Waterjet cutting, at its simplest, is the process of a high-pressure jet of water cutting into a material. The technology often compliments other machining techniques such as milling, laser, EDM and plasma. No hazardous material or vapors form during the waterjet process, and neither do heat-affected zones or mechanical stresses. Waterjet can cut whisper-thin details in stone, glass and metals; quickly drill holes in titanium; cut food; and even kill pathogens in beverages and dips.
Pumps and Other Mechanics
All waterjet machines have a pump that pressurizes the water for delivery to the cutting head, where it converts to a supersonic stream. Two major types of pump exist: direct drive-based pumps and intensifier-based pumps.
Direct drive pumps act like a pressure washer, with a triplex pump actuating three plungers directly from the electric motor. The maximum continuous operating pressure is 10 to 25% lower than comparable intensifier pumps, but this still puts them between 20,000 and 50,000 psi.
Intensifier-based pumps make up the majority of ultrahigh-pressure pumps (that is, pumps over 30,000 psi). These pumps contain two fluid circuits, one for the water and the other for hydraulics. The inlet water filters take in ordinary tap water through first a 1-micron cartridge filter, then a 0.45-micron filter. This water goes to the booster pump, where pressure is maintained at around 90 psi, before it travels to the intensifier pump. Here, pressure increases to 60,000 psi. Before the water finally leaves the pump unit to travel through the plumbing to the cutting head, the water passes through the shock attenuator. This device dampens pressure fluctuations to increase consistency and eliminate pulsing, which leaves marks on the workpiece.
In the hydraulic circuit, an electric motor between pulls oil from a reservoir and pressurizes it. The pressurized oil travels to the manifold, where the manifold’s valves create the stroking action of the intensifier by alternating the injection of hydraulic oil between sides of a biscuit and plunger assembly. As the plunger has a smaller face than the biscuit, the oil pressure “intensifies” the pressure on the water.
The intensifier is a reciprocating pump, meaning the biscuit and plunger assembly delivers high-pressure water out of one side of the intensifier while low-pressure water fills the other side. Recirculation also allows the hydraulic oil to cool when it returns to the reservoir. Check valves ensure that the low-pressure and high-pressure water can travel only in one direction. High-pressure cylinders and end caps encasing the plunger and biscuit assembly must meet special requirements to withstand the force and constant pressure cycles of the process. The whole system is designed to fail gradually, with leaks traveling to special “weep holes” that operators can monitor to better schedule periodic maintenance.
Special high-pressure plumbing delivers the water to the cutting head. This plumbing can also provide freedom of movement to the cutting head, depending on the size of the tubing. Stainless steel is the material of choice for these pipes, which come in three common sizes. At 1/4 inch in diameter, steel tubing is flexible enough to plumb motion equipment but not recommended for transporting high-pressure water over long distances. As this kind of tubing is easily bent, even into coils, lengths of 10 to 20 feet can enable X, Y and Z movement. Larger, 3/8-inch tubing 3/8 inch typically delivers water from the pump to the base of the motion equipment. While it can bend, it is not normally for plumbing motion equipment. The largest tubing, measuring 9/16 inch, is most suitable for transporting high-pressure water over long distances, with the larger diameter helping reduce pressure loss. Tubing of this size pairs well with large pumps because larger volumes of high-pressure water also risk a larger potential pressure loss. This size of tubing cannot bend, however, and requires fittings for corners.
Read more: Machining 101: What is Waterjet Cutting?