A rotation drill rig, air compressor and the "DTH drill string" are the essential unit for DTH drilling while the "DTH drill string" is always made up of DTH drill rods, DTH hammer and DTH bits. As one of key component, the DTH hammer acts as a role of converting the air pressure to the impact energy, simultaneously transferring impact energy and rotation force to DTH bits. The optimum range of hole size for blast hole drilling with DTH is 90 mm to 254 mm (3 -10). Smaller blast holes are generally drilled using tophammer, and larger holes generally use rotary machines. In other applications, like foundation drilling, DTH hammers can be used with single bit in hole sizes up to . 750 mm (30). With multiple hammer units CD (Cluster drills) drill holes up to 70 or 1778 mm. As a rule of thumb, the smallest hole diameter a DTH hammer can drill is its nominal size. A 4 inch hammer will drill a 4 inch (102 mm) hole. The limiting factor is the outside diameter of the hammer, because, as hole diameter reduces, airflow is restricted. Maximum hole size for production drilling is the nominal hammer size plus 1 inch, so for a 4 inch hammer the maximum hole size is 5 inch (127-130 mm). Choosing the right hammer is largely determined by hole size and type of rock formation. Ideally, the size of the hammer should match the required hole dimension as closely as possible, leaving just enough space for cuttings to evacuate the hole.