Drilling Techniques

Part of Machine Tools

Practical methods for accurate, efficient hole-making in wood and metal using hand and machine tools.

Why This Matters

Drilling sounds trivial until you need to drill a 20mm hole 100mm deep in steel and keep it on center within half a millimeter. Wrong technique wastes bits, ruins workpieces, and — in a world without replaceable drill bits from a hardware store — sets back manufacturing capability significantly. Understanding the geometry of cutting, the properties of different work materials, and the feel of a properly feeding drill separates someone who can make precision parts from someone who cannot.

In survival and rebuilding contexts, drilling is required constantly: fastening timber frames, making pump cylinders, fitting axle bushings, machining gear blanks, creating firearm components. Many of these holes must be precisely located, accurately sized, and cleanly cut. Developing good drilling technique is therefore a core industrial skill, not an afterthought.

The techniques here apply to both hand-powered drilling (brace and bit, bow drill) and machine drilling (drill press, lathe with drill chuck), adjusting for the level of technology available.

Bit Selection and Geometry

Drill bit geometry determines what material it cuts well. The key variables are point angle, helix angle (flute twist), and clearance angle behind the cutting edge.

For wood: Auger bits and brad-point bits work best. The brad-point has a central spur that registers on the layout mark before the outer spurs score the circumference, preventing tear-out. The helix is steep (40-45 degrees) to clear chips rapidly. Auger bits have a threaded lead-screw tip that pulls the bit into the wood — excellent for deep holes but requires controlled feeding to avoid splitting.

For soft metals (brass, aluminum, copper): Use a standard twist drill but reduce the helix angle or use a flat-bottom drill to prevent the bit from grabbing. Standard drills in soft metal tend to self-feed — the cutting edges dig in too fast and the bit chatters or grabs. Reduce the rake angle by lightly stoning the cutting face.

For steel: High-carbon or high-speed steel twist drills with a 118 degree point angle for general work, 135 degrees for harder steels. Keep the cutting edges equal in length — an asymmetric grind will make the drill cut oversize and wander.

Layout and Center Punching

All precision drilling starts with accurate layout. Scribe the hole location with a sharp scriber, then center-punch it with a sharp, hardened punch. The punch indent gives the drill tip a seat to start in, preventing wandering during the critical first contact.

For hole locations relative to an edge, use a combination square and scriber. For patterns of holes (bolt circles, flange patterns), lay out with dividers or a compass. Double-check dimensions before punching — a mispunched center is difficult to correct.

When the punch mark is made, hold it to the light and examine it. It should be a clean, conical depression centered exactly on the intersection of your scribe lines. If it is off, use the punch at an angle to push the depression toward the correct position before deepening it with a straight blow.

Drilling in Metal: Step-by-Step

Pilot drilling: For holes larger than 8mm, always drill a pilot hole first — typically half the final diameter. The pilot removes the center of the chip formation area where the drill’s chisel edge (the web) merely pushes metal rather than cutting it. The chisel edge is the hardest point to break through, so removing it with a small pilot dramatically reduces thrust required and improves accuracy.

Cutting speed and feed: Soft materials (aluminum, brass) cut fast — high RPM with moderate feed. Steel requires lower speed and steady, consistent feed pressure. Cast iron requires lower speed still and produces powdery chips. The cutting speed formula: surface speed (meters per minute) equals pi times diameter times RPM divided by 1000. Aim for 20-30 m/min in mild steel.

Cutting fluid: Apply generously. In steel, use oil (cutting oil, motor oil, or even lard works). In aluminum, use kerosene or cutting oil. In brass, dry is acceptable. Fluid lubricates the cutting edge, carries away heat, and flushes chips from the flutes.

Chip clearing: Withdraw the drill from the hole every 5-10mm in deep holes to clear chips. Packed chips cause overheating, drill breakage, and oversized holes. For very deep holes (deeper than 5 times diameter), use a peck drilling rhythm: advance 3-5mm, withdraw fully, re-enter.

Reaming and Finishing Holes

Drilled holes are rarely round or to precise size — they tend to be 0.1-0.3mm oversize and slightly out-of-round due to bit runout and cutting asymmetry. For holes requiring press-fit accuracy (bearing bores, pin fits), ream after drilling.

A reamer is a multi-fluted, lightly tapered cutting tool that removes a small amount of material (0.1-0.2mm) in a single pass, leaving a smooth, accurately sized hole. Reamers must be run at half the RPM of the drill that made the pilot hole and fed steadily without stopping. Stopping a reamer in the cut leaves a witness mark. Apply cutting fluid liberally.

Without a reamer, a hole can be sized by wrapping emery cloth around a smooth rod and working it in the hole, or by carefully filing with a small round file — slow but feasible for non-critical applications.

Common Problems and Fixes

Drill walks off center: Insufficient center punch, no pilot hole, or drill bit ground asymmetrically. Fix by re-punching and using a smaller starting drill.

Drill breaks in the hole: Excessive feed rate, no cutting fluid, chips packed in flutes. Prevention is better than extraction. If a drill breaks, try turning it counter-clockwise with pliers — drills often unscrew from broken-off sections. Otherwise, drill around the broken piece with a larger drill, or use an extractor if hardened tool steel extractors are available.

Oversized hole: Asymmetric grind on the drill. Re-grind the drill ensuring both cutting lips are equal length and at equal angles. Check with a drill point gauge if available.

Rough or torn hole in wood: Bit is dull, or you drilled too fast without a brad-point. Use a sharp bit and back the work with scrap wood to prevent tear-out on exit.