The selection of hammer size is largely determined by the hole diameter and type of rock formation. The optimum blasthole range for DTH drilling is 3½ to 10 inches. Smaller holes are typically drilled with a top hammer rather than DTH hammer, and larger holes generally use the rotary drilling technique, mainly with the focus on hole straightness.
As a general rule, the smallest hole diameter a DTH hammer can drill is its nominal size. A 4-inch hammer is optimally designed to drill a 4-inch hole. The closer the hole diameter is to the hammer’s diameter, the more restricted the hole’s evacuating 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 nominal hammer diameter plus 1 inch. For instance, a 4-inch hammer’s maximum bit size is regarded to be 5 inches in diameter. Pipe diameter should be close to the hammer diameter to provide optimum flushing, reducing the chances of getting stuck in the hole.
Pipe (tubes) made from cold-drawn tubing provide a superior surface finish and tolerance compared to tubes made from hot-rolled tubing. A better finish reduces 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 optimum 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.
The bodies of quality bits are precision machined from alloy steel, heat treated to a specified hardness, providing surface compression for fatigue resistance, and then fitted with precision carbide buttons.
Convex-faced, ballistic-button designs are preferable for fastest cuttings removal. This bit design cuts clean with minimal re-crushing, making it ideal for soft to medium, non-abrasive formations.
In hard and abrasive formations, flat-front designs offer best bit life, especially those that feature strong gauge rows with large spherical buttons that are easy to regrind and maintain. An alternative in these conditions is a concave design with spherical buttons.
Concave bit faces work well in medium to hard, fractured formations. In this type of application, a concave bit minimizes hole deviation.
A fundamental question is whether one needs a reliable, low-cost hammer or a premium hammer. Buying more than one less-expensive hammer to achieve the same drill feet of a single premium hammer is false economy.
Manufacturers design DTH hammers to address the challenges presented by the full array of rock types and applications. For deeper hole applications, hammers are designed to work with different air requirements as bailing velocity requirements change.
Quarrying projects are long-term jobs that will easily outlast the hammer. Frequently replacing hammers would be cost-prohibitive. Dimensional stone quarrying demands consistently straight holes. DTH hammers offer more precision to hole straightness than top hammers.
Mineral exploration generally requires robust hammers capable of running high pressures, often in dirty, remote environments. Operators tend to prefer simple, reliable designs in this application.
Reverse circulation with specialized DTH hammers is less expensive than diamond coring technique. The RC hammers use the same engineered technology and components as standard DTH hammers, but use RC tubes with inner and outer walls for direct airflow to allow the cuttings to pass up through the pipe to a cyclone for collection bags, rather than the typical hole cleaning up the hole’s annulus.
Like quarries, open pit mines typically have high equipment utilization, drilling most of the working day.
Some premium hammers for these applications can now be rebuilt twice before replacement.
Properly training drillers to recognize and maintain optimum drilling parameters prevents premature failure of hammers.
Hammer value involves more than component material and design. Additional factors must be taken into consideration. The market offers a range of quality in both economy and premium hammer selections. The hammer must not only be durable enough to be reliable throughout its anticipated lifespan, but its cost of operation figures in. The greater the hammer efficiency, the better the savings in fuel and energy, resulting in lower cost per drilled foot.
Additionally, drillers should consider the manufacturer’s product support. A consultation from the manufacturer provides advice on DTH requirements. A knowledgeable product support specialist will also help determine which hammer provides the highest profitability for a project.