WJM is appropriate for cutting plastics, foods, rubber insulation, automotive carpeting and headliners, and the majority textiles. Harder materials such as glass, ceramics, concrete, and tough composites can be cut by addition of abrasives to the water jet during abrasive water jet machining (AWJM), which was first developed in the year of 1974 to clean metal prior to surface treatment of the metal. The addition of abrasives to the water jet enhanced the material removal rate (MRR) and produced cutting speeds between 51 and 460 mm/min. commonly, AWJM cuts 10 times faster than the conventional machining methods of composite materials. Zheng et al. (2002) claimed that the abrasive water jet is hundreds, if not thousands, of times additional powerful than the pure water jet.
AWJM uses a small pressure of 4.2 bar to accelerate a large volume of a water (70 percent) and abrasive (30 percent) combination up to a velocity of 30 m/s. Silicon carbides, corundum, and glass beads of grain size 10 to 150 μm are often used as abrasive materials (Fig. 1). Using such a method, burrs of 0.35 mm height and 0.02 mm width left in steel component subsequent to grinding are removed by the erosive effect of the abrasives while water acts as an abrasive carrier that dampens its impact effect on the surface. The introduction of compressed air to the water jet enhances the deburring action.
fig.1 AWJM elements
In AWJM, the water jet stream accelerates abrasive particles, not the water, to cause the material removal. After the pure water jet is created, abrasives are added using either the injection or suspension methods shown in Fig. 2. The main parameters of the abrasives are the material structure and hardness, the mechanical behavior, grain shape, grain size, and distribution.
The basic machining system of AWJM incorporates the following elements.
_ Water delivery
_ Abrasive hopper and feeder
_ Mixing chamber
_ Cutting nozzles
Fig.2 Injection and suspension jets.
Typical process variables include pressure, nozzle diameter, and standoff distance, abrasive type, grit number, and work piece feed rate. An abrasive water jet cuts through 356.6-mm-thick slabs of concrete or 76.6-mm-thick tool steel plates at 38 mm/min in a single pass. The produced surface roughness ranges among 3.8 and 6.4 μm, while tolerances of ±0.13 mm are obtainable. Repeatability of ±0.04 mm, squareness of 0.043 mm/m, and straightness of 0.05 mm per axis are expected. Foundry sands are frequently used for cutting of gates and risers. However, garnet, which is the most ordinary abrasive material, is 30 percent more effective than sand.
During machining of glass a cutting rate of 16.4 mm3/min is achieved, which is 4 to 6 times that for metals. Surface roughness depends on the work piece material, grit size, and type of abrasives. A material with a high removal rate produces large surface roughness. For this reason, fine grains are used for machining soft metals to obtain the same roughness as hard ones. The decrease of surface roughness, at a smaller grain size, is related to the decreased depth of cut and the undeformed chip cross section. In addition the larger the number of grains per unit slurry volume, the additional that fall on a unit surface area.
A carrier liquid consisting of water with anticorrosive additives has a much superior density than air. This contributes to higher acceleration of the grains with a consequent higher grain speed and increased metal removal rate. Moreover, the carrier liquid spreads over the surface filling its cavities and forming a film that impedes the striking action of the grains. Bulges and the tops of surface irregularity are the first to be affected, and the surface excellence improves. Kaczmarek (1976) showed that the use of water air jet permits one to find, on average, a roughness number higher by one, as compared with the effect of an air jet. In high-speed WJM of Inconel, Hashish (1992) concluded that the roughness raises at higher feed rates as well as at lower slurry flow rates.
Advanced water jet and AWJ machines are now existing where the computer loads a computer-aided design (CAD) drawing from another system. The computer determines the start and ends and the sequence of operations. The operator then feeds the material type and tool offset data. The computer determines the feed rate and performs cutting. Other machining systems function with a modem and CAD/computer-aided manufacturing (CAM) capabilities that permits transfer from CATIA, AUTOCAD, IGES, and DXF formats. The computer runs a program that determines, in seconds, how to minimize the waste when cutting from blocks or plates.