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Level 42

26 - NONTRADITIONAL MACHINING AND THERMAL CUTTING


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Importance of nontraditional machining
Reasons for importance of nontraditional material removal processes are: (1) the need to shape new metal alloys and non-metals that are difficult to machine by conventional processes; (2) the requirement of unusual and complex work-part geometries; and (3) the need to avoid surface damage which is often associated with conventional machining.
Categories of non-traditional machining
The four categories are (1) mechanical, but not including conventional machining; (2) electrical; (3) thermal; and (4) chemical.
Ultrasonic machining
In ultrasonic machining, abrasives contained in a slurry are driven at high velocity against the work by a tool vibrating at low amplitude and high frequency. The tool oscillates in a direction perpendicular to the work surface, and is fed slowly into the work, so that the shape of the tool is formed in the part. The abrasives, impinging against the work surface, perform the chip removal.
Water jet cutting process
Water jet cutting uses a high-pressure, high-velocity stream of water directed at the work surface to cut the work, a jet generated with a pressure of up to 400 mega-pascals, exiting a diamond nozzle with speeds of up to 900 meters per second.
Abrasive water jet process
Water jet cutting cuts with a narrow, high velocity water stream; Abrasive water jet cutting adds abrasive grits to the water stream, and Abrasive Jet Cutting cuts with abrasive particles that have been added to a high velocity air stream, a processes usually used to finish and polish stuff.
Abrasive flow machining
Abrasive flow machining works by streaming abrasive particles mixed in with a viscoelastic polymer through complex and twisted passages, pushing the polymer and abrasives mix with pressures up to 20 mega-pascals, moving the material back and forth through the twisted passages of engines and castings, while removing sharp edges and achieving a desired surface finish.
Electrochemical machining types
The three types are: [1] electrochemical machining - fast flowing electrolyte removes molecules from the part in a process similar to electroplating yet the electrolyte flow is so fast that the removed molecules do not get deposited on the tool, a processes used to machine hard metals or create irregularly shaped holes. [2] De-burring - electrochemical machining method to remove burrs or round sharp corners. [3] Electrochemical grinding - the wheel, in addition to grinding, also removes 95% material using an electrochemical method, thus lengthening the life of the grinding wheel, a processes used to sharpening cemented carbide tools, surgical needles, and other fragile shit.
Disadvantages of electrochemical machining
Disadvantages of electrochemical machining include (1) cost of electrical power to operate the process, and (2) cost of disposal of electrolyte sludge. Expensive lightning creates toxic chemicals.
Electric discharge machining
Electric discharge machining (EDM) is one of the most widely used nontraditional processes. The shape of the finished work surface is produced by a formed electrode tool. The sparks occur across a small gap between tool and work surface. The EDM process must take place in the presence of a dielectric fluid, which creates a path for each discharge as the fluid becomes ionized in the gap. The discharges are generated by a pulsating direct current power supply connected to the work and the tool. Two important process parameters in EDM are discharge current and frequency of discharges. As either of these parameters is increased, metal removal rate increases.
Current increase in EDM
As discharge current increases, metal removal rate increases and surface finish is degraded - stronger charge removes more metal; brighter lightning leaves deeper scars.
Overcut
Overcut refers to the gap between the electrode tool in electric discharge machining on each side of the tool and the machined hole, cavity, or kerf in wire electric discharge machining.
Material layers after EDM
The three layers after EDM are the following: (1) Spheres attached to the surface made of part material and electrode material that have spattered the surface. This layer is easily removed. (2) Recast or white layer where EDM has altered the workpiece metallurgical structure. It can be reduced by specifying the proper settings and removed by polishing. (3) Heat affected zone or annealed layer. It has only been heated, not melted.
Electron beam machining.
Electron beam machining is used for a variety of high-precision cutting applications. on any known material.Electron beam machining uses a high velocity stream of electrons focused on the workpiece surface to remove material by melting and vaporization. An electron beam gun generates a continuous stream of electrons that is accelerated to approximately 75% of the speed of light and focused through an electromagnetic lens on the work surface. The lens is capable of reducing the area of the beam to a diameter as small as 0.025mm(0.001 in). On impinging the surface, the kinetic energy of the electrons is converted into thermal energy of extremely high density that melts or vaporizes the material in a very localized area.
Laser beam machining
Laser beam machining (LBM) uses the light energy from a laser to remove material by vaporization and ablation. In laser beam machining, the energy of the coherent light beam is concentrated not only optically but also in terms of time. The light beam is pulsed so that the released energy results in an impulse against the work surface that produces a combination of evaporation and melting, with the melted material evacuating the surface at high velocity.
Plasma arc cutting
Plasma arc cutting (PAC) uses a plasma stream operating at temperatures in the range 10,000 14,000 degrees Celsius to cut metal by melting. The cutting action operates by directing the high-velocity plasma stream at the work, thus melting it and blowing the molten metal through the kerf. The plasma arc is generated between an electrode inside the torch and the anode workpiece.The plasma flows through a water-cooled nozzle that constricts and directs the stream to the desired location on the work. The resulting plasma jet is a high-velocity, well-collimated stream with extremely high temperatures at its center, hot enough to cut through metal in some cases 150 millimeters thick.
Disadvantages of plasma arc cutting.
Two disadvantages of plasma arc cutting are: (1) rough surface on cut edge and (2) metallurgical damage to cut surface.
Air carbon arc cutting
In air carbon arc cutting the electric arc is generated between a carbon electrode and the metallic work, and a high-velocity air jet is used to blow away the melted portion of the metal. Spattering of the molten metal is a hazard and a disadvantage of the process.
Oxyfuel-cutting process
Oxyfuel-cutting, also known as flame cutting use the heat of combustion of certain fuel gases combined with the exothermic reaction of the metal with oxygen. The primary mechanism of material removal in oxyfuel cutting is the chemical reaction of oxygen with the base metal. The purpose of the oxyfuel combustion is to raise the temperature in the region of cutting to support the reaction. These processes are commonly used to cut ferrous metal plates.
Fuels of oxyfuel-cutting process
Principal fuels for oxyfuel-cutting process are acetylene, methylacetylene-propadiene, propylene, propane, and natural gas.
Steps in chemical machining
The four principal steps in chemical machining are: (1) Cleaning - clean the workpiece in order to ensure that the material will be removed from it in an equal manner. (2) Masking - cover the portions of the workpiece which you don't want to be etched away. (3) Etching - immerse the material in the chemicals and let them work their magic while you relax or get high on the vapors. And (4) Demasking - remove the mascant and clean the part.
Masking methods in chemical machining
The three masking methods in chemical machining are (1) cut and peel, (2) screen resist, and (3) photographic resist.
Photoresist
A photoresist is a masking material that is sensitive to light. When exposed, it chemically transforms and can be removed from the surface of the work, leaving the desired surface unprotected by the maskant, thus allowing creation of intricate patterns by applying a negative, projecting ultra violet light, an letting the magic chemicals do their work.
Nontraditional process performance
Nontraditional processes usually have a low removal rate and high specific energy, thus generally making them more expensive than traditional methods such as drilling, milling, and turning, thus it is best to apply them when traditional methods are not practical or economic.