The Thread Types of Rock Drill Bits

How Rock Drills Work

• Rock drills operate differently from regular drills. Masonry drills bits, which are not designed to go as deep as rock bits, slowly pull material out of a hole with the spinning action of its threads. A rock drill, however, has a combination of fluid or air regularly coming out the front or sides of the bit. This fluid pushes freshly pulverized material through slots in the edge of the drill bit, slowly flooding the rock hole and enabling the bit to go deeper. Rock drills come in various sizes and diameters, from 1 inch wide handheld units to motor-driven drills that create holes multiple feet wide.

Down-the-Hole Hammer Thread

• Though not a type of "thread," down-the-hole (DTH) hammer threads are flat and best-suited for boring deep holes in rocks. The bit must be fitted to a hammer drilling unit fitted to pump air or liquid out the front of the bit. The bit itself is flat and circular with a few holes for the pushing mixture, and texture to help with pulverizing the rock while it drills. These kinds of bits are generally used for tasks requiring deep holes drilled into solid rock.

Chisel Thread

• More drill-like than the DTH, chisel threads have a single horizontal taper at the head and varying chisel-like designs at the sides. Manufacturers usually design these side elements to work with specific conditions, but in general, these chisel threads are used for hard rock drilling tasks no deeper than 15 feet.

Cross-Thread

• The cross-thread is so named because, when viewed head-on, the pattern forms a cross. These patterns vary in design, but generally taper seven to 12 degrees, similar to a regular chisel thread. These threads have less of a design along the sides, and are typically used to finish up holes that a DTH or chisel thread creates. Cross-threaded rock bits are particularly useful in fissural, or sedimentary, rock.