Description and Applications. The EDM process machines hard materials into complicated shapes with accurate dimensions. EDM requires an electrically conductive workpiece. Process performance is unaffected by the hardness, toughness, and strength of the material. However, process performance is a function of the melting temperature and thermal conductivity. EDM is currently widely used in aerospace, machinery, and die and mold industries. There are two types of EDM processes:
Die-sinking EDM uses a preshaped tool electrode to generate an inverted image of the tool on the workpiece; commonly used to generate complex-shaped cavities and to drill holes in different geometric shapes and sizes on hard and high-strength materials.
Wire EDM (WEDM) uses a metal wire as the tool electrode; it can generate two- or three-dimensional shapes on the workpiece for making punch dies and other mechanical parts.
Principle of Operation. EDM removes workpiece materials by harnessing thermal energy produced by pulsed spark discharges across a gap between tool and workpiece. A spark discharge generates a very small plasma channel having a high energy density and a very high temperature (up to 10,000 C) that melts and evaporates a small amount of workpiece material. The spark discharges always occur at the highest electrical potential point that moves randomly over the machining gap during machining. With continuous discrete spark discharges, the workpiece material is uniformly removed around the tool electrode. The gap size in EDM is in the range of 400 in. to 0.02 in. (0.01 to 0.5 mm), and is determined by the pulse peak voltage, the peak discharge current, and the type of dielectric fluid.
EDM Power System. The discharge energy during EDM is provided by a direct current pulse power generator. The EDM power system can be classified into RC, LC, RLC, and transistorized types. The transistorized EDM power systems provide square waveform pulses with the pulse on-time usually ranging from 1 to 2000 msec, peak voltage ranging from 40 to 400V, and peak discharge current ranging from 0.5 to 500 A. With the RC, LC, or RLC type power system, the discharge energy comes from a capacitor that is connected in parallel with the machining gap. As a result of the low impedance of plasma channel, the discharge duration is very short (less than 5 msec), and the discharge current is very high, up to 1000 A. The peak voltage is in the same range of transistorized power systems.
The transistorized power systems are usually used in die-sinking EDM operations because of their lower tool wear. Capacitive power systems are used for small hole drilling, machining of advanced materials, and micro-EDM because of higher material removal rate and better process stability. WEDM power generator usually is a transistor-controlled capacitive power system that reduces the wire rupture risk. In this power system, the discharge frequency can be controlled by adjusting the on-time and off-time of the transistors that control the charging pulse for the capacitor connected in parallel with the machining gap.
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Key System Components. The machining gap between tool and workpiece during EDM must be submerged in an electrically nonconductive dielectric fluid. In die-sinking EDM, kerosene is often used as a dielectric fluid because it provides lower tool wear, higher accuracy, and better surface quality. Deionized water is always used as a dielectric fluid in WEDM to provide a larger gap size and lower wire temperature in order to reduce the wire rupture risk. This fluid also serves to flush debris from the gap and thus helps maintain surface quality.
Copper and graphite are commonly used as die-sinking EDM tool materials because of the high electrical conductivity and high melting temperature and the ease of being fabricated into complicated shapes. The wire electrode for WEDM is usually made of copper, brass, or molybdenum in a diameter ranging from 0.01 to 0.5 mm. Stratified copper wire coated with zinc brass with diameter of 0.25 mm is often used.
In the traditional die-sinking EDM process, the tool is fabricated into a required shape and mounted on a ram that moves vertically. The spark discharges can only occur under a particular gap size that determines the strength of electric field to break down the dielectric. A servo control mechanism is equipped to monitor the gap voltage and to drive the machine ram moving up or down to obtain a dischargeable gap size and maintain continuous sparking. Because the average gap voltage is approximately proportional to the gap size, the servo system controls the ram position to keep the average gap voltage as close as possible to a preset voltage, known as the servo reference voltage.
In a WED machine, the wire electrode is held vertically by two wire guides located separately above and beneath the workpiece, with the wire traveling longitudinally during machining. The workpiece is usually mounted on an x-y table. The trajectory of the relative movement between wire and workpiece in the x-y coordinate space is controlled by a CNC servo system according to a preprogrammed cutting passage. The CNC servo system also adjusts the machining gap size in real time, similar to the die-sinking EDM operation. The dielectric fluid is sprayed from above and beneath the workpiece into the machining gap with two nozzles.
The power generators in WED machines usually are transistor-controlled RC or RLC systems that provide higher machining rate and larger gap size to reduce wire rupture risks. In some WED machines, the machining gap is submerged into the dielectric fluid to avoid wire vibration to obtain a better accuracy. The upper wire guide is also controlled by the CNC system in many WED machines. During machining, the upper wire guide and the x-y table simultaneously move along their own preprogrammed trajectories to produce a taper and/or twist surface on the workpiece.
Machining Parameters. The polarity of tool and workpiece in EDM is determined in accordance with the machining parameters. When the discharge duration is less than 20 sec, more material is removed on the anode than that on the cathode. However, if the discharge duration is longer than 30 |1 sec, the material-removal rate on the cathode is higher than that on the anode. Therefore, with a transistorized power system, if the pulse on-time is longer than 30 sec, the tool is connected as anode and the workpiece is connected as cathode. When the on-time is less than about 20 |1 sec, the polarity must be reversed. With an RC, LC, or RLC power system, since the discharge duration is always shorter than 20 sec, the reversed polarity is used.
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With transistorized EDM power systems, the machining rate and surface finish are primarily influenced by the peak current. The machining rate increases with the peak current. The relationship between the machining rate and pulse on-time is nonlinear, and an optimal pulse on-time exists. Reducing peak current improves the surface finish, but decreases the machining rate.
Capabilities and Process Limitations. Die-sinking ED machines with transistorized power systems under good gap-flushing conditions can attain a material-removal rate as high as 12 mm3/min/amp (for a steel workpiece). The wire cut EDM process can cut ferrous materials at a rate over 100 mm2/min. A surface roughness value of 0.01 in. (0.2 mm) can be obtained with a very low discharge current. The tool wear ratio can be controlled within 1% during rough machining and semifinishing with the transistorized power generator. Dimensional tolerance of 118 in. (3 m) and taper accuracy of 20 to 40 in. (0.5 to 1 m/mm) with both die-sinking and WEDM can be obtained.
EDM can machine materials having electrical conductivity of 10-2 W-1 cm-1 or higher. An average current density more than 4 A/cm2 tends to cause substantial tool wear and unstable machining, and may lead to dielectric fire. This factor largely limits the productivity of EDM. During machining a deep cavity using die-sinking EDM under difficult flush condition, arc discharges occur, and the resultant thermal damage on workpiece substantially limits the productivity and the machined surface quality. Dielectric properties also impose additional constraints.
Ultrasonic Machining
Description and Applications. Ultrasonic machining (USM) is a process that uses the high velocity and alternating impact of abrasive particles on the workpiece to remove material. The abrasive particles are mixed in a slurry that fills the machining gap between the tool and workpiece. The alternating movement of abrasive particles is driven by the vibration of the frontal surface of tool at an ultrasonic frequency. The ultrasonic machining process can machine hard and brittle materials.
USM is often used for machining of cavities and drilling of holes on hard and brittle materials including hardened steels, glasses, ceramics, etc. Rotary ultrasonic machining (RUM) is a new application that uses a diamond grinding wheel as the tool for drilling, milling, and threading operations. During RUM, the tool is rotating at a high speed up to 5000 rpm and vibrating axially at ultrasonic frequency. This process is able to drill holes with diameter from 0.02 to 1.6 in. (0.5 to 40 mm) at depths up to 12 in. (300 mm). The material removal rate of 6 mm3/sec can be obtained with the RUM process. The tolerance of 300 in. (0.007 mm) can be easily achieved with both conventional and rotary ultrasonic processes.
Principle of Operation. In the USM process, the machining gap between tool and workpiece is filled with an abrasive slurry composed of an oil mixed with abrasive particles, with the frontal surface of the tool vibrating at ultrasonic frequency to provide the machining energy. The inverted shape of the tool is gradually generated on the workpiece. Material removal by the USM process is very complex. When the machining gap is small, the material may be removed as the frontal surface of the tool moves toward the workpiece, hitting an abrasive particle that impacts the workpiece surface. Material can also be removed by the impact of the abrasive particles when the machining gap is relatively large. In this case, the abrasive particles are accelerated by the pressure of slurry due to the ultrasonic vibration of the frontal surface of the tool. Also, ultrasonic-induced alternating pressure and cavitation in the slurry assist material removal.
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Key System Components. The ultrasonic vibration in USM is generated by an ultrasonic generator. The ultrasonic generator consists of a signal generator, a transducer, and a concentrator. The signal generator produces an electrical signal whose voltage and/or current is changing at an ultrasonic frequency to drive the transducer. The frequency of the electrical signal can be adjusted in the range of 10 to 40 kHz.
The transducer converts the electrical voltage or current into the mechanical vibration. Two types of transducers are commonly used in USM: magnetostrictive and piezoelectric.
The magnetostrictive transducer was extensively used prior to 1970. This transducer is constructed by surrounding a number of sheets of magnetostrictive material with a coil. When the strength of the electric current in the coil changes at an ultrasonic frequency, a mechanical ultrasonic vibration is generated in the magnetostrictive material. This transducer has a low energy conversion efficiency, usually less than 30%.
The piezoelectric ultrasonic transducer is commonly used today. The geometrical dimensions of this transducer vary with the change in the applied electric field. A mechanical ultrasonic vibration is generated when the strength of the electric voltage applied across the transducer material changes at an ultrasonic frequency. This transducer has an extremely high energy conversion efficiency, up to 95%. The amplitude of the ultrasonic vibration generated directly by the transducer is very small, about 400 in. (0.01 mm). A concentrator is used for amplifying the amplitude into a level that is acceptable for USM. The transducer is mounted on the larger end of the concentrator; the tool is mounted on the smaller end.
In the ultrasonic machine, the ultrasonic generator is held vertically on the ram that moves vertically, and the workpiece is mounted on an x-y table that determines the relative position between tool and workpiece. During machining, a force providing pressure between the tool and workpiece is added through the ram mechanism.
Process Parameters and Limitations. The material-removal rate during USM increases with an increase in the amplitude of ultrasonic vibration, grain size of the abrasive particles, and pressure between the tool and workpiece. The surface finish is essentially determined by the grain size for a given workpiece material; i.e., the smaller the grain size, the better surface finish. The abrasive grains used in USM are usually in the range of 100 to 900 mesh number.
The USM process is limited by the softness of the material. Workpiece materials softer than Rockwell C40 result in prohibitively long cycles. The best machining rate can be obtained on materials harder than Rockwell C60.
