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High-pressure jetcutting.

As the search for inexpensive and environmentally safe manufacturing processes has become more urgent, material removal through the use of high-velocity water jets has attracted the interest of engineers. This new book from ASME Press deals with water jet formation, the mechanics and applications of water jetcutting, and facility design.

A narrow liquid jet flowing at a high velocity from a small-diameter hole can act on a material with a force sufficient to break microscopic particles off its main bulk. This property of liquid jets motivated researchers in different countries to develop machining methods in which the jet kinetic energy is transformed into mechanical work for cutting. In general, when it is used as a cutting tool, the jet velocity is supersonic.

Jetcutting is relatively new, and for some applications, it is very effective; but it has been insufficiently studied and seldom used for the machining of different materials. Now, jetcutting equipment is being developed and introduced to industry in the Commonwealth of Independent States (C.I.S.), formerly the Soviet Union, and elsewhere. Specific problems in the formation of liquid jets and energy transformation in them are related to the technology of material machining by supersonic jets of various compositions and designs and to the operation of special equipment.

Successful studies of the use of jetcutting in different materials have been undertaken in the C.I.S., the United States, Great Britain, Germany, Japan, and other countries. Liquid jets can be used for the cutting of paper, cloth, wood, leather, rubber, plastic and ceramic materials, nonferrous alloys, and steel. Hydraulic machines with 8 to 80 kilowatts of power are used for jetcutting. These machines provide jet outflow pressures of at least 150 to 1000 MPa and jet velocities of 540 to 1400 meters per second, significantly faster than the speed of sound in air.

Nozzles with exit-hole diameters from 0.05 to 0.5 millimeter are used for qualitative and productive machining. The nozzle diameter used depends on the machined material thickness and its physical and mechanical properties. The rate of fluid flow through the nozzle is relatively low, 500 to 2500 cubic centimeters per minute, and is dependent on the jet outflow parameters.

The first information on the possibility of using super-high-pressure jets as a cutting tool for the machining of various materials appeared in the Soviet Union. However, jetcutting was first patented by the staff of McCartney Manufacturing Co., a division of the Ingersoll-Rand Corp., in the United States.

Supersonic liquid jets can also be used for applications other than the cutting of nonmetallic material sheets. For example, one U.S. company applies water jets for the cleaning of aluminum and other light alloy castings. This allows complete elimination of mechanical damage to the blank surface. Other U.S., British, German, and Japanese companies use liquid jetcutting to reduce the cost of sheet machining and to address problems such as the location and spacing of facing materials; material crushing; burr removal; hole punching; casting cutting; cleaning of chemical equipment, internal surfaces of pipes, and ship bottoms; and finishing of blank surfaces in barely accessible places.

A supersonic liquid jet is a very precise tool--the width of a cut may be 0.1 to 0.8 millimeter. This allows for the machining of blanks with complex profiles and arbitrary angles of curvature, and it reduces chip volume by a factor of 15 to 20 compared with traditional machining.

The use of supersonic liquid jets allows automation of the machining process if it is necessary; eliminates mechanical cutting tools whose working edges are subject to wear from the technological process; saves on personnel and equipment costs for manufacturing and recutting of tools; improves machining quality; reduces chip volume, noise, and dusting; and enables cutting of details with complex profiles and different sizes.

The multitude of operations available using liquid jetcutting and the variety of jet compositions and requirements have resulted in a large number of methods and schemes of jetcutting and the necessity for classifying them.

Existing methods and schemes of liquid jetcutting depend on a number of factors, which can be divided according to the following groups: operation; machined material; working fluid composition; jet action on a material; and jet direction relative to a material.

Structure and Hydrodynamics of High-Pressure Narrow Jets

Regimes and parameters of hydraulic machining, as well as the productivity of jetcutting machines using a thin supersonic liquid jet as a cutter, depend on the quality of the jet and on variations of its hydrodynamic parameters. Therefore, knowledge of the principles of jet dynamics and jet structure is necessary for proper equipment operations, study, and application of the jetcutting process and the design of new equipment.

One of the principal conditions of a successful application of high-pressure liquid jets for the cutting of various materials is the generation from an exhaust nozzle of a jet with the appropriate hydrodynamic properties. The concept of jet compactness, that is, the jet's ability to maintain kinetic energy at a certain distance from the nozzle and to avoid disintegration, is an accepted criterion for the estimation of jet hydrodynamic properties.

The shape, dimensions, and surface quality of the internal exhaust nozzle profile affect the compactness of the jet, the size of its intact section, and the exhaust velocity. The potential energy of the fluid is transformed in the nozzle into kinetic energy.

The profile of the nozzle can be cylindrical, conical, conoidal, combined (for example, conical-cylindrical), or complex (for example, bicubical). The quality of the jet depends on the shape of the nozzle. According to the Bernoulli equation, for any nozzle shape [p.sub.t] = [p.sub.s] + [p.sub.d] + [f.sub.fr] (1) where [p.sub.t] is the total pressure; [p.sub.s] is the static jet pressure, [p.sub.s] = [p.sub.t]/[p.sub.f]; [p.sub.d] is the dynamic pressure of the jet, [p.sub.d] = [U.sub.2]/2g; [f.sub.fr] represents friction pressure losses in the exhaust due to hydraulic resistance; [p.sub.f] is the fluid density; U is the jet exhaust velocity; and g is gravitational acceleration.

General Physical Model of Jetcutting

Material machining by a narrow supersonic liquid jet is a complicated process with specific characteristic features. This process has not been completely understood because of its complexity and the fact that its practical application is relatively new. Systematization and substantiation of microfracture mechanics in the process of jetcutting and the identification of interactions between different phenomena during this process are important for the achievement of maximum productivity, accuracy, and machining quality at various technological parameters.

A physical scheme of material machining by a supersonic liquid jet provides an understanding of physical, thermal, chemical, and other laws of the jetcutting procedure. In this process, chip removal and particle separation, that is, gradual material destruction (spalling or shear), are achieved as a result of the motion of the cutting tool jet about the machined surface.

The process of jetcutting differs from traditional methods of material machining in that it uses a liquid jet as a cutting tool. The jet exiting the nozzle through the air under an outflow pressure exceeding 50 MPa is supersonic, that is, the velocity exceeds 332 meters per second. The action of the jet (as an external load) with such velocity on the material must be considered as an impact. An impact is characterized as a deformation of material with an initial velocity of an impactor, more than 0.416 meter per second. It must be emphasized that at the instant of impact, a sudden change in jet velocity results in a significant increase in jet pressure. This causes abrupt changes in the velocity at any point in the deformed liquid jet.

The stresses quickly increase to their critical levels in machined materials, subject to such velocities and pressures. Usually, this results in brittle fracture, where the rupture strength appears to be smaller than the yield limit. In this case, d[epsilon]/dt >[sigma][xi], where [epsilon] is the maximum strain, t is the duration of loading, [sigma] is the fracture stress, and [xi] is a coefficient of internal bonding.

The most effective jet for jetcutting has a small diameter ([d.sub.n] = 0.1 to 0.3 mm) and a high outflow pressure (p = 100 to 500 MPa). However, jets with such parameters have not been sufficiently investigated.

Studies of the effect of the outflow pressure of small-diameter jets on the jet force at the surface showed that this force increases according to a parabolic, nearly linear relationship with increases in the pressure.

Conditions and Parameters

Knowledge of optimum values of principal parameters that characterize jetcutting, that is, jet outflow pressure and nozzle diameter, is necessary for the effective application of hydraulic jets as cutting tools for manufacturing details from nonmetallic material sheets (mostly polymers).

A series of experiments was performed to determine the optimum jetcutting conditions for providing qualitative cutting of materials by liquid jets. These experiments were concerned with the destructive capacity of liquid jets acting on materials of various properties and structures.

The critical pressure for jetcutting polymeric materials depends on the physical and mechanical properties of the machined material as well as on the diameter of the exhaust nozzle hole. The critical pressure decreases when this diameter is decreased. This is because the area of the material surface subject to the destructive load decreases when the nozzle diameter is decreased, while the relative cutting pressure increases.

The character of the surface failure of different materials at the minimum critical pressure differs. The structure of a machined material strongly determines the effect of a liquid jet and the character of the particles broken off of the material.

Because jetcutting at the minimum critical pressure is not effective and is of poor quality, it is necessary to increase the outflow pressure. Increasing this pressure results in an increase in the jet penetration depth, while the character of failure remains unchanged.

Machinability

The effectiveness of the liquid jetcutting of various materials is predetermined by their physical and mechanical properties expressed through strength characteristics (parameters) that simultaneously affect jetcutting. The interaction of these characteristics prevents the isolation of the effects of any one of them and the independent study of separate characteristics.

Two principal failure types are distinguished during jetcutting. One (breaking) is due to tensile stresses. The other (shear) is related to shearing stresses. A high loading rate (>500 m/s) results in brittle material failure, which is usually associated with the first type of failure. Therefore, in such cases it is necessary to account for the tensile strength [sigma]. The breaking resistance of brittle materials can be conveniently characterized by the bending strength [sigma]b. The second type of failure is characterized by compressive strength, because the failure of compressed brittle materials occurs in shear. Therefore, the shear resistance of brittle bodies can be determined through the compressive strength limit [sigma]c. The principal parameter determined for impact loading is the specific impact viscosity a, which characterizes structural material properties.

Plastic deformations are almost absent during material failure from jetcutting. Material failure occurs elastically at certain stress levels. Therefore, accounting for the significant elasticity of polymers, it can be assumed that their elastic properties, which are characterized by the elastic modulus E, have a great effect on machinability.

Relation Between Jetcutting Productivity and Jet Outflow Parameters

Productivity and power requirements are important characteristics of any machining process. Jetcutting productivity is expressed by the velocity of the material feed about the tool (jet). The material-feed velocity depends on a number of factors; the principal factors are the physical and mechanical properties of the machined material and its thickness. Understanding the effect of these parameters on the material-feed velocity and defining a mathematical relationship between them makes it easier to determine cutting regimes for hydraulic machining of various materials and to calculate and design new equipment. Jet outflow parameters determine the jet's kinetic energy, that is, its destructive capacity and material destruction time in the cutting zone.

The maximum material-feed velocity about a jet providing qualitative cutting without local breaks, spalling, and delamination in the zone adjacent to the cutting was determined. The machined material was located a distance [L.sub.opt] from the nozzle calculated by Equation 1.

The increase of the material-feed velocity with an increase of the jet outflow pressure is due to the greater jet kinetic energy at the higher outflow velocity and a larger fluid mass, which causes particle separation.

Working Fluid Composition

The choice of working fluid is one of the principal questions that has to be resolved during the development of jetcutting technology for machining a given material. The working fluid used in jetcutting must have a low viscosity to provide limited power losses in the flow during its motion through feeding channels and the nozzle; minimal corrosive action on metallic parts in the equipment; and low toxicity, that is, no danger for skin, lungs, and vision. The availability and cost of the fluid must also be considered. A fluid with the indicated properties should help generate the necessary hydrodynamic properties in the high-speed small-diameter jet to provide maximum productivity and the best machining quality with minimal power requirements for jet formation.

Clean water, which is the cheapest, most available, and least harmful fluid, is usually used as the working fluid for jetcutting. However, water has a corrosive effect on metallic equipment parts and prevents the use of standard pumps in hydraulic systems. Water is also unacceptable for machining some special materials; in some cases, water has a negative effect on the technological parameters of material machining.

The necessity of satisfying the working fluid requirements indicated during the machining of different materials requires the application of working fluids of different composition. Additives are used to provide maximum jet compactness and to increase the outflow velocity from the nozzle. Hence, a multitude of various fluids and additives are used in addition to water, including various alcohols, oil products, and glycerol solutions.

The application of various oils as a destructive agent provides good machining of details and materials that can dissolve in water, for example, ceramic metallized strip-packages used in radio engineering. However, the application of oils requires the installation of ventilators in the production zone due to the high outflow temperature of an oil jet (up to 260[degrees]C) and significant oil evaporation during throttling from a small-diameter nozzle. This narrows the field of application of oils.

Particular attention should be paid to fluids whose parameters significantly differ from water and to additives that, when combined with water, form solutions of various viscosities, surface tension forces, evaporation pressures, and chemical structures. The maximum increases of liquid jetcutting capacity can be achieved by adding abrasive particles.

Determination of the influence of various fluid properties on the cutting effectiveness of a narrow supersonic jet is based on observations of jet compactness, knowledge of jet velocity and material-feed velocity about the nozzle, measurements of cut depths in the material, and residual material wetness immediately after cutting.

Fluid density was found to have a significant effect on the quality and productivity of machining. Cutting effectiveness is only slightly affected by changes of surface stresses due to additives, which do not change other water characteristics. Fluid viscosity considered separately from other properties also has only a limited effect on jetcutting effectiveness, although in some cases jet dissipation in the air decreases. An increase of fluid viscosity can reduce jetcutting effectiveness due to friction losses during fluid motion through feeding channels to the nozzle. However, the cut depth is not significantly affected by additives that reduce friction without noticeable viscosity changes. Therefore, jetcutting effectiveness does not depend on fluid parameters at high jet velocities and small nozzle diameters.

One method of significantly improving jetcutting productivity and extending its technological capabilities is the introduction of abrasive additives into the liquid jet.

The interaction of fluid particles with a machined material is different from that of abrasive particles on a machined material. The material-cutting regimes selected for a high-pressure abrasive-liquid jet allow study of the physical laws of the process and, thus, the development of a mathematical model. This makes optimization of the process possible, that is, a required surface quality can be obtained with maximum productivity.

The cutting of material sheets with high-pressure abrasive-liquid jets can be carried out using various jet machines and technological methods. Ejection nozzle heads are usually used to create the high-pressure abrasive-liquid jets. However, the exit section in this type of nozzle head is subject to intensive wear. A special abrasive with spherical particles can be used to reduce the wear on the exit section. However, this would decrease jetcutting productivity.

The abrasive must be mixed with the high-pressure liquid jet outside the nozzle head to provide effective jetcutting with a high-pressure abrasive-liquid jet. To achieve this, the abrasive can be placed on the external surface of the machined material prior to cutting and an abrasive suspension supplied to the zone of interaction between the high-pressure liquid jet and the material. However, in this case, the abrasive particles do not obtain sufficient kinetic energy because they are not accelerated along with the liquid jet prior to interaction with the machined material. In addition, some abrasive particles are washed off the machined surface by reflected portions of the jet. These factors reduce jetcutting productivity of high-pressure abrasive-liquid jets.

High-Pressure Systems

Numerous problems are encountered in designing equipment for machining materials with narrow supersonic liquid jets. First, a sufficient amount of high-pressure fluid (100 to 1000 MPa) must be provided. Reliable seals are needed to retain necessary volumes of fluid in hydraulic systems subject to high pressure. It is also desirable to completely automate and mechanize all jetcutting processes.

Jetcutting equipment must include a principal motion drive, consisting of a high-pressure joint providing continuous jet outflow with a constant velocity; a nozzle and a nozzle head generating a jet with parameters resulting in maximum machining productivity and the best quality; feeding elements connecting the high-pressure joint and the nozzle; feeding drives providing the relative motion of the machined material about the jet and machining of details of required profiles; and an automatic control system.

The hydraulic high-pressure system can be developed based on conventional schemes of hydraulic drives with volume or throttle control. However, an additional design is required, because pressures on the order of 100 to 1000 MPa cannot be created using standard pumps and control and distribution equipment.

The majority of hydraulic systems work at a pressure that does not exceed 40 to 50 MPa. In this case, changes in compressibility, viscosity, and modulus of elasticity in the working fluid and blank deformations can usually be neglected. However, these factors have to be accounted for at a higher pressure because they affect the service-ability and output parameters of jetcutting machines. Therefore, high-pressure systems providing a reliable outflow of supersonic liquid jets can only be developed using special pumps and hydraulic boosters.

Modern techniques are available with the necessary means for cutting different materials with a high-pressure liquid jet. In addition, various elements of equipment necessary for jetcutting are being used in different branches of industry.

The high-pressure hydraulic systems require special pumps that can work for a long time with the pressure of a working fluid equal to 100 to 1000 MPa as well as distributing, measuring, and auxiliary equipment operating with sufficient serviceability at a high working fluid pressure. All elements of the hydraulic system must be connected to each other by strong and reliable hoses and accessories.

Jetcutting machines and devices are used according to operation instructions. Workers involved with jetcutting equipment must be instructed on the peculiarities of the jetcutting process and high-pressure equipment operation. Machines and devices can be used only with the thorough observation of fire-safety rules.
COPYRIGHT 1992 American Society of Mechanical Engineers
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Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Author:Tikhomirov, R.A.; Petukhov, E.N.; Babanin, V.F.; Starikov, I.D.; Kovalev, V.A.
Publication:Mechanical Engineering-CIME
Date:Jun 1, 1992
Words:3305
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