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Lesson 6.4CutChapter 6 · Cutting and non-traditional machining

Waterjet cutting

Very high-pressure water, usually with abrasive, erodes a cut

Waterjet cutting slices material with a hair-thin stream of water pushed through a tiny jewel at enormous pressure. Mixed with fine garnet sand, the jet cuts steel, titanium, stone and glass; pure water alone cuts foam, rubber and even food. Because nothing gets hot, the edge keeps its original properties, which makes waterjet the cutter for materials that heat would damage.

Waterjet cutting: a real photo
Photo: Public domain, via Wikimedia Commons
Water at about 4,000 bar, loaded with garnet sand, erodes a narrow cutCutting head cut open; side section along the cut, not to scale1High-pressure pumpabout 4,000 bar (60,000 psi)high-pressure tubingGarnet abrasivefine, hard sand2Orifice: a jewel with a tiny holewater leaves at up to about 900 m/s3Mixing chamber pulls in garnet4Mixing tube aims the abrasive jetKerf (cut slot): already cutWorkpiece, not yet cutCutting direction6Catcher tank: deep water absorbs the spent jet5WorkpieceMachine partsWaterGarnet abrasive
Abrasive waterjet cutting head over a workpiece, side section along the cutThe figure is a side section along the cut, with the cutting head cut open; the head moves right. Circled numbers match the steps below.

How it works

  1. Pressurize. A pump, usually an intensifier (a piston pump that multiplies pressure), raises water to about 4,000 bar (60,000 psi), some machines even higher.

  2. Make the jet. The water passes through an orifice: a sapphire, ruby or diamond jewel with a hole a few tenths of a millimeter wide. Pressure turns into speed, up to about 900 m/s.

  3. Add abrasive. The fast jet crosses a mixing chamber and sucks in garnet sand from a hopper, the way a spray bottle draws up liquid.

  4. Aim and erode. A long, hard mixing tube (or focusing tube) straightens the mix into a jet about 1 mm wide. Millions of grains strike the work and erode a narrow kerf (the cut slot) through it.

  5. Catch the jet. Below the work, a tank of deep water slows the spent jet and collects the used sand and debris.

  6. Follow the path. The CNC moves the head along the outline. The jet bends back at the bottom of thick cuts, so the machine slows at corners to keep them sharp.

Key facts

Materials
Almost anything: steel, titanium, aluminum, stone, glass, ceramics, composites (abrasive); foam, rubber, textiles, paper, food (pure water)
Typical parts
Flat profiles, gaskets, stone countertops and inlays, glass panels, aerospace brackets
Part size
Tables up to several meters; metal up to roughly 150 mm thick
Precision
Often about ±0.1 mm; thick parts show a slight taper
Surface finish
Matte, sand-blasted look; smooth near the top, faint wavy lines near the bottom; no heat-affected zone
Batch size it suits
One part to medium series
Tooling and machine cost
No tooling; machines from tens to several hundred thousand dollars, plus garnet and pump upkeep
Speed
Slower than laser or plasma; thick metal can take minutes per meter of cut

Strengths and limits

Strengths

  • No heat: no warping, hardening or burned edges
  • Cuts almost any material, including stone, glass and composites
  • Thick parts, up to roughly 150 mm of metal
  • One machine cuts many materials without tool changes

Limits

  • Slower and costlier per meter of cut than thermal methods
  • Garnet, pump seals and orifices are costly consumables
  • Kerf of about 1 mm and a slight taper in thick parts
  • Wet and noisy; parts must be dried and the tank cleaned of sludge

Where it's used

  • AerospaceTitanium and composite parts that heat would damage
  • Stone and glassKitchen countertops, floor inlays, glass panels and mosaics
  • Machine shopsThick plate blanks, prototypes, hard alloy parts
  • AutomotiveInterior carpets, headliners and seals (pure water)
  • FoodPortioning cakes, candy bars and frozen fish (pure water)
  • Gaskets and foamRubber gaskets, foam packaging inserts

How to recognize it

  • A matte, finely sand-blasted cut face with no heat tint at all.
  • Faint curved lines near the bottom of thick cuts, bending backward from the cut direction.
  • A slight taper: the cut is often a touch wider at the top than at the bottom.
  • Intricate shapes in stone or glass, such as floor inlays, that no saw could make.

Compare with

Laser cutting6.2faster and more precise in thin sheet, but it heats the edge and cannot cut stone or thick glass.Plasma cutting6.3much faster and cheaper in thick steel, but it leaves a heat-affected, beveled edge and cuts only metal.Ultrasonic machining6.9also erodes with abrasive grains, but drills shapes in very hard, brittle materials instead of cutting outlines.

Watch it

Fireball Tool · The basics of a waterjet table, for beginners

Waterjet Channel - Daniel Adair · The pump, orifice, mixing tube and catcher tank explained

Secrets of Everyday Things · Why pressure alone is not enough to cut steel

OMAXCorp · A tilting head that corrects the taper, up close

See real photos

Remember

  • Pressure becomes speed at a tiny jewel orifice; garnet sand gives the jet its cutting power.
  • No heat, so it cuts stone, glass, composites and heat-sensitive metals cleanly.
  • Slower and wetter than laser or plasma, with about 1 mm kerf and slight taper.

More in Cutting and non-traditional machining

All 100 processes
6.1Punching and shearingA punch shears shapes out of sheet against a dieOpen the lesson6.2Laser cuttingFocused light melts or vaporizes a narrow cutOpen the lesson6.3Plasma cuttingA superheated gas jet melts through conductive plateOpen the lesson6.5Wire EDMSparks from a thin moving wire cut through-profilesOpen the lesson6.6Sinker EDMSparks copy a shaped electrode into a blind cavityOpen the lesson6.7Electrochemical machining (ECM)Electrolysis dissolves metal in the shape of the toolOpen the lesson6.8Chemical machining and photochemical etchingA light-printed mask lets an etchant dissolve only bare metalOpen the lesson6.9Ultrasonic machiningA vibrating tool hammers abrasive grains into the surfaceOpen the lesson

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