02-11-2016, 11:16 AM
1463471499-WJM2.ppt (Size: 949.5 KB / Downloads: 362)
Introduction
Key element in WJM – a jet of water.
Water jet travels at velocities as high as 900 m/s (approximately Mach 3).
When the water stream strikes a workpiece surface, the erosive force of water removes the material rapidly.
The water, in this case, acts like a saw and cuts a narrow groove in the workpiece material.
WJM – Components & Functions
1. Hydraulic Pump
Powered from a 30 kilowatt (kW) electric motor
Supplies oil at pressures as high as 117 bars.
Compressed oil drives a reciprocating plunger pump termed an intensifier.
The hydraulic pump offers complete flexibility for water jet cutting and cleaning applications.
It also supports single or multiple cutting stations for increased machining productivity.
2. Intensifier
Accepts the water at low pressure(typically 4 bar) and expels it, through an accumulator, at higher pressures of 3800 bar.
The intensifier converts 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 ultrahigh-pressure output.
During the plunger inlet stroke, filtered water enters the high-pressure cylinder through the check value assembly.
After the plunger reverses direction, the water is compressed and exits at ultrahigh pressure.
3. Accumulator
Maintains the continuous flow of the high-pressure water and eliminates pressure fluctuations.
It relies on the compressibility of water (12 percent at 3800 bar) in order to maintain a uniform discharge pressure and water jet velocity, when the intensifier piston changes its direction.
4. 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 manually or through a remote-control valve specially designed for this purpose.
5. Jet Cutting Nozzle
Nozzle provides a coherent water jet stream for optimum cutting of low-density, soft material that is considered unmachinable 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 through multistage filtration, which removes 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 multiaxis robotic installations.
6. Catcher
Acts as a reservoir for collecting the machining debris entrained in the water jet.
Moreover, it reduces the noise levels [105 decibels (dB)] associated with the reduction in the velocity of the water jet from Mach 3 to subsonic levels.
Jet Nozzle
Standoff distance - Gap between the jet nozzle (0.1–0.3 mm diameter) and the workpiece.
Typical range - 2.5 – 6 mm.
However for materials used in printed circuit boards, it may be increased to 13 to 19 mm.
But larger the standoff distance, smaller would be the depth of cut.
When cutting fiber-reinforced plastics, reports showed that the increase in machining rate and use of the small nozzle diameter increased the width of the damaged layer.
Jet Fluid
Typical pressures used are 150 to 1000 MPa to provide 8 to 80 kW of power.
For a given nozzle diameter, increase in pressure allows more power to be used in the machining process, which in turn increases the depth of the cut.
Jet velocities range between 540 to 1400 m/s.
The quality of cutting improves at higher pressures by widening the diameter of the jet and by lowering the traverse speed.
Under such conditions, materials of greater thicknesses and densities can be cut.
Moreover, the larger the pump pressure, the greater will be the depth of the cut.
The fluid used must possess low viscosity to minimize the energy losses and be noncorrosive, nontoxic, common, and inexpensive.
Water is commonly used for cutting alloy steels.
Alcohol is used for cutting meat, while cooking oils are recommended for cutting frozen foods.
WJM – 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 completely 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, harder 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.