Water jet cutting

Principle of water jet cutting

Water under high pressure Pure water nozzle Abrasive Abrasive focusing nozzle guide Cutting beam material to be cut

Cutting sheet metal with a water jet

The water jet cutting is a manufacturing process from the main group of the separation . There it is common with the laser beam fusion cutting , the flame cutting and the plasma fusion cutting the group of abrasive associated methods. A distinction is made between water jet cutting with pure water and abrasive cutting, in which a hard, powdery material - the abrasive - is added to the water . The associated machine tool is the water jet cutting machine .

With pure water jet cutting, soft materials such as plastics, foils, foams or paper are separated. Abrasive cutting is used on hard materials such as steel, ceramic or glass. It is of particular importance when cutting composite materials that cannot usually be cut satisfactorily using conventional methods. It is also very environmentally friendly.

history

Water jets were used in mining in the early 20th century to remove gravel or clay deposits. In the gold mines of California, gold veins were separated from stones and earth. From 1930, American and Russian engineers used it to clean castings. At that time, pressures of only 100  bar were used. The first patent went to Norman Franz for a machine that worked at 700 bar. In the late 1960s it was used in the aircraft industry to separate parts that are sensitive to heat, such as fiber composites , honeycomb and laminated materials . From 1974 onwards, hard particles were used as additives in the water jet, which significantly increased the quality of the workpieces and the cost-effectiveness of the process and thus led to its breakthrough in industrial applications. In 1975/76 building materials, plastics and corrugated cardboard were separated using the process.

Mechanisms of action and physical principles

The material removal during waterjet cutting is based on the high pressure that the jet causes on the surface of the workpiece. This is an important difference to competing processes based on thermal energy such as laser beam, plasma and oxy-fuel cutting. The water jet only separates microscopic particles near the surface. There is therefore no stretching of the workpiece due to heat or machining forces. The water flowing transversely from the point of action also causes shear forces which also contribute to material removal. In the case of hard , brittle materials such as ceramics or cast iron , the compressive forces lead to microcracks on the surface that propagate and unite and thus detach particles. With soft, tough ( ductile ) materials such as steel , the material can initially deform plastically without peeling off. This can lead to work hardening , which leads to embrittlement of the material and thus allows material to be removed. In addition, changes in the crystal structure of metals can occur. The deformations promote dislocations and accumulations of voids in the grid, which also lead to the formation of cracks.

When the cutting jet penetrates deeper into the material, it pushes a spoil cushion in front of it and loses energy due to the friction at the kerf. The achievable quality of the kerf, measured as roughness , therefore decreases continuously. At the joint , similar to flame cutting, there is a typical pattern in the form of a grooved structure, which is also known as " groove wake ".

The beam power at the nozzle exit may from the cross section of the nozzle, the density of the liquid and the beam velocity are calculated to ${\displaystyle P}$ ${\displaystyle A}$ ${\displaystyle \rho }$ ${\displaystyle v}$

${\displaystyle P=\rho A{\frac {v^{3}}{2}}}$.

The jet speed therefore has a very high influence on the performance, which is proportional to . The jet speed corresponds to neglecting pipe and nozzle friction ${\displaystyle v^{3}}$

${\displaystyle v={\sqrt {\frac {2p}{\rho }}}.}$

Accordingly, the main influences on the jet power are the diameter of the nozzle and the pressure. It should be noted, however, that at the very high pressures, the water can no longer be regarded as approximately incompressible. From 1 to 4000 bar, for example, water is compressed by 13.2% of its volume thanks to its compressibility.

System components

→ Main article : water jet cutting machine

The machines essentially consist of three components:

• A water treatment for desalination and filtration to reduce the wear and tear of the components,
• the high pressure generation by means of pumps and
• the actual beam generation with a nozzle whose diameter is between 0.1 mm and 0.5 mm.

technology

In industrial practice, pressures between 1000 and 4000 bar and speeds of around 900 m / s are used. Pump pressure, nozzle spacing, nozzle diameter and the feed rate are used as process parameters . They determine the energy input at the point of action and also the work result in terms of the surface quality and productivity achieved . If abrasives are also added to the jet, there are further parameters: the type of solid (hardness), the grain size , the mass flow , the focus diameter and the focus length. The solids lead to a wider joint, but increase the separability.

The working or cutting pressure on the workpiece surface determines the cutting depth . A pressure of at least 600 bar is necessary to ensure material removal at all. Then the cutting depth increases linearly with increasing pressure. Cutting pressures of up to 6200 bar are used. The nozzle diameter is directly proportional to the depth of cut, while the nozzle spacing is inversely proportional.

Pure water cutting

The workpiece is separated by a jet of pure water. Mainly soft but also tough materials are processed with pure water. These include plastic films , textiles , elastomers , thermoplastics , paper , fibers , foam and insulation materials and food . At a pressure of 4000 bar, for example, textiles up to a thickness of 30 mm can be separated. Pure water cutting is environmentally friendly: there are no chips , grinding dust, toxic gases or air pollution. Cooling lubricants are unnecessary and the water used can be used as a cycle material. With pure water cutting, the jet has a very small diameter and does not tend to form undesirable droplets. This allows the best result to be achieved with low material thicknesses. Mainly machines with several nozzles are used, which run on one or more traverses.

material	Thickness in mm	Feed in m / min
rubber	025th	03
leather	005	03
Paper / cardboard	000.2-5	20-500
Plastics ( PVC )	005	02.5
fiberglass reinforced plastic	005	00.15
Carbon fiber reinforced plastic	005	00.1
plywood	005	05
Insulation materials	100	08th

Abrasive cutting

In order to generate an abrasive water jet from the pure water jet, an abrasive agent is added to the cutting head in an additional mixing chamber. Abrasive cutting is used for hard or thick workpieces. Garnet or olive sand is usually used as an abrasive, and sometimes corundum is used for softer materials. This allows stone , bulletproof glass , ceramics , graphite , wood , marble and all metals to be separated. Laminates made of materials with different melting points can only be separated cleanly using this process. The machining of steels up to a thickness of 50 mm or other metals up to 120 mm is possible. The high jet speed creates a vacuum in the cutting head, which sucks the abrasive into the mixing chamber and mixes it with the water. The mixture is focused and accelerated by the abrasive nozzle. The jet diameter is about 0.2 mm larger than with pure water cutting. Instead, the cutting ability increases with the hardness of the abrasive used.

material	Thickness in mm	Feed in m / min
steel	25th	0.5
titanium	20th	0.3
aluminum	30th	0.75
Fiber-plastic composite	05	6th
Natural stone	50	0.075
Ceramics	30th	0.05
Glass	35	0.04
concrete	50	0.075

Error on the workpiece

Groove error (following error)

Groove defects affect the surface quality of the joint. The groove error increases with increasing cutting speed. The influence is relatively small on straight cuts, but can become very large in places with great curvature such as corners. Therefore corners should be cut at a slower speed.

V-shaped cut surfaces

V-shaped cut surfaces are created similarly to plasma fusion cutting. At high cutting speeds, the joint is wider on the top than on the bottom. At low speeds it is the other way around. In between there is a speed at which the cut surfaces run parallel. The angle error decreases:

• the smaller the nozzle spacing is
• the harder the workpiece material is
• the more even the abrasive is
• the smaller the workpiece thickness
• the better the focus of the nozzle.

application

In addition to cutting, water jets are also used for deburring , plastering and water jets (cleaning surfaces).

Water jet cutting is used when the materials to be processed are temperature-sensitive. The fine jet enables very filigree and complex contours to be cut. The cut can start at any point on the workpiece and does not necessarily have to start at the edge in the case of sheet metal or foils. Materials that have a light-reflecting surface are difficult to process with lasers; however, they do not cause any problems with water jets. Carbon or glass fiber reinforced plastics can be processed particularly well with water jet cutting compared to machining processes that lead to the destruction of the materials. In contrast to fixed tools, the water jet cannot jam. Because of the low processing temperatures, no toxic fumes are generated when processing plastics.

The inclined cutting edge, which leads to relatively poor shape and position tolerances , is a disadvantage .

Individual evidence

1. ^ Fritz, Schulze: Fertigungstechnik , Springer, 2015, 11th edition, p. 408.
2. ↑ Risse: Manufacturing processes in mechantronics, Feinwerk- und Präzisionsgerätetechnik , Springer, 2012, p. 133 f.
3. ↑ König, Klocke: Manufacturing Process 3 - Ablation, Generation and Laser Material Processing , 4th Edition, 2007, p. 321.
4. ↑ Risse: Manufacturing processes in mechantronics, Feinwerk- und Präzisionsgerätetechnik , Springer, 2012, p. 136f.
5. ^ Fritz, Schulze: Fertigungstechnik , Springer, 2015, 11th edition, p. 408.
6. ^ Fritz, Schulze: Fertigungstechnik , Springer, 2015, 11th edition, p. 408.
7. ↑ König, Klocke: Manufacturing Process 3 - Ablation, Generation and Laser Material Processing , 4th Edition, 2007, p. 322.
8. ↑ Risse: Manufacturing processes in mechantronics, Feinwerk- und Präzisionsgerätetechnik , Springer, 2012, p. 136.
9. ↑ König, Klocke: Manufacturing Process 3 - Ablation, Generation and Laser Material Processing , 4th Edition, 2007, p. 323 f.
10. ^ Fritz, Schulze: Fertigungstechnik , Springer, 2015, 11th edition, p. 409.
11. ↑ König, Klocke: Manufacturing Process 3 - Ablation, Generation and Laser Material Processing , 4th Edition, 2007, p. 326 f.
12. ↑ Risse: Manufacturing processes in mechantronics, Feinwerk- und Präzisionsgerätetechnik , Springer, 2012, p. 138 f.
13. ^ Fritz, Schulze: Fertigungstechnik , Springer, 2015, 11th edition, p. 410.
14. ↑ König, Klocke: Manufacturing Process 3 - Ablation, Generation and Laser Material Processing , 4th Edition, 2007, p. 327.
15. ↑ König, Klocke: Manufacturing Process 3 - Ablation, Generation and Laser Material Processing , 4th Edition, 2007, p. 327.
16. ^ Fritz, Schulze: Fertigungstechnik , Springer, 2015, 11th edition, p. 411.
17. ↑ Abrasive cutting technology. Retrieved January 23, 2017 . 
18. ↑ König, Klocke: Manufacturing Process 3 - Ablation, Generation and Laser Material Processing , 4th Edition, 2007, p. 327.
19. ↑ König, Klocke: Manufacturing Process 3 - Ablation, Generation and Laser Material Processing , 4th Edition, 2007, p. 327.
20. ^ Fritz, Schulze: Fertigungstechnik , Springer, 2015, 11th edition, p. 412.
21. ^ Fritz, Schulze: Fertigungstechnik , Springer, 2015, 11th edition, p. 412 f.
22. ↑ König, Klocke: Manufacturing Process 3 - Ablation, Generation and Laser Material Processing , 4th Edition, 2007, p. 321.

Web links

Water jet cutting explained on Youtube