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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 that is in asphalt on the interstates and the state roads was 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. They needed to get them out fast. 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 be drilling with one. There are three basic categories all drills fall into: rotary, percussive drifter, and DTH (Down-the-hole) percussive hammer.

Rotary Drilling
There are three categories of directional drilling. The first of these three is Rotary Drilling. All Directional drills including Ditch Witch ATs, Vermeer's RS 6 and all mud motors would be classified under rotary drilling, whether it is a rotational motor turning the drill bit or a fluid. Rotary drills (both Vertical and Horizontal) rely on high rotational speeds and thrust to penatrate the rock. The harder the rock the greater the thrust required to get the same effect and the smaller the bit the better chance of premature bearing failure. Rotary drilling is cost effective in soft to medium non-abrasive type rock but is less cost effective in medium to hard, abrasive type rock. Rotary drilling is very rarely used in the Stone Crusher and Blast Hole industry today.
Drifter Percussive Drilling
The second catergory of drills is called Drifter Percussive Drilling. These drills do not rely on high thrust or torque for cutting action. They operate from percussive energy that comes from a hammering action from the carraige. The vibration is then trasmitted through the drill string to the button carbide bit that tears the rock apart. 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 perrcussive energy must travel to engage the bit. They are mostly used in the mining industry where vibration from blasting material has to be kept to a minimum. These are drills that helped build our great interstate systems cutting through the mountians (and we've all seen their handy work in the pre-split rock walls that they leave behind). Drifter Percussive Drilling is cost effective but limited by depth and the size hole they can drill.
The third category of drilling is Down-the-hole (DTH) Percussive Drilling. Unlike the Drifter Drill (where the hammering action is intiated at the end of the drill string) The DTH hammering action begins at the end of the bit which results in low lose of energy. This is unique in that the more air pressure they hold the faster they will cut. It does not require high rotational torque or high thrust like the Rotary Drill to penetrate the rock.  As the high pressure air passes through the hammer with the Button Bit engaged on the face of the rock, the piston hits the back of the bit forcing the bit into the rock at approximately 2000 strikes per minute. The air then passes through the end of the bit removing the rock cuttings through the bore hole and decompressing the air. This is called Up-hole Velocity and is measured in feet per minute (fpm). Down-the-hole Percussive Hammers are cost effective for drilling hard rock. They are used for water well drilling, blast hole drilling, surface mining, as well as gas exploration. This type of drill was introduced to the Directional Rock Drilling industry about eight years ago and is fast becoming the preferred rock-drilling method in the southeast United States becasue of the very prevalent granite and gneiss rock in this region. The system can be adapted to most current directional drilling machines. It can drill at speeds never seen until now. It eliminates most of the mud mixing and clean up which is perfect for the 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 and you do not even need to run the mud pump on the drill or anchor it.