Waterjet Cutting (WJC) is a non-traditional method in which a high velocity jet is used to machine a material. Often it is assisted by abrasives and & is known as Abrasive. Waterjet Cutting (AWJC). The process offers a wide range of advantages. It is omni-directional and enables fully automated 3-D cutting and has unlimited creative potential. The resulting kerf width is small and hence improved yield. Forces involved are minimum and non-crushed edges even on very soft materials. Work hardening is eliminated and heat affected zone (HAZ) is absent. Environment pollution is minimised since airborne dust and fumes are absent. No deburring is required on the processed part. It is a versatile process and can cut a variety of materials. However, initial cost of equipment is high. In addition excess waviness/striations on side wall surface may limit the depth of cutting in practice..
STRUCTURE AND MODE OF OPERATION
Figure 1 shows a schematic diagram of AWJC. Water is pressurised to about 400 MPa. A high velocity jet of 900 m/sec is obtained when the pressurised water passes through a nozzle, usually of sapphire, of diameter 0.6 mm. Abrasives delivered from a feed system is precisely controlled for its flow. These particles are entrained by the Waterjet stream and energy is transferred from water to the abrasive particles. The resulting AWJ is directed on to the target. Alternately abrasive suspension jets are used, in which premixing of abrasives with water is done to achieve their suspension (fluidised). These abrasive suspension jets are found to be economical. The distance between the target and the tip of the nozzle, Known as stand-off distance, is found to be critical in machining operations. Maximum material removal takes place at optimum stand-off distance. A catcher tank traps the unspent energy of the jet and the eroded particles. Large catcher tanks are used, or compact tube like catchers that move with nozzle are used for Omni-directional cutting.
Typical kerf width may range between 0.1. and 0.75. mm. Tolerances achievable are around 0.025 mm. The side wall of the cut surface is found to have two distinct zones, a smooth upper zone and a wavy bottom zone. Control of operating conditions during cutting ( jet pressure, abrasive flow rate and jet traverse rate) can be used to modulate the striations on the cut surface. However, motion control and vibrations due to rack/pinion that may be used in the system could lead to striations and hence design of system elements used for precision machining operations is considered very important.
A variety of abrasives are used in abrasive Waterjet cutting. Garnet is often used as abrasive material. Hard abrasives cut faster and deeper and also have less breakdown in mixing chamber. Sharp, angular abrasives cut
more efficiently than rounded ones because of penetration and shearing especially on difficult or thick materials. Heavier particles transfer higher energy and hence cut faster. The presence of impurities can reduce the effectiveness of the abrasive, for example, silica being lighter than garnet, if present, may reduce the cutting efficiency. Consistency in particle size distribution and range will result in even performance in cutting. Defects in abrasive mineral grains lead to unexpected behaviour of garnet. Removal of Fines and dust in abrasives used is critical to avoid clogging. Oversized abrasive particles, if present, lead to blocking and clogging in the nozzle. Hence, largest particle should be less than half the size of the nozzle. There are a number of variables to consider while selecting an abrasive. The most important is the abrasives' effect on the cost of cutting a unit length. This may be determined by experimentation and calculation of cost.
A researcher interested on the behaviour of abrasive may resort to micropetrography analysis of abrasives at various stages of AWJC starting with the fresh abrasives to those consumed in cutting the target material. Water soluble abrasives (eg. Aqua Dyne) are preferred in certain applications. Table 1 shows some materials and the cutting speeds used in AWJC.
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