In Waterjet, Fast Cutting Found at Lower Pressure

In abrasive waterjet machining, does higher pump pressure translate to faster cutting?
Power is proportional to flow times pressure, he notes. As a result, “for a given electrical input power, any increase in pressure must be matched by a decrease in volume flow rate. This means that a higher-pressure pump must use a nozzle with a smaller orifice.” Thus, the 50-hp pump that would use a 0.014-inch nozzle orifice at 60,000 psi is constrained to a 0.010-inch orifice at 90,000 psi,

Water jet cutting machine

In waterjet applications in soft materials where the water alone does the cutting, a smaller-diameter jet stream might be more effective. But in abrasive waterjet applications, it’s the abrasive doing the cutting rather than the water. The water’s role is to accelerate the abrasive particles. A smaller-diameter stream “actually carries less momentum for entraining and accelerating abrasive particles than a lower pressure jet of the same horsepower

Water cutting products

Efficiency might be the better indicator of cutting performance. That is, how well motor power is converted into power at the nozzle. A direct-drive pump delivering 60,000 psi features lower efficiency losses than pumps designed to achieve higher pressure, Dr. Olsen says. This means less of the power of the motor is wasted as heat. He says a 50-hp direct-drive pump can deliver about 45 hp to the nozzle. Higher-pressure pumps potentially impose greater power losses than this. They also impose costs related to the added strain of the higher pressure on the machine’s valves, tubes and fittings. Because of all of these costs, he argues that there is significant value in discovering just how much cutting performance a 60,000-psi waterjet machine can deliver

Why Not Waterjet

Manufacturing is always looking for ways to lower its costs and increase its throughput while maintaining quality. The drivers for these continuous improvements come from customers who are themselves driven by market forces beyond their control. The impact is a general and universal acceleration of the manufacturing process--from design to delivery--across the manufacturing landscape.

In response, shops of all stripes are looking for ways to satisfy these customer-driven demands. For metalcutting shops, the search often takes them beyond the realm of what many consider traditional metalcutting technology.

Water cutting machine

Abrasive waterjet technology is one of the newer cutting systems available in today's market. They are quick, flexible and effective in meeting a wide variety of challenges among manufacturers. Years of development have resulted in improved wearability and control technology, making waterjet cutting systems competitive to other cutting methods.

Materials And Capabilities
The list of materials that a waterjet system will penetrate is significant. To date, applications have been used with foam, GIO phenolic, steel, armor plating, urethane, titanium, Kevlar, aluminum, linen phenolic, brass, neoprene, copper, stainless steel, spectra, fiberglass, corrugated cardboard, acrylic, ceramic tile, wood, rubber, glass, marble and granite.

Waterjet systems are also an attractive alternative to surface preparation. They strike a surface with such impact that coatings are instantly removed without the side effects of noxious dust that other methods may create.

The advantage of a waterjet cutting system is not limited to the wide variety of materials through which it cuts. Its effectiveness comes not only from what it does, but also from what it does not do.

Waterjet systems are an ideal process solution for cutting materials that are heat sensitive. Because it produces no HAZ (heat affected zone), waterjet is a candidate for applications where thermal induced micro-fractures or distortions are unacceptable. Waterjet's relatively cool cutting makes it temper neutral. Its cutting action will neither harden nor anneal an application material.

Environmental concerns related to the cutting of hazardous materials such as asbestos and fiberglass are reduced through the use of a waterjet cutting system. Airborne contaminants and fumes are significantly reduced or eliminated.

The erosion process of abrasive waterjet cutting creates no burring or rough edges. Often the necessity of additional finishing operations is eliminated.

There are no start holes created and parts can be optimally placed to fully utilize a piece of material. It also eliminates the distortion from compression that is created by traditional die cutting methods.

Five-Axis Waterjet Cuts Virtually any Material

Jet Edge’s Idro line of five-axis precision waterjet cutting machines is capable of cutting virtually any material, the company says. The line is available in three sizes: 5.5 × 6.5 ft., 5.5 × 13 ft. and 6.5 × 13 ft. nominal (1,700 × 2,000 mm, 1,700 × 4,000 mm and 2,000 × 4,000 mm). All models feature the IKC five-axis waterjet cutting head, which is capable of making inclined cuts and controlling kerf to ensure optimal part quality. The cutting head features 600-degree rotation, a maximum angle of ±60 degrees and dynamic precision from ±0.2 to ±0.5 mm/ m, depending on the tilt of head.

Five-Axis Waterjet Cuts

The line’s high-precision, ground, rack-and-pinion X and Y axes, as well as its ballscrew-driven, 5.9" (150-mm) Z axis, ensure accuracy, the company says. It maintains a cutting tolerance ±0.004" (0.1 mm/m) and repeatability of ±0.001" (0.025 mm). The machine also supports a contouring and rapid feed rate of 0 to 1,575 ipm (0 to 40,000 mm/min.) Motion components are protected by steel covers with labyrinth lip seals to ensure lasting performance.

Five-Axis Waterjet Cuts

The line is equipped with programmable contact height sensing and one five-axis cutting head. A second five-axis or three-axis cutting head is also available. Other standard features include a stainless steel tank; automatic safety guards with clear windows on front and back; and a dredge conveyor for abrasive removal. Options include a rotating axis for pipe cutting, a fire jet etching system, a drill and twin shuttles. According to the company, free software updates are included for the life of the machine. The machine’s rigid, fixed, upright bridge structure can be moved out of the way for forklift or crane loading.

Waterjet In Action

The advantages that Airbus cites—faster cutting and avoiding damage to the workpiece—are among the main benefits aircraft manufacturers seek through machining composites with waterjet. Other benefits include:

- No dust. Fine dust from machining composites can infiltrate controls and other electronic equipment in the shop. It also makes the shop grimy and unappealing. With waterjet, this dust is contained and controlled. For the most part, the dust is carried away with the water, from which it can later be removed.

Waterjet In Action

- No heat-affected zone. Delamination and fiber pullout are not the only dangers of mechanical cutting. Another is heat, which might melt the matrix of CFRP. But waterjet is inherently a cool process. Heat generation is slight, and the water transports the heat away.

- No rigid clamping. An unsupported edge of a CFRP workpiece is prone to vibrate during milling. For this reason, machining of composite structures on routers or similar machines often involves elaborate tooling designed to carefully and rigidly clamp the work at every trimmed edge. Vacuum fixturing built to the precise contours of the part is common. But with abrasive waterjet, the force of cutting is slight. The force also pushes down against the support beneath the part. Therefore, while programmable workholding is sometimes used (see below), rigid custom tooling is not required.

Water cut in Action

Aircraft Equipment
That “programmable workholding” is one of various features Airbus specified to allow its waterjet machines to be adapted to the particular needs of complex aircraft parts. Work is supported atop a flexible header system consisting of an array of effectors that resembles a bed of nails. Each effector’s vertical position is set independently, so the overall array can follow the contours of the part. Changing from one part number’s positions to the programmed effector positions for a different part takes only about 2 minutes, Mr. Saberton says. This compares very well to the hours that might be required to move one custom fixture off the machine and replace it with another hard fixture. Mr. Morazo-Perez says Airbus initially questioned this approach to workholding—worrying in particular whether the flexible tooling would stand up to the water over time. The company became convinced after visiting shops using similar programmable workholding on waterjet machines in the U.S.

Other machine features particular to aircraft abrasive waterjet machining include:

- Side-fire nozzles. Aircraft skins can include small ribs and stringers that impede access. The machine’s ability to switch to a nozzle that redirects the jet to the side can be useful for cutting within these tight spaces.

- C-catcher. The contours of aircraft structures can double back upon themselves, meaning the jet of water exiting the cut might hit some other surface of the part. To prevent this, a C-shaped catcher (see photo) can intercept the exit side of the jet. A consumable inside of this device absorbs the energy of the jet so the water can be captured and reclaimed.

- Rotary spindle. As Airbus indicated, rotary-tool machining remains useful in many composites applications. In fact, certain features must be machined in this way. While waterjet can machine a hole, for example, it can’t machine a countersink. Therefore, some abrasive waterjet machines incorporate rotary spindles for such needs as this. The rotary spindle is also useful for marking tools.

Waterjet Basics

This non-traditional application of abrasive waterjet may interest some to whom waterjet machining is an unfamiliar process, so here is a brief primer:

Abrasive waterjet machining can be thought of as a water-driven grinding process. A narrow, high-pressure jet of water carries particles of garnet abrasive at speeds ranging from 1,000 to 2,400 feet per second. This garnet does the cutting. And there are few materials, metal or non-metal, that cannot be cut in this way. In metalworking, abrasive waterjet machining often finds work cutting hard steels, aerospace alloys, and other materials that are difficult to machine through a more conventional, mechanical cutting process. However, waterjet machining also offers other advantages. One is that it's a "cool" process. Because there is no heat-affected zone, waterjet is a candidate for applications where thermal-induced micro-fractures or distortions in the part are unacceptable.

Abrasive waterjet machining - Process

This article uses "waterjet" and "abrasive waterjet" interchangeably, but it's worth noting that the terms are not necessarily synonymous in larger contexts. There is such a thing as pure waterjet cutting used outside the metalworking industry. But in the context of machining metals and other hard materials, "waterjet machining" can be assumed to mean abrasive waterjet machining, with the hard garnet particles added to the jet.



Historically, controlling a waterjet's depth of cut has been difficult. That's why waterjet machining is typically used in applications where depth of cut doesn't matter. These include applications where the jet passes all the way though the workpiece, like slotting, drilling, and machining cut-outs. Machining when the jet does not pass all the way through has traditionally been limited to "artistic" purposes, like etching and labeling.

Waterjet milling does add control over depth of cut. Thanks to this addition, waterjet cutting when the jet does not pass all the way through can be used not just for appearance effects, but also to produce features that are functional.