Auger Drill Bits
Auger drill bits are used with a carpenter’s brace for drilling holes into wood. At the working end of the twist, there are two sharp points called spurs, which score a circle equal in diameter to the hole, and two radial cutting edges that cut shavings within the scored circle. The twist is helical and carries the shavings away from the cutters. The tang is square and tapered and fits in the chuck on the brace. Expansive auger bits have adjustable blades with cutting edges and spurs that can be extended radially to cut large holes. Metal-cutting twist drills can drill holes in wood, but they cannot produce as clean a hole as an auger bit or as large a hole as an expansive auger.
Much larger augers are used to boreholes in the soil for the emplacement of fence posts, telephone poles, and the like. Augers are sometimes used in surface coal mining operations to recover a portion of the seam exposed at the bottom of the last highwall (exposed but unexcavated face). The coal auger may be compared to an auger used to bore a hole in a piece of wood. As the auger rotates, it penetrates deeper into the wood, and the wood cuttings back out of the hole along the spiral. The same is true with the coal auger. As the coal discharges from the spiral, it is collected for transportation to the preparation plant or market. Additional auger lengths are added as the cutting head of the auger penetrates deeper into the coal. The length drilled depends on the pitch and thickness of the coal seam and the physical characteristics of the strata immediately above it. The penetration can be deeper for competent roof strata and more level seams. Webs of intact coal must be left between adjacent holes to support the overburden (overlying rock) and partly to separate individual holes. Coal is also left above and below the hole. Maintaining structural integrity results in rather low coal recovery, typically less than 35 percent of the area under augering.