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Unformatted text preview: u/g mines. Soft, medium-hard
rocks in tunnels, u/g
& surface mines. Soft & medium-hard
rocks in tunnels, u/g
and surface mines. Figure 4.3 Rock drills classification based on application of mechanical energy to rock. is discharged against the bottom of the hole and the surface of the rock is crushed into
drill cuttings. These cuttings are in turn transported up the hole by means of flushing
air that is supplied through the flushing hole in the drill string. As the drill is rotated
the whole bottom area is worked upon. The rock drill and drill string are arranged on
feeding device. The feed force keeps the drill constantly in contact with the rock surface in order to utilize the impact power to the maximum.
In good drilling conditions use of these drills, is an obvious choice due to low energy
consumption and investments on drill-strings.1 In surface mines and civil construction
sites 76–127 mm (3 –5 ) hole diameters is the usual range.1 4.5.2 DOWN-THE-HOLE (DTH) DRILLING
In this system the down-the-hole hammer and its impact mechanism operate down the
hole. The piston strikes directly on the bit, and no energy is lost through joints in
the drill string (fig. 4.2(a)). The drill tubes (rods, steels) convey compressed air to the
impact mechanism and transmit rotation torque and feed force. The exhaust air blows
the holes and cleans it and carries the cuttings up the hole. The drills, which are known
by the various trade names such as ‘down-the-hole drill’, ‘in-the-hole-drill’ have been,
referred here as DTH drills.
DTH drills differ from the conventional drills by virtue of placement of the drill in
the drill string. The DTH drill follows immediately behind the bit into the hole, rather
than remaining on the feed as with the ordinary drifters and jackhammers. Thus, no
energy is dissipated through the steel or couplings, and the penetration rate is nearly
constant, regardless the depth of the hole. Since the drill must operate on compressed
air and tolerates only small amounts of water, cuttings are flushed either by air with
water-mist injection, or by standard mine air with a dust collector.
This is very simple method for the operators for deep and straight hole drilling.1 In
surface mines 85–165 mm (3.4 –6.5 ) hole diameters is the usual range.1 © 2005 by Taylor & Francis Group, LLC DRILLING 65 4.5.3 ROTARY DRILLING
Rotary crushing is a drilling method, which was originally used for drilling oil wells,
but it is now days also employed for the blast hole drilling in large open pits and hard
species of rocks. It is used for a rock having the compressive strength upto 5000 bar
(72,500 psi). In rotary drilling energy is transmitted via drill rod, which rotates at the
same time as the drill bit is forced down by high feed force (fig. 4.2(a)). All rotary
drilling requires high feed pressure and slow rotation. The relationship between these
two parameters varies with the type of rock. In soft formations low pressure and
higher rotation rate and vice versa, are the logics usually followed. In general, if the
rock hardness is less than 4.0 on Moh’s scale, the rotary drilling has established its
advantages, except when the rock is abrasive. The rotary drills can be operated using
either compressed air or electrical power.
When drilling is done by rotary crushing method, the energy is transmitted to the
drill via a pipe which is rotated, and presses the bit against the rock (fig. 4.2(b)). The
cemented carbide buttons press the rock and break off the chips, in principle in
the same manner, as percussive drilling.
When drilling is done by rotary cutting method the energy is transmitted to the insert
via a drill tube, which is rotated and presses the inserts against the rock. The edge of
insert then generates a pressur...
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