Another type of drill is the air operated "hammer bit" which is very similar in principle to the jackhammer. The hammer is attached to the bottom of the drill pipe and operates within the hole driven by compressed air, delivering between 800 to 1200 blows per minute. It is ideally suited to very hard formations and it is not unusual to achieve an average drilling rate of 5 to 10 metres (8 to 16 ft) per hour when drilling a 152 mm (6") diameter hole in rock.
The compressed air operating the hammer passes through the ports in the bit and cleans the cuttings from the hole as well as cooling the bit. The cutting face of the bit incorporates tungsten carbide inserts to increase the life of the tool.
Modern hammer drills of the 152 mm to 204 mm (6" to 8") diameter bit size normally used for water bores are designed to operate on air pressure of up to 14 Kg/Cm2 (200 PSI) and need an air volume of at least 17M3/minute (600 CFM).
The "down-the-hole-hammer" has revolutionised hard rock drilling and has enabled water supplies to be established from rock aquifers previously regarded as being too hard to drill economically.
A major advantage of the rotary air drilling method whether drag bit, rock bit or hammer bit is that water is blown to the surface as soon as the water bearing strata is encountered. This allows the driller to obtain an indication of the available supply of water as the drilling progresses.
