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Air Rotary

There are actually two types of air drilling: direct rotary air and air hammer. In both, air is used

as the drilling fluid, as opposed to a water-based fluid such as in mud rotary drilling. Both arerestricted to use in consolidated or semi-consolidated formations.

In North Carolina, air drilling is used almost exclusively for drilling in the saprolite and igneousand metamorphic rock of the Piedmont and Mountains. In many cases, hollow-stem augers arefirst used to drill through the saprolite. With the augers in place to stabilize the borehole, an airdrilling rig will drill a socket into competent bedrock. The casing will be set and the augers willbe pulled as the annular space around the casing is grouted. After the grout sets up, the air rigwill return and the open hole portion of the well will be drilled through the casing.

In direct rotary air drilling, air is pumped down the drill pipe and escapes into the borehole

through holes in the bit. As the bit turns, it grinds the cuttings into small pieces, which thecompressed air carries to the top of the borehole. Direct rotary air drilling is normally used forsofter formation materials, such as limestone, sandstone, and some of the softer metamorphicrocks. It is not as commonly used in North Carolina as air hammer drilling.

In air hammer drilling, compressed air is pumped through the drill pipe to an air hammer bit inthe borehole. The pneumatic bit strikes the rock very rapidly. During drilling the pipe string isrotated by the drilling rig to aid in keeping the borehole straight. The compressed air that escapesat the bottom of the air hammer carries the pulverized cuttings to the surface. Air hammerdrilling is very rapid in even the hardest materials. It is the most commonly used air drillingmethod in North Carolina due to its higher efficiency while drilling in our hard metamorphic and

igneous rocks.

Air drilling offers many advantages over other drilling methods. While the initial set-up costs foran air rig are fairly expensive, this method is much quicker than other drilling methods fordrilling in the hard rock of North Carolina. Additionally, since it does not use water-baseddrilling fluids, there is less chance of cuttings or drilling fluids clogging the aquifer materialsthan with mud rotary drilling. Because of its higher drilling efficiency, air drilling is often muchless expensive than other methods of drilling.

The hammer drill is similar to a standard electric drill, with the exception that it is provided

with a hammer action for drillingmasonry. The hammer action may be engaged or disengaged asrequired. Most electric hammer drills are rated (input power) at between 600 and 1100 watts.The efficiency is usually 50-60% i.e. 1000 watts of input is converted into 500-600 watts ofoutput (rotation of the drill and hammering action).

The hammer action is provided by two cam plates that make the chuck rapidly pulse forward andbackward as the drill spins on its axis. This pulsing (hammering) action is measured in BlowsPer Minute (BPM) with 10,000 or more BPMs being common. Because the combined mass of

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the chuck and bit is comparable to that of the body of the drill, the energy transfer is inefficientand can sometimes make it difficult for larger bits to penetrate harder materials such as pouredconcrete. The operator experiences considerable vibration, and the cams are generally made fromhardened steel to avoid them wearing out quickly. In practice, drills are restricted to standardmasonry bits up to 13 mm (1/2 inch) in diameter. A typical application for a hammer drill is

installing electrical boxes, conduit straps or shelves in concrete.

In contrast to the cam-type hammer drill, a rotary/pneumatic hammer drill accelerates only thebit. This is accomplished through a piston design, rather than a spinning cam. Rotary hammershave much less vibration and penetrate most building materials. They can also be used as "drillonly" or as "hammer only" which extends their usefulness for tasks such as chipping brick orconcrete. Hole drilling progress is greatly superior to cam-type hammer drills, and these drills aregenerally used for holes of 19 mm (3/4 inch) or greater in size. A typical application for a rotaryhammer drill is boring large holes for lag bolts in foundations, or installing large lead anchors inconcrete for handrails or benches.

A standard hammer drill accepts 6 mm (1/4 inch) and 13 mm (1/2 inch) drill bits, while a rotaryhammer usesSDSor Spline Shank bits. These heavy bits are adept at pulverising the masonryand drill into this hard material with relative ease.

However, there is a big difference in cost. In the UK a cam hammer typically costs 12 or more,while a rotary/pneumatic costs 35 or more. In the US a typical hammer drill costs between $70and $120, and a rotary hammer between $150 and $500 (depending on bit size)

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