chain drill

A chain drill is a rotating tool used to bore holes in rock and soil, particularly in geotechnical and mining applications. Instead of a solid bit, it carries a loop of chain fitted with hardened steel teeth or cutters. As the chain rotates at high speed, the teeth strike and scrape the rock face, breaking it away incrementally. The resulting debris is flushed out by drilling fluid or air circulated through the borehole, and the loosened cuttings are hoisted to surface.

The chain itself is typically case-hardened alloy steel, and the cutting teeth are replaceable inserts pressed or bolted into the chain links. Tooth patterns vary by rock type: softer formations use single-point teeth spaced farther apart, while harder rock demands closer spacing and tougher, smaller inserts. Chain diameter ranges from about 40 mm to over 200 mm depending on the drill rig's power and the borehole diameter needed.

Variants and operation

Chain drills come in several configurations. The open-chain drill has teeth projecting radially outward and works best in soft to medium rock. The closed-chain or core-barrel type has teeth arranged around the inner wall and cuts an annular groove, leaving a core in the center that can be recovered for geological logging. Some designs use replaceable chain segments rather than continuous loops, allowing worn sections to be swapped without removing the entire string.

The tool is rotated by the drill pipe itself, which transmits torque from the surface drilling rig. Rotation speed typically ranges from 30 to 80 rpm for large boreholes, climbing to 200+ rpm for smaller holes in softer rock. Weight on the chain is controlled by the driller and usually kept between 1 and 10 tonnes, depending on rock strength and bit condition. If the chain jams, the torque limiter in the drive head will slip or shear to prevent damage to the rig.

Chain drills compete with core barrels and percussion tools but hold distinct advantages in stable rock where core recovery is unnecessary and in formations where percussion produces excessive vibration. Disadvantages include slower penetration rates in very hard rock, higher cost per tooth replacement compared to some tricone bits, and the need for controlled flushing to prevent tooth loading and binding. Tooth wear is the primary consumable cost, with replacement intervals typically measured in tens of metres of drilling depth.