Drilling & Holemaking
Drilling is the machining process that makes a hole in solid material with a rotating end-cutting tool fed along its own axis. It is the most common operation in machining — estimates put the share of cutting tools it consumes at a third to half of all chip-making work — because nearly every part carries a hole, and no other process makes one from solid faster. Holemaking is the wider family that starts with drilling: the rough hole is then finished by reaming, boring, or held round for tapping, each operation the subject of its own wiki entry. This one is about the drilling cut itself — the tool that makes it, the parameters that drive it, and why a simple hole is surprisingly easy to get wrong.
How a drill cuts
The workhorse is the twist drill, and understanding it explains most of what makes drilling distinctive. A twist drill is, in effect, two single-point cutting edges — the cutting lips — arranged on the end of a rotating bar so that together they sweep the full diameter of the hole. As the drill is fed axially each lip peels a chip, and the chips spiral up the helical flutes ground along the body, which exist to carry them out of the hole. Three things follow. First, the cutting speed is not uniform across the lip: it peaks at the outside diameter and falls to zero at the centre, so the very middle of the drill cannot cut in the ordinary way. There the lips meet in a blunt zone called the chisel edge, which does not slice but displaces and extrudes the metal sideways under high pressure, contributing a disproportionate share of the thrust that drives the drill into the work. Second, because drilling cuts on the end, the chips must travel the whole depth of the hole to escape — there is no open side for them to leave by, and that is the central difficulty of the process. Third, the hole is made by the periphery of the drill, so its diameter, roundness and straightness are set by the tool and how truly it runs, not by the machine’s positioning alone.
Speed, feed and thrust
Drilling parameters follow the same logic as every cutting process, with a driller’s twist. Cutting speed is set on the drill’s outside diameter — the only place that truly cuts — and translated to spindle speed; because speed falls toward the centre, the practical limit is how fast the periphery can run without burning. Feed is expressed per revolution, and here is the twist: since both lips cut each revolution, the feed per revolution is shared between two edges, so each lip’s chip is half the feed per revolution. Drills therefore remove metal quickly, but the whole feed force is borne by the end of a long, slender bar that tends to bow and wander unless rigidly guided. Thrust, torque and heat all rise with feed and diameter, which is why small drills spin fast with light feeds, and why hole quality is so sensitive to the feed at entry and exit, where the drill is least supported.
Chip evacuation and depth
Because the chips must climb out of the hole, the depth of a drilled hole is not free. Drills are rated by depth-to-diameter ratio — the hole depth divided by the drill diameter. An ordinary jobber-length twist drill clears chips cleanly only to about three or four diameters deep; beyond that the flutes fill, chips pack and weld, and the drill overheats, breaks or cuts oversize. The classic answer is peck drilling: a drilling cycle that withdraws the drill partway out of the hole at intervals to break and clear chips before continuing. Pecking makes deep holes possible, but it is a compromise — every retraction and re-entry is wasted motion, the repeated re-engagement dulls the drill faster, and each re-entry can start the drill off true enough to open the hole oversize. Production answers exist: parabolic-flute drills carry wider flutes that clear chips to greater depths in softer materials; coolant-fed drills push cutting fluid through the tool to flush chips out along the flutes and extend depth capability considerably; and for the deepest work — many times the diameter — a gundrill with a single cutting edge and a coolant hole through its centre pushes chips ahead of it rather than pulling them up flutes. The principle is always the same: the hole is only as good as the chip’s way out.
Why holes go wrong
Drilling is easy to start and hard to make accurate, and the failure modes are characteristic. A drill begins by plunging its chisel edge into the work, and at that moment nothing guides it: if the point is off the spindle centreline, or the lips are unequal, or the surface is a slope or a casting skin, the drill walks — skids sideways before the lips fully engage — and the hole starts where the drill chose, not where the program said. Once cutting, the drill is a slender column in compression and can bow and wander, drifting off axis in a long hole, and it deflects away from hard spots or an off-centre pre-drilled hole. The symptoms are the drilled hole’s chronic complaints: a bell-mouthed entry, a hole oversize beyond what the drill’s nominal diameter should allow, a hole not straight or not round, and a ragged burr at breakthrough, where the last web of material tears rather than cuts. The defences are procedural: a centre drill or spot drill makes a true, guided start; the drill is kept short, sharp and with equal lips, running true in a good holder; the feed is reduced at entry and exit; and pecking is used only where chips demand it. A hole that must be truly round, straight and to size is therefore rarely the end of the story — drilling is followed by a finishing operation such as reaming or boring, and by tapping when the hole is to take a thread.
Drilling in the job flow
Most drilling on modern shop floors happens inside a milling or turning program rather than on a dedicated machine. A machining centre drills the hole patterns of a prismatic part — drilled, spot-drilled, tapped and reamed holes alike — between its milling moves, using the same spindle and a magazine full of drills, which is why holemaking is such a large share of its work; a lathe drills along its axis from the turret or tailstock, producing the bores that turning alone cannot. Because drilling is a chip-making process, its speeds and feeds come from the same feeds and speeds logic as every cut, its chips obey the rules of chip formation, and its tools are the same carbide and HSS as all cutting tools; runout, too, is felt keenly here, since a drill that runs out cuts a hole oversize. Within the practice of CNC machining, drilling is the process that makes the holes a milling machine or a turning machine fills with threads, pins, bearings and fasteners — the most common operation, and the one whose quiet failures a good machinist never stops watching.