Why almandine garnet is used almost exclusively
Almandine garnet has become the standard because it satisfies four properties at once: sufficient hardness, high density for momentum transfer, sharp-edged fracture without problematic dust generation, and an unproblematic occupational-health classification. Alternatives such as corundum cut faster but wear the focusing tube considerably more; quartz-bearing media are excluded on occupational-safety grounds.
| Property | Guidance Value | Significance for the Process |
|---|---|---|
| Mohs hardness | 7.5–8.0 | Hard enough for metal, soft enough for acceptable focusing-tube wear |
| Density | 3.9–4.1 g/cm³ | Determines momentum transfer in the mixing chamber |
| Grain shape | sharp-edged, fractured | Angularity affects cutting action more than hardness |
| Moisture content | < 0.5% | Higher values cause bridging and metering faults |
| Price range | €0.25–0.60/kg | Depends on origin, processing, and order volume |
Ranges for commercially available qualities. The supplier's data sheets govern.
Grit size: the mesh rating and its effect
| Grit | Approx. Particle Size | Effect | Typical Use |
|---|---|---|---|
| Mesh 50 | 0.30 mm | High cutting speed, rougher edge, higher focusing-tube wear | Thick parts, separation cuts |
| Mesh 80 | 0.18 mm | Balanced performance, edge quality, and wear | Standard grit for most applications |
| Mesh 120 | 0.125 mm | Finer edge, lower traverse speed | Thin sheet, higher quality requirement |
| Mesh 220 | 0.063 mm | Very fine edge, marked loss of performance | Fine cuts, sensitive materials |
The mesh number denotes the sieve opening, not the particle size directly: a higher number means a finer grit.
When edge quality problems appear, grit size is often changed first. The reverse order is more sensible: check the condition of the water orifice and focusing tube first, then adjust traverse speed and abrasive mass flow, and only then change the grit. A grit change requires new cutting data for the entire part range.
Mass flow as an independent control variable
Abrasive mass flow and cutting performance are not linearly related. Performance rises up to an optimum; beyond it, the excess grit disrupts jet formation in the mixing chamber and performance falls again, while cost and focusing-tube wear continue to rise. The optimum is configuration-dependent and should be determined on the actual part.
per cutting head, configuration-dependent
at 0.30 mm water orifice and mesh 80
at 0.35 kg/min continuous cutting
abrasive including disposal, of consumable cost
Recycling and disposal
The grit used leaves the process fractured and mixed with the removed material fraction. Two paths are open: disposal as waste, or reprocessing in a recycling plant with separation, washing, drying, and classification. Economics depend less on the price of new material than on plant utilisation and local disposal cost.
- Establish a reliable volume estimateConsumption per operating hour times planned annual hours, plus removed material
- Clarify the waste classificationThe removed material determines the classification, not the garnet
- Plan interim storageSilo or container, drainage, accessibility for the disposal contractor
- Compare recycling against new materialInvestment, operating effort, and quality loss versus new-material price plus disposal
- Remove finesRecycled abrasive without fines removal quietly degrades cutting performance
Frequently asked questions about abrasive
Why does cutting performance fall even though more abrasive is metered in?
Above the optimum, the additional grit disrupts acceleration in the mixing chamber. Part of the material is no longer brought up to speed and instead obstructs the jet. The effect is reproducible and can be made visible within minutes using a cutting series with increasing mass flow on the same part.
Is cheaper abrasive economical?
Only if grain shape, moisture, and fines content are right. High fines content increases consumption at equal performance, high moisture disrupts metering, and irregular grain shape lowers cutting performance. The comparison must therefore be made on cost per metre cut, not price per kilogram.
Can abrasive be omitted?
Yes, for soft and fibrous materials. Gaskets, foams, food products, many plastics, and thin composites can be cut with pure water. For metals, stone, glass, and thicker composites, abrasive is required. The distinction is covered under Pure Water or Abrasive.
- Flow International, Fundamentals of Waterjet Technology – https://www.flowwaterjet.com/explore/basics-of-a-waterjet-pump