Abrasive Water-Jet Machining

            Abrasive Water-Jet Machining
                            (AWJM)

The use of high-pressure water jets, which may contain abrasive powder, for cutting and removing materials. For example, water accelerated up to twice the speed of sound [343 m/s (1125 ft/s) at 20°C (68°F)] can penetrate and cut rock in a few seconds.
                                        

Among the methods of cutting metal and nonmetallic materials, pure and abrasive water-jet cutting techniques have a distinct advantage because of their versatility and speed. They can cut all materials, including hard-to-machine materials such as superalloy, Kevlar, and boron carbide. They can also easily cut aerospace materials such as graphite composite and titanium, and brittle materials such as advanced ceramics, granite, marble, and glass (see illustration). The pure water jet is used by the food industry to cut candy and chocolate bars, meats, vegetables, and fruits. It is being tested for orthopedic surgery applications in bone cutting and scaling the flesh from bones. Other biomedical applications include a nonsurgical water-jet system for rapidly removing clots from blood vessels, and a water jet for corneal surgery.

The advantages of pure and abrasive water-jet cutting are (1) absence of thermal distortion and work hardening; (2) noncontact during cutting, thus eliminating tool wear and contact force; and (3) omnidirectional cutting, allowing the cutting of complex shapes and contours.
Although the use of the water-jet system is rapidly growing, the technique has some drawbacks and limitations. Water-jet technology has not yet developed fully for high-tolerance and -precision machining. The initial capital investment for the system, including the motion-control equipment and operating costs, is relatively high. The noise level (80 adjusted decibels) is somewhat high, but the system can be specially designed to isolate the noise source.
                             

The water-jet pump and its delivery system are designed to produce a high-velocity jet stream within a relatively short trajectory distance, since the kinetic energy of the water and abrasive particles is directly proportional to the square of the jet velocity. In abrasive jet cutting applications, the abrasives entrained in the jet stream usually attain approximately 80% of the water-droplet velocity at the nozzle tip. The jet cuts the material by a rapid erosion process, when its force exceeds the compressive strength of the material. Since the area eroded by the abrasive is also swept by the water stream, the heat generated during the cutting is dissipated immediately, resulting in a small rise in temperature (less than 90°F or 50°C) in the workpiece. Therefore, no thermal distortion or work hardening is associated with water-jet cutting. The cutting by rapid erosion also significantly reduces the actual force exerted on the material, enabling the water jet to cut fragile or deformable materials such as glass and honeycomb structures. See also Jet flow; Metal, mechanical properties of; Shear.

                                  

      

                         final product of Abrasive Water-Jet Machining

           

                                         Abrasive waterjet

While cutting with water is possible, the addition of an abrasive turned the waterjet into a modern machining tool. This began in 1935 when the idea of adding an abrasive to the water stream was developed by Elmo Smith for the field of liquid ablative blasting. Smith’s design was further refined by Leslie Tirrell of the Hydroblast Corporation in 1937, resulting in a nozzle design that created a cohesive mix of high-pressure water and abrasive. Smith and Tirrell’s nozzle design set the groundwork for an efficient abrasive waterjet cutting tool. Producing a commercially viable abrasive waterjet nozzle came next, and an engineering research team at Flow Industries built on their high-pressure pump research, as well as Tirrell’s original nozzle design from the 1930s and Schwacha’s research in high-pressure waterjet cutting from the 1950s, to develop the modern abrasive waterjet nozzle. The final step was creating a durable mixing tube that could withstand the power of the high-pressure abrasivejet, and it was Boride Products (now Kennametal) development of their ROCTEC line of ceramic tungsten carbide composite tubes that significantly increased the operational life of the abrasivejet nozzle.



              

                                               Process

There are six main process characteristics to water jet cutting:

1. Uses a high velocity stream of abrasive particles suspended in a stream of Ultra High Pressure Water (30,000–90,000 psi) which is produced by a water jet intensifier pump.
2. Is used for machining a large array of materials, including heat-sensitive, delicate or very hard materials.
3. Produces no heat damage to workpiece surface or edges.
4. Nozzles are typically made of sintered boride.
5. Produces a taper of less than 1 degree on most cuts, which can be reduced or eliminated entirely by slowing down the cut process.
6. Distance of nozzle from workpiece affects the size of the kerf and the removal rate of material. Typical distance is .125″ (3.175 mm).

Temperature is not as much of a factor.