Waterjet cutting

In waterjet cutting, a workpiece is separated by a high-pressure water jet . A high-pressure pump is used to generate a water jet with 3500-6000 bar. In this process, the water jet separates superficial and microscopic particles.

Either filtered water (pure water cutting) or an abrasive (e.g. sand) is used in addition to the water.

Water pressure of 3,500 to 6,000 bar is common for waterjet cutting.

There are 2 types of waterjet cutting. The pure water cutting and the abrasive waterjet cutting. In abrasive waterjet cutting, sharp-edged cutting sand (abrasive sand) is added to achieve a higher cutting performance.

Pure water cutting is particularly suitable for soft materials such as foam, soft wood, and more. Abrasive waterjet cutting, on the other hand, is for harder materials, such as metal, stone, glass, and more.

In CNC waterjet cutting, software and machine merge into one tool. In this process, the workpiece is usually manufactured without the employee’s supervision, thus creating new resources.

This manufacturing method is used, among other things, for the production of prototypes, in toolmaking, but also for the production of small and large series.

– Import sample digitally (alternatively photograph and digitise analogue)
– Make any necessary corrections
– Select material and material thickness
– Software calculates own costs and recommended sales price
– Generate work report (material & processing time are documented)

Thanks to STM SmartCut, the flexibility of the waterjet can be fully exploited. At the same time, suggested strategies and the intuitive user interface make it very easy to use.

The software communicates directly between the waterjet cutting system and a standard office PC as the CNC control unit. StM SmartCut allows faster processing of cutting requirements and requires no special CNC or EDP knowledge.

Waterjet cutting technology goes back to a German dentist who obtained a patent for drilling teeth at the beginning of the 19th century.

The technology of waterjet cutting in today’s industrial production, on the other hand, is based on the principle of the kinetic energy that a drop of water has when it emerges from the water nozzle at a certain speed. The water nozzle has the crucial task of concentrating the water under high pressure. The water nozzle is just large enough so that the pressure does not drop. Typically, the nozzle diameter for waterjet cutting is 0.1 – 0.5 mm.

The water droplets emerge at a certain speed depending on the pressure. At 4000 bar, for example, the jet velocity is 640 m/sec. At this speed, the water droplets separate a wide variety of soft materials such as plastics, foams, cork, felt and similar materials.

If the waterjet cutting process is to be used to cut metals, ceramics, glass or other hard materials, sand (abrasive) must be added to a mixing chamber with negative pressure after the waterjet nozzle.

Garnet sand with a grain size of 0.1 to 0.3 mm is used for this purpose. The garnet sand is very hard, sharp-edged and heavy enough to achieve a very good cutting result. At the same time, it is non-toxic and can be easily recycled and subsequently disposed of.

Typically, the water jet cutting process uses a 0.35 mm water nozzle at 4000 bar and 450 g sand/minute for cutting.

The cutting technology or the cutting process with waterjet is also very flexible, since any material starting with glass, stone, wood, ceramics, up to foam, felt, cork and metals can be cut in different thicknesses without heat input.

Waterjet cutting is extremely versatile and can cut a wide range of materials including metal, plastic, stone, glass and composites. For soft material, cutting with pure water is sufficient. For harder materials, so-called abrasive sand is added.

The list of materials that can be cut with a waterjet is very long. Metal, stone, glass, wood, plastic, composite, carbon & fiberglass, foam, food, and more.

Waterjet cutting works with almost all materials up to 300mm material thickness. For pure water cutting of plastics and foams, material thicknesses of over 300mm are also possible.

The distortion of the cut material is brought about in conventional cutting processes by thermal influences (e.g. plasma cutting or laser cutting). Waterjet cutting is a cold cutting process and enables hard and soft materials to be separated without material deformation and without burrs.

Over 80% of waterjet users worldwide cut to ±0.1mm or higher accuracy.

The resulting taper is a function of the cutting speed. The largest possible taper is equal to the maximum cutting width on the material surface (Ex: 0.8 mm) and 0.0 mm on the material underside.

As you reduce the cutting speed, the taper also gets smaller until you get parallel sides. The usual taper for precision cuts is between 0.05 and 0.10 mm.

The kerf is approximately as wide as the inside diameter of the focusing tube, in most applications between 0.8 and 1 mm.

The following minimum water quality requirements apply:
– pH value 7.0 – 8.5
– Carbonate hardness 20 – 60 ppm corresponds to 2 – 6 ° dH
– Calcium carbonate (Ca) 35 – 107 ppm
– Chloride content (Cl) 100 mg/l
– Inlet temperature for feed water 10 – 25 °C
– Electrical conductivity at +25°C 450 µS/cm
– Filtrate dry residue 350 mg/l
– Free, dissolved chlorine 1 mg/l
– Feed water pre-pressure 0.2 – 2.5 MPa
*** Translated with www.DeepL.com/Translator (free version) ***

Deviations from these values lead to shorter service lives of the high-pressure seals of the pump and the water nozzles. Therefore, a water analysis and, if necessary, appropriate water treatment is recommended.

This depends on the performance of the HP pump or water nozzle. Usually between 2.22 – 14.42 l/min.

Wasserdüse:
– 0.1: 0.32 l/min
– 0.15: 0.71 l/min
– 0.2: 1.22 l/min
– 0.25: 1.87 l/min
– 0.3: 2.66 l/min
– 0.35: 3.57 l/min
– 0.4: 4.60 l/min
– 0.5: 6.82 l/min

The biggest advantage is the reduction of cutting noise to below 75 dBA. The disadvantage of underwater cutting is that it is difficult or impossible to see the workpiece during cutting.

Workpiece handling also takes place under water – unless you have a water level control in the water tank. Furthermore, it should be noted that the cutting process suffers a power reduction of approx. 5% at 1mm overlap with water.

In abrasive waterjet cutting, a high-pressure waterjet is combined with abrasive abrasives. The abrasives, often garnet sand or aluminium oxide, increase the cutting performance and enable the cutting of hard materials such as metal, stone or glass.

Garnet sand is by far the most widely used abrasive. It convinces in the areas of acquisition costs, cutting speed, mixing head service life and minimal health risks. Other abrasives contain olive sand, aluminum oxide and some artificial materials.

This depends on the working pressure and the water nozzle used. Usually between 150-450g/min. At 400 MPa working pressure is needed for optimum cutting performance:

– Nozzle 0.15 mm / focus 0.6mm 150g/min
– Nozzle 0.25 mm / focus 0.8mm 350g/min
– Nozzle 0.35 mm / focus 1.0mm 450g/min

Laser cutting is a very productive process. Nevertheless, the waterjet has some advantages over the laser:

– No problems with reflective materials such as brass and aluminium
– No heat influence, so there is no burning and no change in the material structure due to heat
– The water jet can be used to cut heat-sensitive materials such as plastic, rubber, composite materials, glass, stone and very hard ceramics
– Only the cutting speed needs to be adjusted when changing materials
– Additional cutting heads can easily be attached for extended production
– Distance between nozzle and material is less critical
– Waterjet systems are cheaper to purchase

Waterjet cutting is a modern alternative to laser cutting.

A wide variety of materials can be cut flexibly with waterjet. These range from glass, plastics, wood, ceramics and metals to special metals such as sintered metal, bimetal, spring steel, titanium, Inconel or platinum, and many more.

The processing of the material takes place without the application of heat and thus without any structural changes. No further finishing of the workpiece is necessary. Compared to cutting with laser cutting systems, waterjet processing can also cut very different material thicknesses.

Plasma cutting is a heat process. It adds large amounts of heat to the workpiece and leaves a heat-affected zone.

The surface created by a water jet is generally better. There is no burr on the material, so finishing is not necessary.

The abrasive waterjet is less restricted in terms of cutting thickness and several working areas of one jet can be close together.

EDM is very accurate, but also very slow. An electrically conductive material is mandatory and causes a change in the material structure due to heat.

Waterjet cutting is fast, requires no conductive material and there is no change to the material structure.

If you want to cut through the material edge and holes or create blind holes and threads, the abrasive waterjet is usually much faster, easier to programme and also cheaper than a milling cutter.

This is mainly due to the fact that the material is cut through in one go and that no metal chips are produced.

Waterjet cutting has a higher accuracy than can be achieved with a flame or plasma cutting system. In addition, waterjet cutting systems cut higher material thicknesses than laser systems.

Waterjet cutting is not suitable for materials that dissolve or react chemically on contact with water, such as some types of rubber, certain plastics or materials that are softened by water.

Similarly, very hard or brittle materials are sometimes difficult to cut because the water jet may not be able to apply enough cutting power. In such cases, other cutting methods such as laser cutting or wire erosion may be more suitable.

The support grids or workpiece supports last for many cutting hours, provided you don’t always cut on the same spot. The grids can be moved, exchanged and reversed, as with laser or plasma cutting systems.

The high-pressure seals must be replaced if they leak! At a constant working pressure of 400 MPa, the seals should be replaced after 400 to 1,000 cutting hours.

Focuses made of good ceramic carbide last about 100 hours. Focuses made of the highest ceramic carbide quality last approx. 30% longer than focuses made of other materials.

Such a focusing tube can last up to 150 cutting hours if the diameter is increased by 0.5 mm. Many of our customers already use somewhat used foci for the production of parts where high accuracy is not required.

One sapphire nozzle lasts 25 cutting hours. A diamond nozzle (the highest nozzle quality) lasts much longer, but costs many times more.

In most cases, contract cutting on abrasive waterjet cutting machines is charged between € 120.00 and € 220.00 per hour.

The costs per hour start at € 15.00 for a small cutting head (nozzle: 0.15 mm / focus 0.6 mm) and go up to € 35.00 for a large cutting head (nozzle 0.35 mm / focus 1.0 mm).

The amount includes all machine-related costs. Working time, acquisition costs and depreciation must still be added.

The abrasive currently costs about 450 euros per ton, depending on the quality.

The cost of laser cutting and waterjet cutting can vary depending on the specific project. For thin materials or for large series production, laser cutting is often cheaper than waterjet cutting.

However, waterjet cutting offers greater material versatility and is often a better choice for certain materials, thicker workpieces or complex shapes.

However, the actual cost depends on the requirements and characteristics of each project.

Residual moisture is not a problem for materials with a smooth surface. The component is blown off with compressed air.

For materials that actively attract and bind water, such as wood, open-pored foams and textiles, the material should be dried afterwards. In practice, blower dryers are used for this purpose.

You use several cutting heads if you want to produce many identical parts.

Either a second cutting head is attached to the existing Z-axis, or a second Z-axis is installed to which additional cutting heads can then be attached.

It is important that each individual cutting head must be supplied with the same pressure and flow rate in order to realise a uniform cut.

With most materials, the waterjet creates its own starting hole (launching point). For some composite materials, the pump pressure must be reduced and a special drilling spindle used.

A connected load of at least 11 kW must be taken into account for each installed cutting valve .

Since the force applied to the workpiece is very small (less than 1 kg for precision cuts and less than 5 kg for average cuts), costly clamping devices are not required. Most users use simple light weights to keep their workpieces stable.