What is Churn drill?

The churn drill is a large drilling machine that bores large diameter holes in the ground. In mining, they were used to drill into the soft carbonate rocks of lead and zinc hosted regions to extract bulk samples of the ore. Churn drills are also called percussion drills as they function by lifting and dropping a heavy chisel-like bit which breaks the rock as it falls. Churn drills are most effective in soft- to medium-density rock of relative shallow depth.