In this article we will discuss about:- 1. Meaning of Wire-Cut EDM 2. Features of Wire-Cut EDM 3. Spark Machining.
Meaning of Wire-Cut EDM:
The wire-cut EDM uses a very thin wire 0.02 to 0.3 mm in diameter as an electrode and machines a workpiece with electrical discharge like a bandsaw by moving either the workpiece or wire. Erosion of the metal utilizing the phenomenon of spark discharge is the very same as in conventional EDM. The prominent feature of a moving wire is that a complicated cutout can be easily machined without using a forming electrode.
Fig. 10.21 shows in schematic form the principle of wire-cut EDM process. Wire-cut EDM machine basically consists of a machine proper composed of a workpiece contour movement control unit (NC unit or copying unit), workpiece mounting table and wire drive section for accurately moving the wire at constant tension; a machining power supply which applies electrical energy to the wire electrode; and a unit which supplies a dielectric fluid (distilled water) with constant specific resistance.
Features of Wire-Cut EDM:
The various features of wire-cut EDM process are:
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(1) Forming electrode adapted to product shape is not required:
Since a thin wire of copper or tungsten is used as the electrode to machine the workpiece as programmed, there is no need for forming electrodes which are traditionally made by cutting and grinding an expensive alloy of silver and tungsten or copper and tungsten.
This feature helps reduce man-hour requirements and insures greater economy due to use of an inexpensive electrical material. The consumption of wire on the EDM machine is about 0.03 kg/ hr. The 0.2 mm dia copper wire which is most frequently used for wire-cut EDM can generally machine for 50 to 60 hours.
(2) Electrode wear is negligible:
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Since the wire electrode is constantly fed during machining, its wear can be practically ignored.
(3) Machined surfaces are smooth:
Since a new portion of wire electrode is constantly supplied at a speed of about 10 to 30 mm/sec, machining can be continued without any accumulation of chips and gases. Since a very thin electrode is used, an extremely small amount of discharge energy suffices for one spark and the workpiece is machined under conditions for finishing as specified for conventional EDM. This easily provides uniform surfaces of about 5 to 10 µm Rmax and machined dies can be used for production purposes without any polishing.
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(4) Geometrical and dimensional tolerances are tight:
Wire-cut EDM is not a sinking process with a formed electrode as in conventional EDM. The table is driven in increments of 1 µm per pulse, enabling two-dimensional contours to be machined to close tolerance and repeatability. It is generally considered that a total machining tolerance of 1/100 to 2/100 mm is possible and better temperature control and machining know-how may provide a machining tolerance as close as 5/ 1000 mm.
(5) Relative tolerance between punch and die is extremely high and die life is extended:
A programmed full- scale figure can be enlarged or reduced in 1 µm increments of compensation value. This capability allows the punch and dies to be machined with a minimum required clearance, which has been traditionally done by skill and fitting the punch and die.
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The results are a reduction in local stress concentration in the die during press stamping and an extension of die life by seven to over ten times. Of course, the punch plate stripper and punch fit snugly, which another major advantage in die is making.
(6) Straight holes can be produced to close tolerance:
The wire is maintained at optimum tension by a unique wire tension control mechanism. This prevents tapers, barrel- shaped holes, machining streaks, wire breakage and wire vibration. The effect of indirect secondary electrical discharge by the product of electrical discharge (chip) is lessened. Attachments can be mounted for relieving and taper machining.
(7) EDM machine can be operated unattended for long time at high operating rate:
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Distilled water is used as the dielectric fluid and all machine motions are controlled by NC. As a result, the wire-cut EDM machine can be operated unattended day and night without any fire hazards.
(8) Machining is done without requiring any skills:
Complicated and very minute shapes can be efficiently machined by unskilled operators using programs alone. On top of that, the machining time required can be accurately predicted to facilitate process and delivery control.
Spark Machining of Non-Conducting Materials:
Hard and brittle materials can be machined by AJM, USM, EBM and LBM processes. Another process found suitable is spark machining. Electric field available between two electrodes in an ECM cell is utilised and non-conducting material to be machined is placed in the gap. Sparks can be generated between an electrolyte and a tool electrode if the tool-electrolyte contact area is small and a sufficient voltage is applied between the electrodes.
The spark produced can remove material from non-conducting materials like glass, ceramic. Material removal rate is low and depth of cut produced is small. It is best suited for shallow cutting, finishing operations and micro-drilling of non-conducting materials.