What is Waterjet cutting?
Pure Water Jet Cutting
Waterjet cutting technology is based on the use of one or several jets of water with very high power density. Up to 30kW of power within a fine water jet, of the size of a needle. Compressed water at a pressure of 400MPa or more is conveyed in stainless steel tubing to a nozzle, usually of 0.1 - 0.3mm in diameter. As the water escapes the nozzle it approaches a velocity of 900m/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 litres per minute.
The water jet cuts soft materials by displacing or eroding material from the work piece. Plastics (both solid and foamed), rubber, floor mats, other non-woven materials, plywood, mdf 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 work piece; this is especially advantageous in the case of lead which shouldn't enter the environment.
Fibre materials are another group of materials that can be cut to advantage with a pure water jet. Examples are fibreglass and carbon fibre. When the stream of water strikes this type of material, micro cracks become pressurised. The pressure causes cracks to propagate and a network of cracks causes material to quickly disintegrate and be washed away by the water stream. The combined capacity of water jets to cut soft plastics as well as brittle fibre material is especially useful in the cutting of fibre-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 water stream. Hence this feature also reduces a problem in the working environment where these products are machined.
Abrasive Water Jet Cutting
During the 1980's a breakthrough was made in water jet technology. By adding abrasives to the water jet , a completely new process was generated which led to an enormous expansion of applications for water jet cutting. Practically any material could now be cut and cutting of metal and concrete of dimensions of up to 300mm can be achieved. 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.
The principal of the AWJ cutting head is shown in the drawing above. 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 tube or focusing tube. The watejet formed in the orifice is gradually broken into small droplets in the cutting head. In the mixing chamber, these droplets transfer energy to the abrasive particles which are fed into the cutting head and accelerated to high speeds. Abrasives and water are then focused into the 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 ca 4% water, 1% abrasive and the rest is air. The erosive power of an abrasive water jet makes it considerably more powerful than a water only jet. The diameter of the focusing tube, which is usually about 0.5 - 1.2mm, although dimensions as small as 0.3mm exist. The cutting result depends on a large number of parameters that influence both power supply and efficiency, which in turn determines cutting speed and cut quality.
The advantages usually attributed to AWJ cutting are
· It is extremely viable process that can cut practically any material even at great thicknesses.
· The same tool setup can be used for almost all materials saving setup costs.
· No heat affected zone on the cut surface.
· Virtually no mechanical damage on the cut surface.
· Narrow cut width of the jet (about 0.5 to 1.2mm) depending on material saving material costs.
· Narrow omnidirectional jet can cut complex geometries.
· Low cutting forces (<50 N) means low requirements for clamping and fixturing.
· Can cut thin materials because of the low cutting forces.
· Cuts thick material including 300mm steel and titanium.
· Minor or no burr formation.
· Multiple cutting heads for parallel cutting, thus raising productivity therefore saving money.
· Subsequent machining processes can often be eliminated.
When Can I Use AWJ Cutting
Abrasive waterjet cutting is used mainly for manufacturing with materials that can't be cut easily and well by thermal cutting processes. The cutting of two-dimensional geometries in flat sheet and plate materials of any thickness up to 300mm is the most widespread application of abrasive waterjet cutting. Abrasive waterjet cutting offers several advantages over other cutting methods in:
· 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 +/- 0.25mm in materials up to 100mm thick, but tolerances of +/- 0.1mm are achievable.
· 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.
· Avoid subsequent finishing of the work piece.
· As the cutting forces are small, clamping requirements are relatively simple.
A flexible cutting method
The capacity of the AWJ beam to cut virtually all materials offers very flexible method. With AWJ the same tool can be used to cut most materials, even without adjustments other than the 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 larger series.
Machining with minimum material waste to near-net-shape
The narrow kerf, about 1mm wide means that very little material is wasted in cutting. The use of nesting software means that the sheet material can be utilized as efficiently as possible. This type of software helps to lay out all of the parts in such a way that together they cover as much of the sheet or plate as possible, thereby reducing the amount of waste and reducing costs.
Suitable method for hard-to-cut materials (exotic materials and composites)
Materials can have many diverse properties which make them difficult to cut, for example they can be:
· Very hard
· Very soft with low bearing strength
· A combination of hard and soft
· Heat sensitive.
Some materials have properties, for example hardness or hard constituents, which mean that they can't be cut with conventional tools. The AWJ cutting process can effectively cut very hard material, such as engineering ceramics. Hard and brittle material is also suitable for AWJ cutting, since the forces used are low, which makes the material less likely to break.
· In general most materials including very thick ones, can be cut with abrasive waterjet cutting.
· An AWJ can cut most materials, including very high-strength materials. If a material is to be heat treated, it's recommended to carry out the cutting after the heat treatment.
· Materials which are traditionally difficult to work need not be so with a AWJ. e.g. Titanium alloys are easier to cut than steel.
· For AWJ cutting, the composition of an alloy, or possible material inclusions or reinforcements, doesn't play a major role. Cutting speed in composition materials (alloys) is usually the same as that in the base material. The hardness of the base material is the most influential factor for each group of materials.
Design / Drafting Tips
· Most 2-D geometries can be cut using a jet diameter of 0.5 to 1.5mm depending on the application.
· Mark the drawing to indicate any surface which doesn't have close tolerances. This can save money by allowing faster cutting.
· Avoid small radii and internal sharp corners if they're not required as they reduce the cutting speed and make the contouring operation more expensive.
· Use objects that are as simple as possible.
· Draw as precisely as possible in the middle of the tolerance range.
· Draw in the CAD coordinate system.
· Use a 1:1 scale in the drawing.
· Be sure that all corners are closed.
· Be sure to remove any possible double lines that may be hidden in the drawing.
The abrasive media commonly used is garnet. Garnet is a natural mineral (a semi-precious stone) either found in alluvial deposits (natural sand) or rock inclusions that are crushed and separated. The waterjet abrasives are sieved to a narrow particle size and washed clean to remove any dust.
Opportunities in Manufacturing
When product design is based on the requirements of function, then the choice of material, economic considerations and design technique must take into account the manufacturing process, environmental requirements and efficient use of materials. Material technology is developing continuously, which offers new ways for engineers to improve their product designs. There is a trend towards using high-strength materials, for making lighter and stronger constructions possible. This, in its turn presents challenges to manufacturing engineers, who must devise suitable tools to master these new materials which are often difficult to work. It can be said, somewhat simplified, that the traditional machining processes (forming, shearing and cutting) are based on the principle that tools made of high-strength materials are used to form or cut softer more yielding materials. Many modern high-strength materials now have material properties for which it has not been possible to find strong enough tool materials to allow machining in the traditional way; this is why non-traditional methods are sometimes needed, as they are not as limited by the mechanical strength. Each mechanical process interacts with a given material in different ways and leaves, thereby, a characteristic "print", which influences both the geometry and material characteristics.
Comparison of industrial cutting methods
The term industrial cutting here means the cutting of parts from bulk material, often sheet material of various dimensions. Traditionally this has been done by thermal processes, such as oxyfuel flame cutting, high definition plasma cutting and laser cutting. In recent years abrasive waterjet cutting has been developed as an important complement to the thermal methods.
Abrasive waterjet cutting has the capability to cut virtually any material and dimensions for which thermal methods have been traditionally used. In most cases where the damage, in the form of a recast layer, hardening, annealing and local changes in the material structure due to heat effects which results from thermal cutting is acceptable, these methods tare faster. The AWJ cutting process however, attracts attention because of its cold cut, which doesn't damage the original material and is generally distinguished by its good precision. According to the cutting standard ISO 9013, AWJ cutting is usually to be found within the two best cut quality categories. For all methods in general, the tolerances which can be obtained are reduced with increasing thickness of the work piece. Since the precision of an AWJ cut is not reduced by increasing thickness at the same rate as the thermal cutting methods generally are, the AWJ method stands out comparatively a precision method, especially for thicker materials.
When comparing costs for alternative cutting methods, it's important to consider the whole picture. For example, AWJ cutting has the advantage that finishing work in subsequent operations cab often be avoided, with savings of both time and money. With these characteristics, abrasive waterjet cutting has become partly a competitive method, but perhaps more important, also a complement to thermal cutting methods.
From an environmental perspective, abrasive waterjet cutting 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 gasses as well as electromagnetic and ultraviolet radiation formed by some thermal methods can be avoided, which is desirable for work environments.