How to choose DTH hammers for Rock mining

The choice of hammer size is mainly determined by the hole diameter and type of rock formation. The optimum blast hole range for DTH drilling is 3.5 to 10 inches. Smaller holes are usually drilled with a top hammer instead of DTH hammer, and bigger holes normally use the rotary drilling technique, mainly with the main target on hole straightness.

As a common rule, the smallest hole diameter a DTH hammer can drill is its minimal size. A 4-inch hammer is best designed to drill a 4-inch hole. The quicker the hole diameter is to the hammer's diameter, the more regulated the hole's clearing airflow is. Drilling holes at the nominal size leaves adequate space for cuttings to evacuate the hole up the annulus between the hammer and drill pipe diameters and the internal diameter of the hole's wall.

Maximum bit size for production drilling is the minimal hammer diameter plus 1 inch. For instance, a 4-inch hammer's maximum bit size is held to be 5 inches in diameter. Pipe diameter must be close to the hammer diameter to provide optimum flushing, reducing the chances of getting stuck in the hole.

Pipe (tubes) prepared from cold-drawn tubing provide a greater surface finish and tolerance compared to tubes made from hot-rolled tubing. A better finish decreases the risk from metal chips from the tubes, called scaling. Scaling that flows through the hammer is a major cause of premature hammer failure.

Friction-welded joints add strength. Heat treating the threads of end-pieces ensures finest durability and strength of the thread profile. Preserving the thread profile keeps coupling and uncoupling smooth, without adding time to the average rate of penetration.