WATER JET CUTTING

WATER JET CUTTING

 INTRODUCTIONDoff                                                                                                                                                                                                                                                   In the battle to reduce costs, engineering and manufacturing departments are    constantly on the lookout for an edge. The water jet process provides many unique capabilities and advantages that can prove very effective in the cost battle. Learning more about the water jet technology will give us an opportunity to put these cost-cutting capabilities to work. Beyond cost cutting, the water jet process is recognized as the most versatile and fastest growing process in the world. Water jets are used in high production applications across the globe. They compliment other technologies such as milling, laser, EDM, plasma and routers. No poisonous gases or liquids are used in water jet cutting, and water jets do not create hazardous materials or vapors. No heat effected zones or mechanical stresses are left on a water jet cut surface. It is truly a versatile, productive, cold cutting process. The water jet has shown that it can do things that other technologies simply cannot. From cutting whisper, thin details in stone, glass and metals; to rapid whole drilling of titanium; for cutting of food, to the killing of pathogens in beverages and dips, the water jet has proven itself unique.

THEORY OF WATERJET CUTTING

         

          Most water jet cutting theories explain water jet cutting as a form of micro erosion as described here .Water jet cutting works by forcing a large volume of water through a small orifice in the nozzle. The constant volume of water traveling through a reduced cross sectional area causes the particles to rapidly accelerate. This accelerated stream leaving the nozzle impacts the material to be cut. The extreme pressure of the accelerated water particles contacts a small area of the work piece. In this small area thework piece develops small cracks due to stream impact. The water jet washes away the material that "erodes" from the surface of the work piece. The crack caused by the water jet impact is now exposed to the water jet. The extreme pressure and impact of particles in the following stream cause the small crack to propagate until the material is cut through.

  

 WORKING PRINCIPLE

            For cutting thicker materials some abrasives like garnet is added to pure water jet. This is called abrasive water jet and cutting using abrasive jet is called abrasive jet cutting. The high-pressure abrasive jet cuts thicker and harder materials. Abrasive jet cutting is advancement in the field of water jet cutting

 WATERJET CUTTING PROCESS

            Pure water jet is the original water cutting method. Water jet cutting uses only a pressurized stream of water to cut through material. This type of cutting is limited to material with naturally occurring small cracks or softer materials like disposable diapers, tissue paper, and automotive interiors. In the cases of tissue paper and disposable diapers the water jet process creates less moisture on the material than touching or breathing on it. The figure shows the water jet cutting process.

               In this process water is increased in pressure by high-pressure pump to about 40000-60000 PSI and is forced through the orifice on to the target material. This high-pressure water on striking the surface performs the machining operation. The potential energy contained in the water is converted in the process to kinetic energy, i.e., into jet velocity, thus achieving its "cutting" effect.

PURE WATERJET

          

The basic water jet process involves water flowing from a pump, through plumbing, and out a c

 utting head.

 In water jet cutting, the material removal process can be described as a supersonic erosion process. It is not pressure, but stream velocity that tears away microscopic pieces or grains of material. Pressure and velocity are two distinct forms of energy. The pump’s water pressure is converted to the other form of energy, water velocity by a tiny jewel. A jewel is affixed to the end of the plumbing tubing. The jewel has a tiny hole in it. The pressurized water passes through this tiny opening changing the pressure to velocity. At approximately 40,000 psi the resulting stream that passes out of the orifice is traveling at Mach 2. And at 60,000 psi the speed is over Mach 3.

                                     

 ABRASIVE WATER JET CUTTING

The cutting ability of water jet machining can be improved drastically by adding hard and sharp abrasive particles into the water jet. Thus, WJM is typically used to cut so called “softer” and “easy-to-machine” materials like thin sheets and foils, non-ferrous metallic alloys, wood, textiles, honeycomb, polymers, frozen meat, leather etc, but the domain of “harder and “difficult-to-machine” materials like thick plates of steels, aluminum and other commercial materials, metal matrix and ceramic matrix composites, reinforced plastics, layered composites etc are reserved for AWJM.

Other than cutting (machining) high pressure water jet also finds application in paint removal, cleaning, surgery, peening to remove residual stress etc. AWJM can as well be used besides cutting for pocket milling, turning ,drilling etc .One of the strategic areas where robotic AWJM is finding critical application is dismantling of nuclear plants.

MACHINE

Any standard abrasive water jet machining (AWJM) system using entrained AWJM methodology consists of following modules.

• LP booster pump

• Hydraulic unit

• Additive Mixer

• Intensifier

• Accumulator

• Flexible high pressure transmission line

• On-off valve

• Orifice

• Mixing Chamber

• Focussing tube or inserts

• Catcher

• CNC table

• Abrasive metering device

•Catcher

MIXING

Schematic view of mixing process

 Mixing means gradual entrainment of abrasive particles within the water jet and finally the abrasive water jet comes out of the focusing tube or the nozzle.

During mixing process, the abrasive particles are gradually accelerated due to transfer of momentum from the water phase to abrasive phase and when the jet finally leaves the focusing tube, both phases, water and abrasive, are assumed to be at same velocity.


 APPLICATIONS OF WATERJET CUTTING

 Flexible water jet cutting technology is used in practically all sectors of industry: Aerospace, residential and industrial construction, mechanical engineering, the glass industry, the wood, textiles and paper industries, the automotive and its supplier industries, and the electrical, electronic and foodstuffs industries. Unlike traditional thermal cutting methods, water jet cutting technology wins friends with its high level of cost-effectiveness and flexibility. The most diverse materials, from metal via plastics up to and including granite, can be quickly and precisely worked using a high-pressure jet of water. Material thickness of 150 mm or more present no difficulties to our cutting processes. The water jet achieves optimum cut-edge qualities on both simple and extremely complex contours.

General 2D applications

• Sheet metal: Stainless steel, carbon steel, high-alloy nickel steels, aluminum, titanium, copper
• Building: Decorative stone, marble, granite, tiles, plasterboard, glass and  mineral      wool
• Glass: Laminated glass, safety glass, and bulletproof glass
• Foodstuffs: Baked goods, deep-frozen products and fish
• Paper: Cardboard, corrugated cardboard, printing papers
• Miscellaneous: Plywood, leather, textiles, composites, rubber, plastics, sealing materials and foams .

3D and robot applications:

• Abrasive: Titanium, aluminum and stainless steel motor-vehicle components, turbine blades, decorative stone or marble.
• Pure water: Motor-vehicle elements such as carpets, door-trims, fenders, dashboards, instrument panels, rear shelves.

Application

The applications and materials, which are generally machined using WJ and AWJ, are given below:

1.      Paint removal

2.      Cleaning

3.      Cutting soft materials

4.      Cutting frozen meat

5.      Textile, Leather industry

6.      Mass Immunization

7.      Surgery

8.      Preening

9.      Cutting

10.  Pocket Milling

11.  Drilling

12.  Turning

13.  Nuclear Plant Dismantling

Materials

• Steels

• Non-ferrous alloys

• Ti alloys, Ni- alloys

• Polymers

• Honeycombs

• Metal Matrix Composite

• Ceramic Matrix Composite

• Concrete

• Stone – Granite

• Wood

• Reinforced plastics

• Metal Polymer Laminates

• Glass Fiber Metal Laminates

ADVANTAGES

 • The AWJ easily machines titanium, nickel alloys and very hard brittle materials

• It is a cold cutting process, machining without creating thermal distortions or thermally driven changes to the metallurgical and crystal structure (no brittle recast or micro-cracks as left by laser and EDM)

• The work piece sees very low tool loads with the abrasive-water jet and machining delicate part features is an option

• Glass and ceramic materials can be machined to complex shapes without the sub-surface flaws created during grinding

• Drilling small, deep holes is easily performed with the AWJ

• AWJ cuts more materials and is faster than EDM

• AWJ cuts more materials and is faster than laser with thick materials

The AWJ has many advantages over other machining technologies, but has remained a niche market in part because of limitations involved in machining to close tolerances. The capabilities at Ormond make it feasible to address closer tolerance work and take advantage of AWJ benefits over a broader range of applications.

  DISADVANTAGES

•      One of the main disadvantages of water jet cutting is that a limited number of materials can be cut economically. While it is possible to cut tool steels, and other hard materials, the cutting rate has to be greatly reduced, and the time to cut a part can be very long. Because of this, water jet cutting can be very costly and outweigh the advantages.

•       Another disadvantage is that very thick parts can not be cut with water jet cutting and still hold dimensional accuracy. If the part is too thick, the jet may dissipate some, and cause it to cut on a diagonal, or to have a wider cut at the bottom of the part than the top. It can also cause a rough wave pattern on the cut surface.

•      Taper is also a problem with water jet cutting in very thick materials. Taper is when the jet exits the part at a different angle than it enters the part, and can cause dimensional inaccuracy. Decreasing the speed of the head may reduce this, although it can still be a problem.

WATER JETS VS. OTHER MACHINING PROCESSES

Flame Cutting:

Water jets would make a great compliment to a flame cutting where more precision or higher quality is required, or for parts where heating is not good, or where there is a need to cut a wider range of materials.

Milling:

Water jets are used a lot for complimenting or replacing milling operations.  They are used for roughing out parts prior to milling, for replacing milling entirely, or for providing secondary machining on parts that just came off the mill.  For this reason, many traditional machine shops are adding water jet capability to provide a competitive edge.

Punch Press:

Some stamping houses are using water jets for fast turn-around, or for low quantity or prototyping work.  Water jets make a great complimentary tool for punch presses and the like because they offer a wider range of capability for similar parts.

 

 Environmental issues

Nowadays, every manufacturing process is being re-evaluated in terms of its impact on the environment. For example, use of conventional coolants in machining and grinding is being looked upon critically from the point of view of its impact on environment. The environmental issues relevant to AWJM are  :-

• Water recycling

• spent water disposal

• Chip recovery

• Abrasive recovery and reuse

Environmental issues and concerns have lead the researchers to use such mediums and abrasives that do not require disposal, recycling or lead to pollution. Work is going on in the area of high-pressure cryogenic jet machining (Fig. 16) where liquid nitrogen replaces the water phase and dry ice crystals (solid CO₂ crystals) replace the abrasive phase leading to no need of disposal or waste generation. The removed work material in the form of microchips can be collected much easily reducing the chances of environmental degradation.

FUTURE SCOPE

Since its development, water jet machining has seen many improvements in its design. Water jet cutting technology is one of the fastest growing major machine tool processes in the world due to its versatility and ease of operation. Manufacturers are realizing that there are virtually no limits to what water jets are capable of cutting and machining. Machine shops of all sizes are realizing greater efficiency and productivity by implementing UHP water jets in their operations. Water jets are becoming the machine tool of choice for many shops. Since abrasive water jet (AWJ) technology was first invented by Flow in the early 1980s, the technology has rapidly evolved with continuous research and development.

What makes water jets so popular? Water jets require few secondary operations, produce net-shaped parts with no heat-affected zone, heat distortion, or mechanical stresses caused by other cutting methods, can cut with narrow kerfs, and can provide better usage of raw material since parts can be tightly nested. As a result of the Flow Master PC control system and intuitive operation, water jets are extremely easy to use. Typically, operators can be trained in hours and are producing high quality parts in hours. Additionally, water jets can cut virtually any material, leaving a satin-smooth edge. These benefits add up to significant cost savings per part in industries that have traditionally defined productivity by cost per hour.

The latest development in the field of water jet cutting is the use of super water for cutting, which enhances both abrasive and non-abrasive water jet cutting.

CONCLUSION

—  Relatively new technology has caught on quickly and is replacing century-old methods for manufacturing.

—  Water Jet Cutting is not only used in typical machining applications, but also in food and soft-goods industries.

—  As material and pump technology advances faster cutting rates, longer component life and tighter tolerances will be achievable.

—  Water Jet Machining paves the way for new machining processes that embrace simplicity and have a small environmental impact.

 REFERENCE

en.wikipedia.org

www.o-keating.com