It’s only water
September 1st 2005

Here Peter Longman of KMT Waterjet Systems explains why more companies are coming to realise the many benefits of waterjet cutting

More and more companies are coming to realise the benefits of the highly accurate, fast, environmentally compatible waterjet cutting method. Waterjet has the flexibility of being able to cut a broad spectrum of materials with no heat-affected zone, no hardening or material stresses, without producing any dust or smoke, leaving a smooth cut edge with no burrs.

So how is waterjet cutting able to achieve such versatility? Every cutting method relies on the input of energy into the material in order to overcome the chemical bindings present in its structure.

When looking at the operating principles of waterjet cutting it can be classified as a mechanical method. The energy of the rapidly moving jet – either of pure water or an abrasive mixture – is applied to the workpiece, causing microerosion of the material. During the operation the cutting water works as a cooling agent which also contributes to the high quality cut.

The pump develops a pressure in the primary oil circuit – achieved through the use of a variable displacement pump – which is then multiplied by a factor of 20 in the reciprocating intensifier assembly.

Today intensifier units with power ratings between 11 and 75kW (15-100HP) that generate output pressures of over 4000bar (60 000 psi) with flow rates between 1.2 and 7.6 litres/min can be supplied. Pumps can also be connected together to create even higher flow rates.

From the intensifier, the water passes through special high-pressure tubing, which is partially flexible to allow movements of the cutting head. The head itself consists of a pneumatically controlled needle valve and a nozzle tube, the end of which has a diamond or sapphire orifice, sealed in a cone. The orifice effectively converts energy again from pressure to velocity to produce an outgoing stream of water travelling at over twice the speed of sound.

In most cases the cutting medium consists of nothing more than tap water. In other cases a conventional water softener may be added to meet the specification.

During the cutting process, the workpiece is placed on a lattice above a water tank that is usually about one metre deep to help capture and disperse the energy of the waterjet as it exits the workpiece.

When cutting harder materials that require the hydro abrasive method, an abrasive cutting head is used. This is mounted below the orifice (0.1-0.35mm) and consists of a mixing chamber where the high velocity jet causes a vacuum that draws in air and abrasive sand (either garnet or olivine) through an entry port. In order to bring the mixture into alignment, a focusing tube (0.5-1.1 mm) of three times the diameter of the water orifice is required, out of which passes the sharp jet of water and abrasive mixture.

When considering investment in a waterjet cutting machine the capital cost needs to be understood. Waterjet compares favourably with laser cutters: for a similar sized machine, a waterjet cutting machine is likely to be around half the capital cost. However, its versatility is the key as it will allow the owner to cut a far more diverse range of materials and take on new customers from new market sectors. Its flexibility is also not to be overlooked and with multi-heads, it is also more cost effective than laser when cutting aluminium.

Unlike traditional thermal cutting methods, water-jet can cut the most diverse materials quickly and precisely using a high-pressure jet of water. Material thicknesses of 150mm or more present no difficulties. The water-jet achieves optimum cut-edge qualities on both simple and complex contours.

Water-jet cutting falls into general 2D and 3D applications. 2D includes the cutting of sheet metal, construction materials, glass, foodstuffs, paper, cardboard and miscellaneous materials such as: plywood, leather, textiles, composites, rubber, plastics, sealing materials and foams.

3D and robot applications cover abrasive cutting of titanium, aluminium and stainless steel motor-vehicle components; turbine blades, decorative stone or marble.

Pure water cutting can be used for motorvehicle components such as carpets, door-trims, fenders, dashboards, instrument panels, rear shelves.

The essential benefits of the water-jet cutting system are the fact that there are no heat-affected zones, no thermal hardening and no material stresses. From a health and safety point of view there are also no gases and no production of dust or fumes.

The production benefits of waterjet include the fact that there is no need to sharpen tools while cutting at high speed.

Such high cutting accuracy and high surface quality can be achieved that secondary operations such as deburring can be omitted and re-working becomes unnecessary. Burr-free cut edges, a narrow cutting gap and low tangential forces exerted on the component also provide significant production advantages.

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