§1Anatomy of a twist drill
A twist drill is a cylinder with two helical flutes cut into it, leaving two cutting lips at the point and two narrow lands running up the body. It cuts at the tip and clears chips up the flutes.
Each part earns its keep. The two lips at the point do the cutting, meeting at the centre in the chisel edge. The two helical flutes both form the cutting edges and act as channels to carry chips up and out of the deepening hole — a drill that cannot clear its chips packs and seizes. The thin margins on the lands guide the drill and burnish the hole wall. And the web, the solid core between the flutes, gives the drill its stiffness, thickening toward the shank for strength. Reading these features explains both what the drill does well and its two weaknesses — the non-cutting chisel edge (§3) and the tendency to wander at entry (§6).
Contents§2Point angle and helix
Two angles define a drill’s cutting action: the included point angle at the tip, and the helix angle of the flutes.
The point angle is the total included angle of the two lips — 118° is the general-purpose standard, suited to steel and most metals, while a blunter 135° point is used for hard and tough materials, spreading the load and starting better on hard surfaces. A sharper point suits soft materials such as plastics and aluminium. The helix angle of the flutes sets how aggressively the drill pulls into the work and how well it clears chips: a standard helix suits general steel, a fast (steep) helix clears the stringy chips of soft metals, and a slow helix gives a stronger edge for hard materials and brass. The two angles are ground to match the material, and a correctly pointed drill is the difference between clean holes and a struggling, overheating tool.
Contents§3The chisel edge and thrust
At the very centre of the drill, the two lips meet in the chisel edge — and here the drill does not cut at all. It extrudes, and that is the source of most of the drilling thrust.
Cutting speed falls to zero at the axis, so the chisel edge cannot shear metal; instead it squeezes the material aside under pressure, which takes a large axial force. The chisel edge, though a small part of the drill, accounts for a big share of the thrust needed to push the drill in — and it is the reason a drill wanders when started, since the blunt centre skates before the lips engage. Two standard cures follow: web thinning, grinding the chisel edge shorter to reduce the extruding zone and the thrust; and spot drilling or centre punching first, giving the chisel edge a cone to sit in so the drill starts on location. Understanding the chisel edge explains both why drilling is thrust-limited and why holes need starting.
Contents§4Speed and feed
Drilling uses the same cutting speed and spindle-speed relationship as the rest of machining, with the feed reckoned per revolution as the drill advances.
A 10 mm high-speed-steel drill in steel at a cutting speed of 25 m/min turns at N = 1000 × 25/(π × 10) = 796 rev/min. At a feed of 0.15 mm/rev it sinks at 0.15 × 796 = 119 mm/min. Note the speed is the peripheral speed at the drill’s outer corner; at the centre it is zero, which is why the chisel edge cannot cut. Feed must scale with drill size — a big drill takes a heavier feed per revolution than a small one — and too high a speed burns the corners where the cutting speed is greatest, the first place a drill fails.
§5Counterbores, countersinks and spotfaces
A drilled hole is often only the start; three related tools enlarge its mouth to seat a fastener neatly.
| Tool | Makes | For |
|---|---|---|
| Counterbore | a flat-bottomed cylindrical recess | a socket-head cap screw to sit flush or below |
| Countersink | a conical recess (often 82° or 90°) | a flat-head screw to sit flush |
| Spotface | a shallow flat around the hole | a flat seat for a bolt head or nut/washer |
| A counterbore is piloted — a spigot on its tip runs in the drilled hole to keep the recess concentric — and sized to the fastener: an M10 socket-head cap screw, for example, seats in a counterbore of about 17–18 mm diameter. The three finish a hole so the fastener seats square, flush and concentric, which is why they follow the drill on most fixing holes. | ||
§6Drilling accurately
A twist drill is quick but wanders; accurate holes come from starting it right and helping it clear.
Because the chisel edge skates before the lips bite, a drill left to start on its own drifts off the mark and cuts oversize and out of round. The remedies are routine: spot drill or centre punch first so the point starts on location; keep the drill sharp and correctly pointed, since an unevenly ground point cuts oversize and pulls to one side; peck drill deep holes — withdrawing periodically to clear chips — so the flutes do not pack and seize; and use cutting fluid to cool the corners and flush chips. Where the hole must be precisely sized or located, the drill is treated as a roughing tool only, and a bore or a reamer (its own page) gives the final size — the drill makes the hole, the finishing tool makes it right.
Contents§7Quick reference
The working core of the page on one card rack.
Parts
2 lips · chisel edge
flutes clear chips · web stiffens
Point angle
118° general · 135° hard
Chisel edge
extrudes, not cuts
→ thrust & wander
Speed/feed
N = 1000 V/(π D)
feed scales with size
Finishing
counterbore · countersink · spotface