Water jet cutting machine
The water jet has been used for digging in gravel and clay deposits since around 1900 . In Soviet and US mines , the possibility of using a high pressure water jet for coal and ore mining was investigated in the 1930s . When composite materials were introduced in aircraft construction in the 1960s , the Boeing company suggested the use of a water jet to process such materials. The company Ingersoll Rand , today KMT (Sweden), 1971 delivered the first operational water jet cutting machine. At the end of the 1990s a new process came onto the market, which is now called Dynamic Waterjet . With this patented process, the angle errors caused by the beam expansion are corrected at the cutting edge, which enables precise cutting even of thick workpieces.
With water jet cutting, the material to be processed is separated by a high pressure water jet and usually with the addition of sharp-edged cutting sand. This jet generates a pressure of mostly 4000 up to max. 6000 bar and reaches exit speeds of up to 1000 m / s. The food to be cut barely heats up. Therefore, in contrast to laser cutting, the process is also suitable for cutting hardened steel. Due to the high pressure, the cutting water is germ-free ( high pressure sterilization ). The water quality must meet certain minimum requirements. In order to achieve optimal pump and component service life, special processing may be necessary, which, due to the extremely high pressures, requires a lot of experience and sensitivity; Standard processes such as water softeners or reverse osmosis systems often create more problems than they solve. Due to the high exit speed of the water, a sound pressure of up to 120 dB occurs when cutting. By cutting under water, for example by raising the water level in the jet catcher and surrounding the nozzle with a water bell, the noise emission can be significantly reduced.
With the high pressure water jet process almost all materials can be processed, from soft, porous foam to hard sapphire . The focus is on plastics processing, metal processing, leather processing and stone processing. The possibility of swiveling the cutting head ( 3D processing) means that almost any complicated shape can be cut in space using a cutting vector control.
Accuracies of up to 0.005 mm / m processing length can be achieved. However, the processing room must be air-conditioned for this.
In addition to cutting, water jet cutting is also used for deburring .
An important advantage of cutting with the abrasive water jet is the absence of unwanted structural changes (material structure changes; see iron-carbon diagram ) at the cutting edges. Abrasive water jet cutting is therefore used with great success in materials research and construction. Waterjet cutting finds industrial applications when cutting dissimilar workpieces such as B. ceramic- metal mixtures or when trimming composite layers of plastic dashboards for cars by means of a five-axis robot including a water jet.
A water jet cutting machine consists of various components that can be combined in different ways. Components include: accumulator, high-pressure piping, CNC-controlled guide machine, pressure intensifier, oil tank, oil pump, electric motor, valve and nozzle.
The machine frame, which is usually assembled from tubular steel of different formats, carries the individual axes of the machine. The guides on high-quality machines are annealed, milled, ground or scraped to reduce stress. Then the straightness of the frame and the guideways is checked using a laser interferometer. The frame is aligned using fixators or dowel elements. The standard design for waterjet cutting is the so-called portal design (as a flat bed for standard tasks or as a high portal system for workpieces with extreme dimensions), on which two axes carry the crossbeam without a connection. With these types, machines of almost any size can be implemented (spans of the portal up to about 5000 mm). In the case of portal machines, the two guide axes move in a so-called gantry network and are therefore coupled via the CNC control (two axes behave like one). In addition to the portal, there is also the design variant as a support arm, in which the crossbeam is only guided on one side. This design is cheaper to manufacture and has an advantage due to the particularly good accessibility to the cutting area from three sides. For a long time, however, it was technically inferior to the portal design and less suitable for precision cutting because the support arm design generally tended to oscillate. Current high-quality support arm systems are designed to be so rigid in terms of vibration using FE calculations that the same tight tolerances can be achieved as with a portal machine.
The residual energy of the water jet that remains after the cutting work has been performed can be dissipated in various ways. The most common variant is a water basin that acts as a "jet catcher". The water basin should have a sufficient water column of over 600 mm so that the residual energy of the water jet can be converted into heat. The water basin should be set up without a mechanical connection to the guide machine, as even larger water basins can heat up in the double-digit range after a few hours of cutting time. If the lead machine and the water basin are now one unit, such warming leads to changes in the machine geometry. The consequences are straightness errors, material expansion and thus inaccuracies in the manufacture of parts. Deviations in the range of several tenths of a millimeter are not uncommon. However, the heating has an influence on the behavior of the workpiece and must be taken into account as soon as possible in the case of higher accuracy requirements (online CNC correction).
In addition to the use of fixed water basins, there is also another variant, namely the so-called "catcher". In water jet cutting, a catcher refers to a mobile, narrow water catchment basin that moves synchronously with the movement of the cutting axis. These catchers are often filled with ceramic balls that are supposed to convert the residual energy. The biggest disadvantage of these catchers are extreme airborne noise emissions from the non-water-covered upper side, as well as a high proportion of splashing water. As the amount of water in circulation is much lower, the heating in catcher operation is much faster, but it is decoupled from the workpiece.
high pressure pump
The high pressure pump is used to generate a high pressure water jet that is as pulsation-free as possible. The simplest versions are operated with compressed air, which supplies the high pressure via a pressure intensifier . Due to the poor efficiency , however, this only applies to systems in the low-end range. As a rule, high-pressure pumps that use a hydraulic unit are used for waterjet cutting (the efficiency is around 65%). These units hydraulically generate a pre-pressure of up to approx. 200 bar. With better pumps, the pressure can be reduced as required using a proportional valve. The compressed oil is pumped into the hydraulic cylinder of the high pressure booster. Here the oil acts on a piston rod with a transmission ratio of about 20: 1 - 40: 1 (hydraulic surface to water surface). In this way, pressures up to around 6500 bar can be generated. The high-pressure water that leaves the high-pressure cylinder enters a so-called pulsation damper. This is a high-pressure gas accumulator, a "buffer cylinder" (usually with a volume of one or two liters), which is intended to dampen pressure fluctuations when the hydraulic piston is reversed. The larger the buffer bottle, the better the cutting performance and quality. HP pumps can have several high pressure intensifiers and buffer bottles. The output of today's systems is between about 11 and 149 kW . The delivery rate can be up to 15.2 liters per minute. Most recently, cutting pump units have been used that generate a pressure of 4100 bar by means of plunger pumps . The detour via the hydraulics can be dispensed with here. With these water jet cutting pump units, the high pressure pump is driven directly, so that an efficiency of over 90% is achieved. Due to the triplex characteristics, the pulsation is so low that a pulsation damper can also be dispensed with. In the performance range, this technology can convert up to 750 kW drive power and generate a volume flow of up to 100 l / min at 3800 bar.
The cutting water mixed with cutting material and abrasives must be removed from the jet shredder. This is done either continuously through disposal or manually at intervals. The continuous disposal consists either of a scraper conveyor, which removes the cutting agent residues from the jet shredder, or of a water circulation, which separates the residues from the jet shredder. The water from the jet shredder is then filtered and fed back into the cutting basin.
Water jet cutting systems are consistently equipped with CNC controls. In addition to the simplest versions that only allow a plotter control, higher-quality machines have controls that interpolate all axes and also perform an adaptive feed rate reduction depending on the cutting process. In addition to a CAD interface, there is often a CAM connection. For several years now, PC controls have also been on the market that offer the advantage of easy training for operators who are not familiar with CNC.
- Andreas Risse: Manufacturing processes in mechatronics, precision engineering and precision device technology . Vieweg + Teubner Verlag, Wiesbaden 2012, ISBN 978-3-8348-8312-4 , water jet cutting, p. 133 ff ., urn : nbn: de: 1111-201207241495 ( books.google.de ).
- Michaela Hörbinger: water jet cutting . Process possibilities and comparison with alternative industrial separation processes. Bachelor thesis . : Bachelor + Master Publishing, Hamburg 2015, ISBN 978-3-95820-401-0 , water jet cutting machines, p. 13 ff . ( books.google.de ).
- How do waterjet cutting machines work? on zhsf-edu.ch
- Linear technology helps ensure cutting precision and speed. In: vogel.de. Konstruktionspraxis.vogel.de, accessed on November 16, 2015 .
- Dynamic Waterjet: The Story of a Problem Worth Solving . flowwaterjetblog.com, April 10, 2015; accessed on February 19, 2019
- WPFI - water jet cutting - water jet cutting machine - water cutting. In: hydroschneidetechnik.de. Fischer Werkzeugbau, accessed on November 16, 2015 .