Die-less cutting -Part 2 (Water Jet)
2.Waterjet Cutting Systems
Waterjet processing has developed from a technology originally used for processing paper and food product and some most advanced aerospace components.
The principle of cutting with water involves generating a high velocity stream of water which erodes away the material being processed. To achieve this water will be forced through a nozzle under a force of 4000 bars. Water under high pressure (High potential energy) while passing through Nozzle, it is converted into Kinetic energy (900 m/s). The cutting stream is as fine as human hair.
The first commercial applications early to mid 1970’s, and involved the cutting of corrugated cardboard. The largest water jet cuttings are used for disposable diapers, tissue paper, and automotive interiors. Water jet systems are more expensive than mechanical knife systems.
Water jet cutting is of two types
- Pure water jet (Without abrasive)
- Abrasive water jets (With abrasive)
a. Pure Water jet
Pure water jet is well suited for soft and pliable materials. The fluid used must have a low viscosity to minimize energy loses and it should be
- Be non-corrosive
- Common and inexpensive
Application of pure water jet
- Leather and Textiles
- Foamed plastics
- Cardboard, corrugated board
- Foodstuffs (eg. Ice cream)
Attributes of pure water jet
- Typical fluid is water. But additives such as alcohols, oils, products and glycerol are used, when they can be dissolved in water.
- Typical jet size <0.8mm faster than speed of sound. (540 m/s – 1400 m/s)
- Typical pressures are 150-1000 MPa and use 8-80 KW.
- Typical fluid volume is 0.5 to 2.5 l/min.
- Good for slotting.
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 of material. As the water exists out from the nozzle, the water is warmed as it is accelerated to high speed. Frictional forces warm the stream as it is exits the orifice. Water temperature is then raised 2 to 3 degrees for each 1,000 PSI. The quick the March 3 jet is approximately 170oF to 180o F.
Water Jet cutting system contains three basic components.
- Intensifier (To create High pressure)
- Nozzle (To form fine and concentrated velocity)
- Receiver (To collect used water)
1. Intensifier: An intensifier is simple, hydraulically driven, reciprocating plunger pump. Low-pressure oil is alternatively applied to each side of a large piston, causing it to move back and forth. The oil is supplied by a variable volume, pressure compensated pump driven by an electric motor. The reciprocating movement is achieved by directional valves. The movement of the large piston drives small diameter, high-pressure plungers which act against water in the small high-pressure cylinders. By varying the oil pressure on the large piston, it is possible to adjust the high-pressure output from some minimum value to a maximum system rating.
A filter is provided on the incoming water supply line. This filter provides sufficient protection from small impurities and foreign particles that might cause damage or reduce the life of high-pressure pump seals. No other processing is required unless there is an unusual accumulation of calcium, iron, or other mineral deposits with in the system. A second filter is placed after the accumulator to ensure further protection of the down stream nozzle.
2.Nozzle: The pressurised water is piped through high-pressure lines to the cutting point. In the nozzle, the pressure is relieved water jet is used for cutting. Nozzle diameters generally between 0.1 to 0.4 mm. The three common types of orifice (Nozzle) material are (sapphire, ruby, diamond) each has the unique attributes. Sapphire is the most common orifice. It is a single crystal jewel. It has a fairly good quality stream, and has a life, with good water approximately 50 to 100 cutting hours. Sapphire is used for pure water jet applications. Ruby can also be used in abrasive water jet applications. The cost of ruby is same as sapphire. Diamond has considerably longer run life (800 to 2,000 hours). Diamond is especially used where 24 hours per day operation is required. Diamond and other orifice types, can sometimes be ultrasonically cleaned and reused.
3. Receiver: In addition to collecting spent cutting fluid and small particles from the material being cut, the receiver provides sound reduction from noise created by Jet velocities, and also eliminates undue hazard which results from the exposed jet.
Operation: A typical system using a booster pump tackles normal tap water and filters and boosts the inlet line water pressure up to 8-10 bar. During the suction stroke of the intensifier this water enters one side of the intensifier through a check valve assembly and is drawn into high-pressure cylinder. The compressed water is now forced through the outlet check valve assembly into high-pressure line leading to the accumulator. The accumulator is fed by both sides of the intensifier on alternate compression strokes. The high-pressure water passes through a high-pressure filter unit to operating value (at a rate of 0 to 4 liters per second). The water stream then passes through the material being cut and into a receiver tube.
Advantages of Water Jet cutting:
- Die costs: This set will reduce the costs of dies required for cutting patterns.
- Pattern development costs: With CAD system reduce the need of preparation of patterns dies etc. You can completely remove the development costs.
3.Maintannece of Dies/Spares: Depending on the toughness or density of the material to cut, you may have to periodically sharpen and replace the dies/ spare dies. Once can avoid all these costs by using the cutting system.
- Material handling: Because of no dies involved, the cutting room can switch from product to product without any effort. No rack of dies and no material handling. So indirect labour cost can be reduced.
- Flexibility of production: It is easy to shift the production/cutting system from one area to another. All nesting and production master scheduling can be done at one place and can send the information to another plant.
- Reduction of work-in-progress inventory: With standard machinery water jet, lot sizes can vary from small to large without any changes in operating efficiency. This will reduce the large work-in-process inventory.
- Change over time: If any changes in the design the whole process (dies preparation) has to be repeated.
- Saving time and money :One of the main advantages of waterjet processing is its ability to save time and money. The elimination of thermal and mechanical stress was a major concern for the application
- Faster Cutting Speeds
- Can cut thicker, Thin or Harder Materials
- Replacement Nozzles are inexpensive
- Able to cut soft, light material (fibre glass insulation etc)
- 13.Non-heat cutting
Video courtesy: Leather cutting machine / water-jet / CNC / multi-head
b. Abrasive Water jet
Abrasive water jets were introduced to manufacturing in the early 1980s. Abrasive water jet is used to cut hard, brittle materials.
An abrasive waterjet can cut almost any material, including plastics, composites, laminates and glass, Aluminum, copper, steel and titanium can be cut up to 18 inches thick.
A series of high-pressure lines, pumps and intensifiers force a stream of filtered water through a tiny orifice. The stream, which is pressurized up to 4000 bars, mixes with fine abrasive in a tube to provide extraordinary cutting power. Since the tool doesn’t contact the material and produces virtually no heat, distortion-free cutting is achieved in even the thinnest of materials. Abrasive is added after the pure water stream is created. Then the abrasive particles are accelerated, like a bullet in a riffle mixing tube. The most common abrasive is garnet. Garnet is hard, tough and inexpensive, like colored sandpaper found at the hardware store; different mesh sizes are used for different.
The type of abrasive used depends on the material to be cut. So three factors decides the type of abrasive
- Hardness: The material to be cut should not harder than the adhesive.
- Grain size: the nozzle diameter must be greater than the maximum grain size. Finer abrasive produced a finer smooth cut.
- Surface: The abrasive particles must of sharp crystalline structure since the cutting process here is equivalent to micro chipping abrasion process.
Abrasive waterjet cutting eliminates stress and heat-affected zones to the material and never hardens. The abrasive waterjet process never changes the metallurgy of any alloy. Abrasive waterjet cutting also cuts with a narrow kerf or width of cut. This, too, enables close nesting of parts, maximizing material usage.
The abrasive water jets differ from the pure water jet in just few ways. In pure water jet supersonic stream erodes the material. In the abrasive water jet, the water jet stream abrasive particles and those particles, erodes the material and more powerful than a pure water jet.
Abrasive waterjet Attributes:
- No heat affected zones
- Thin stream ( 0.02 to .5 inch dia)
- Thin material cutting
- Stack cutting
- Little material loss
- Low cutting pressure ( below 1 lb)
- One jet setup for nearly all abrasive jobs.
- Quickly shifted from water jet to abrasive jet
- Little or no burr
Advantage of Abrasive jet
The advantages of pure water jet are applicable here also and
1.Taper: Abrasive waterjet cutting can produce more taper than other processes, including laser cutting. One of the biggest drawbacks with abrasive waterjet cutting is the taper created during the process.
2.Complicated Geometry: Abrasive waterjet processing is more capable of cutting more geometric shapes than other methods.
3.Heat-affected zone, re-cast and stresses: Unlike other methods like plasma or laser cutting, waterjet processing does not produce a heat-affected zone. HAZ occurs in metals when the temperature rises above the critical transformation point. Waterjet processing also does not cause stresses or distortion in material.
Cover Image resource: cutEXPERT doublejet
Source: Class notes prepared by A.V.Suresh, Ex.Sr . Technical officer, CFTI, Agra for students, while he was working in CFTI, Agra