FAQ

Cutting with high pressure – The special feature of waterjet cutting

One of the special features of waterjet cutting is certainly the basic technique of this process: cutting with high pressure.

The water jet is generated via a high-pressure pump. The material to be processed is separated either with pure water or by adding sharp-edged cutting sand. In the field of waterjet cutting, the high-pressure pump (HDP) is therefore often referred to as ‘the heart’ of the waterjet cutting system.

At STM, all waterjet cutting machines are equipped with a high-pressure pump if required. For this purpose, STM has called on the support of competent partners with years of experience. With pumps perfectly matched to the system, STM enables its customers to benefit from know-how, experience and expertise in a modular waterjet cutting system, thus offering everything from a single source.

No matter if pump with pressure intensifier or direct drive pump, at STM you get a fully functional waterjet cutting system incl. suitable high-pressure technology.

When purchasing or replacing a (waterjet) cutting system for the first time, the material to be processed primarily determines the technical requirements. For waterjet cutting with high pressure, a distinction is made between two basic process variants:

High-pressure cutting with pure water – also called ‘clean water’ – and cutting with water with the addition of abrasive sand. A water pressure of 3,500 to 6,000 bar is common here.

Due to the numerous areas of application for cutting with high pressure, STM will provide you with advice tailored to your application that meets your requirements, while at the same time representing a waterjet solution that is as budget-friendly as possible.

Thanks to the modular design, there are hardly any limits to high-pressure waterjet cutting, as hardware and software can be upgraded or retrofitted at any time.

Wondering how to cut with a water jet?

To be precise, the cutting power is provided by the high water pressure. The water jet is only the tool of high pressure. The speed of the water jet plays the main role in this modern separation process.

In creating this high-precision tool, water is accelerated at 4000 to 6000 bar to Mach 2 to Mach 3 (two to three times the speed of airborne sound). This allows the waterjet to develop its unique cutting performance.

Pure water can be used to cut soft materials such as foam, food or plastic. If abrasive sand (usually garnet sand) is added to the water jet, even the hardest materials such as stone, metals or glass can be separated. In addition, when cutting with waterjet, you have to pay attention to many other components, such as the right cutting head or the high-pressure valve.

At STM, we will be happy to advise you on the right waterjet cutting system for your individual requirements. For example, even particularly thick materials (up to 300 mm) can be cut with an STM waterjet cutting system.

The distortion of the cut material is caused in conventional cutting processes by thermal influences (e.g. in plasma cutting or laser cutting). Cutting without distortion, on the other hand, is made possible by waterjet cutting. This saves valuable production time during the post-processing of cut workpieces.

Cutting with a waterjet cutting system is the classic cold cutting process and enables a wide variety of hard and soft materials to be cut without burrs without any material deformation.

In CNC waterjet cutting with a waterjet cutting system, software and machine merge into one tool with which craftsmen and industrial productions can work or manufacture digitally. This happens with CNC waterjet cutting in an individual, flexible and easy-to-use way, thus combining digital and analog working methods.

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.

The process is simple:

The sample piece is either photographed in analog and thus digitized by the STM SmartCut software or already digitally, in common formats, and processed. (.dwg., .dxf, .step, .ai, .iges or also formats from Trumpf or Bystronic are possible)

Necessary corrections can be made easily and intuitively digitally via the software. In addition, with CNC waterjet cutting, the material to be processed as well as the existing material thickness for the future workpiece can be selected. The program immediately offers a calculation for the production’s own costs and the recommended retail price (SRP).

For transparent order processing, a work report can be generated at the end of production, documenting the material as well as the processing time, among other things. This enables more precise, documented manufacturing and opportunities for effective optimization.

In CNC waterjet cutting, the workpiece is usually produced without the employee’s supervision, creating new resources.

Waterjet cutting is a modern alternative to laser cutting. The advantages are obvious:

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. This means that there are no heat-affected zones and thus no additional heat-induced distortion. Waterjet cutting is therefore an extremely popular alternative to laser cutting, as the cold cutting process does not require any further post-processing or post-treatment of the workpiece, due to material changes caused by the cutting process.

Compared to cutting with laser cutting systems, waterjet processing can also cut very different material thicknesses. These range from 0.1 mm up to 300 mm material thickness. These can be cut in this span practically without retooling the cutting head, just by changing the parameters in the STM cutting software.

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, 3.4 liters of water and 450 g of sand/minute for cutting.

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

Over 80% of waterjet users worldwide cut with ±0.1mm or less accuracy.

The biggest advantage is the reduction of cutting noise to below 75 dBA. One negative side of underwater cutting is that you have little to no visibility of 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 said that the cutting process suffers a power reduction of approx. 5% at 1mm overlap with water.

Up to 200 mm thick steel and titanium are cut on a production basis. There are cases where thicknesses of 300 mm and 400 mm have been cut. However, the majority of users cut material with thicknesses between 10 mm and 60 mm.

This depends on the performance of the HP pump or water nozzle.

The kerf is approx. 15% wider than the focusing tube diameter. With a 0.8 mm focusing tube, the kerf is 1.0 mm wide. As the focusing tube diameter increases, the kerf also widens.

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.

With most materials, the waterjet creates its own starting hole (launching point) without any further effort. For some composites, the pump pressure must be reduced and a special vacuum gating device must be used.

From 700 mm of water in the water tank.

The following minimum water quality requirements exist:

• 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 inlet pressure 0.2 – 2.5 MPa

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

A connected load of 37 kW must be taken into account per installed abrasive cutting head.

Use multiple cutting heads if you want to create 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. The important thing here is that each individual cutting head must be supplied with the same pressure and flow rate in order to be able to realize a uniform cut.

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 1,200 cutting hours.

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 health hazard. Other abrasives contain olive sand, aluminum oxide and some artificial materials.

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

The support grids 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.

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, with an expansion of the diameter by 0.5 mm, can perform up to 150 cutting hours. 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 quality nozzle) lasts much longer, but costs 30 times more and is typically only used for pure waterjet cutting.

STM SmartCut allows the flexibility of the waterjet to 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.

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.

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

• No strength restriction
• No problems with reflective materials such as brass and aluminum
• No heat influence, therefore there is no burning and no change of material structure by heat
• The water jet can be used to treat heat-sensitive materials such as plastic, rubber or composites, as well as glass, stone and very hard
• Ceramics are cut
• Material change: only the cutting speed is changed. Neither gas nor optics need to be replaced
• Additional cutting heads can be easily attached for expanded production
• Maintenance of laser equipment is more specialized and difficult to perform
• Distance between nozzle and material is far from critical
• Waterjet systems are less capital intensive to purchase than lasers

EDM is very accurate, but also very slow. It also requires an electrically conductive material and evokes a change in the material structure due to heat.

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

This is mainly due to the fact that we cut through in one go and that we do not produce metal chips. Additionally noted, waste is worth more in whole form than in chip form.

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 underside of the material, so finishing is not necessary. The abrasive waterjet is not subject to any restriction on cutting strength and several working areas of a jet can be close together.

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 € 19.00 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. Labor, leasing and depreciation must still be added.

The abrasive currently costs about 450 euros per ton.

It is disconnected. The safety equipment around the machine was therefore planned very conscientiously. Users of abrasive waterjet cutting equipment are subject to more stringent safety requirements than most machinists in other industries.

Noise generation during waterjet cutting depends on the distance of the free jet to the workpiece surface – i.e. the distance the waterjet travels in the air at supersonic speed. If you cut under water, the noise level drops below 75 dBA. When cutting over water, the noise level increases up to 105 dBA, depending on the distance of the cutting head to the workpiece surface. In extreme cases, the entire work surface is covered.

Neither the water, nor the garnet sand contain toxic substances. Therefore, once the suspended solids have settled or been filtered out, the water can be discharged into the canal. The abrasive can be disposed of as construction waste. If the processed material is toxic or hazardous to health, the waste water and abrasive must be disposed of as hazardous waste!

Yes. Most customers use tap water. Before the high-pressure pump, the water must be filtered to 0.5 microns. Deionization as well as osmotic systems are not recommended. Such systems clean the water too much – and too pure water is extremely aggressive, which leads to increased wear of the high-pressure system.

Water as well as abrasive can be cleaned and recycled with the ONECLEAN system. For more information, see the OneClean System subpage.