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Lesson 5.3CutChapter 5 · Machining

Drilling

A spinning twist drill plunges into the part

Drilling makes round holes: a spinning tool with a pointed tip, the twist drill, is pushed straight into the material. The holes for every bolt in a car, the thousands of tiny holes in a circuit board and the hole you make in a wall at home are all drilled. It is the most common machining operation of all.

Drilling: a real photo
Photo: Hardyplants, CC0, via Wikimedia Commons
Two cutting edges at the tip bore the hole while spiral flutes carry chips outLeft: cross-section of a twist drill mid-cut. Right: the three common kinds of finished hole.Mid-cut, in cross-sectionFinished holesTableWorkpiece (steel plate)ClampSpindleChuckTwist drillwith spiral flutesCutting lipstwo edges at the tipDrill spinsFeed: pushed downChips climbthe flutesThroughBlindTappedDrill backs out12345Workpiece and chipsMachine and drillMotion
Drilling · cross-section mid-cut and finished hole types, 5 stepsCircled numbers match the steps below. The left view cuts the drill and plate in half; the right view shows the three kinds of hole you will meet.

Drilling in a minute, by MecMade

How it works

  1. Mark and clamp. The part is clamped so it cannot spin with the drill. A short, stiff spot drill (or a center punch, by hand) first marks the exact spot so the drill does not wander.

  2. Spin the drill. The spindle turns the drill, typically a few hundred rpm (revolutions per minute) for large drills in steel and several thousand for small ones.

  3. Feed it in. The drill is pushed along its own axis. The two cutting lips (the edges of the cone-shaped tip) shave the bottom of the hole, typically 0.05–0.3 mm per revolution.

  4. Clear the chips. Chips ride up the flutes (the spiral grooves) and spill out at the top. Coolant cools the tip, and deep holes are drilled in short pecks (in-and-out moves) to clear chips.

  5. Break through and back out. A through hole ends when the tip exits the far side, often leaving a burr (a rough lip) to remove. A blind hole stops at depth with a cone-shaped bottom; tapping can then cut threads inside.

Key facts

Materials
Almost everything: metals, plastics, wood, composites, circuit boards
Typical parts
Holes for bolts, pins, rivets, fluid passages and electrical vias
Part size
Holes from about 0.1 mm (circuit boards) to over 50 mm; deep-hole drills reach depths of 100 diameters or more
Precision
Typically ±0.05–0.1 mm on diameter; reaming (a light finishing cut) brings it to about ±0.01 mm
Surface finish
Typically Ra 1.6–6.3 µm (Ra = average roughness; 1 µm is a thousandth of a millimeter) inside the hole
Batch size it suits
Any: one hole by hand to millions on automated lines
Tooling and machine cost
Drills cost a few dollars to a few hundred; a drill press costs hundreds of dollars, CNC machines tens of thousands and more
Speed
Seconds per hole; a circuit-board drill makes several holes per second

Strengths and limits

Strengths

  • Fast, cheap and simple
  • Standard drills in every size, no custom tooling
  • Works on almost any material
  • Easy to automate, with many holes per minute

Limits

  • Only round holes, and only straight ones
  • Holes tend to be slightly oversize and not perfectly straight
  • Deep holes are hard: chips jam and the drill can wander or break
  • Leaves burrs, and precise holes need reaming or boring afterward

Where it's used

  • AutomotiveBolt holes in engine blocks and wheels, oil passages
  • AerospaceHundreds of thousands of rivet holes in an airliner's skin
  • ElectronicsHoles and vias in printed circuit boards
  • ConstructionHoles for anchors and bolts in steel beams and concrete
  • EnergyLong cooling and fluid passages in molds, turbine parts and heat exchangers

How to recognize it

  • Round holes with fine circular marks on the inside wall.
  • A cone-shaped bottom in holes that do not go all the way through (the drill's point angle, often about 118°).
  • Small burrs or a chamfer (a beveled edge) where the drill entered or left.

Compare with

Milling5.2a mill can move sideways to make slots and pockets; a drill only goes straight in.Turning5.1on a lathe the part spins and a fixed drill makes holes on the axis; boring then enlarges them precisely.Thread rolling and tapping3.6tapping cuts threads inside a drilled hole so a bolt can screw in.

Watch it

Casual DIY · Drill press basics for beginners

Sandvik Coromant · Industrial drilling on a CNC machine, in close-up

MTDCNC · How very deep holes are drilled on a machining center

Newbury Electronics · A circuit board factory tour, including CNC drilling of tiny holes

See real photos

Remember

  • A drill spins and plunges straight in; its two tip edges do the cutting.
  • The spiral flutes are chip chutes, which is why deep holes need pecking and coolant.
  • Drilled holes are fast but rough; reaming, boring or tapping finish them.

More in Machining

All 100 processes
5.1ShortTurning (lathe)The part spins; a single-point tool cuts itOpen the lesson5.2ShortMillingA spinning multi-edge cutter moves over a fixed partOpen the lesson5.4BroachingA long toothed bar is pulled through or across in one strokeOpen the lesson5.5Gear cutting (hobbing, shaping)Hobs or shaper cutters generate gear teethOpen the lesson5.6KnurlingHardened wheels press a grip pattern into a turning partOpen the lesson5.7GrindingA fast wheel of hard grains shaves the surfaceOpen the lesson5.8HoningStones turn and stroke inside a boreOpen the lesson5.9Superfinishing and polishingFine stones or buffs remove the last microscopic peaksOpen the lesson

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