06-12-2012, 06:01 PM
ADVANCED MACHINING PROCESSES
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INTRODUCTION
While making a part from raw material, one may require bulk removal of material, forming cavities/holes and finally finishing as per the parts requirements. Many advanced finishing processes have been employed to make circular and/or noncircular cavities and holes in difficult-to-machine materials. Some of the processes employed for hole making are electro-discharge machining, laser beam machining, electron beam machining, shaped tube electro-chemical machining and electrochemical spark machining. With the demand for stringent technological and functional requirements of the parts from the micro- to nanometer range, ultra precision finishing processes have evolved to meet the needs of the manufacturing scientists and engineers. The traditional finishing processes of this category have various limitations, for example, complex shapes, miniature sizes, and three-dimensional (3D) parts cannot be processed/finished economically and rapidly by traditional machining/finishing processes. This led to the development of advanced finishing techniques, namely abrasive flow machining, magnetic abrasive finishing, magnetic float polishing, magneto-rheological abrasive finishing and ion beam machining.
Traditionally, abrasives either in loose or bonded form whose geometry varies continuously in an unpredictable manner during the process are used for final finishing purposes.
Nowadays, new advances in materials syntheses have enabled the production of ultra-fine abrasives in the nanometer range. With such abrasives, it has become possible to achieve nanometer surface finishes and dimensional tolerances. There are processes (ion beam machining and elastic emission machining) that can give ultra-precision finish of the order of size of an atom or molecule of a substance. In some cases, the surface finish (center line average (CLA) value) obtained has been reported to be even smaller than the size of an atom. Various processes have been employed for finishing purposes, like abrasive flow machining (AFM), magnetic abrasive flow machining (MAFM), magnetic abrasive finishing (MAF), magnetic float polishing (MFP), and magneto-rheological abrasive flow finishing (MRAFF), elastic emission machining (EEM) and ion beam machining (IBM).
Thermal advanced machining processes
Application of AMP is quite common in making holes in difficult-to-machine materials as well as shaping and sizing a part. Sometimes holes with a high aspect ratio or a large number of holes in a work piece without burrs and without residual stresses are needed. Some processes are good only for electrically conductive materials while others are excellent for making thousands of holes in a square centimeter area of metallic as well as non-metallic materials. Processes such as EDM,travelling-wire EDM, LBM and EBM fall into the category of thermal AMP in which material removal takes place by melting or melting and vaporization.
The energy source for material removal is in the form of heat.
ABRASIVE WATER JET CUTTING (AWJC)
The abrasive water jet cutting (AWJC) process is a high-potential process applicable to both metals as well as non-metals. In this process, a high-velocity water jet mixed with fine abrasive particles hits the work piece surface. The velocity of the abrasive mixed water jet is very high, hence the kinetic energy with which the abrasive particles and the water jet hit the work piece surface is very high (as high as 900 m/s in special cases) and hence it leads to the erosion of the work surface. Here, a part of the momentum of water jet is transferred to the abrasives; hence the velocity of abrasives rises rapidly.
Depending upon the type of the work piece material being cut and the depth at which cutting is taking place, material removal occurs due to erosion, shear or failure under a rapidly changing localized stress field. The pressure at which a water jet operates is about 400 MPa, which is sufficient to produce a jet velocity of 900 m/s. Such a high-velocity jet is able to cut materials such as ceramics, composites, rocks, metals etc. [6]. Material removal by erosion takes place in the upper part of the work piece while it occurs by deformation wear at the lower part of the work piece being cut. The AWJC process can easily cut both electrically non-conductive and conductive, and difficult-to-machine materials.
ELECTRIC DISCHARGE MACHINING (EDM) AND WIRE EDM
The working principle of EDM process can be understood from figure. Dielectric flows through the gap between the electrodes (usually with the tool as the cathode and the work piece as the anode), which are connected to a pulsed direct current (DC) power supply. This produces sparks between the electrodes, which melt and sometimes vaporize material from both the tool and the work piece. To improve the accuracy of axisymmetric profiles, orbital EDM is advocated. Figure shows an inter-electrode gap between the tool and the work piece in which dielectric is flushed at high pressure. As shown in Fig, once the power supply is on, the capacitor keeps charging until the breakdown voltage (Vb) is attained and then sparking takes place at a point of least electrical resistance. After each discharge, the capacitor recharges and the spark energy is shared mainly by work piece, tool, dielectric and debris (removed material). Radiation losses are also present.
The flowing dielectric in the IEG cools the tool and work piece, cleans the IEG and localizes the spark energy into a small cross-sectional area.
LASER BEAM MACHINING (LBM)
The acronym laser means light amplification by stimulated emission of radiation. In laser beam machining, a laser beam is focused onto the target/work piece surface resulting in an energy density of the order of 103 W/mm2 (or more in some cases), which is enough to melt and vaporize materials such as diamond. Holes drilled using a laser beam system is normally not straight sided, as shown in Figure. Laser light is monochromatic, coherent and gives very low divergence. An LBM machine has high capital and operating costs, and very low machining efficiency (<1%). Industrial lasers operate either in continuous wave (CW) or in pulse mode. CW lasers are used for processes such as welding, laser chemical vapor deposition (LCVD) and surface hardening. These applications require uninterrupted supply of energy for melting and phase transformation.