Types of Drills
The drills of machining are the rotating, end-cutting tools that make round holes, and the drilling they do is governed by which drill is chosen for which start. A drill cuts with its end, its two or more lips shearing the metal away as it feeds into the work, and every hole that is not reamed, bored or milled is made by a drill first. But the single word hides a family of distinct tools, each with a different job in the life of a hole. The twist drill is the standard, the tool that actually cuts the hole. The spot drill starts a hole true on a curved or angled surface. The centre drill makes the small, true starting hole that a twist drill or a lathe centre needs. And the step drill opens a stepped or oversized hole in thin material. This entry sets out the main types of drills a machinist uses, how each is built and when each is reached for.
The twist drill
The twist drill is the drill — the helical, pointed cutter that makes the common hole, and the tool that the name “drill” usually means. Its body is a cylinder with flutes spiralling up it; the flutes form the cutting lips at the point and carry the chips away from the cut. At the point, the two lips meet at the point angle — the included angle of the cone — and between their inner ends lies the chisel edge, the short transverse edge at the very centre that does not cut but pushes into the work, which is why a twist drill can wander when it starts on a curved surface and why deep drilling needs care. The point angle is chosen for the material: the general-purpose 118 degrees for most steels and aluminium, the stronger 135 degrees with its thinned web for the harder steels, the flatter points for brass and the plastics. The standard twist drill is made in high speed steel or in solid carbide, two, three or four fluted, its geometry ground to cut the hole cleanly and to clear the chip.
The spot drill
The spot drill is the short, stiff drill whose only job is to start a twist drill true. A twist drill, with its pushing chisel edge, wanders when it meets a curved or sloping surface; it will walk off the point and the hole will be out of position. The spot drill answers by cutting a shallow, accurate spot — a small cone, no deeper than its own diameter — at the exact position of the hole, giving the twist drill a flat, true seat to start from. The spot drill’s point angle is matched to the drill that follows: a 90-degree spot for a 90-degree drill point (in practice the common 118-degree drill is usually spotted with a 90-degree spot drill, whose flat-bottomed effect seats the tip), or the 120- and 142-degree spots for the drills that follow. Because it is short and rigid and cuts only its shallow cone, the spot drill holds position to a fine accuracy and does not deflect — the tool that turns “drill the hole at this point” from hope into certainty, as the drilling entry describes.
The centre drill
The centre drill is the small, two-part drill that makes the starting hole for a lathe centre and for accurate drilling. Its name comes from its work in the lathe and the cylindrical grinder: the small pilot cuts a pilot hole, and the 60-degree countersink behind it cuts the conical seat in which the machine’s live or dead centre sits, holding the rotating work between centres. In milling and drilling the centre drill plays a second role: because its pilot is short and its 60-degree countersink makes a small, stiff, accurate start, it is used to start small holes that must be true and to guide a twist drill on its way. The distinction from a spot drill is real though the tools look alike: the spot drill makes a flat-bottomed cone matched to a following drill’s point, while the centre drill cuts the true 60-degree centre that a lathe centre will sit in. Both are started, brief and shallow, and both are the machinist’s answer to the truth of the hole’s start.
The step drill
The step drill is the specialist of thin material, a single drill ground with several diameters stepping up its point, so that one tool drills and opens a hole through successive sizes. As the step drill feeds in, its first, smallest step cuts the hole, then each wider step opens it in turn, until the final step reaches the size wanted; a single pass can thus make a hole of one size and open its mouth to another, or cut a countersink-like seat in one operation. The step drill is at home in sheet metal and thin stock — the sheet, the plate, the plastic panel — where a long twist drill would snatch or chatter as it broke through, and where the step’s broad, flat shoulders ream the hole and often leave a deburred edge as they finish. It is not a deep-hole tool: its steps cut only as deep as the widest step, so it suits thin work and the opened hole, not the deep bore. For the sheet-metal shop it is the tool that makes the tidy, sized hole in one quick pass.
Choosing among the drills
The choice among the drill types follows the hole that is being made, and the sequence is usually the pattern of the drilling operation itself. For a hole that must be in the right place and true — the ordinary machined hole — the machinist spots it with a spot drill or starts it with a centre drill, then follows with a twist drill of the hole’s size, cutting to depth with the pecking or the chip-clearing the process calls for. For a hole on a curved surface or at an angle, the spot is essential; for a hole that a lathe centre must support, the centre drill cuts the seat; for a hole in sheet, the step drill does in one pass what a twist and a countersink would do in two. Each drill is a cutting tool ground for its moment in the hole’s making, mounted in the spindle of the machining centre or the lathe, run at the speeds and feeds its material calls for — and between them, the round hole is started true, cut clean and made to size.