Page 17 - Electric Drives and Electromechanical Systems

Chapter 1 Electromechanical systems 9

Electrochemical machining is effectively the reverse of electroplating. Metal is
removed from the workpiece, which takes up the exact shape of the tool. This technique
has the advantage of producing very accurate copies of the tool, with no tool wear, and it
is widely used in the manufacture of moulds for the plastics industry and aerospace
components. The principal features of the process are shown in Fig. 1.4B. A voltage is
applied between the tool and the workpiece, and material is removed from the work-
piece in the presence of an electrolyte. With a high level of electrolyte flow, which is
normally supplied via small holes in the tooling, the waste product is flushed from the
gap and held in solution prior to being filtered out in the electrolyte-supply plant. While
the voltage between the tool and the workpiece is in the range 8e20 V, the currents will
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be considerable, as the metal removal rate is typically 1600 mm min 1 per 1000 A. To
achieve satisfactory machining, the gap between the tool and the workpiece must be
kept in the range 0.1e0.2 mm. While no direct machining force is required, the feed drive
must overcome the forces due to the high electrolyte pressure in the gap. Due to the high
currents involved, considerable damage would occur if the feed-rate was higher than the
metal removal rate, allowing the die and the blank tool collided. To ensure this does not
occur, the voltage across the gap is closely monitored, and is used to modify the pre-
defined feed rates, and, in the event of a collision, to remove the machining power.
In electrodischarge machining Fig. 1.4C a controlled spark is generated using a
special-purpose power supply between the workpiece and the electrode. Because of the
high temperature (10 000 C) small pieces of the workpiece and the tool are vaporised;

the blast caused by the spark removes the waste so that it can be flushed away by the
electrolyte. The choice of the electrode (for example, copper, carbon) and the dielectric
(for example, mineral oil, paraffin, or deionised water) is determined by the material
being machined and the quality of the finish required. As material from the workpiece is
removed, the electrode is advanced to achieve a constant discharge voltage.
Due to the nature of the process, the electrode position oscilates at the pulse fre-
quency, and this requires a drive with a high dynamic response; in many cases a hy-
draulic drive is used, though these are now being superseded by electrical systems.
Several different configurations can be used, including wire machining, small-hole
drilling, and die sinking. In electrodischarge wire machining, the electrode is a moving
wire, which can be moved relative to the workpiece in up to five axes; this allows the
production of complex shapes that could not be easily produced by any other means.
Water jet machining involves the use of a very-high pressure of water directed at the
material being cut. The water is typically pressurised to between 1300 and 4000 bar and
with a nozzle diameter of 0.18e0.4 mm, a water velocity of over 800 m s 1 results. With a
suitable feed rate, the water will cleanly cut through a wide range of materials, including
paper, wood and fibreglass. If an abrasive powder, such as silicon carbide, is added to the
water a substantial increase in performance is possible though at a cost of increased
nozzle ware. With the addition of an abrasive powder, steel plate over 50 mm thick can
easily be cut. The key advantages of this process include very low side forces, which
allows the user to machine a part with walls as thin as 0.5 mm without damage, allowing