Cutting Head Anatomy — 4 Main Components
- ORIFICE (pure water stream generator) — Converts the pump pressure into supersonic velocity. A diamond or sapphire aperture 0.1-0.4 mm in diameter.
- MIXING CHAMBER — The low-pressure zone where abrasive garnet is drawn in through a side inlet. The garnet is entrained by the Venturi effect.
- FOCUSING TUBE (concentrator) — The hardened tungsten carbide tube in which the water and garnet mixture is aligned. Internal diameter 0.76-1.2 mm.
- GUARD / SHROUD — A shield that contains the splash-back generated at the cutting point; optional, but recommended for service life.
The Moment the Waterjet Is Created — Step by Step
- Water at 4,000-6,000 bar reaches the cutting head from the high-pressure pump.
- It passes through a very narrow aperture (0.25-0.35 mm) in the orifice — the pressure drops and the velocity rises to 3-4 times the speed of sound (~1,000 m/s).
- A low pressure forms in the mixing chamber (the Venturi effect); garnet is drawn in through the side inlet.
- The water and garnet mixture enters the focusing tube — it is precisely aligned and compressed, and a coherent stream is formed.
- The stream leaves the focusing tube (~0.9-1.2 mm in diameter) and holds its accuracy over a distance of 8-10 cm.
- The stream erodes the material by micro-erosion; the machined contour is left in the workpiece.
The Orifice (Water Passage) — In Detail
- Material: synthetic diamond (the most common, 100-200 hours of life) or sapphire (40-80 hours, cheaper)
- Standard diameter: 0.25 mm (general), 0.30 mm (high speed), 0.35 mm (thick work)
- Signs of wear: a 15%+ drop in cutting speed, a scattered stream, "water leaking out into the open"
- Replacement interval: diamond 100-200 hours, sapphire 40-80 hours
- Poor water quality (high TDS, particles) shortens the life by up to 50% → water treatment is essential
Diamond or Sapphire?
Diamond is expensive but lasts 2-3 times longer → more economical over the long run. Sapphire suits a start-up operation and low volumes. Plants running several machines move to diamond as standard.
The Focusing Tube
The focusing tube gives the cutting stream its "shape and coherence". A wrongly chosen focusing tube leads to a loss of accuracy, higher garnet consumption and a drop in cut quality.
| Internal Diameter | Length | Use |
|---|---|---|
| 0.76 mm | 76 mm | Fine work, precise detail (material ≤25 mm) |
| 0.89 mm | 102 mm | General use — the most common choice |
| 1.02 mm | 102 mm | Medium to thick work (25-100 mm) |
| 1.17 mm | 127 mm | Thick work, high speed (>100 mm) |
- Material: tungsten carbide (hardened)
- Life: 40-80 operating hours (wears 2-4 times faster than the orifice)
- Signs of wear: the internal diameter widens → the stream scatters and surface quality deteriorates
- A worn tube is tolerable up to 1.3-1.4 mm, after which it must be REPLACED
Garnet (Abrasive) Flow
- Standard flow: 0.3-0.5 kg/min (depending on material and speed)
- Grain size: 80 mesh (fine) → 120 mesh (quality-led), 50 mesh (coarse and fast)
- Conveying: fed by air pressure; moisture causes clumping (use silica gel)
- Hourly consumption: 18-30 kg/hour in normal use
- Too much garnet → excess consumption and a drop in cut quality
- Too little garnet → the cut stalls and the tube wears
Wear Parts — Maintenance Schedule
| Part | Typical Life | Replacement Signal |
|---|---|---|
| Diamond orifice | 100-200 hours | Loss of speed, scattered stream |
| Sapphire orifice | 40-80 hours | Loss of speed, leaks |
| Focusing tube | 40-80 hours | Internal diameter >1.3 mm, deteriorating quality |
| Mixing chamber ring | 300-500 hours | Leaks, pressure loss |
| UHP seal set | Annually | Leaks, pressure fluctuation |
No Intervention While the UHP Line Is Live
Even with the pump switched off, the 4,000-6,000 bar pressure waiting in the line is lethal. Before removing any hose or seal the bleed valve must always be opened, and work may only begin once the gauge reads zero. Details: the USEL Care safety instructions.
Quality and Speed Optimisation
Cut quality is divided into five classes (Q1 = the roughest, Q5 = mirror quality). Speed and quality are inversely proportional:
- Q1 (rough) — the fastest, for rough cutting
- Q2 (general) — standard industrial cutting
- Q3 (good) — precision work, visible surfaces (the most common)
- Q4 (high quality) — assembly-ready parts
- Q5 (mirror) — polished quality, requiring no further work
As quality rises the speed falls by 30-70% — choosing the optimum balance for the project is critical to the economics.