ELECTRICAL DISCHARGE MACHINING Machinery's Handbook, 31st Edition
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ELECTRICAL DISCHARGE MACHINING Generally called EDM, electrical discharge machining uses an electrode to remove metal from a workpiece by generating electric sparks between conducting surfaces. The two main types of EDM are termed sinker or plunge, used for making mold or die cavi- ties, and wire, used to cut shapes such as are needed for stamping dies. For die sinking, the electrode usually is made from copper or graphite and is shaped as a positive replica of the shape to be formed on or in the workpiece. A typical EDM sinker machine, shown diagrammatically in Fig. 1, resembles a vertical milling machine, with the electrode attached to the vertical slide. The slide is moved down and up by an electronic, servo- controlled drive unit that controls the spacing between the electrode and the workpiece on the table. The table can be adjusted in three directions, often under numerical control, to positions that bring a workpiece surface to within 0.0005-0.030 inch (0.013–0.76 mm) from the electrode surface, where a spark is generated.
Upper Guide
W
+ y
– u – v + v
+ u
+ x
– x
– y
Die Block
Sinker Electrode
Brass Wire Electrode
Servo System
–
+ –
Power Supply
Tank Containing Dielectric Fluid
+
Lower Guide
Workpiece
Fig. 1. Sinker or Plunge Type EDM Machines are Used to Sink Cavities in Molds and Dies.
Fig. 2. Wire Type EDM Machines are Used to Cut Stamping Die Profiles.
Wire EDM, shown diagrammatically in Fig. 2, are numerically controlled and somewhat resemble a bandsaw with the saw blade replaced by a fine brass or copper wire, which forms the electrode. This wire is wound off one reel, passed through tensioning and guide rollers, then through the workpiece and through lower guide rollers before being wound onto another reel for storage and eventual recycling. One set of guide rollers, usually the lower, can be moved on two axes at 90 degrees apart under numerical control to adjust the angle of the wire when profiles of varying angles are to be produced. The table also is movable in two directions under numerical control to adjust the position of the workpiece relative to the wire. Provision must be made for the cut-out part to be supported when it is freed from the workpiece so that it does not pinch and break the wire. EDM applied to grinding machines is termed EDG. The process uses a graphite wheel as an electrode, and wheels can be up to 12 inches (30.48 cm) in diameter by 6 inches (15.24 cm) wide. The wheel periphery is dressed to the profile required on the workpiece, and the wheel profile can then be transferred to the workpiece as it is traversed past the wheel, which rotates but does not touch the work. EDG machines are highly specialized and are mainly used for producing complex profiles on polycrystaline diamond cutting tools and for shaping carbide tooling such as form tools, thread chasers, dies, and crushing rolls. EDM Terms * .— Anode: The positive terminal of an electrolytic cell or battery. In EDM, incorrectly applied to the tool or electrode. * Source: Hansvedt Industries
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