How Waterjet Works
Abrasive waterjet machines cut metals and other materials using a high velocity stream of water mixed with garnet abrasive. The garnet does the cutting -- the water is strictly a medium to accelerate the garnet. With the water pressurized to 50,000 psi, the pure water jet exits the diamond focusing orifice at about Mach 2. A siphon action draws in a metered stream of garnet abrasive, which exits the cutting nozzle as a .03" (.75mm) diameter stream, and precisely abrades whatever it passes over, as if a high-powered round jeweler's saw. For common materials, the feed rate varies from perhaps 3 to 100 ipm. Lower pressure is used to cut delicate materials. Garnet is an aggressive abrasive; high pressure water is a perpetual engineering lesson in fatigue failure. All components of the high pressure system are consumables - it's amazing the process has become so refined and predictable.
Holes of any size are cut by starting at one point, then proceeding in a closed loop around the inside edge of the hole until returning to the starting point, leaving a loose scrap two kerf-widths smaller than the hole. The software offsets the cut position to cut to the precise part edge, locating the kerf in the waste. This is analogous to the tool offset for an end mill, and is adjusted to compensate as the abrasive mix tube wears.
Square holes and any other shapes are cut as easy as round - waterjet enables a new way of designing for fabrication, with mortise-tenon joints, holes and parts any shape you can draw, plug welds, poka-yoke - opportunity is often wasted when using waterjet to cut parts to the same geometry as used for plasma or shear cutting.
Square holes have an actual inside corner radius of about .015", which must be factored into design clearances for waterjet parts intended to fit together.
Waterjet, for present purposes, is a "2D" process - cuts are perpendicular to the surface of flat sheet and plate materials, and extend all the way through the material. The material remains the same thickness it started as. Etching, i.e. cutting to a controlled shallow depth, is possible in some materials. Etching is useful for visual marking, and in some cases bending - waterjet etching is not "3D" machining.
Waterjet's advantage for metal is the "cold" cut - a precise cut without the heat affected zone (HAZ) encountered with laser, plasma or flame cutting. With exceptions, waterjet also cuts plastics, glass, stone, tile, wood and other materials, in many cases efficiently cutting materials where no other practical tool exists.