مهندس محی الدین اله دادی
Take ordinary tap water and pressurize it to 60,000 psi (4,000 bar) and force it through a very small hole. Mix the water with garnet abrasive and you have a very thin stream of water traveling very fast that will rapidly erode most materials.
Some waterjets are "pure waterjets" and don"t add the garnet abrasive. These are used to cut softer materials, such as food, rubber, and foam.
What can waterjets cut? What can"t they cut?
Waterjets can cut just about any material that can be made into a sheet and placed in front of them.
The most popular materials are metals (especially aluminum, because it"s relatively soft and cuts quickly), because waterjets can cut intricate shapes to a high precision quickly and economically. Since metals are the most common material cut by machining shops, waterjets tend to cut a lot of metal.
Waterjets also commonly cut stone and glass, because the waterjet can get intricate shapes not possible using traditional machining methods. These materials are popular with artists who like to work with these materials and waterjets because it lets them create almost anything they can envision.
Among the very few materials that waterjets cannot cut are diamonds and tempered glass. Diamonds are too hard to cut (and there may be a few other very hard materials that can"t be cut). Tempered glass will shatter when it is cut with a waterjet (tempered glass is designed to shatter when it"s disturbed and is frequently used in windshields for this very reason).
A few advanced ceramics are so hard that it"s not economical to cut them. Some composite materials (layers of different materials sandwiched together) can"t be cut because the water can seep between the layers and "delaminate" the material. Many composite materials cut just fine, though, and there are some techniques to cutting laminated materials.
What do they cost?
Waterjets typically come as complete systems, including the high-pressure water pump, a system to precisely position the waterjet nozzle, a tank to catch the waste water, and an abrasive feed system. Prices run from $50,000 to 300,000, with $150,000 being about average for a mid-range waterjet system.
Prices can run considerably higher than this for custom systems or very large waterjet cutting systems.
Waterjet systems are not currently something for the home workshop. You"ll find them in use in machining shops and industrial workshops. Among other factors, you need industrial levels of electricity to power the pumps (which can pull as much as 50 amps; some pumps require 250 amps to get started).
For the hobbyist interest in waterjets, the more economical approach is to work with a job shop to make the parts. Most job shops can accept computer drawings you create to make exactly the part you want.
Waterjets are fast, flexible, reasonably precise, and in the last few years have become friendly and easy to use. They use the technology of high-pressure water being forced through a small hole (typically called the "orifice" or "jewel") to concentrate an extreme amount of energy in a small area. The restriction of the tiny orifice creates high pressure and a high-velocity beam, much like putting your finger over the end of a garden hose.
Pure waterjets use the beam of water exiting the orifice to cut soft material like diapers, candy bars, and thin soft wood, but are not effective for cutting harder materials.
Typical design of a pure waterjet nozzle
The inlet water for a pure waterjet is pressurized between 20,000 and 60,000 Pounds Per Square Inch (PSI) (1300 to 6200 bar). This is forced through a tiny hole in the jewel, which is typically 0.007" to 0.020" in diameter (0.18 to 0.4 mm). This creates a very high-velocity, very thin beam of water (which is why some people refer to waterjets as "water lasers") traveling as close to the speed of sound (about 600 mph or 960 km/hr).
An abrasivejet starts out the same as a pure waterjet. As the thin stream of water leaves the jewel, however, abrasive is added to the the stream and mixed. The high-velocity water exiting the jewel creates a vacuum which pulls abrasive from the abrasive line, which then mixes with the water in the mixing tube. The beam of water accelerates abrasive particles to speeds fast enough to cut through much harder materials.
(Left): A diagram of an abrasivejet nozzle. (Right): Photograph of the same nozzle, with the guard removed, cutting out some parts.
The cutting action of an abrasivejet is two-fold. The force of the water and abrasive erodes the material, even if the jet is stationary (which is how the material is initially pierced). The cutting action is greatly enhanced if the abrasivejet stream is moved across the material and the ideal speed of movement depends on a variety of factors, including the material, the shape of the part, the water pressure and the type of abrasive. Controlling the speed of the abrasivejet nozzle is crucial to efficient and economical machining.
(Left) A typical waterjet nozzle to the left of an abrasivejet nozzle.
Waterjet nozzle fired into the air
In this video below, a waterjet nozzle is raised a few inches above the work surface, and fired for a few seconds into the air. Keep in mind that there is about 30 horsepower going through that little stream of water.
Cutting 1/2" aluminum at Westec trade show
This video was taken at the Westec trade show some years ago (1997 or so). The video shows cutting from various angles, with narration.
Cutting a foam mouse pad
A large mouse-pad, that is designed to wrap around a keyboard, is cut from foam.
"The reason I chose to film foam being cut is because it is fast enough that the video can be worth watching, and you can see a complete part from start to finish," said Carl Olsen. "I also used an abrasive nozzle, which was really unnecessary. I could have used a water-only nozzle which would have made a nicer edge finish, but I did not have one handy. Also, because this was foam, I could have cut it much faster, but I chose a slower speed so that I could have time to react, should it want to float away on me."
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