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EDM Machining, Electrical-Discharge Milling and Grinding
ELECTRICAL DISCHARGE MACHINING (EDM), also referred to as SPARK-EROSION or ELECTRODISCHARGE MACHINING, SPARK ERODING, DIE SINKING or WIRE EROSION, is a NON-CONVENTIONAL MANUFACTURING process where erosion of metals takes place and desired shape is obtained using electrical discharges in the form of sparks. We also offer some varieties of EDM, namely NO-WEAR EDM, WIRE EDM (WEDM), EDM GRINDING (EDG), DIE-SINKING EDM, ELECTRICAL-DISCHARGE MILLING, micro-EDM, m-EDM and ELECTROCHEMICAL-DISCHARGE GRINDING (ECDG). Our EDM systems consist of shaped tools/electrode and the workpiece connected to DC power supplies and inserted in a electrically nonconducting dielectric fluid. After 1940 electrical discharge machining has become one of the most important and popular production technologies in manufacturing industries.
When the distance between the two electrodes is reduced, the intensity of the electric field in the volume between the electrodes becomes greater than the strength of the dielectric in some points, which breaks, eventually forming a bridge for current to flow between the two electrodes. An intense electrical arc is generated causing significant heating to melt a portion of the workpiece and some of the tooling material. As a result, material is removed from both the electrodes. At the same time, the dielectric fluid is heated rapidly, resulting in evaporation of the fluid in the arc gap. Once the current flow stops or it is stopped heat is removed from the gas bubble by the surrounding dielectric fluid and the bubble cavitates (collapses). The shock wave created by the bubble’s collapse and the flow of dielectric fluid flush debris from the workpiece surface and entrain any molten workpiece material into the dielectric fluid. The repetition rate for these discharges are between 50 to 500 kHz, voltages between 50 to 380 V and currents between 0.1 and 500 Amperes. New liquid dielectric such as mineral oils, kerosene or distilled & deionized water is usually conveyed into the inter-electrode volume carrying away the solid particles (in the form of debris) and the insulating proprieties of the dielectric is restored. After a current flow, the potential difference between the two electrodes is restored to what it was before the breakdown, so a new liquid dielectric breakdown can occur. Our modern electrical discharge machines (EDM) offer numerically controlled movements and are equipped with pumps and filtering systems for the dielectric fluids.
Electrical discharge machining (EDM) is a machining method mainly used for hard metals or those that would be very difficult to machine with conventional techniques. EDM typically works with any materials that are electrical conductors, although methods for machining insulating ceramics with EDM have also been proposed. The melting point and latent heat of melting are properties that determine the volume of metal removed per discharge. The higher these values, the slower the material removal rate. Because the electrical discharge machining process does not involve any mechanical energy, the hardness, strength, and toughness of the workpiece do not affect the removal rate. Discharge frequency or energy per discharge, the voltage and current are varied to control material removal rates. Rate of material removal and surface roughness increase with increasing current density and decreasing spark frequency. We can cut intricate contours or cavities in pre-hardened steel using EDM without the need for heat treatment to soften and re-harden them. We can use this method with any metal or metal alloys like titanium, hastelloy, kovar, and inconel. Applications of the EDM process include shaping of polycrystalline diamond tools. EDM is considered a non-traditional or non-conventional machining method along with processes such as electrochemical machining (ECM), water jet cutting (WJ, AWJ), laser cutting. On the other hand the conventional machining methods include turning, milling, grinding, drilling and other process whose material removal mechanism is essentially based on mechanical forces. Electrodes for electrical-discharge machining (EDM) are made of graphite, brass, copper and copper-tungsten alloy. Electrode diameters down to 0.1mm are possible. Since tool wear is an undesired phenomenon adversely affecting dimensional accuracy in EDM, we take advantage of a process called NO-WEAR EDM, by reversing polarity and using copper tools to minimize tool wear.
Ideally speaking, the electrical-discharge machining (EDM) can be considered a series of breakdown and restoration of the dielectric liquid between the electrodes. In reality however, the removal of the debris from the inter-electrode area is almost always partial. This causes the electrical proprieties of the dielectric in the inter-electrodes area to be different from their nominal values and vary with time. The inter-electrode distance, (spark-gap), is adjusted by the control algorithms of the specific machine used. The spark-gap in EDM can unfortunately sometimes be short-circuited by the debris. The control system of the electrode may fail to react quickly enough to prevent the two electrodes (tool and workpiece) from short circuiting. This unwanted short circuit contributes to material removal differently from the ideal case. We pay utmost importance to flushing action in order to restore the insulating properties of the dielectric so that the current always happens in the point of the inter-electrode area, thereby minimizing the possibility of unwanted change of shape (damage) of the tool-electrode and workpiece. To obtain a specific geometry, the EDM tool is guided along the desired path very close to the workpiece without touching it, We pay utmost attention to the performance of motion control in use. This way, a large number of current discharges / sparks take place, and each contributes to the removal of material from both tool and workpiece, where small craters are formed. The size of the craters is a function of the technological parameters set for the specific job at hand and dimensions may range from the nanoscale (such as in the case of micro-EDM operations) to some hundreds of micrometers in roughing conditions. These small craters on the tool cause gradual erosion of the electrode called “tool wear”. To counteract the detrimental effect of the wear on the geometry of the workpiece we continuously replace the tool-electrode during a machining operation. Sometimes we achieve this by using a continuously replaced wire as electrode ( this EDM process is also called WIRE EDM ). Sometimes we use the tool-electrode in such a way that only a small portion of it is actually engaged in the machining process and this portion is changed on a regular basis. This is, for instance, the case when using a rotating disk as a tool-electrode. This process is called EDM GRINDING. Yet another technique we deploy consists of using a set of electrodes with different sizes and shapes during the same EDM operation to compensate for wear. We call this multiple electrode technique, and is most commonly used when the tool electrode replicates in negative the desired shape and is advanced towards the blank along a single direction, usually the vertical direction (i.e. z-axis). This resembles the sink of the tool into the dielectric liquid in which the workpiece is immersed, and therefore it is referred to as DIE-SINKING EDM (sometimes called CONVENTIONAL EDM or RAM EDM). The machines for this operation are called SINKER EDM. The electrodes for this type of EDM have complex forms. If the final geometry is obtained using a usually simple-shaped electrode moved along several directions and is also subject to rotations, we call it EDM MILLING. The amount of wear is strictly dependent on the technological parameters used in the operation ( polarity, maximum current, open circuit voltage). For example, in micro-EDM, also known as m-EDM, these parameters are usually set at values which generates severe wear. Therefore, wear is a major problem in that area which we minimize using our accumulated know-how. For example to minimize wear to graphite electrodes, a digital generator, controllable within milliseconds, reverses polarity as electro-erosion takes place. This results in an effect similar to electroplating that continuously deposits the eroded graphite back on the electrode. In another method, a so-called ''Zero Wear'' circuit we minimize how often the discharge starts and stops, keeping it on for as long a time as possible. The material removal rate in electrical-discharge machining can be estimated from:
MRR = 4 x 10 exp(4) x I x Tw exp (-1.23)
Here MRR is in mm3/min, I is current in Amperes, Tw is workpiece melting point in K-273.15K. The exp stands for exponent.
On the other hand, the wear rate Wt of the electrode can be obtained from:
Wt = ( 1.1 x 10exp(11) ) x I x Ttexp(-2.38)
Here Wt is in mm3/min and Tt is melting point of the electrode material in K-273.15K
Finally, the wear ratio of the workpiece to electrode R can be obtained from:
R = 2.25 x Trexp(-2.38)
Here Tr is the ratio of melting points of workpiece to electrode.
SINKER EDM :
Sinker EDM, also referred to as CAVITY TYPE EDM or VOLUME EDM, consists of an electrode and workpiece submerged in an insulating liquid. The electrode and workpiece are connected to a power supply. The power supply generates an electrical potential between the two. As the electrode approaches the workpiece, dielectric breakdown occurs in the fluid, forming a plasma channel, and a small spark jumps. The sparks usually strike one at a time because it is highly unlikely that different locations in the inter-electrode space have identical local electrical characteristics which would enable a spark to occur in all such locations simultaneously. Hundreds of thousands of these sparks happen at random points between the electrode and the workpiece per second. As the base metal erodes, and the spark gap subsequently increases, the electrode is lowered automatically by our CNC machine so that the process can continue uninterrupted. Our equipment has controlling cycles known as ''on time'' and ''off time''. The on time setting determines the length or duration of the spark. A longer on time produces a deeper cavity for that spark and all subsequent sparks for that cycle, creating a rougher finish on the workpiece and vice versa. The off time is the period of time that one spark is replaced by another. A longer off time permits the dielectric fluid to flush through a nozzle to clean out the eroded debris, thereby avoiding a short circuit. These settings are adjusted in micro seconds.
WIRE EDM :
In WIRE ELECTRICAL DISCHARGE MACHINING (WEDM), also called WIRE-CUT EDM or WIRE CUTTING, we feed a thin single-strand metal wire of brass through the workpiece, which is submerged in a tank of dielectric fluid. Wire EDM is an important variation of EDM. We occasionally use wire-cut EDM to cut plates as thick as 300mm and to make punches, tools, and dies from hard metals that are difficult to machine with other manufacturing methods. In this process which resembles to contour cutting with a band saw, the wire, which is constantly fed from a spool, is held between upper and lower diamond guides. The CNC-controlled guides move in the x–y plane and the upper guide can also move independently in the z–u–v axis, giving rise to the ability to cut tapered and transitioning shapes (such as circle on the bottom and square at the top). The upper guide can control axis movements in x–y–u–v–i–j–k–l–. This allows the WEDM to cut very intricate and delicate shapes. The average cutting kerf of our equipment that achieves the best economic cost and machining time is 0.335 mm using Ø 0.25 brass, copper or tungsten wire. However the upper and lower diamond guides of our CNC equipment are accurate to about 0.004 mm, and can have a cutting path or kerf as small as 0.021 mm using Ø 0.02 mm wire. So really narrow cuts are possible. The cutting width is greater than the width of the wire because sparking occurs from the sides of the wire to the workpiece, causing erosion. This ''overcut'' is necessary, for many applications it is predictable and therefore can be compensated for ( in micro-EDM this is not often the case). The wire spools are long—an 8 kg spool of 0.25 mm wire is just over 19 kilometers in length. Wire diameter can be as small as 20 micrometres and the geometry precision is in the neighborhood of +/- 1 micrometer. We generally use the wire only once and recycle it because it is relatively inexpensive. It travels at a constant velocity of 0.15 to 9m/min and a constant kerf (slot) is maintained during a cut. In the wire-cut EDM process we use water as the dielectric fluid, controlling its resistivity and other electrical properties with filters and de-ionizer units. The water flushes the cut debris away from the cutting zone. Flushing is an important factor in determining the maximum feed rate for a given material thickness and therefore we keep it consistent. Cutting speed in wire EDM is stated in terms of the cross-sectional area cut per unit time, such as 18,000 mm2/hr for 50mm thick D2 tool steel. The linear cutting speed for this case would be 18,000/50 = 360mm/hr The material removal rate in wire EDM is:
MRR = Vf x h x b
Here MRR is in mm3/min, Vf is the feed rate of the wire into workpiece in mm/min, h is thickness or height in mm, and b is the kerf, which is:
b = dw + 2s
Here dw is wire diameter and s is gap between wire and workpiece in mm.
Along with tighter tolerances, our modern multi axis EDM wire-cutting machining centers have added features such as multi heads for cutting two parts at the same time, controls for preventing wire breakage, automatic self-threading features in case of wire breakage, and programmed machining strategies to optimize the operation, straight and angular cutting capabilities.
Wire-EDM offers us low residual stresses, because it does not require high cutting forces for removal of material. When the energy/power per pulse is relatively low (as in finishing operations), little change in the mechanical properties of a material is expected due to low residual stresses.
ELECTRICAL-DISCHARGE GRINDING (EDG) : The grinding wheels do not contain abrasives, they are made of graphite or brass. Repetitive sparks between the rotating wheel and workpiece remove material from workpiece surfaces. The material removal rate is:
MRR = K x I
Here MRR is in mm3/min, I is current in Amperes, and K is workpiece material factor in mm3/A-min. We frequently use electrical-discharge grinding to saw narrow slits on components. We sometimes combine EDG (Electrical-Discharge Grinding) process with ECG (Electrochemical Grinding) process where material is removed by chemical action, the electrical discharges from the graphite wheel breaking up the oxide film and washed away by the electrolyte. The process is called ELECTROCHEMICAL-DISCHARGE GRINDING (ECDG). Even though the ECDG process consumes relatively more power, it is a faster process than the EDG. We mostly grind carbide tools using this technique.
Applications of Electrical Discharge Machining:
We use the EDM process in mold-making, tool and die manufacturing, as well as for making prototype and production parts, especially for the aerospace, automobile and electronics industries in which production quantities are relatively low. In Sinker EDM, a graphite, copper tungsten or pure copper electrode is machined into the desired (negative) shape and fed into the workpiece on the end of a vertical ram.
Coinage die making:
For the creation of dies for producing jewelry and badges by the coinage (stamping) process, the positive master may be made from sterling silver, since (with appropriate machine settings) the master is significantly eroded and is used only once. The resultant negative die is then hardened and used in a drop hammer to produce stamped flats from cutout sheet blanks of bronze, silver, or low proof gold alloy. For badges these flats may be further shaped to a curved surface by another die. This type of EDM is usually performed submerged in an oil-based dielectric. The finished object may be further refined by hard (glass) or soft (paint) enameling and/or electroplated with pure gold or nickel. Softer materials such as silver may be hand engraved as a refinement.
Drilling of Small Holes:
On our wire-cut EDM machines, we use small hole drilling EDM to make a through hole in a workpiece through which to thread the wire for the wire-cut EDM operation. Separate EDM heads specifically for small hole drilling are mounted on our wire-cut machines which allow large hardened plates to have finished parts eroded from them as needed and without pre-drilling. We also use small hole EDM to drill rows of holes into the edges of turbine blades used in jet engines. Gas flow through these small holes allows the engines to use higher temperatures than otherwise possible. The high-temperature, very hard, single crystal alloys these blades are made of makes conventional machining of these holes with high aspect ratio extremely difficult and even impossible. Other application areas for small hole EDM is to create microscopic orifices for fuel system components. Besides the integrated EDM heads, we deploy stand-alone small hole drilling EDM machines with x–y axes to machine blind or through holes. EDM drills bore holes with a long brass or copper tube electrode that rotates in a chuck with a constant flow of distilled or deionized water flowing through the electrode as a flushing agent and dielectric. Some small-hole drilling EDMs are able to drill through 100 mm of soft or even hardened steel in less than 10 seconds. Holes between 0.3 mm and 6.1 mm can be achieved in this drilling operation.
Metal disintegration machining:
We also have special EDM machines for the specific purpose of removing broken tools (drill bits or taps) from work pieces. This process is called ''metal disintegration machining''.
Advantages and Disadvantages Electrical-Discharge Machining:
Advantages of EDM include machining of:
- Complex shapes that would otherwise be difficult to produce with conventional cutting tools
- Extremely hard material to very close tolerances
- Very small work pieces where conventional cutting tools may damage the part from excess cutting tool pressure.
- There is no direct contact between tool and work piece. Therefore delicate sections and weak materials can be machined without any distortion.
- A good surface finish can be obtained.
- Very fine holes can be easily drilled.
Disadvantages of EDM include:
- The slow rate of material removal.
- The additional time and cost used for creating electrodes for ram/sinker EDM.
- Reproducing sharp corners on the workpiece is difficult due to electrode wear.
- Power consumption is high.
- ''Overcut'' is formed.
- Excessive tool wear occurs during machining.
- Electrically non-conductive materials can be machined only with specific set-up of the process.