Spot Drilling & Centre Drilling

Spot drilling and centre drilling are the short, shallow operations that start a hole true, the first cuts in the making of an accurate hole. A twist drill can make a hole, but it cannot always start one where it should: the drill’s point pushes into the work with its blunt chisel edge, and on a curved surface, a sloping face or a surface that is not square to the drill it will wander — sliding off the point before it bites, so the hole starts off its true position. Spot drilling and centre drilling answer the wander by cutting a small, accurate start first. The spot drill cuts a shallow, flat-bottomed cone that seats the twist drill’s point exactly on location. The centre drill cuts the small 60-degree cone that seats a lathe centre, and that also starts a small drill truly. Both are quick, both are shallow, and both are the difference between a hole that is where it should be and a hole that is not. This entry sets out how each is done.

The start a drill needs

The reason for these operations lies in the way a drill begins to cut, and the drilling entry sets out the process around it. A twist drill cuts with the two lips of its point, but at the very centre of the point lies the chisel edge, the short transverse edge that does not cut but pushes the metal aside; as the drill begins, this chisel edge bears on the work before the lips bite, and if the surface it bears on is not square to the drill, the chisel edge skids and the drill walks off its mark. On the flat, square end of a facing cut the drill usually starts well enough, but on a round bar, a curved or angled surface, or a surface with a hard skin, the drill needs a flat, true seat to start from. That seat is what spot and centre drilling provide: a small, pre-cut cone whose centre is exactly where the hole must be, into which the drill’s point drops and from which its lips start cutting at once, on location and on axis.

The spot drill and its spot

Spot drilling is the operation that locates a hole, and it is done with the short, rigid spot drill that this wiki’s tooling entry describes. The spot drill is fed in at the hole’s position and cuts a shallow spot — a small cone, no deeper than is needed to seat the following drill’s point. The spot’s angle is chosen to suit the drill that follows, and a common practice is to spot at an angle close to the drill’s point — the 90-degree spot under the general-purpose 118-degree drill is the everyday choice — so that the drill’s lips start cutting together and it begins on centre. The spot drill is short and stiff, so it cuts its spot without deflecting, and it holds position to a fine accuracy — which is why the hole’s location is set by the spot, and the drill that follows is trusted to keep it. On the machining centre the spot is cut in the same setup as the drill, the program stopping at the spot before the drill takes over.

The centre drill and its centre

Centre drilling is the operation that makes a centre, and it is done with the two-part centre drill — the small drill with a pilot followed by a 60-degree countersink. Its first and classic use is on the lathe and the cylindrical grinder: the centre drill cuts the 60-degree centre in the end of the work, the conical seat in which the machine’s live or dead centre sits, so that the work can be turned or ground between centres — held at both ends on the true centres that the centre drill cut. Its second use is in general drilling: because the centre drill’s short pilot and its 60-degree countersink make a small, stiff, accurate start, it is used to start small holes that must be true, cutting a start that a larger drill then follows. The centre drill differs from the spot drill in what it makes — the true 60-degree centre for a lathe centre, rather than the flat-bottomed spot for a drill point — and in its delicacy: the centre drill is small and its pilot is easily broken if it is fed too hard or run too fast, so it is used gently, at a modest speed and a light feed.

Spotting and centring in practice

The two operations are used where the accuracy of the hole’s start matters, and the practice of each is a matter of depth and care. The spotting depth is the first thing to get right: the spot should be deep enough to seat the drill’s point fully, so that its lips start together, but no deeper than that — a spot drilled too deep can interfere with the hole’s mouth or leave a countersink where none is wanted. The centring depth is set by the use: for a lathe centre, the 60-degree cone must be deep and clean enough to seat the centre firmly; for a drill start, the pilot hole is all that is needed. Both operations are run with the feeds and speeds of a small, rigid cutter — a moderate speed and a light feed, the operator or the program watching the depth rather than hurrying the cut — and both are usually followed at once by the drill they started for. The two operations are quick, but they are the operations that decide where the hole is.

When a start is needed

Spotting and centring are not used for every hole, and knowing when they are needed is part of the machinist’s judgement. A hole that is not critical in position, drilled in a flat, square face, may be drilled directly — the drill’s own point starting cleanly on the flat surface. But a hole whose position matters, drilled into a curved or angled surface, a hole started where the drill would otherwise skid, or a small hole whose drill would bend before it bites, is spotted or centred first; and a part that must be held between centres has its ends centre-drilled before anything else. The rule the shop works to is simple: when the drill cannot be trusted to start where it should, give it a start that it can trust — the shallow, accurate cone of the spot or the centre, cut by the short, stiff tool that puts the hole exactly where the drawing puts it.