Water Jet Machining | Unconventional Machining Process

Introduction

The input element in water jet machining (WJM) is a water jet, which travels at velocities as high as 900 m/s (approximately Mach 3). When the stream hits a work piece surface, the erosive force of water eliminates the material rapidly. The water, in this casing, acts as a saw and cuts a narrow groove the work piece material.

01Water Jet Machining

                           Water Jet Machining                             

wjm1

Water Jet Machine

The Water Jet machining system

Figure.1 shows the WJM arrangement and the main parts of which it is composed.

(i)Hydraulic pump. The hydraulic pump is powered from a 30- kilowatt (kW) electric motor and provides oil at pressures as high as 117 bars in order to drive a reciprocating plunger pump termed an intensifier. The hydraulic pump offers total flexibility for water jet cutting and cleaning applications. It also supports single or many cutting stations for increased machining productivity.

01Water Jet Machining

fig.1 Schematic illustration of WJM system.

Intensifier. The intensifier admits the water at low pressure (typically 4 bar) and expels it, through an accumulator, at higher pressures of 3800 bar. The intensifier transfers the energy from the low-pressure hydraulic fluid into ultrahigh-pressure water. The hydraulic system provides fluid power to a reciprocating piston in the intensifier center section.

A limit switch, located at each end of the piston travel, signals the electronic controls to shift the directional control valve and reverses the piston direction. The intensifier assembly, with a plunger on each side of the piston, generates pressure in both directions. As one side of the intensifier is in the inlet stroke, the opposite side is generating ultra huge-pressure output.

During the plunger inlet stroke, filtered water enters the high-pressure cylinder throughout the check value assembly. After the plunger reverses direction, the water is compressed and exits at ultrahigh pressure.

Accumulator. The accumulator maintains the continuous flow of the high-pressure water and removes pressure fluctuations. It relies on the compressibility of water (12 percent at 3800 bar) in order to maintain a consistent discharge pressure and water jet velocity, when the intensifier piston changes its direction.

High-pressure tubing. High-pressure tubing transports pressurized water to the cutting head. Typical tube diameters are 6 to 14 mm. The equipment allows for flexible movement of the cutting head. The cutting action is controlled either physically or through a remote-control valve specially designed for this purpose.

Jet cutting nozzle. The nozzle provides a consistent water jet stream for optimum cutting of low-density, soft material that is considered un machinable by conventional methods. Nozzles are normally made from synthetic sapphire. About 200 h of operation are expected from a nozzle, which becomes damaged by particles of dirt and the accumulation of mineral deposits on the orifice due to erosive water hardness. A longer nozzle life can be obtained throughout multistage filtration, which eliminates undesired solids of size greater than 0.45 μm. The compact design of the water jet cutting head promotes integration with motion control systems ranging from two-axis (XY) tables to sophisticated multi axis robotic installations.

01WJM terminology

Fig.2 WJM terminology.

Catcher. The catcher performs as a reservoir for collecting the machining debris entrained in the water jet. Furthermore, it reduces the noise levels [105 decibels (dB)] associated with the decrease in the velocity of the water jet from Mach 3 to subsonic levels.

Water jet machining performance

Fig.3 Factors affecting WJM performance.

Process parameters:

Jet nozzle. The standoff distance, shown in Fig. 2, is the gap between the jet nozzle (0.1–0.3 mm diameter) and the work piece (2.5–6 mm). However, for materials used in printed circuit boards, it might be increased to 13 to 19 mm. For a nozzle of 0.12-mm diameter and cutting rate of 1.1 millimeters per second (mm/s), McGeough (1988) reported the reduce of the depth of cut at a larger standoff distance. When cutting fiber-reinforced plastics, reports showed that the augment in machining rate and use of the small nozzle diameter increased the width of the damaged layer.

Jet fluid. Typical pressures reported by McGeough (1988) are 150 to 1000 MPa, which provide 8 to 80 kW of power. For a known nozzle diameter, the increase in pressure allows more power to be used in the machining process, which in turn amplifies the depth of the cut. Jet velocities range between 540 to 1400 m/s. The quality of cutting develops at higher pressures by widening the diameter of the jet and by lowering the traverse speed. Under such conditions, materials of superior thicknesses and densities can be cut. Furthermore, the larger the pump pressure, the greater will be the depth of the cut. The fluid used must possess low viscosity to reduce the energy losses and be noncorrosive, nontoxic, common, and inexpensive. Water is generally used for cutting alloy steels. Alcohol is used for cutting meat, while cooking oils are advised for cutting frozen foods. Figure.3 summarizes different parameters affecting the performance of WJM.

Target material. Brittle materials will fracture, while ductile ones will cut well. Material thicknesses range from 0.8 to 25 mm or more.

Applications

WJM is used on metals, paper, cloth, leather, rubber, plastics, food, and ceramics. It is a versatile and cost-effective cutting process that can be used as an alternative to traditional machining methods. It eliminates heat-affected zones, toxic fumes, recast layers, work hardening, and thermal stresses. It is the most flexible and effective cleaning solution available for a variety of industrial needs. In general, the cut surface has a sandblast appearance. Moreover, unbreakable materials exhibit a better edge finish. Typical surface finishes ranges from 1.6 μm root mean square (RMS) to very coarse depending on the application. Tolerances are in the range of 25 μm on thin material. Both the produced surface roughness and tolerance depend on the machining speed.

Cutting. WJM is limited to fiberglass and corrugated wood.

Drilling. The process drills precision-angled and -shaped holes in a variety of materials for which other processes such as EDM or EBM are too expensive or too slow.

Machining of fiber-reinforced plastics. In this case the thermal material damage is negligible. The tool, being effectively pointed, accurately cuts contours. The main drawback is the deflection of the water jet by the fiber embedded in the matrix, which protrudes after machining. The feed rate attainable depends on the surface quality required.

Cutting of rocks. Water jet cutting of a 51-mm-deep slot in granite using two oscillating jets at 275 MPa during 14 passes at a 25.4-mm/s feed rate has been reported by McGeough (1988). Moreover an oscillating nozzle system operating at the same feed rate and pressure of 172 MPa, with the standoff distance adjusted every pass was used to cut a 178-mm-deep slot in sandstone.

Deburring. The method uses large pressures to remove large burrs (3 mm height) in 12-mm-diameter drilled holes in a hollow molybdenum-chromium steel shaft at 15 s using 700-bar pressure and a flow rate of 27 L/min. In this method burrs are broken off by the impact of water. A higher pressure (4000 bar) and a lower flow rate (2.5 L/min) are used to remove burrs from nonmetallic materials.

Cutting of printed circuit boards. Using a small-diameter water jet mounted near to the part edge, a printed circuit board (PCB) can be cut at a speed that exceeds 8 m/min, to the accuracy of }0.13 mm. Boards of various shapes for use in portable radios and cassette players can be cut using computer numerical control (CNC) technology.

Surface treatment. The process finds many applications including:

_ Removing deposits and residues without toxic chemicals, which eliminates costly cleanup and disposal problems

_ Surface cleaning of pipes and castings, decorative finishing, nuclear decontamination, food utensil cleaning, degreasing, polishing, preparation for precise inspection, and surface texturing

_ Economical surface preparation and coating removal

_ Removing corrosion, spray residue, soluble salts, chemicals, and surface damage prior to recoating or painting

Wire stripping. The process is able to remove the wire insulating material without damaging the metal or removing the tinning on the copper wire. The processing time can be decreased to about 20 percent of the manual stripping method (Metals Handbook, 1989).


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