Other types of drill bits and use

Other methods of rotary drilling differ from the diamond drilling method essentially in the type of drilling bit used. The drill bits used are as follows:

The saw-toothed crown. The drilling tool is a saw-toothed steel crown or cutter. The teeth are set alternately inward and outward to give the necessary clearance. The speed of rotation is only 5 to 10 r.p.m. and the drill bits are suitable only for drilling through rocks of medium hardness; only holes of diameter not less than 150 mm are possible.

Rock roller bits. These are suitable for hole diameter between 75 mm and 300 mm. In mining areas these are commonly used for drilling large diameter holes (125 to 225 mm) to a depth of 6 to 18 m for blast holes in mechanised quarries. Flushing of the hole with compressed air instead of by water under pressure is the common practice with this type of bit. Rock roller bits can be used for deep hole drilling with speed and are suitable for mostly vertical downward holes. (Fig. 3.5,b).

Chilled steel shots: These shots are prepared by heating very finely divided steel particles to a very high temperature and then suddenly cooling them in ice cold water. Chilled shots are used in conjunction with a plain steel shell or cylinder with a diagonal slot near the bottom. They are fed through the hollow drill rods and pass to the bottom of the hole where they get caught between the bottom end of the cutting shell and the rock. As the shell and the drill rods rotate, the chilled steel shots cut the rock by a milling action. The method is suitable for vertical and large diameter holes of 100 mm to 750 mm. It is also called Calyx drilling, but is not much favoured these days as diamond drill bits have gained wide popularity and are available in large diameters upto 250 mm.

Calyx drilling proved to be a significant step during the development of drilling techniques.

Except the rock roller bits, all the other drilling tools used for rotary drilling provide pores of the strata passed through.

Underground drilling

To drill a hole from underground workings for purposes of prospecting, stowing, tapping water or gas or any, other object, the drill equipment has necessarily to be of smaller dimensions. This restricts the size and weight of the machine and therefore puts limitations on the size of the bore hole and its length (or depth). The power available underground may be electricity at 440/550 volts, or compressed air (in most of the metalliferous mines.) Flame proof diesel engines are rarely employed as power sources. At some underground working places the water supply for drilling may be on a very limited scale and not as plentiful as on the surface.

Underground drills for exploration have often to be shifted to blind ends of roadways with narrow dimensions. They are, therefore, usually with skid plates. Components of aluminium alloy are nowadays increasingly used for such drills to reduce weight.

Wafer Development Society, Hyderabad, manufactures drills of the following sizes. The drills are available with hydraulic, semi-hydraulic and pneumatic operating systems with an option of diesel or electric prime-mover. Underground drills can be provided with DTH and Drifter attachment. Single-tube core barrel

RANGE OF DRILLS

Model	Hole dia.	Depth, m	Application	mounting
WRC- 650	150- 170 mm .	30- 60	Open cast mines	Crawler
WDC- 150	150-165 mm	30- 60	Open cast mines	Crawler
WDC- 100	100- 115 mm	30- 60	Open cast mines	Crawler
SUPERTRACK 15	100- 115 mm	30- 60	Open cast mines	Crawler
WDM -400	150- 165mm	30- 60	Open cast/ underground	Skid
WDM -100	100- 115mm	30- 60	Open cast/ underground	Tyre
WDM-400 M	[100-115mm]	30- 60
DTH& Driffer	[  50 -100 mm]	30- 45	Underground	Skid
WDM- 100U	100- 115 mm	30- 60	Underground	Trolley
WDM- 100UF	50- 102 mm	30- 45	Underground	Skid
WDM- 110U	100- 115 mm	30- 60	Underground	Skid
WDM- 100FM	100- 115mm	30- 60	Underground	Skid