Cutting technology Water jet cutting - Function, methods and application examples
In water jet cutting, workpieces are separated from each other by a high-pressure water jet. For this purpose, either specially treated water or an abrasive agent is used.
History of water jet cutting
Water jet cutting came into being at the end of the 19th and beginning of the 20th century. Initially, the process was used to remove clay and gravel deposits. A little later, water jet cutting was used in the US American gold mines to remove stones and earth from the gold veins. In the 1930s, American and Russian engineers used the process to clean castings. The pressure used for water jet cutting was only 100 bar at that time. Norman Franz, professor at the University of British Columbia, secured the first patent for a machine used for water jet cutting at a pressure of 700 bar.
In the 1960s, the aircraft manufacturer Boeing became aware of water jet cutting because it promised optimum processing of the new composite materials introduced at the time. McCartney Manufacturing, an Ingersoll-Rand subsidiary, began using water jet cutting commercially in 1971 to process paper tubes. At that time, the company worked exclusively with pure water jet cutting, preferring materials for the aerospace industry as well as paper diapers. Ingersoll-Rand's high-pressure pumps managed to build up a pressure of up to 3800 bar for water jet cutting. Their subsidiary Bestmatik from Sweden designed a special cutting table to process wooden puzzles precisely using water jet cutting.
It quickly turned out that although pure water jet cutting is ideal for soft materials with a maximum of medium hardness, materials such as steel, ceramics, glass and stone are left out. Attempts to improve water jet cutting with an abrasive were finally crowned with success in the early 1980s. Ingersoll-Rand added abrasive water jet cutting to its product range in 1984. At the end of the 1990s, the manufacturer Flow optimised the process again. The so-called Dynamic water jet offers even higher precision and the possibility of cutting even very thick workpieces.
The water jet cutting process
1. What preparation or pre-treatment is necessary?
In water jet cutting, the material is cut either by a high-pressure water jet alone or by the addition of an abrasive. Water jet cutting requires a high pressure, which can be up to 6200 bar, which results in an outlet speed of 1000 m/s. The water jet cutting process is carried out by means of a high-pressure water jet alone or by the addition of an abrasive medium. There is only a small amount of heating. The high pressure produces a sterility of the used water, whereby this must fulfill certain minimum requirements. In order for the pump to work optimally, the water may have to be specially treated.
Since standard water treatment processes such as softening or reverse osmosis only work to a limited extent at this high pressure, this is work that must be carried out by an experienced specialist. The sound pressure of up to 120 dB, which is generated during water jet cutting by the high exit velocity, must also be taken into account. Emissions can be significantly reduced if water jet cutting is carried out under water, the nozzle is covered with a water bell or the water level in the jet catcher is raised.
2. Components of water jet cutting
A water jet cutting machine consists of the following main components, which can be combined in different ways:
- Guiding machine (CNC-controlled)
- High-pressure piping
- Oil tank and oil pump
- Pressure intensifier
- Electric motor
- Water treatment for desalting and filtering
- Nozzle with diameter between 0.1 and 0.5 mm and valve
In addition, a machine for water jet cutting is constructed as follows:
Machine frame: The axes of the machine are supported by steel tubes. The guides are milled, scraped, ground or annealed with low tension. A laser interferometer checks the frame and the guide for correct alignment and aligns them with dowels. For water jet cutting, a portal design is generally preferred, whereby a distinction is made between flat boards for normal work and high portal systems for special dimensions. In this way, it is also possible to design very large machines with a gantry span of up to 5000 mm. The two guide axes can be coupled via a CNC control, which means that both axes act as one. There are also variants as support arms with a one-sided cross beam guide. This simplifies access to the cutting area, and the purchase price of these machines is also lower. In the past, water jet cutting worked more precisely with the portal design, as there were fewer vibrations. However, newer support arm systems are now vibration-reduced, so you can achieve the same performance as with a portal machine.
Jet destroyer: After the water jet has fulfilled its task, there is still a high residual energy in the jet itself. Water basins are often used as a kind of jet catcher to reduce this energy. A water column of 600 mm is recommended so that this residual energy can be converted into heat. This water basin should be separated from the machine itself, otherwise the water can heat up to two-digit temperatures after a few hours of work. If the pool and the machine are coupled, there may be long-term displacements that affect the cutting precision. In addition, there are the so-called catcher, which in water jet cutting refers to a catch basin that is adapted to the movements of the cutting axis. Catchers have an extreme sound emission, in addition a lot of splash water occurs. The residual energy is converted by integrated ceramic balls. There are only small amounts of water in circulation. For this reason, the heating takes place faster, but is decoupled from the workpiece.
High-pressure pump: A high-pressure pump is used to ensure that the water jet is as pulsation-free as possible when cutting water. Simple models are only operated with compressed air, which can lead to poor efficiency. High pressure pumps, which generate a pressure of up to 200 bar using oil hydraulics, are more common for water jet cutting. A proportional valve enables pressure control. Compressed oil is first pumped into the high-pressure booster. There it acts with transmission ratios of 20:1-40:1 on the water surface. This enables a pressure of up to 6200 bar. The water then passes into a pulsation damper, which functions like a high-pressure gas accumulator as a kind of buffer cylinder with 1 to 2 l volume. This dampens pressure fluctuations when the hydraulic piston is reversed. The larger these buffers are designed, the better the cutting performance. Today's systems have an output of between 11 and 149 kW at a flow rate of up to 15.2 l per minute. Newer machines use cutting pump units with plunger pumps that can directly generate a pressure of 4100 bar. This eliminates the need for hydraulics and the pulsation damper due to the low pulsation. The volume flow generated is up to 100 l/m at a pressure of 3800 bar and a drive power of 7450 kW. In addition, there are different drive concepts for the high-pressure pumps for water jet cutting in comparison.
Disposal of the cutting water: In the abrasive process, the cutting water is mixed with various substances, which is why the water must be removed from the jet shredder. Scratch conveyors are available for this purpose, which continuously remove residues from the jet shredder. There are also models in which the water is removed manually. After the water has been filtered, it can be used again for water jet cutting. Abrasive recycling can save costs and resources in water jet cutting.
Control components: water jet cutting is carried out exclusively with a CNC control. Simple machines have a plotter control. On high quality models you will find controls with adapted feed speeds at which the axes are also interpolated. In addition to a CAD interface, these machines also have a CAM connection for water jet cutting. Some models are equipped with a PC control, which allows even CNC-untrained users to perform water jet cutting.
What types of water jet cutting are there?
In water jet cutting, materials are separated using a high-pressure water jet. In industry, a pressure of between 1000 and 4000 bar is used, but it is possible – depending on the consistency of the material to be cut – to increase the pressure up to 6200 bar. The pressure exerted on the surface of the workpiece determines the depth of the cut. Under a pressure of 600 bar no material removal is possible at all. The depth of cut increases linearly with the pressure. The diameter of the nozzle used must be selected in proportion to the cutting depth, whereas the distance between the nozzles must be inversely proportional.
On average, the outlet speed is 900 to 1000 m/s. The nozzle diameter is proportional to the cutting depth. A typical feature of this process, which is gentle on the material, is that it generates only little heat. In pure water jet cutting, only water is used for cutting. The final result is determined by the parameters pump pressure, nozzle distance, nozzle diameter and feed speed. The choice of these parameters determines the energy input directly at the point of action and thus also the quality of the surface and the achieved productivity of the cut. Abrasive water jet cutting is used for harder materials. An abrasive agent is also added here, which requires further parameters in the process such as focus diameter, focus length, hardness, grain size and mass flow. The joint becomes wider as a result, but the separability is increased.
1. Pure water jet cutting
With pure water, both soft and stronger, tough materials can be processed. In this process, the individual parts are separated by pure, filtered water. Since no dust or chips are generated or toxic gases are released, this variant of water jet cutting is environmentally friendly. After completion of the cutting process, the water can continue to be used and finally fed into the water cycle. The diameter of the water jet is relatively small, which prevents the formation of drops. It can be less than 0.1 mm. Especially with materials of only small thickness, optimum, precisely fitting cutting results can be achieved. As a rule, machines with several nozzles are used, which are mounted on one or more traverses. If a pressure of 4000 bar is built up during the cutting process, it is possible to accurately separate textiles up to a thickness of 30 mm.
2. Abrasive water jet cutting
An abrasive water jet can be generated from a pure water jet. Three additional components are added to the cutting head:
- Pure water focusing nozzle
- Abrasive mixing chamber
- Abrasive focusing nozzle
The addition of an abrasive agent is also necessary. First, the pure water nozzle ensures that highly compressed water is formed into a jet. At a speed of 1000 m/s, this jet shoots through the abrasive mixing chamber, creating a vacuum in the cutting head. Through a minimal opening in the cutting head, the actual cutting or abrasive agent can now be sucked into the mixing chamber. There it mixes with the water jet and exits, focused by the nozzle, for cutting. On average, this jet is 0.2 mm larger than with pure water jet cutting. Garnet sand or olive sand is often used as an abrasive. The harder the abrasive used, the higher the cutting capacity. Sometimes corundum is used, especially for softer materials. Steel can be cut up to a thickness of 50 mm and other metals up to a thickness of 120 mm.
The water jet cutting of impressively thick metal sheets is shown by Schmidt in this video.
Other procedures of water jet cutting
Micro water jet cutting offers a significantly reduced water jet diameter, which results in a cutting accuracy of +/-0.01 mm. This also reduces the cutting width for pure water jet cutting to 0.08 mm and for abrasive water jet cutting to 0.2 mm. water jet cutting in 3D enables you to cut three-dimensionally, allowing you to produce very complex parts individually or in series. The cutting head can be rotated in this process.
Possible alternatives to water jet cutting
Direct contact with water is not desired for a large number of materials. These alternatives to water jet cutting are at your disposal:
Plasma cutting: A special gas is used in this system, which becomes plasma at 30,000 °C. The gas is then used to cut the water. Compressed air leads the molten material out of the kerf. The process is popular with steel up to 15 mm thick. The cutting speed is a great advantage of this process.
Laser cutting: The laser cutting process uses light to focus energy on the material to be processed. Thus the material evaporates selectively. The cutting gap of high-quality machines is almost invisible. Complex and acute-angled cuts are thus feasible. It is one of the most precise processes ever.
Plotter: With the aid of a tangential or drag knife, the material is cut with a knife plotter. Cutting inserts are interchangeable so that a variety of materials can be cut. The acquisition costs are low. However, milling and new cutting inserts incur costs in the long term. A plotter is only interesting for you if you have to cut very thin materials such as foils and textiles.
water jet cutting - the application examples
Water jet cutting with pure water is preferred for the following materials:
- Fibrous materials
- Sealing materials
- Insulating materials
With abrasive water jet cutting, the following materials can be separated cleanly:
- Laminates made of materials with different melting points
- Glass, also bulletproof glass
- Different rocks
For water jet cutting with pure water, the following material thicknesses can be cut:
- Paper and cardboard: 0.2 to 5 mm
- Plastics (also glass fibre reinforced and carbon fibre reinforced plastics: 5 mm)
- Plywood: 5 mm
- Leather: 5 mm
- Rubber: 25 mm
- Insulation materials: 100 mm
In the abrasive process, the following thicknesses can be separated in water jet cutting:
- Composite of fibre-plastic: 5 mm
- Titanium: 20 mm
- Steel: 25 mm
- Aluminium: 30 mm
- Ceramic: 30 mm
- Glass: 35 mm
- Concrete: 50 mm
- Natural stone: 50 mm
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