Gun barrels, curling stones and shaking bits: the hidden craft of drilling holes
A gun drill can bore a hole more than 300 times deeper than it is wide and stay dead straight. The trick, first worked out for rifle barrels, is a bit that centres itself, riding on bearing pads that press against the walls of the very hole it is cutting while it spins.
Ordinary drilling spins a bit, usually with several cutting points, at hundreds or thousands of revolutions per minute, shaving off chips called swarf that escape up the spiral flutes. The shape of those chips hints at how easily a metal machines: long ribbons suggest it cuts well. Rock is different. There the bit mostly hammers in short rapid blows, driven either from outside the hole or by a mechanism down inside it, and rigs that bore sideways are known as drifter drills.
Keeping a hole where it belongs takes care. Bits tend to wander off line, especially long thin ones, so machinists drill at right angles to the surface, start from a small guide hole, or clamp the bit's path with a jig. Anything deeper than ten times its diameter counts as deep-hole drilling and needs special tooling; vibration there is a major fault that can snap the tool. On computer-controlled machines, peck drilling plunges partway, pulls back to clear chips, and repeats. Brass and other soft metals can grab a bit and make it chatter, so a tiny flat is ground on the cutting edge to push the metal instead of biting it.
Some methods sound unlikely. Trepanning cuts out a solid disc like a compass tracing a circle, making holes up to 915 millimetres across in plate, beams or even the granite of curling stones. Microdrilling handles holes under half a millimetre, where spindles run past 10,000 revolutions per minute. Vibration drilling, first studied in the 1950s by V.N. Poduraev at Moscow's Bauman University, shakes the bit along its axis so chips break into bits small enough to flush away; piezoelectric versions reach 2 kilohertz.
Orbital drilling spins the cutter on its own axis while also circling it around the hole's centre. Changing the offset lets one tool make many diameters, and the lower push leaves metal holes free of burrs and stops layered composites from peeling apart.
Source: Drilling
