and horizontal rotary drills rely on high rotational speeds and high thrusts to penetrate the rock. The harder the rock the greater the thrust is required to get the same effect. The smaller the bit the better chance of premature failed bearings. Rotary drilling can be cost-effective in soft to medium nonabrasive type rock but not on the high side of medium to the hard abrasive type rock. Rotary drilling is very rarely used in the stone crusher blast hole industry today.

Second Category
The second category of drills is called 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 then transmitted through the drill string to a button carbide bit that in turn is engaged into the rock. They almost always pass low-pressure air through the drill string to evacuate the cuttings. This method of drilling rock is good for short holes 100 feet or less because the deeper the hole the farther the percussive energy must travel to engage the bit.

They are used in the mining industry, mostly where vibration from blasting material has to be kept at a minimum, due to the smaller holes that they drill. These are the drills that helped build our great interstate systems by cutting through the mountains, and we've all seen their handy work in the pre-split rock walls they leave behind. Drifter percussive drilling is very cost-effective, but limited by the depth and the size hole they can drill.

Third Category
The third category of drilling is DTH-down the hole—percussive hammer. Unlike the drifter drill, the hammering action passes through the drill string. The DTH hammering action takes place at the end of the button carbide bit only, with little loss of energy. They are unique in that the more air pressure they can hold the faster they will cut. It does not require high rotational torque or much thrust like the rotary bit to penetrate the rock. As the high pressure air passes through the hammer with the button bit engaged into the face of the rock, the piston will hit the back of the bit forcing the bit into the rock at 2000 blows per minute. The air then passes through the end of the bit removing the rock cuttings through the borehole back to atmospheric air pressure. We call this up hole velocity and measure in feet per minute.

Down the hole percussive hammers are very cost-effective for drilling hard rock. They are used for water well drilling, blast hole drilling, surface mining, exploration for gas, and was introduced to the directional rock drilling industry about three years ago. It is fast becoming the preferred method in the Southeast United States for directional rock drilling due to the extremely hard abrasive granite and gneiss rock in this region. The system can be adapted to most directional machines. It can drill at speeds never seen until now while eliminating most of the mud mixing and clean up which is perfect for environmentally sensitive jobs. The hammering action that takes place at the end of the drill string only requires around 600 pounds of thrust. The drill is usually at low idle, you do not even run the mud pump on the drill or have to anchor it.

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