Industrial Waterjet
Characteristic to the waterjet technique is the use of compressed water as an energy carrier up to the machining zone. Yes, water is a compressible media. Perhaps many remember from their education that they were told water was incompressible. This is a simplification that with "normal" pressure levels provides small calculation errors. With pressure up toward 400 MPa (approx. 4000 bar) water compresses almost 12%.
When the water, after having been transported from the pressure increaser (the water pump), reaches a nozzle the inner energy is released and transformed into kinetic energy under the formation of a water jet with very high speed. By using a fine nozzle very high speeds are created in the stream and we achieve power concentrations of almost 180 kW/mm2. It is with the help of the highly concentrated power supply that we can split and cut material. The mechanisms that work in the so called clean water cutting, where only the water stream performs the cut is based on the water stream's braking against the material building up pressure against the material surface. For dense material this pressure must exceed the material's deformation resistance in order to bring about the desired cutting process. In certain harder materials with micro cracks, the water can penetrate down in and pressurise these, which then grow with final material cutting as a result. Even hard and dense materials can be cut with pure water through other acting erosion mechanisms, such as drop erosion or cavitation. Research surrounding the use of these mechanisms with controlled material cutting has undergone for a long time but has proved difficult to control. However, there is a strong interested in being able to bring about effective material cutting without additives, for example in the nuclear power industry.
The idea of adding abrasive agent to the water stream lead to a technology break through and today it is the so called abrasive water cutting that is developed most both technically and market-wise. The technique is able to cut most materials. The abrasive agent is added in a cutting head, which in its construction has similarities with a blaster pistol. The figure below show the schematic construction of a cutting head for abrasive water cutting.
After the water jet is created in the primary nozzle it passes through a mixing chamber and then out through a second nozzle. Like with a jet pump, we create an underpressure in the mixing chamber with whose help we can feed in an abrasive agent to the stream via an opening in the mixing chamber. This disturbs the sensitive stream which is broken up into drops, which in its turn hits and accelerates the abrasive grain to speed up to almost half of the stream's speed. In the second nozzle the particles are focused and the drops to an abrasive water jet with a diameter of roughly 1 mm. As is understood from this description, we no longer have a water jet in an actual context, but rather a particle stream which volume-wise is comprised of 95% air.
In the reflection above, we discussed the importance of achieving high power concentrations. Through the abrasive cutting head, we have divided the available energy over an approximately ten times greater surface. To this, we can mention that we in the cutting head have had an efficiency around 10% with the creation of the AWJ stream. Despite this we have gained a cutting process that so effectively can handle cutting the most difficult materials, even those that are harder than the abrasive agent itself. How then can this be possible? The answer is particle erosion. When particles go into contact with the material this takes place in the smallest contact surfaces and when the energy is released here, the power concentration becomes much more powerful so that even hard construction ceramics locally present plastic deformation. The cutting occurs through a combination of several erosion mechanisms in co-operation, which can explain that almost all materials can be cut in this manner. Furthermore, the particles have the advantage of being able to carry the energy with them farther down in the cut.
As we see by the reasoning above there is, despite the method's capacity, today very great potential for further improvement in the technique. With understanding for where and why losses occur and how energy is used, the individual user can himself critically scrutinize and optimise his cutting process.
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