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Lesson 5.2CutChapter 5 · Machining

Milling

A spinning multi-edge cutter moves over a fixed part

Milling carves a part with a spinning cutter that has several teeth, while the workpiece is moved past it. It makes flat faces, slots, pockets and almost any 3D shape: engine block faces, plastic injection molds and the aluminum bodies of many laptops are milled. It is the most versatile machining process.

Milling: a real photo
Photo: Impressionmanufacturer, CC BY-SA 4.0, via Wikimedia Commons
A spinning cutter with several teeth is steered through a clamped blockLeft: side view of a vertical mill cutting a step. Right: seen from above, the cutter follows a path.Side viewSeen from aboveTableViseWorkpiece (steel block)Spindle headmotor insideEnd mill (cutter)Cutter spinsDepth of cutChipsFeed: the table moves the part under the cutterPocketbeing cutProgrammed toolpathCutter (end mill)The machine steers the cutterin X, Y and Z (depth)12345Workpiece and chipsMachine and cutterMotionRemoved layerToolpath
Milling · side view and toolpath seen from above, 5 stepsCircled numbers match the steps below. The left view shows the cut from the side; the right view shows how the cutter is steered, seen from above.

Milling in a minute, by MecMade

How it works

  1. Clamp the block. A vise (a screw-driven clamp) or bolts hold the workpiece firmly on the machine table, because cutting forces are large.

  2. Spin the cutter. The spindle turns a cutter with several cutting edges (teeth). Speeds run from a few hundred rpm (revolutions per minute) for large cutters in steel to 20,000 or more for small ones in aluminum.

  3. Set the depth. The cutter is lowered to the depth of cut, from a few tenths of a millimeter for finishing to several millimeters for roughing.

  4. Feed the part. The table moves the part under the cutter. Each tooth takes a small bite, typically 0.05–0.2 mm thick, and flings it off as a chip.

  5. Follow a path. A CNC program (computer numerical control: the machine follows coded coordinates) steers the cutter in X, Y and Z. It traces outlines and clears pockets in many passes, roughing first and finishing last, and swaps cutters automatically.

Key facts

Materials
Metals (aluminum, steel, titanium, brass), plastics, wood, composites
Typical parts
Brackets, housings, engine blocks, molds and dies, gearbox cases, prototypes
Part size
From a few millimeters to several meters on large gantry machines (aircraft wing parts)
Precision
Typically ±0.01–0.05 mm; about ±0.005 mm on precise machines
Surface finish
Typically Ra 0.8–3.2 µm (Ra = average roughness; 1 µm is a thousandth of a millimeter), with visible toolpath lines
Batch size it suits
One-offs and prototypes up to thousands of parts
Tooling and machine cost
Cutters cost tens to hundreds of dollars; machines run from a few thousand dollars (manual) to hundreds of thousands or more (5-axis CNC)
Speed
Minutes to hours per part, depending on how much material must come out

Strengths and limits

Strengths

  • Makes almost any shape: flat faces, slots, pockets, 3D surfaces
  • No part-specific tooling: a new design only needs a new program
  • Accurate and repeatable, easy to automate
  • Works on most solid materials

Limits

  • Slower and costlier per part than casting or molding at high volume
  • Can turn most of the starting block into chips
  • Inside corners always keep a radius, because the cutter is round
  • Deep, narrow cavities are hard: long, thin cutters bend and vibrate

Where it's used

  • AerospaceWing ribs and spars, engine casings, brackets machined from solid aluminum or titanium
  • AutomotiveEngine blocks and cylinder heads (flat faces), gearbox housings, prototypes
  • ToolingInjection molds, die-casting dies, stamping dies
  • ElectronicsLaptop and phone bodies, heat sinks, enclosures
  • MedicalKnee and hip implants, surgical instruments

How to recognize it

  • Fine parallel lines or overlapping arcs on flat faces: the paths of the cutter.
  • Rounded inside corners on pockets, matching the cutter's radius.
  • Large circular swirls on big flat faces, and tiny ridges (scallops) on curved 3D surfaces.

Compare with

Turning5.1the part spins instead of the tool, so turning makes round parts faster and cheaper.Drilling5.3a drill only plunges straight in to make holes; a mill also moves sideways.Sinker EDM6.6erodes with sparks instead of cutting, reaching sharp inside corners and hardened steel that milling struggles with.

Watch it

Autodesk Fusion · Milling basics for beginners

Sandvik Coromant · A close-up of a milling cutter in action

DMG MORI · A 5-axis machining center (a CNC mill that can also tilt the part), shown by its maker

Practical Machinist · Real 5-axis cuts carving complex parts

See real photos

Remember

  • In milling the cutter spins and the machine moves the part or the cutter along a programmed path.
  • Each tooth takes a small bite, so a cutter with more teeth can feed faster.
  • Rounded inside corners and fine path lines are the signature of a milled part.

More in Machining

All 100 processes
5.1ShortTurning (lathe)The part spins; a single-point tool cuts itOpen the lesson5.3ShortDrillingA spinning twist drill plunges into the 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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