Drill Bits
Part of Machine Tools
Understanding drill bit geometry, materials, and sharpening — so you can maintain, resharpen, and improvise cutting tools for drilling metal and wood.
Why This Matters
Drill bits are consumables in any working shop — they dull, break, and wear. In a world without supply chains, the ability to resharpen dull bits, repurpose old ones, and understand what makes a bit cut efficiently is the difference between a functional workshop and one that grinds to a halt when the last drill bit breaks.
More fundamentally, understanding drill geometry lets you drill cleanly in difficult materials: hard steel, cast iron, stone, glass, and hard wood each require different approaches. A bit geometry that cuts mild steel perfectly will rub and burn in hardened steel, wander in soft brass, and chatter in cast iron. Adapting drill geometry to material is a skill that multiplies the effectiveness of every bit in your kit.
This knowledge also enables improvised drill making — hand-forged drill bits from tool steel can cut holes in emergencies where no commercial bit is available. The geometry required is straightforward; the execution requires heat treatment knowledge but no specialized equipment.
Twist Drill Geometry
The standard twist drill has three critical angular parameters:
Point angle: The included angle at the tip between the two cutting lips. Standard: 118° for general purpose on steel, cast iron, and most materials. Harder materials (stainless steel, high alloy steel): 135° — shallower, less aggressive entry. Softer materials (aluminum, plastic, wood): 90-100° — more aggressive, faster penetration.
Lip clearance angle (relief angle): The angle behind the cutting lip that allows the freshly cut metal to clear. Without clearance, the drill rubs rather than cuts. Standard: 12-15° behind the cutting lip. Too little clearance: drill rubs, overheats, won’t penetrate. Too much clearance: cutting lip is thin and weak, chips easily.
Helix angle: The angle of the flutes (spiral grooves) relative to the drill axis. Standard: 25-30°. Higher helix (faster spiral): removes chips quickly, good for soft metals and plastics. Lower helix: stronger, better chip support, used for hard materials.
Web thickness: The core of the drill at the point. Standard bits have increasing web thickness toward the shank (for strength). A thick web requires a chisel edge at the tip (the section of the point between the cutting lips that doesn’t cut — it scrapes, requiring high drilling force). Thinning the web (grinding away the center of the point) reduces thrust force dramatically and improves centering.
Drill Materials and Their Applications
High-speed steel (HSS): Standard for all general-purpose drill bits. Contains tungsten, molybdenum, and vanadium. Retains hardness to about 600°C. The workhorse material; 90% of all twist drills are HSS. Use with cutting fluid (oil or water-based) for extended life in steel.
Cobalt HSS (M35, M42): Contains 5-8% cobalt. More heat-resistant than standard HSS, better for stainless steel, high-nickel alloys, and hard steels. Recognizable by gold color of some variants.
Carbide-tipped: Tungsten carbide tip brazed onto a steel body, or solid carbide. Extremely hard and wear-resistant; drills hardened steel, cast iron, abrasive materials. Brittle — chips easily if drill wanders or is used without rigid setup. Expensive; only for materials that wear out HSS quickly.
Tool steel (salvaged from old files, chisels): Old files are tool steel (W1 or similar) that can be reforged into drill bits if commercial bits are unavailable. Heat treating is essential. Lower performance than modern HSS but functional for drilling wood, soft metal, and moderate-hardness steel.
Sharpening Twist Drills
Resharpening a dull twist drill is one of the most valuable workshop skills. A correctly sharpened drill cuts cleanly and lasts much longer; an incorrectly sharpened one wanders, produces oversize holes, and breaks.
Manual sharpening on a bench grinder:
- Hold the drill at 59° to the wheel face (this produces the 118° included point angle)
- Position the cutting lip horizontal
- Press the cutting lip against the wheel while rotating the drill slightly downward and to the right — this creates the clearance angle behind the cutting lip
- Grind a few light passes; check the lip angle and length with a drill gauge or careful visual inspection
- Repeat for the other cutting lip, maintaining identical geometry
The critical check: Both cutting lips must be the same length and at the same angle. If one lip is longer than the other, the drill cuts an oversize hole and puts asymmetric load on the drill (which can cause it to wander or break). Compare by looking at the point straight-on — both lips should be symmetric.
Web thinning: For drilling hard steel or large-diameter holes, grind the center of the point to thin the web and reduce the chisel edge. This reduces drilling force significantly. Use a small grinding wheel or the corner of the bench wheel.
Checking results: After sharpening, test-drill in the target material. A correctly sharpened drill starts without wandering (centered hole), cuts a clean chip, and doesn’t require excessive force. The chips from steel should be blue-silver spirals; if they’re pale or dusty, the bit is still dull or rubbing.
Improvised Drill Making
When no drill bits are available:
Spade bit from flat steel: Cut a spade shape from 3-4mm mild steel, hardened tool steel (old file), or leaf spring. File the tip to a central point with two cutting lips at 118°. Harden (heat to bright red, quench in oil) and temper (heat to blue, 300°C, quench). Clamp in a vise or drill chuck for use in a hand brace. Works for wood and soft materials.
Punched hole and reaming: For softer materials, a hardened punch can pierce a rough hole which is then opened to size with a round file or a rat-tail file turned in a brace.
D-bit (half-round drill): Turn a steel rod to the desired drill diameter on a lathe. File or grind away one half of the cylindrical end to create a D-shaped cross section. The flat face is the cutting face; harden the cutting edge. This is a simple drill for accurate holes in soft metals — it follows the center of the original hole accurately.
Center punch method for hard stone/tile: Use a sharp hardened steel punch with a pointed tip. Strike repeatedly with a hammer while slightly rotating the punch between strikes. This works for ceramics, brick, and stone where twist drills fail. Carbide-tipped punches (or chert/flint tips) work even better.
Specialty Drill Types
Step drill: Single drill with multiple diameter steps. Drills progressively larger holes in sheet metal in a single operation without pilot holes. Can be improvised by grinding a larger taper on an existing bit.
Pilot drill + countersink: A small lead drill centered in a countersink tool. Drills the pilot hole and countersinks for a flat-head screw in one pass.
Spade bit for wood: Large flat bits for boring large holes in wood. Much easier to make or improvise than a twist bit of equivalent diameter. A simple flat steel blank with a central point and two cutting lips serves adequately for rough carpentry.
Auger bit: Long spiral bit for wood, with a central lead screw that pulls the bit through the material. The lead screw is the difficult part to improvise; otherwise, similar in principle to a twist bit for wood.