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.
Pure Waterjet Machines
Pure waterjet machines are the original waterjet cutters, with a history dating back to the early 1970s. They create less moisture on material than touching or breathing on it, making them suitable for production of such products as automotive interiors and disposable diapers. The streams are very thin — 0.004 inch to 0.010 inch in diameter — and provide extremely detailed geometries with very little material loss. Cutting forces are extremely low, and fixturing is often simple. These machines are best for 24-hour operation.
When considering cutting heads for pure waterjet machines, it is important to remember that stream velocity is what tears away microscopic pieces or grains of material, not pressure. To achieve this high velocity, pressurized water passes through a tiny hole in a jewel — typically a sapphire, ruby or diamond — affixed to the end of the nozzle. Typical cutting uses an orifice diameter of 0.004 inch to 0.010 inch, while special applications such as waterblasting concrete can uses sizes up to 0.10 inch. At 40,000 psi, the stream from this orifice travels at about Mach 2, while the velocity exceeds Mach 3 at 60,000 psi.
Different jewels have different specialties in waterjet cutting. Sapphires are the most common, general purpose material. They last for approximately 50 to 100 cutting hours, although abrasive waterjet applications halve those times. Rubies are unsuitable for pure waterjet cutting, but the stream they produce is excellent for abrasive cutting. During abrasive cutting, rubies last about 50 to 100 cutting hours. Diamonds are far more expensive than sapphires and rubies, but last between 800 and 2,000 cutting hours. This makes diamonds particularly well-suited to 24-hour operation. Diamond orifices can also be ultrasonically cleaned and reused in some circumstances.
Abrasive Waterjet Machines
In an abrasive waterjet machine, the mechanism of material removal is not the stream itself. Rather, the stream accelerates abrasive particles to erode the material. Thousands of times more powerful than a pure waterjet cutter, these machines can cut hard materials such as metals, stone, composites and ceramics.
Abrasive streams are larger than their pure waterjet cousins, measuring between 0.020 inch and 0.050 inch in diameter. They can cut stacks and materials up to 10 inches thick, while still creating no heat-affected zones or mechanical stresses. In spite of their increased strength, cutting forces for abrasive streams still measure less than one pound. Nearly all abrasive jet jobs use one jet setup, and can easily switch from single to multi-head use or even abrasive waterjet to pure waterjet.
The abrasives are hard, specially screened and sized sands — typically garnet. Different mesh sizes are useful for different jobs. Smooth surfaces can be achieved with 120 Mesh abrasives, while 80 Mesh abrasives prove more useful for general purpose applications. Abrasives with 50 Mesh cut faster, but leave a slightly rougher surface.
Although waterjets are simpler to operate than many other machines, the mixing tube requires operator attention. This tube is like a rifle barrel in its acceleration potential, and is available in different sizes with different replacement lives. Long-lasting mixing tubes were a revolutionary innovation for abrasive waterjet cutting, but the tubes are still brittle — if the cutting head comes into contact with clamps, weights or the target material, the tube will likely brake. Broken tubes cannot be repaired, so keeping costs low requires minimizing replacements. Modern machines often feature automatic collision detection to prevent crashes with the mixing tube.
Standoff distance between the mixing tube and the target material is typically 0.010 inch to 0.200 inch, but operators must remember that standoff greater than 0.080 inch can cause frosting to appear atop the cut edge of the part. Cutting underwater and other techniques can reduce or eliminate this frosting.
Originally, mixing tubes were manufactured from tungsten carbide, and only achieved between four and six cutting hours of life. Today’s low-cost composite tubes achieve 35 to 60 cutting hours of life, and are recommended for rough cutting or training new operators. Composite carbide tubes extend this life to between 80 and 90 cutting hours. Premium composite carbide tubes have 100 to 150 cutting hours of life, are suitable for precision and everyday work, and exhibit the most predictable and concentric wear.
In addition to providing motion, waterjet machine tools must include a method of holding both the workpiece and a system for catching and collecting water and debris from the machining operation.
Stationary and one-dimension machines are the simplest waterjets. Stationary waterjets typically see use in aerospace to trim composites. The operator feeds the material through the stream like a bandsaw, while a catcher collects the stream and debris. Most stationary waterjets are pure waterjets, but not all. Slitters are a variation of stationary machine where products such as paper feed through the machine, with the waterjet slitting the product into specific widths. Cross-cutters are machines that move along one axis. They often work in conjunction with slitters to make a grid-like pattern on such products as vending machine brownie cakes. The slitter cuts the product to a specific width, while the cross-cutter cuts across a product fed beneath it.
Operators should not use abrasive waterjets of this type manually. It is difficult to move the object being cut at a specific and consistent speed, and extremely dangerous. Many manufacturers will not even quote machines for these setups.
XY tables, also called flatstock machines, are the most common two-dimensional waterjet cutters. Pure waterjets cut gaskets, plastics, rubber and foam,…
Read More:Machining 101: What is Waterjet Cutting?