Finepart Sweden : Waterjet Cutter – How it works

What is a waterjet cutter?

Waterjet cutting technology is based on the use of one or several jets of water with very high power density. Up to 30 kW of power within a fine, high pressure waterjet of the size of a needle is an amazing thought. The waterjet can be used either to cut directly (pure water) or to accelerate abrasive particles that erode the material (Abrasive Waterjet -AWJ).

High water pressure

High pressure water, at 400 MPa or more, is conveyed in stainless steel tubing to a nozzle, usually of 0.1 - 0.3 mm in diameter. As the water escapes the nozzle, it approaches a velocity of 900 m/s. The flow of water leaving the nozzle is determined by the pressure generating the jet, which is normally in the range of 2 to 3 liters per minute.

Figure 2.1 Water jet cutting. By this method soft material can be cut, as well as hard brittle material with inherent microcracks, for example fibreglass. Photographs by courtesy of KMT Cutting Systems AB.

The benefits usually attributed to waterjet cutting are:

• It is an extremely viable process that can cut practically any material (even at great thicknesses)
• The same tool setup can be used for almost all materials
• No heat effects on the cut surface
• Virtually no mechanical damage to the cut surface
• Narrow cut width of the jet (about 0.5 to 1.2 mm) depending on the size of the focusing tube, which saves material
• Narrow omnidirectional jet can cut complex geometries
• Low cutting forces (< 50 N) means low requirements for clamping and fixturing
• Can cut thin material (with the advantage of low cutting force)
• Cut thick material (300 mm thick steel and titanium are being cut)
• Can cut stacked material (however, gaps between layers should be avoided by clamping)
• Minor or no burr formation
• Easy to use multiple cutting heads for parallel cutting, thus raising productivity
• The need for subsequent machining processes can often be eliminated

From an environmental perspective, an abrasive waterjet cutter is attractive because with natural materials, sand and water, it uses nature's own tool for manufacturing, namely erosion.

The AWJ cuts without significantly heating the material; consequently, the hazardous metal gases as well as electromagnetic and ultraviolet radiation formed by some thermal methods can be avoided, which is desirable for work environments. It is, however, important to keep in mind that used abrasives will contain an addition of erosion fragments from the cut material.

Requirements about how the produced waste material should be handled and disposed of vary within the EU at present. There may also be local regulations for waste material handling and disposal.

Types Of Waterjet Systems

There are three main branches for waterjet cutting systems that are used for different types of products depending on material, size and required precision.

• Pure Waterjet
• Abrasive Waterjet
• Micro Abrasive Waterjet

Pure Waterjet

The water jet cuts soft materials by displacing or eroding material from the workpiece.

Plastics (both solid and foamed), rubber, floor mats and other non-woven materials, plywood and corrugated cardboard are examples of materials which can be cut in this way with a pure water jet.

Even softer metals, such as tin and lead, can be cut this way with the advantage that the material gives way under the pressure of the water jet, while still remaining as a burr on the workpiece; this is especially advantageous in the case of lead which should not be allowed to enter the wastewater system.

Thin and light materials are often cut at high speeds, but even 100 mm thick rubber can be cut, although at lower speeds. This technique of cutting with a stream of water only is known as pure waterjet cutting. (See principle in Figure 2.1).

Figure 2.2: The cutting mechanisms of pure waterjet cutting. Soft material gives way to the stagnation pressure of the water jet when it rises above the deformation resistance. Also hard, brittle materials with microcracks can be cut by means of pressurizing the cracks.

Fibre materials are another group of materials that can be cut to advantage with a pure water jet.

Examples:

• Fiberglass
• Carbon fiber
• Brittle fiber material
• Plastics
• Fiber-reinforced plastics

When the stream of water strikes this type of material, the cracks become pressurized. The pressure causes the cracks to propagate, and a network of cracks causes material to quickly disintegrate and be washed away by the water.

The cutting of glass fiber wool was actually the first Swedish application of waterjet cutting; it was used at the Gullfiber Company in Billesholm as early as 1974. In fact this has received international attention as one of the early industrial applications of waterjet cutting.

The combined capacity of waterjets to cut soft plastics as well as brittle fiber material is especially useful in the cutting of fiber-reinforced plastics (FRP). In this application the waterjet cutting technique offers important advantages: it gives a fine cut and also eliminates dust generation since the material removed becomes bound to the stream of water. Hence, this feature also reduces a problem in the working environment where these products are machined.

The Breakthrough of Abrasive Waterjet (AWJ) Cutting

During the 1980s, a breakthrough was made in waterjet technology. By adding abrasives to the waterjet, a completely new process was generated, which led to an enormous expansion of applications for waterjet cutting.

Practically any material could now be cut (see Figure 2.4), and cutting of metal and concrete of dimensions up to 300 mm or more has been reported. The new method, now known as abrasive waterjet cutting (AWJ), was quickly developed, since it was based primarily on the same high-pressure components as the pure waterjet cutting with the addition of an AWJ cutting head, by which the abrasive could be added, by means of an abrasive feed system.

Figure 2.4: The cutting of a composite material made from a combination of hard, soft, brittle, and ductile materials. From the top we see stainless steel, ceramic tile, brass, wood, titanium, rubber, copper, granite and, at the bottom, aluminium. The front surface was cut with the abrasive waterjet.

The Operating Principle of the Abrasive Cutting Head

The principle of the abrasive waterjet (AWJ) cutting head is described schematically in Figure 2.5.

The main parts of the cutting head are a straight inlet tube that stabilizes the stream of incoming water, a primary nozzle or orifice, a mixing chamber with one or more feeding ports for abrasives, and a mixing tube or focusing tube.

The waterjet formed in the orifice is gradually broken into small drops in the cutting head. In the mixing chamber, these drops transfer energy to the abrasive particles (usually garnet grit) which are fed into the cutting head and accelerated to high speeds.

Abrasives and water are then focused into an abrasive waterjet in the mixing tube.

The abrasive waterjet thereby becomes a stream of particles without any real core of water. By volume the mixture is about 4 % water, 1 % abrasive and the rest is air.

It is highly critical that both nozzles should be aligned to avoid losing power in the process. The method extends the same equipment as that used for pure waterjet cutting, with the addition of an abrasive feeder and an AWJ cutting head.

Figure 2.5: The abrasive waterjet (AWJ) cutting head.

As a simplification, this can be compared with a conventional sand blast nozzle assembly. However, water is significantly more effective in momentum transfer than a gaseous medium: the specific weights of water and abrasive are closer.

When the stream of water passes through the cutting head, a partial vacuum is formed (the Venturi effect), which makes it possible to feed the abrasive into the cutting head. The accelerated abrasive is mixed with the drops of water to form a stream of particles without an actual core of water.

Focusing tube (abrasive nozzle)

The erosive power of an abrasive water jet makes it considerably more powerful than a water-only jet. The diameter of the AWJ beam, and thus the width of the cut, is established by the diameter of the focusing tube, which is usually about 0.5 - 1.2 mm, although dimensions as small as 0.2 mm exist. Jet width below 0.5 mm would be considered a Fine Abrasive Waterjet (FAW) and is the most recent development of the waterjet process.

Micro Abrasive Waterjet

The concept of micro abrasive waterjet was initially launched by Donald S. Miller in 1998 (?), who demonstrated cutting extremely fine parts with an abrasive waterjet of 50 µm jet diameter. This technology was industrialized about a decade later by the introduction of commercially available micro abrasive waterjet machining centers.

The concept of bringing the benefits of waterjet technology into fine mechanics/ mechatronics offers important opportunities for advanced product development in a wide scope of applications. Today, ultra-precise waterjet cutting is a fast growing branch of waterjet technology with market awareness of the new capabilities being the main market driver.

Benefits of micro abrasive waterjet

• Shares the generic benefits of waterjet technology, cutting any material without distorting the material properties
• Cut very small radii and narrow slots
• Cuts parts down to +/-10 µm tolerance
• Cut features down to 100 µm width
• Fine abrasives produce fine surface finish down to ca 1 µm in Ra-value

The micro abrasive waterjet machine systems

The concept of micro abrasive waterjets relies on a combination of a very accurate machine motion system and precise fixturing of the parts to ensure that the fine features are cut to dimensions with fine tolerances typically down to ca +/- 10µm or even better in fortunate cases. Whereas most traditional waterjet cutters are being built as cutting tables, the micro abrasive waterjet machines rely on design concepts we find on precision machine tools. Typically, the micro abrasive waterjet machining center is calibrated to hold a positional accuracy of +/-2.5 µm.

To achieve high precision parts, it is important to be able to fine tune the dimensions. Cutting speed and radius compensation is used actively to adjust dimensions to perfection. Whereas traditional waterjet machines often use multiple cutting heads, this is not used to a wide extent in micro abrasive waterjet machines as it prevents exact adjustment of the dimensions of each part.

5-axis manipulation in micro abrasive waterjet machines

The traditional way of reducing taper in a three-axis machine is to reduce the speed. This is effective in most materials, but for very hard materials there will typically be a residual taper that is difficult to remove.

For this a five-axis manipulation can be used, where the nozzle is tilted to compensate for errors.

To achieve a taper-free cut with a three-axis machine normally requires cutting at speeds nearing 10% of the speed of a separation cut. This means that the surface quality will be fine at significantly higher speeds that are being used to eliminate form errors. Therefore, the concept of tilting the jet while cutting precision parts has a great potential of reducing cutting times. With twice the cutting speed, the added cost of five axis manipulation can often be justified by a quick return of investment.

Traditional Waterjets vs Micro Abrasive Waterjets

Micro abrasive waterjets generally refer to jet diameters from 300 µm and smaller. Most operate by principles of a miniaturized traditional AWJ cutting head design. Typically, these cutting heads have a design optimizing precision over cutting speed. Also, other designs are being used to facilitate even finer jets.

The micro abrasive waterjet is up to 10 times more accurate than the traditional waterjet and is suitable for fine mechanical products. The traditional waterjet is less accurate, gives a rougher surface and is used for rough cutting.

The miniaturized cutting system requires premium quality abrasive. Albeit, more expensive the consumption is typically one tenth of what we see in traditional waterjets. A 200µm jet typically consumes ca 0.2 liter water and 10 grams of abrasive per minute. Even though costs of the precision powder are higher compared to traditional waterjet abrasives, the running cost is low.

What Can I Cut Using an Abrasive Waterjet?

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The abrasive waterjet process is capable of cutting virtually all materials.

• Food
• Rubber
• Plastic
• Fiber wool
• Non-woven materials
• Corrugated cardboard

The feature (working principle?) of the waterjet allows cutting of very soft material or ultra-hard or anything in between. Materials can be categorized into different groups and in the table below you will find a summary of why the waterjet process is capable of cutting the specific group of material.

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Abrasive waterjet cutting is used mainly for manufacturing with materials that can not be cut easily and well by thermal cutting processes. It is also used for cutting single parts and for producing small and medium-sized series. This method is, of course, also suitable for larger series, for example, when a cold cutting process is an important requirement to retain certain desirable surface and material characteristics.

Choice of Material

• An AWJ can cut most materials, including very high-strength material. If a material is to be heat treated (e.g. quenched and/or tempered), it is recommended to carry out the cutting after the heat treatment. (This applies to through hardened material.)
• Laminated and/or brittle material can be difficult to cut and require a more experienced operator.
• Keep in mind that materials which are traditionally difficult to work with need not be so with abrasive waterjet cutting. Titanium alloys, for example, are easier to cut than steel.

• For AWJ cutting, the composition of an alloy, or possible material inclusions or reinforcements, does not play a major role. Cutting speed in composition materials (alloys) is usually the same as that used in the base material. The hardness of the base material is the most influential factor for each group of materials.
• In general, most materials, including very thick ones, can be cut with abrasive waterjet cutting.

Cutting 2-D geometries in all materials

The cutting of two-dimensional geometries in flat sheet and plate materials of any

thickness up to 300 mm is the most widespread application of abrasive waterjet cutting. However many companies, for various reasons, advertize the method as suitable up to 100 mm.

Two-dimensional AWJ cutting offers several advantages over other cutting methods in:

• It is very quick and easy to set up and program the system for different
• applications, since everything is cut with the same tool;
• Closed contours can be cut, as the AWJ is able to pierce a starting hole;
• Generally, machining can be done to within a tolerance of ± 0.1 mm in materials up to 100 mm thick;
• As the cutting forces are small, clamping requirements are relatively simple;
• The narrow jet diameter facilitates cutting complex geometries in relatively small parts ;
• The method produces a cold cut without any significant thermal or mechanical damage.

A flexible cutting method

The capacity of the AWJ beam to cut virtually all materials offers a very flexible method: the same tool can be used to cut most materials, even without adjustments other than cutting speed.

With modern CAD/CAM software, parts can be machined easily and quickly; the software helps to adapt both the tool paths and traverse rate to the desired quality of cut. This makes the technique suitable also for manufacturing prototypes and smaller series.

The waterjet job shop is able to cut almost any material; this flexibility makes it easy to turn to them for the most diverse cutting needs. If you later wish to produce something on a larger scale, you would naturally need to decide whether waterjet cutting or another method is the most cost effective choice.

Handling complex geometries

Since the small jet diameter makes possible a narrow cut width, this pointed tool offers good conditions for cutting complex shapes, even when the dimensions are relatively small.

Abrasive waterjet cutting process

Although the capability of the abrasive waterjet is capable of cutting virtually all materials, a fundamental understanding of operating parameters can be quite helpful for optimizing the cutting performance as well as the economy of the process. Below the main operating parameters listed in figure 2.6 are discussed.

Figure 2.6: The performance of an abrasive waterjet is controlled by a rather large number of parameters includingthe above mentioned main parameters. Cutting capability is ultimately determined by the jet-material interactionresulting from the process parameter selection and the cutting conditions obtained from the machining parametersettings. The cutting results obtained are, for example, cutting speed, depth of cut, kerf width, surface finish ormaterial removal rate.

Machining parameters

• traverse rate
• standoff
• jet attack angle
• numbers of traverse

Hydraulic parameters

• water pressure
• waterjet diameter

Work material parameters

• hardness
• ductility - brittleness
• fracture strength
• flow stress
• strain hardening
• strain rate sensitivity
• modulus of elasticity
• thermal shock resistance
• …etc.

Abrasive parameters

• abrasive material
• abrasive particle size
• abrasive particle shape

Mixing parameters

• abrasive flow rate
• focusing tube geometry
• (incl. diameter, length)

Multi Axis Cutting

In recent years, cutting tables with five-axis control of cutting heads have been

introduced. These machines generally feature the capacity to tilt and sometimes to rotate the cutting head, which means that beveled cuts can be made along contours. This enables new capabilities such as the following:

• Cutting three-dimensionally, which makes it possible to cut holes and contours in forgings, castings and arched or formed sheet stampings
• Contouring and beveling for preparation of a welding joint performed in one operation
• Compensation for angularity of the kerf wall. (While one kerf wall becomes angular, the angular inaccuracy of the other wall will be doubled, resulting in a scrap side of the cut.)

Challenges of multiple-axis cutting

A problem inherent in multiple-axis cutting is the increased complexity of the programming, which means more advanced CAM programs are required, and there are higher demands on the software users.

In addition, the more the jet is tilted towards the surface at an attack angle other than 90 degrees, the more important a constant stand-off distance becomes. This problem is accentuated because the true stand-off distance at the center axis of the jet becomes more difficult to measure accurately as the jet leans to one side. Although this type of machine is still somewhat exotic, the advantages it offers are likely to make it more popular in the near future.

More common, however, are the 5-axis machine systems made for cutting flat plates.

This type of cutting table can be used to cut beveled edges, for example in welding preparation. They can also compensate for the small taper in the cut that is normally produced in the cutting process. This feature is called Taper Angle Control (TAC).

By making a simple cutting test of three different cutting speeds (fine, medium and rough cut) in the target material the operator receives a sample part. Angularities at these three cutting speeds are then measured and inserted as data into the CAM program.

With this input from the operator the CAM software will produce a program for 5-axis control compensating for the natural kerf taper angle.

Waterjet cutting control and programming

A majority of cutting applications are made in two-dimensional paths, which requires a controller that at least interpolates 2 servo axes simultaneously.

On waterjet cutting machines 3 axis control systems are the base standard. To enable cutting with compensation of form errors or to produce more complex 3D geometries, a 5-axis controller is required.

Full 5 axis control normally introduces the concept of programming motion in respect to a tool center point (TCP). This means that the 5-axis controller enables simultaneous synchronized motion of 5 axes that allows moving along a 3D path in the workspace with a possibility to also vary the jet angle towards the TCP while moving along the path.

To obtain high-precision parts with the micro abrasive waterjet, a smooth and precise path motion control is required.

To achieve smooth motion, high-end controllers with very fine interpolation in the nanometer range are used. A closed position feedback loop is used and absolute measuring scales at high resolution are required. These need to be insensitive to thermal influence and in order to achieve this, floating scales of Invar can be used which have virtually no thermal expansion.

When programming micro parts it is important that the CAD/CAM software is adapted to minute parts. The software must handle parts at a resolution that is high enough to meet the precision requirements of the part. Numerical truncation errors may accumulate leading to less accurate parts.

The jet dynamics associated with the material removal processes require the use of CAD/CAM software specialized for the waterjet process. Such software takes into account the physical behavior of the jet as it follows the path along specific geometrical shapes.

In modern CAD-CAM systems it is simple to utilize the 5-axis control system for form error compensation. Parts are programmed as 2D parts and all necessary angular compensation is applied using a compensation database using experimentally assessed data.

The Waterjet Cut Quality & Precision

Cut quality classification (quality)

With appropriate selection of cutting speed and parameter settings, the abrasive waterjet process can produce parts with a choice of cut surfaces that ranges from rough separating cuts to precise taper-free cuts. An industrial praxis for cut quality classification has become widely accepted among AWJ systems manufacturers and operators. Although this may vary slightly among practitioners, cut quality is usually divided into five classes ranging from an extra rough quality index, q = 1, to an extra fine quality index, q = 5.

Figure 5.5 : Steel cut to the quality classes q = 1 to q = 5 according to the Zeng & Kim equation. Notethat quality indexes q = 4 and 5, which are both very fine, differ in better angularity and less burr forq = 5.

Micro abrasive waterjet has cut quality classes 1-9 Explanation and images