P&C - Machine processes Flashcards
Explain the benefits of using chemical milling
In chemical milling, shallow cavities are produced on plates, sheets, forgings, and extrusions, generally for overall reduction of weight.
This process has been used on a wide variety of metals, with depths of metal removal as large as 12mm. Selective attack by the chemical reagent on different areas of the workpiece surfaces is controlled by removable layers of material, called masking, or by partial immersion in the reagent.
The procedure for chemical milling consists of the following steps;
- If the part to be machined has residual stresses from prior processing, the stresses should first be relieved in order to prevent warping after chemical milling.
- The surfaces are thoroughly degreased and cleaned to ensure good adhesion of the masking material and uniform material removal. Scale from heat treatment should also be removed.
- The masking material is applied. Masking with tapes or paints (maskants) is a common practice, although elastomers (rubber and neoprene) and plastics (polyvinyl chloride, polyethylene, and polystyrene) are also used. The maskant material should not react with the chemical reagent.
- The making that covers various regions that require etching is peeled off by the scribe-and-peel technique.
- The exposed surfaces are etched with etchants such as sodium hydroxide (for aluminium), solutions of hydrochloric and nitric acids (for steels), or iron chloride (for stainless steels). Temperature control and stirring during chemical milling is important in order to obtain a uniform depth of material removed.
- After machining, the parts should be washed thoroughly to prevent further reactions with any etchant residues.
- The rest of the masking material is removed and the part is cleaned and inspected.
- Additional finishing operations may be performed on chemically milled parts.
- This sequence of operations can be repeated to produce stepped cavities and various contours.
Chemical milling is used in the aerospace industry to remove shallow layers of material from large aircraft components, missile skin panels, and extruded parts for airframes. Tank capacities for reagents are as large as 3.7m X 15m. The process is also used to fabricate microelectronic devices. Some surface damage may result from chemical milling because of preferential etching and intergranular attack, which adversely affect surface properties. The chemical milling of welded and brazed structures may result in uneven material removal. Chemical milling of castings may result in uneven surfaces caused by porosity in and nonuniformity of the material.
Design Considerations for Chemical Machining
- Because the etchant attacks all exposed surfaces continuously, designs involving sharp corners, deep and narrow cavities, severe tapers, folded seams, or porous workpiece materials should be avoided
- Because the etchant attacks the material in both vertical and horizontal directions, undercuts may develop
- In order to improve the production rate, the bulk of the workpiece should be shaped by other processes (such as machining) prior to chemical machining.
- Dimensional variations can occur because of size changes in artwork due to humidity and temperature. The variation can be minimised by properly selecting artwork media and controlling the environment in the artwork generation and the production area in the plant.
- Many product designs are now made with computer-aided design systems. However, product drawings must be translated into a protocol that is compatible with the equipment for photochemical artwork generation.
Summary of steps:
Relieve residual stresses to prevent warping
Clean the material surface
Apply masking material
Remove the masking on regions that require etching
Apply the reagents
Wash the part
Remove remaining masking
Explain the benefits of using Photochemical blanking
Photochemical Blanking Summary: Uses chemicals and photographic processes to remove material, usually from a thin sheet
Can produce complex shapes on metals as thin as .0025 mm without forming burrs
Procedure:
Prepare the design at a magnification of up to 100x; make a photographic negative and reduce it to the size of the part
Coat the blank with a photosensitive material
Place the negative over the part and expose it to ultraviolet light to harden the exposed photosensitive coating
Dissolve the unexposed coating
Apply the chemical reagent
Remove the masking and wash the part
Photochemical blanking, also called photoetching, is a modification of chemical milling. The material is removed, usually from a flat thin sheet, by photographic techniques. Complex burr-free shapes can be blanked on metals as thin as 0.0025mm. Sometimes called photochemical machining, the process is also used for etching.
The procedure in photochemical blanking consists of the following steps:
- The design of the part to be blanked is prepared at a magnification of up to 100X. A photographic negative is then made and reduced to the size of the finished part. The reduced negative of the design is called artwork. The original (enlarged) drawing allows inherent design errors to be reduced by the amount of reduction (such as 100X) for the final artwork image.
- The sheet blank is coated with a photosensitive material (photoresist) by dipping, spraying, or roller coating, and dried in an oven. This coating is often called the emulsion.
- The negative is placed over the coated blank and exposed to ultraviolet light, which hardens the exposed areas.
- The blank is developed, which dissolves the unexposed areas.
- The blank is then immersed into a bath of reagent (as in chemical milling), or sprayed with the reagent, which etches away the exposed areas.
- The making material is removed, and the part is washed thoroughly to remove all chemical residues.
Process Capabilibies.
Typical applications for photochemical blanking are fine screens, printed-circuit cards, electric-motor laminations, flat screens, and masks for colour television. Although skilled labour is required, tooling costs are low; the process can be automated; it is economical for medium- to high-production volume. Photochemical blanking is capable of making very small parts where traditional blanking dies are difficult to produce. The process is also effective for blanking fragile workpieces and materials. The handling of chemical reagents requires precautions and special considerations to protect the workers against exposure to both liquid chemicals and volatile chemicals. Furthermore, the disposal of chemical by-products from this process is a major drawback, although some by-products can be recycled.
Explain the benefits of using Electrochemical machining
3.3 Electrochemical Machining Summary:
Uses an electrolyte and electrical current to ionize and remove metal atoms
Can machine complex cavities in high-strength materials
Leaves a burr-free surface
Not affected by the strength, hardness or toughness of the material
Design Considerations:
The electrolyte erodes away sharp profiles
It is difficult to control electrolyte flow; irregular cavities may not be formed accurately
Allow for small taper in holes made this way
Electrochemical Machining (ECM) is basically the reverse of electroplating. An electrolyte acts as current carrier and the high rate of electrolyte movement in the tool-workpiece gap washes metal ions away from the workpiece (anode) before they have a chance to plate onto the tool (cathode). Note that the cavity produced is the female mating image of the tool. Modifications of this process are used for turning, facing, slotting, trepanning, and profiling operations in which the electrode becomes the cutting tool. The shaped tool is generally made of brass, copper, bronze, or stainless steel. The electrolyte is a highly conductive inorganic salt solution, such as sodium chloride mixed in water or sodium nitrate. It is pumped at a high rate through the passages in the tool. A dc power supply in the range of 5-25 V maintains current densities, which for most applications are 1.5-8 A/mm2 of active machined surface. Machines having current capacities as high as 40,000A and as low as 5A are available. The penetration rate of the tool is proportional to the current density. Because the metal removal rate is only a function of ion exchange rate, it is not affected by the strength, hardness, or toughness of the workpiece.
Process Capabilities
Electrochemical machining is generally used to machine complex cavities in high-strength materials, particularly in the aerospace industry for the mass production of turbine-blades, jet-engine parts, and nozzles. It is also used to machine forging-die cavities (die sinking) and to produce small holes. The ECM process leaves a burr-free surface; in fact; it can also be used as a deburring process. It does not cause any thermal damage to the part, and the lack of tool forces prevents distortion of the part. Furthermore, there is no tool wear, and the process is capable of producing complex shapes as well as machining hard materials. However, the mechanical properties of components made by ECM should be compared carefully to those of other material-removal methods.
Electrochemical machining systems are now available as numerically-controlled machining centers, with the capability of high production rates, high flexibility, and the maintenance of close dimensional tolerances.
Design Considerations for Electrochemical Machining
- Because of the tendency for the electrolyte to erode away sharp profiles, electrochemical machining is not suited for producing sharp square corners or flat bottoms.
- Controlling the electrolyte flow may be difficult, so irregular cavities may not be produced to the desired shape with acceptable dimensional accuracy.
- Designs should make provision for a small taper for holes and cavities or be machined.
Explain the benefits of using Pulsed electrochemical machining
A form of electrochemical machining; the current is pulsed to eliminate the need for high electrolyte flow Improves fatigue life of the part
Explain the benefits of using Electrochemical grinding
Electrochemical Grinding:
Summary: Uses a rotating cathode embedded with abrasive particles for applications comparable to milling, grinding and sawing
Most of the metal removal is done by the electrolyte, resulting in very low tool wear
Adaptable for honing
Design Considerations: (in addition to those for electrochemical machining)
Avoid sharp inside radii Flat surfaces to be ground should be narrower than the width of the grinding wheel
Electrochemical Grinding (ECG) combines electrochemical machining with conventional grinding. The equipment used is similar to a conventional grinder, except that the wheel is a rotating cathode embedded with abrasive particles. The wheel is metal-bonded with diamond or aluminium-oxide abrasives, and rotates at a surface speed of from 1200 m/min to 2000 m/min. The abrasives have two functions:
(a) to serve as insulators between the wheel and the workpiece,
(b) to mechanically remove electrolytic products from the working area.
A flow of electrolyte solution (usually sodium nitrate) is provided for the electrochemical machining phase of the operation. Current densities range from 1 A/mm2 to 3 A/mm2. The majority of metal removal in ECG is by electrolytic action, and typically less than 5% of metal is removed by the abrasive action of the wheel; therefore, wheel wear is very low. Finishing cuts are usually made by the grinding action but only to produce a surface with good finish and dimensional accuracy. The ECG process is suitable for applications similar to those for milling, grinding, and sawing. It is not adaptable to cavity-sinking operations. This process has been successfully applied to carbides and high-strength alloys. It offers a distinct advantage over traditional diamond-wheel grinding when processing very hard materials, where wheel wear can be high. ECG machines are now available with numerical controls, improving dimensional accuracy, repeatability, and increased productivity.
Electrochemical honing combines the fine abrasive action of honing with electrochemical action. Although the equipment is costly, the process is as much as five times faster than conventional honing, and the tool lasts as much as ten times longer. It is used primarily for finishing internal cylindrical surfaces.
Design considerations for Electrochemical Grinding
In addition to those already listed for electrochemical machining, ECG requires two additional design considerations:
(a) designs should avoid sharp inside radii
(b) if flat surfaces are to be produced, the electrochemically ground surface should be narrower than the width of the grinding wheel.
Explain the benefits of using Electrical-discharge machining
Electrical-discharge machining:
Uses a shaped electrode and electric sparks to remove metal; discharges sparks at about 50-500 kHz A dielectric (nonconductive) fluid removes debris and acts as an insulator until the potential difference is high enough
Can be used on any material that conducts electricity
Design Considerations:
Design parts so that the electrodes can be made economically
Avoid deep slots and narrow openings
Do not require very fine surface finish
Most of the material removal should be done by other processes to speed production
Explain the benefits of using Electrical-discharge grinding
Electrical-discharge grinding: The grinding wheel lacks abrasives and removes material by electrical discharges
Can be combined with electrochemical grinding
Can be used for sawing, in which the saw has no teeth
Explain the benefits of using Electrical-discharge wire cutting
Electrical-discharge wire cutting:
The wire moves through the workpiece like a band saw, removing material by electrical discharge
Dielectric fluid is applied to the work area
The wire is generally used only once; it is inexpensive
Explain the benefits of using laser-beam machining
Laser-beam machining:
Summary:
Uses a concentrated beam of light to vaporize part of the workpiece
Usually produces a rough surface with a heat-affected zone
Can cut holes as small as .005 mm with depth/diameter ratios of 50:1
Design Considerations:
Non-reflective workpiece surfaces are preferable
Sharp corners are difficult to produce; deep cuts produce tapers
Consider the effects of high temperature on the workpiece material
Laser Beam Machining
In laser-beam machining (LBM), the source of energy is a laser (a acronym for Light Amplification by Stimulated Emission of Radiation), which focuses optical energy on the surface of the workpiece. The highly focused, high-density energy melts and evaporates portions of the workpiece in a controlled manner. This process, which does not require a vacuum, is used to machine a variety of metallic and non-metallic materials. There are several types of lasers used in manufacturing operations.
(a) CO2 (pulsed or continuous wave)
(b) Nd: YAG (neodymium:yttrium-aluminium-garnet)
(c) Nd: glass, ruby
(d) Excimer lasers (from the words Excited and Dimer, meaning two mers or two molecules of the same chemical composition).
Importantly physical parameters in LBM are the reflectivity and conductivity of the workpiece surface, and its specific heat and latent heats of melting and evaporation. The lower these quantities, the more efficient the process. The surface produced by LBM is usually rough and has a heat-affected zone which, in critical applications, may have to be removed or heat treated. Kerf width is an important consideration, as it is in other cutting processes, such as sawing, wire EDM, and electron-beam machining.
Laser beams may be used in combination with a gas stream, such as oxygen, nitrogen, or argon (laser-beam torch), for cutting thin sheet materials. High-pressure, inert-gas assisted laser cutting is used for stainless steel and aluminium; it leaves an oxide-free edge that can improve weldability. Gas streams also have the important function of blowing away molten and vaporised material from the workpiece surface.
3.5.1 Process Capabilities
Laser-beam machining is widely used for drilling and cutting metals, non-metallic materials, ceramics, and composite materials. The abrasive nature of composite materials and the cleanliness of the operation have made laser-beam machining an attractive alternative to traditional machining methods. Holes as small as 0.0005 mm with hole depth-to-diameter ratios of 50:1, have been produced in various materials, although a more practical minimum is 0.025mm. Steel plates as thick as 32mm can be cut with laser beams. Laser-beam machining is being used increasingly in the electronics and automotive industries. Bleeder holes for fuel-pump covers and lubrication holes in transmission hubs are, for example, being drilled with lasers. The cooling holes in the first stage vanes of the Boeing 747 jet engines are also produced by lasers. Significant cost savings have been achieved by laser-beam machining, a process that is competing with electrical-discharge machining.
Laser beams are use for the following:
- Welding
- Small-scale and localised heat treating of metals and ceramics to modify their surface mechanical and tribological properties and,
- The marking of parts, such as letter, numbers, codes, etc.
Marking can also be done by processes such as:
(a) with ink
(b) with mechanical devices such as punches, pins, stylus, scroll rolls, or stamping; and
(c) by etching.
Although the equipment is more expensive than that used in other methods, marking and engraving with lasers has become increasingly common due to its accuracy, reproducibility, flexibility, ease of automation, and on-line application in manufacturing. The inherent flexibility of the laser-cutting process, with its fibre-optic beam delivery simple fixturing, and low setup times, and the availability of multi-kW machines and 2D and 3D computer-controlled laser cutting systems are attractive features. Therefore, laser cutting can compete successfully with cutting sheet metal with the traditional punching processes. Extreme caution should be exercised with lasers. Even low-power lasers can cause damage to the retina of the eye if proper precautions are not observed.
- 5.2 Design Considerations for Laser-Beam Machining
(a) Reflectivity of the workpiece surface is an important consideration in laser-beam machining; because they reflect less, dull and unpolished surfaces are preferable.
(b) Designs with sharp corners should be avoided since they can be difficult to produce. Deep cuts produce tapers.
(c) Any adverse effects on the properties of the machined materials caused by the high local temperatures and heat-affected zone should be investigated.
Explain the benefits of using Electron beam machining and Plasma arc cutting
Electron-Beam Machining and Plasma-Arc Cutting
Electron-Beam machine summary:
Vaporizes material using electrons accelerated to 50-80% the speed of light
Produces finer surface finish and narrower cut width than other thermal cutting processes
Requires a vacuum; generates hazardous X rays
Plasma-Arc cutting summary: Uses plasma (ionized gas) to rapidly vaporize material
Material removal rates are much higher than those for laser beam machining and electron beam machining; produces good surface finish and thin cut width
Design Considerations: (in addition to those for laser-beam machining)
Parts should match the size of the vacuum chamber
Consider manufacturing the part as a number of smaller components
The source of energy in electron-beam machining (EBM) is high-velocity electrons, which strike the surface of the workpiece and generate heat. The machines utilise voltages in the range of 50 kV-200 kV to accelerate the electrons to speeds of 50% to 80% of the speed of light. Its applications are similar to those of laser-beam machining, except that EBM requires a vacuum. Consequently, it is used much less than laser beam machining. Electron-beam machining can be used for very accurate cutting of a wide variety of metals. Surface finish is better and kerf width is narrower than that for other thermal cutting processes. The interaction of the electron beam with the workpiece surface produces hazardous x-rays; the equipment should, therefore, be used only by highly trained personnel.
In plasma-arc cutting (PAC), plasma beams (ionised gas) are used to rapidly cut ferrous and nonferrous sheets and plates. The temperatures generated are very high (9400oC), in the torch for oxygen as a plasma gas). Consequently, the process is fast, the kerf width is small, and the surface finish is good. Parts as thick as 150mm can be cut. Material-removal rates are much higher than those associated with the EDM and LBM processes, and parts can be machined with good reproducibility. Plasma arc cutting is highly automated today, using programmable controllers.
- 6.1 Design considerations
(a) the guidelines for LBM generally apply to EBM as well
(b) because vacuum chambers have limited capacity, part sizes should closely match the size of the vacuum chamber for a high-production rate per cycle
(c) if a part requires electron-beam machining on only a small portion of the workpiece, consideration should be given to manufacturing it as a number of smaller components and assembling them after electron-beam machining.
Explain the benefits of using Water jet machining
Water-Jet Machining
Summary: A pressurized jet of water cuts a groove in the material Effective for many nonmetallic materials
Cuts can be started at any location; does not produce heat; produces very little burring
When we put our hand across a jet of water or air, we feel a considerable concentrated force acting on it. This force results from the momentum change of the stream, and, in fact, is the principle on which the operation of water or gas turbines is based.
In water-jet machining (WJM), also called hydrodynamic machining, this force is utilised in cutting and deburring operations. The water jet acts like a saw and cuts a narrow groove in the material. A pressure level of about 400 MPa is generally used for efficient operation, although pressures as high as 1400 MPa can be generated. Jet-nozzle diameters range between 0.05 mm and 1 mm. A variety of materials can be cut, including plastics, fabrics, rubber, wood products, paper, leather, insulating materials, brick, and composite materials. Depending on the materials, thicknesses can range up to 25 mm and higher. Vinyl and foam coverings for automobile dashboards, as well as some body panels, are being cut using multiple-axis, robot-guided water-jet machining equipment. Because it is an efficient and clean operation compared to other cutting processes, it is also used in the food processing industry for cutting and slicing food products.
The advantages of this process are that
(a) Cuts can be started at any location without the need for predrilled holes.
(b) No heat is produced
(c) No deflection of the rest of the workpiece takes place (so the process is suitable for flexible materials)
(d) Little wetting of the workpiece takes place
(e) The burr produced is minimal It is also an environmentally safe manufacturing process.
Explain the benefits of using Abrasive water jet machining
Abrasive waterjet machining: The water jet contains abrasive particles; this increases the material removal rate
Can cut metallic, nonmetallic, and advanced composite materials Suitable for heat-sensitive materials
Explain the benefits of using Abrasive jet machining
Abrasive jet machining:
A high-speed jet of dry air, nitrogen or carbon dioxide carries abrasive particles
Good for cutting hard or brittle materials
Can be used for deburring, cleaning, or removing oxides or surface films
What is chemical machining
Chemical Machining (CM) was developed based on the observation that chemicals attack metals and etch them, thereby removing small amounts of material from the surface. This process is carried out by chemical dissolution, using reagents or etchants, such as acids and alkaline solutions.
Chemical machining is the oldest of the nontraditional machining processes, and has been used to engrave metals and hard stones, in deburring, and more recently in the production of printed circuit boards and microprocessor ships.
Design Considerations:
Avoid sharp corners, deep narrow cavities, steep tapers, folded seams and porous workpieces
Undercuts may develop
Most of the workpiece should be shaped by other processes to speed production
Variations may occur depending on humidity and temperature
Computerized designs must be converted to a format compatible with the photochemical artwork equipment
Discuss the economics of advanced machining processes
Economics of Advanced Machining Processes
Advanced machining processes have unique applications, particularly for difficult-to-machine materials and for parts with complex internal and external profiles. The economic production run for a particular process depends on the cost of tooling and equipment, operating costs, the material-removal rate required, and the level of operator skill required, as well as on secondary and finishing operations that may subsequently be necessary. In chemical machining, an important factor is the cost of reagents, maskants, and disposal, together with the cost of cleaning the parts.
In electrical-discharge machining, the cost of electrodes and the need to periodically replace them can be significant. The rate of material removal, and with it production rate, can vary significantly in these processes. The cost of tooling and equipment also varies considerably, as does the operator skill required. The high capital investment for machines such as electrical and high-energy beam machining should be justified in terms of the production runs and the feasibility of manufacturing the same part by other means, if at all possible.
Summary
- Advanced machining processes have unique capabilities, and involve electrochemical, electrical, laser and high-energy-beam sources of energy. The mechanical properties of the workpiece material are not significant because these processes rely on mechanisms that do not involve the strength, hardness, ductility, or toughness of the material; rather they involve physical, chemical, and electrical properties.
- Chemical and electrical methods of machining are particularly suitable for hard materials and complex shapes. They do not produce forces (and can therefore be used for thin, slender and flexible workpieces), significant temperatures, or residual stresses. However, the effects of these processes on surface integrity must be investigated, as they can damage surfaces considerably, reducing the fatigue life of the product.
- High-energy-beam machining processes basically utilise laser beams, electron beams, and plasma arc. They have important industrial applications, possess high flexibility of operation, and are economically competitive with various other processes.
- Water-jet, abrasive water-jet, and abrasive-jet machining processes can be used for cutting as well as deburring operations. Because they do not utilise hard tooling, they have inherent flexibility of operation.
Future of Advanced Machining
- The need for economical methods of material removal will increase further because of the development of new materials, ceramics, and composites as well as complex shapes that are difficult to machine with traditional processes.
- In spite of their advantages, the effects of advanced machining processes on the properties and service life of workpieces are important considerations, particularly for critical applications.
- The trend in the machinery for advanced machining processes is for computer controls, using multiple-axis robots, as well as exploring possibilities of combining different processes for flexibility and improved productivity.
- Laser-beam and electrical-discharge machining of automotive and various other components is being implemented at an increasing rate.
