The market-based pulverized coal power plant design is based on the utilization of pulverized coal feeding a conventional steam boiler and steam turbine. The plant configuration is based on current state-of-the-art technology, commercially available components, and current industry trends.
Read this topic. Thread Tools Show Printable Version. Advanced Non-traditional Machining Processes V. Advanced Machining Processes By V. Introduction Metal cutting operations and terminology Trends in metal cutting theory and practice Cutting tool materials Modelling and simulation of machining processes and operations Orthogonal and oblique cutting mechanics Chip control Cutting vibrations Heat in metal cutting Cutting fluids Tribology of metal cutting Tool wear and tool life Machinability of engineering materials Machining economics and optimization Advanced machining processes Micromachining Nanotechnology Sensor-assisted machining Virtual and e-machining Surface integrity Troubleshooting for machining Appendix Index.
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The MA action is adopted during grinding, honing, and superfinishing processes that employ either solid grinding wheels or sticks in the form of bonded abrasives Fig.
Furthermore, in lapping, polishing, and buffing, loose abrasives are used as tools in a liquid machining media as shown in Fig. This is because traditional machining is most often based on the removal of material using tools that are harder than the workpiece. For example, the high ratio of the volume of grinding wheel worn per unit volume of metal removed 50— made classical grinding suitable only to a lim- ited extent for production of polycrystalline diamond PCD profile tools. The high cost of machining ceramics and composites and the damage generated during machining are major obstacles to the implementa- tion of these materials.
In addition to the advanced materials, more complex shapes, low-rigidity structures, and micromachined compo- nents with tight tolerances and fine surface quality are often needed. Traditional machining methods are often ineffective in machining these parts. To meet these demands, new processes are developed. These methods play a considerable role in the aircraft, automobile, tool, die, and mold making industries.
The nontraditional machining methods Fig. These can be classified according to the source of energy used to generate such a machining action: mechanical, thermal, chemical, and electrochemical. Ultrasonic machining USM and water jet machining WJM are typical examples of single-action, mechanical, nontraditional machining processes. The machin- ing medium is solid grains suspended in the abrasive slurry in the former, while a fluid is employed in the WJM process.
The introduction of abrasives to the fluid jet enhances the cutting in case of abrasive water jet machining AWJM or ice particles during ice jet machining IJM see Fig. Thermal machining removes the machining allowance by melting or vaporizing the workpiece material.
Many sec- ondary phenomena relating to surface quality occur during machining such as microcracking, formation of heat-affected zones, and striations. For each of these processes, the machining medium is different. Electrochemical machining ECM uses the electrochemical dissolution ECD phase to remove the machining allowance using ion transfer in an electrolytic cell Fig.
The reason for such a combination and the development of a hybrid machining process is mainly to make use of the combined advan- tages and to avoid or reduce some adverse effects the constituent processes produce when they are individually applied.
The perform- ance characteristics of a hybrid process are considerably different from those of the single-phase processes in terms of productivity, accuracy, and surface quality www. Depending on the major machining phase involved in the material removal, hybrid machining can be classified into hybrid chemical and electrochemical processes and hybrid thermal machining. Such a machining action can be combined with the ther- mal assistance by local heating in case of laser-assisted electro- chemical machining ECML.
In other words, the introduction of the mechanical abrasion action assists the ECD machining phase during electrochemical grinding ECG and electrochemical superfinishing ECS. The mechanical action of the fluid jet assists the process of chemical dissolution in electrochemical buffing ECB. Kozak and Rajurkar reported that the mechanical interaction with workpiece material changes the conditions for a better anodic dis- solution process through mechanical depassivation of the surface.
Under such conditions, removing thin layers of oxides and other compounds from the anode surface makes the dissolution and smoothing processes more intensive. Significant effects of the mechanical machining action have been observed with ultrasonic waves. The cavitations generated by such vibrations enhance the ECM by improving electrolyte flushing and hence the material removal from the machined surface. In this case the main material removal mechanism is a thermal one.
The combination of this phase with the ECD phase, MA action, and ultrasonic US vibration generates a family of double action processes. Such a combination enhance the rate of material removal and surface quality in electrochemical discharge grinding ECDG and the other hybrid processes shown in Fig. References El-Kady, E. Kaczmarek, J. Principles of Machining by Cutting, Abrasion, and Erosion. Stevenage, U. Kozak, J. McGeough, J.
Advanced Methods of Machining. London, New York: Chapman and Hall. Micromachining of Engineering Materials. New York: Marcel Dekker, Inc. Tanigushi, N. Todd, J. Chapter 2 Mechanical Processes 2. During that oscillation, the abrasive slurry of B4C or SiC is continuously fed into the machining zone between a soft tool brass or steel and the workpiece.
The abrasive particles are, therefore, ham- mered into the workpiece surface and cause chipping of fine particles from it. Balamuth first discovered USM in during ultrasonic grinding of abrasive powders.
The industrial applications began in the s when the new machine tools appeared. USM is characterized by the absence of any deleterious effect on the metallic structure of the workpiece material. The magnetostrictor is energized at the ultrasonic frequency and produces small-amplitude vibrations.
Such a small vibration is amplified using the constrictor mechanical amplifier that holds the tool. The abrasive slurry is pumped between the oscillating tool and the brittle workpiece. A static pressure is applied in the tool-workpiece interface that maintains the abrasive slurry. The magnetostrictor used in USM, shown in Fig. The magne- tostriction effect was first discovered by Joule at Manchester in Accordingly, a magnetic field undergoing ultrasonic frequencies causes Cooling Leads to water transducer Magnetostriction winding transducer Cooling water Abrasive slurry Concentrator Tool Workpiece Figure 2.
High-frequency winding Armature Magnetostrictor Polarizing winding Magnetostrictor core Amplitude transformer attachment Figure 2. This effect is used to oscillate the USM tool, which is mounted at the end of a magnetostrictor, at ultrasonic frequencies 18 to 20 kHz. The method of operation of a magnetostrictor can be explained as follows. Materials having high magnetostrictive elongation are recommended to be used for a magnetostrictor.
Figure 2. If the transducer is magnetized with a direct current, as shown in Fig. The maximum elon- gation Amax in the magnetostrictor of length l equal to half of the wave- length l Fig. In order to obtain the maximum amplification and a good efficiency, the magnetostrictor must, therefore, be designed to operate at reso- nance where its natural frequency must be equal to the frequency of the magnetic field.
The vibration amplitude is increased by fitting an amplifier acoustic horn into the output end of the magnetostrictor. Depending on the final amplitude required, the amplitude amplification can be achieved by one or more acoustic horns Fig. In order to have the maximum amplitude of vibration resonance the length of the con- centrator is made multiples of one-half the wavelength of sound l in the concentrator horn material.
The choice of the shape of the acoustic horn controls the final amplitude. Five acoustic horns cylindrical, stepped, exponential, hyperbolic cosine, and conical horns have been reported by Youssef The main drawbacks of the magnetostrictive transducer are the high losses encountered, the low efficiency 55 percent , the conse- quent heat up, and the need for cooling.
Higher efficiencies 90—95 percent are possible by using piezoelectric transformers to modern USM machines. Tool tips must have high wear resistance and fatigue strength. For machining glass and tungsten carbide, copper and chromium silver steel tools are recommended. Silver and chromium nickel steel are used for machining sintered carbides.
During USM, tools are fed toward, and held against, the workpiece by means of a static pressure that has to overcome the cutting resistance at the inter- face of the tool and workpiece.
Different tool feed mechanisms are avail- able that utilize pneumatic, periodic switching of a stepping motor or solenoid, compact spring-loaded system, and counterweight techniques as mentioned in claymore. Abrasive slurry is usually composed of 50 percent by volume fine abrasive grains — grit number of boron carbide B4C , aluminum oxide Al2O3 , or silicon carbide SiC in 50 percent water. The abrasive slurry is circulated between the oscillating tool and workpiece.
Under the effect of the static feed force and the ultrasonic vibration, the abrasive particles are hammered into the workpiece surface caus- ing mechanical chipping of minute particles. As machining progresses, the slurry becomes less effective as the particles wear and break down. The expected life ranges from to hours h of ultrasonic exposure Metals Handbook, The slurry is continuously fed to the machining zone in order to ensure effi- cient flushing of debris and keeps the suspension cool during machin- ing.
The performance of USM depends on the manner in which the slurry is fed to the cutting zone. Mechanical abrasion by localized direct hammering of the abrasive grains stuck between the vibrating tool and adjacent work surface. The microchipping by free impacts of particles that fly across the machining gap and strike the workpiece at random locations.
The work surface erosion by cavitation in the slurry stream. The relative contribution of the cavitation effect is reported to be less than 5 percent of the total material removed. The dominant mecha- nism involved in USM of all materials is direct hammering. Soft and elastic materials like mild steel are often plastically deformed first and are later removed at a lower rate.
In case of hard and brittle materials such as glass, the machining rate is high and the role played by free impact can also be noticed. When machining porous materials such as graphite, the mechanism of ero- sion is introduced. The rate of material removal, in USM, depends, first of all, on the frequency of tool vibration, static pressure, the size of the machined area, and the abrasive and workpiece material.
The material removal rate and hence the machinability by USM depends on the brittleness criterion which is the ratio of shearing to breaking strength of a material. According to Table 2. Moreover, because of the low brittleness criterion of steel, which is softer, it is used as a tool material. The amplitude of the tool oscillation has the greatest effect of all the process variables.
The material removal rate increases with a rise in the amplitude of the tool vibration. The vibra- tion amplitude determines the velocity of the abrasive particles at the interface between the tool and workpiece.
Under such circumstances the kinetic energy rises, at larger amplitudes, which enhances the mechan- ical chipping action and consequently increases the removal rate. A greater vibration amplitude may lead to the occurrence of splashing, which causes a reduction of the number of active abrasive grains and results in a decrease in the material removal rate. According to Kaczmarek with regard to the range of grain sizes used in practice, the amplitude of oscillation varies within the limits of 0.
Regarding the effect of vibration frequency on the removal rate, it has been reported by McGeough that the increase in vibration frequency reduces the removal rate. This trend may be related to the small chipping time allowed for each grain such that a lower chipping action prevails caus- ing a decrease in the removal rate. Both the grain size and the vibration ampli- tude have a similar effect on the removal rate. According to McGeough , the removal rate rises at greater grain sizes until the size reaches the vibration amplitude, at which stage, the material removal rate decreases.
When the grain size is large compared to the vibration ampli- tude, there is a difficulty of abrasive renewal in the machining gap. Because of its higher hardness, B4C achieves higher removal rates than silicon carbide SiC when machining a soda glass workpiece. The rate of material removal obtained with silicon carbide is about 15 percent lower when machining glass, 33 percent lower for tool steel, and about 35 percent lower for sintered carbide.
Water is commonly used as the abrasive carrying liquid for the abra- sive slurry while benzene, glycerol, and oils are alternatives. The increase of slurry viscosity reduces the removal rate. The improved flow of slurry results in an enhanced machining rate.
In practice a volumetric con- centration of about 30 to 35 percent of abrasives is recommended. A change of concentration occurs during machining as a result of the abra- sive dust settling on the machine table.
The actual concentration should, therefore, be checked at certain time intervals. The increase of abrasive concentration up to 40 percent enhances the machining rate. More cut- ting edges become available in the machining zone, which raises the chipping rate and consequently the overall removal rate. The machining rate is affected by the ratio of the tool hardness to the workpiece hardness. In this regard, the higher the ratio, the lower will be the material removal rate. For this reason soft and tough materials are recommended for USM tools.
The machining rate is affected by the tool shape and area. An increase in the tool area decreases the machining rate due to the problem of adequately distributing the abrasive slurry over the entire machining zone. It has been reported by McGeough that, for the same machining area, a narrow rectangular shape yields a higher machining rate than a square cross-sectional one. The rise in the static feed pressure enhances the machining rate up to a lim- iting condition, beyond which no further increase occurs.
According to Kaczmarek , at pressures lower than the opti- mum, the force pressing the grains into the material is too small and the volume removed by a particular grain diminishes. Beyond the opti- mum pressure, damping is too strong and the tool ceases to break away from the grains, thus preventing them from changing position, which reduces the removal rate.
Measurements also showed a decrease in the material removal rate with an increase in the hole depth. The reason for this is that the deeper the tool reaches, the more difficult and slower is the exchange of abrasives from underneath the tool.
Generally the form accuracy of machined parts suffers from the following disturbing factors, which cause oversize, conicity, and out of roundness. The process accuracy is measured through the overcut over- size produced during drilling of holes.
The hole oversize measures the difference between the hole diameter, measured at the top surface, and the tool diameter. The side gap between the tool and the machined hole is necessary to enable the abrasives to flow to the machining zone under the oscillating tool. Hence the grain size of the abrasives represents the main factor, which affects the overcut produced. The overcut is consid- ered to be about two to four times greater than the mean grain size when machining glass and tungsten carbide.
It is about three times greater than the mean grain size of B4C mesh numbers — However, the magnitude of the overcut depends on many other process variables including the type of workpiece material and the method of tool feed.
The overcut is usually greater at the entry side than at the exit one due to the cumulative abrasion effect of the fresh and sharp grain particles. The out of roundness arises by the lateral vibra- tions of the tool. Such vibrations may arise due to the out of perpen- dicularity of the tool face and the tool centerline and when the acoustic parts of the machine are misaligned.
The surface finish is closely related to the machining rate in USM. Table 2. The larger the grit size, the faster the cutting but the coarser the surface finish.
A surface finish of 0. However, other factors such as tool surface, amplitude of tool vibration, and material being machined also affect the surface finish. The larger the grit smaller the grain size , the smoother becomes the produced surface.
As mentioned earlier, the larger chipping marks formed on brittle machined materi- als create rougher surfaces than that obtained in the case of machined hard alloy steel. The amplitude of tool oscillation has a smaller effect on the surface finish.
As the amplitude is raised the individual grains are pressed further into the workpiece surface thus causing deeper TABLE 2. Other process variables such as static pressure have a little effect on the surface finish.
Smoother surfaces can also be obtained when the viscosity of the liquid carrier of the abrasive slurry is reduced. It is evident that the sur- face irregularities of the sidewall surfaces of the cavities are consider- ably larger than those of the bottom.
This results from the sidewalls being scratched by grains entering and leaving the machining zone. Cavitation damage to the machined surface occurs when the tool par- ticles penetrate deeper into the workpiece. Under such circumstances it is more difficult to replenish adequately the slurry in these deeper regions and thus a rougher surface is produced.
A modified version of USM is shown in Fig. The process is, therefore, called rotary ultrasonic machining RUM. Cruz et al. RUM ensures high removal rates, lower tool pressures for del- icate parts, improved deep hole drilling, less breakout or through holes, and no core seizing during core drilling.
The process allows the uninterrupted drilling of small-diameter holes, while conventional drilling necessitates a tool retraction, which increases the machining time. Small holes require more time as the rate of machining decreases with the depth of penetration due to the difficulty in main- taining a continuous supply of new slurry at the tool face.
Generally a depth-to-diameter ratio of 2. During USM sink- ing, the material removal is difficult when the machined depth exceeds 5 to 7 mm or when the active section of the tool becomes important.
Under such conditions the removal of the abrasive grits at the interface becomes difficult and hence the material removal process is impossible. Moreover the manufacture of such a tool is generally complex and costly.
Contouring USM Fig. The same figure also shows holes and contours machined using a USM contour machining. Gilmore used USM to pro- 2. Typical ultrasonic machining speeds, in graphite, range from 0. The surface roughness ranges from 0. Small machining forces permit the manufacture of fragile graphite EDM electrodes. Before Figure 2. Ultrasonic polishing occurs by vibrating a brittle tool material such as graphite or glass into the workpiece at an ultrasonic frequency and a relatively low vibration amplitude.
The fine abrasive particles, in the slurry, abrade the high spots of the workpiece surface, typically removing 0. Using such a technique Gilmore reported the surface finish to be as low as 0.
Micro-ultrasonic machining MUSM is a method that utilizes workpiece vibration. According to Egashira and Masuzana vibrating the workpiece allows for freer tool system design because it does not include the set of transducer, horn, and cone. In addition, the complete system is much more simple and compact than conventional USM Fig.
However the high wear resistance of sintered diamond SD tools made it possible to machine multiple holes using a single tool. When the 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. The hydraulic pump is powered from a kilowatt kW electric motor and supplies oil at pressures as high as bars in order to drive a reciprocating plunger pump termed an intensi- fier.
The hydraulic pump offers complete flexibility for water jet cutting and cleaning applications. It also supports single or multiple cutting sta- tions for increased machining productivity. The intensifier accepts the water at low pressure typically 4 bar and expels it, through an accumulator, at higher pressures of bar.
The intensifier converts the energy from the low-pressure hydraulic fluid into ultrahigh-pressure water. The hydraulic system pro- vides 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 direc- tion.
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. The accumulator maintains the continuous flow of the high-pressure water and eliminates pressure fluctuations. It relies on the compressibility of water 12 percent at bar in order to maintain a uniform discharge pressure and water jet velocity, when the intensifier piston changes its direction.
High-pressure tubing transports pres- surized 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. Nozzles are normally made from synthetic sapphire.
About h of operation are expected from a nozzle, which becomes damaged by particles of dirt and the accumula- tion 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. 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.
The catcher acts as a reservoir for collecting the machining debris entrained in the water jet. Moreover, it reduces the noise levels [ decibels dB ] associated with the reduction in the velocity of the water jet from Mach 3 to subsonic levels.
The standoff distance, shown in Fig. However for materials used in printed circuit boards, it may be increased to 13 to 19 mm. For a nozzle of 0. 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. 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 thick- nesses 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 recom- mended for cutting frozen foods. Target material.
Brittle materials will fracture, while ductile ones will cut well. Material thicknesses range from 0. 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 hard- ening, and thermal stresses.
In general the cut surface has a sandblast appearance. Moreover, harder materials exhibit a better edge finish. Typical surface finishes ranges from 1. Both the pro- duced surface roughness and tolerance depend on the machining speed.
WJM is limited to fiberglass and corrugated wood. 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. In this case the thermal material damage is negligible.
The tool, being effectively pointed, accu- rately cuts contours. The main drawback is the deflection of the water Figure 2. Mechanical Processes 37 Figure 2. The feed rate attainable depends on the surface quality required. Water jet cutting of a mm-deep slot in gran- ite using two oscillating jets at MPa during 14 passes at a Moreover an oscillat- ing nozzle system operating at the same feed rate and pressure of MPa, with the standoff distance adjusted every pass was used to cut a mm-deep slot in sandstone.
A higher pressure bar and a lower flow rate 2. Boards of var- ious shapes for use in portable radios and cassette players can be cut using computer numerical control CNC technology. The process can 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, The process differs from sandblasting SB in that AJM has smaller- diameter abrasives and a more finely controlled delivery system.
The workpiece material is removed by the mechanical abrasion MA action of the high-velocity abrasive particles. AJM machining is best suited for machining holes in superhard materials. It is typically used to cut, clean, peen, deburr, deflash, and etch glass, ceramics, or hard metals. Oxygen should never be used because it causes a violent chemical reaction with workpiece chips or abrasives. After filtration and regulation, the gas is passed through a mixing chamber that contains abrasive particles and vibrates at 50 Hz.
Aluminum oxide Al2O3 and silicon carbide powders are used for heavy cleaning, cutting, and deburring. Magnesium carbonate is recommended for use in light cleaning and etching, while sodium bicarbonate is used for fine cleaning and the cut- ting of soft materials. Commercial-grade powders are not suitable because their sizes are not well classified. They may contain silica dust, which can be a health hazard. It is not practical to reuse the abrasive powder because contaminations and worn grit will cause a decline of the machining rate.
The abrasive powder feed rate is controlled by the ampli- tude of vibrations in the mixing chamber.
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