03-07-2012, 11:55 AM
Hot machining & Hybrid non traditional machining Methods
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Introduction
Electrochemical grinding (ECG) utilizes a negatively charged abrasive grinding wheel, electrolyte solution, and a positively charged workpiece,as shown in Fig. 6.2. The process is, therefore, similar to ECM except that the cathode is a specially constructed grinding wheel instead of a cathodic shaped tool like the contour to be machined by ECM. The insulating abrasive material (diamond or aluminum oxide) of the grinding wheel is set in a conductive bonding material. In ECG,the nonconducting abrasive particles act as a spacer between thewheel conductive bond and the anodic workpiece. Depending on the grain size of these particles, a constant interelectrode gap (0.025 mm).
Material removal process
During USMEC, the intensity of the dissolution phase depends on the relative position of the tool with respect to the workpiece. This phase reaches its maximum level at a gap size equal to the size of the statically pressed abrasive grains. Under this particular condition, the nonconductive
abrasive grains form the minimum interelectrode gap size. When a pulsed voltage replaces the straight dc one, it must be synchronized with the tool oscillation in order to maintain an efficient ECD and avoid the formation of spark discharges across the interelectrode gap.The dissolution phase occurs along with the MA caused by the ultrasonicimpact of abrasive grains at the machined surface. Since the anodic dissolution phase is accompanied by the formation of a brittle (passive).
Electrodischarge Grinding
Electrodischarge grinding (EDG) removes conductive materials by rapid
spark discharges between a rotating tool and workpiece that are separated
by a flowing dielectric fluid (Fig. 7.12). The spark gap is normally
held at 0.013 to 0.075 mm by the servomechanism that controls the
motion of the workpiece. The dc power source has capabilities ranging from 30 to 100 A, 2 to 500 kHz, and 30 to 400 V. The conductive wheel,
usually made of graphite, rotates at 30 to 180 m/min in a dielectric bath
of filtered hydrocarbon oil. The workpiece is usually connected to the
positive terminal of the dc power supply. As can be seen from Fig. 7.12,
the workpiece is machined using a stream of electric sparks. Each spark
discharge melts or vaporizes a small amount of metal from the workpiece
surface. Higher machining currents produce faster rates of machining,
rougher finishes, and a deeper heat-affected zone (HAZ) in the workpiece.
Less current is used for the production of smoother and less damaged surfaces.