Beau Engle
 
Electrical Discharge Machining
 
ITDPT 303
 
11/4/2003
 
 
Introduction
   
     In the past fifty years, EDM and wire EDM have made tremendous increases in the manufacturing field.  Puertas & Perez (2003) state “EDM is a widespread technique used in industry for high precision machining of all types of conductive materials such as metals, metallic alloys, graphite, or even some ceramic materials, of any hardness” (p. 791). Also EDM process is used in various fields and industries such as the medical field, construction, automotive, and aeronautics and space. Puertas and Perez (2003) explain that there are two basic types of EDM: "die sinking and wire EDM.  Die sinking EDM reproduces the shape of the tool used (electrode) in the part being machined, whereas in wire EDM a metal wire (electrode) is used to cut a programmed outline into the piece being machined" (p. 791).   This study will be discussing the processes of EDM and wire EDM, their properties and uses, how dielectric fluid plays a role in the process, and advantages and disadvantages that EDM encounters.

Principles of EDM
   
     The basic principle behind EDM machines dates back to the fifties. They basically work in one of two ways: "they cut metal with a special metal wire electrode that is programmed to travel along a preprogrammed path (in the case of wire EDM), or they form a required shape negatively in a metal using a three dimensional electrode (in the case of die sinking EDM machines)" (Dave Wilson, 1999, p.13). EDM uses a charged electrode, and then brings the electrode near a workpiece (oppositely charged). As the tool and the workpiece get close enough, a spark will occur in dielectric fluid.  The spark then creates a hole in both the electrode and the workpiece (www.claymore.engineer.gvsu.edu-278).
     Some of the different types of materials used for electrodes are: copper, tungsten, and graphite. Although all these materials are used as electrodes, graphite is the most commonly used material because it is less effected and warn from the process.  Graphite also is machined very easily, conducts electricity, and does not vaporize. During the process, the discharges that reveal the “spark” create the crater-like surface on the workpiece. (www-ai.ijs.si/SasoDzeroski/ILP2/ilpkdd/apps/edm.html).
     When explaining the workpiece and the tool being used, the terms, anode and cathode are the proper vocabulary terms used.  The tool during the EDM process is shaped to the detailed required. The positive electrode and the negative electrode never touch each other, and a small gap is maintained at all times between the two electrodes by a computer. Both electrodes are inundated into dielectric fluid and create a course for the electric discharge to be made.  Dielectric fluid is normally a clear color, some manufacturers will slightly dye their brand to make it unique.  All EDM fluids have a high dielectric strength.  Dielectric strength is important while a high value of strength is required, too high of a value will force a smaller gap and could lead to a higher wear on the electrode.  Most quality dielectric fluids will be odorless but there are some that do produce an odor which generally means the quality isn't as high.  There are petroleum and sythetic oils for EDM.  Many refined petroleum EDM fluids are just as good as the sythetic oils.  "EDM 3001 Lite, Clear-3 EDM 3033 provide good overall performance in most EDM situations from fine finishing to roughing"(www.edmoils.com). The dielectric fluid allows the tool to cool and removes any waste products remaining.  The dielectric fluid is critical in coming out with a good surface finish because it controls the discharge. (www.bath.ac.uk/spark.htm).
     Furthermore a D.C. power supply is connected to the two electrodes that make their way through the dielectric fluid ionizing it. Heat and temperature reach about 12,000 degrees celcius which will melt and vaporize the material. Hundreds of thousands of discharges happen each second.  The diameter of a piece of hair is what is eroded on both the workpiece and the tool, which is caused by the tremendous heating. The tool wear compared to the workpiece wear can vary from about 1-100 (www.bath.ac.uk/spark.htm).
 
EDM Operation
   
     Operating the EDM machine is not as difficult as the process itself. Puertas & Perez (2003) explain:

"Electrical discharge machining (EDM) is a non-traditional manufacturing process based on removing material from a part by means of a series of repeated electrical discharges (created by electric pulse generators at short intervals) between a tool, called the electrode, and the part being machined in the presence of a dialectric fluid" ( p.791). 

When operating EDM, there are two designs for the tool feed: ram and quill. The ram-feed machines are more heavy duty and less expensive than the quill-feed. The ram-feed uses a hydraulic cylinder for the movement of the head, whereas the quill-feed uses a hydraulic motor to drive a leadscrew. Both are controlled by “advancing and retracting the tool” (Biekert, 1993, p. 301). Not only are the designs different, but there are also a large number of factors to consider within the EDM process, such as the level of generator intensity, the pulse time, and the duty cycle. Puertas & Perez (2003) clarify,

"The level of generator intensity represents the maximum value of the discharge current intensity. The pulse time is the duration of time that the current is allowed to flow per cycle, and the duty cycle is the percentage of the pulse time relative to the total cycle time" (p.793).  
In addition, none of these processes within EDM require force, because the anode never touches the cathode.
    Another factor to consider in the EDM process is the speed of material removal.  The speed of the material removal process as in most cases is measured by cubic inches per hour. The EDM process does not require force. There are several factors that control the material removal rate.  The most important of these is the melting temperature of the workpiece material. The lower the melting temperature, the faster the removal rate. The rate of which the electrode is eroded is also considered. “Erosion rates are not affected by the material hardness but by the melting temperature of the material being used” (Fellers & Hunt, 2001, p.124). To summarize the EDM process for example in electric arc welding the arc melts the metal.  If that metal could be removed continuously from that particular spot where the electrode is located this would be EDM.
 
Wire EDM
     
     As EDM technology has developed, it can now use a moving wire as the electrode, wire EDM. Wire EDM uses brass, tungsten, or copper as its material for the wire electrode.  Second, deionized water is used for the dielectric fluid.  And finallly, almost like the standard EDM, the wire is eroded and slowly fed. Although it is similar to standard EDM, higher currents and lower rest times make this process much faster (www.claymore.engineer.gvsu.edu-278). Dundas (2002) adds: “A 1983 wire EDM machine had a maximum speed of 10 square inches per hour, now machines recently purchased in the past two years can cut as many as 28.5 square inches per hour” ( p. 80). The newer systems also reduce programming time. The type of material being cut, the thickness, and cutting speed are the only thing the operator needs to specify (Dundas, 2002).
     According to www.reliableedm.com, the ten benefits of wire EDM include: efficient production, capabilities, fast turnarounds, reliable repeatability, without EDM it is impossible to machine, reduces costs, stress-free and burr free cutting, exotic materials, tight tolerance and excellent finishes, program files are downloadable, parts for wire EDM accessible.




Uses
  
    The use of EDM has developed so much in recent years that different materials are being machined, the equipment is becoming more compatible, and businesses are thriving off of this new technology. Over the past few years, advances in the field of EDM have allowed the manufacturing of ceramic materials. The main inconvenience with the die-sinking EDM is the electrical conductivity of the ceramic material. There are different variables when using the EDM for ceramic materials, such as: "surface roughness, material removal rate, and electrode wear" (Puertas & Perez, 2003, 791-792).
      Another advancement in EDM comes from Integrity EDM located in Tipton, Indiana. This company is an electrical discharge machining shop, that produces high accuracy components from high nickel-based alloys for aircrafts engines and land-based turbine markets. Integrity produces finished parts at a 40 percent reduction rate in cutting time compared to other technologies, while improving surface characteristics and dimensional accuracy.  The owner and manager of Integrity, Mr. Don Rowland states, “We typically put a large number of very small and sometimes very deep holes into some incredibly hard-to-machine materials” ( as quoted in Jordan, & Steigerwald, 2003, p. 75). After looking at all the traditional cutting methods, plus waterjet, plasm, e-beam and laser, EDM is the most conventional way of drilling holes because of the materials being drilled (Jordan, & Steigerwald, 2003).    
EDM can be used to make fixtures, collets and jet engine blade slots, mold cooling ribs and reinforcing ribs. This fact makes wire and ram EDMs ideal for making magnetic reader heads for missiles, artificial joints, turbine blades and car engine prototypes (bgpeck)
 
 Advantages and Disadvantages
  
    EDM is a method of machining parts that cannot be done by conventional machines. “Since the tool does not touch the workpiece, there are no cutting forces generated; therefore, very fragile parts can be machined” (bgpeck, 2003, p.1). The shape and also the hardness of the materials being used make EDM ideal. The EDM process leaves no burrs and the material is flushed away by the dielectric fluid, and by eliminating extra steps it also lowers costs. EDM makes it ideal for small, lots of parts, allowing to reduce operating expenses, delivery dates, or reduce inventory.   EDM can replace many types of contour grinding operations and eliminate secondary operations such as deburring and polishing. P.1, 2003(bgpeck) EDM is at an advantage when secondary operations are too labor intensive for traditional machines.  EDM allows for cutting complex shapes with out distortion.
  
References
 
Biekert, R. (1993). CIM Technology: fundamentals and applications. South Holland, IL: The Goodheart-Wilcox Company Inc.
 
Dundas, B. (Feb. 2002). EDM for the long run. Modern Machines Shop, 74. Article 6048234. Retrieved on October 22, 2003 from http://weblinks1.epnet.com
 
Electrical discharge machining (LAI). Retrieved on October 22, 2003 from http://www-ai.ijs.si/SasoDzeroski/ILP2/ilpkdd/apps/edm.html
 
Fellers, W. D., & Hunt, W. W. (2001). Manufacturing processes for technology. Columbus, OH: Prentice Hall.
 
Guitrau, E. B. (1997). The EDM Handbook. Cincinnati, OH: Hanser Gardner Publications.
 
Jordan, J., & Steigerwald, S. (Mar. 2003). EDMs help cut machining time on aircraft parts by 40 percent. Modern Machine Shop, 75, Article 9250638. Retrieved on October 22, 2003 from http://weblinks1.epnet.com
 
Hugh, J. (31 August 2001). Manufacturing: engineer on a disk. Retrieved on October 22, 2003 from http://claymore.engineer.gvsu.edu/eod/manufact/manufact-277.html
 
Hugh, J. (31 August, 2001). Manufacturing: engineer on a disk. Retrieved on October 22, 2003 from http://claymore.engineer.gvsu.edu/eod/manufact/manufact-278.html
 
Puertas, I., & Perez, C. J. (2003). Modelling the manufacturing parameters in electrical discharge machining of siliconized silicon carbide. Journal of Engineering and Manufacturing, 217, pp 791-802.
 
Spark discharge machining. Retrieved on October 22, 2003 from http://www.bath.ac.uk/~en9smd/spark.htm
 
Ten benefits of reliable EDM. Reliable EDM Inc. Retrieved November 2, 2003 from http://www.reliableedm.com/benefits_of_edm.htm
 
Tlusty, G. (2000). Manufacturing processes and equipment. Upper Saddle River, NJ: Prentice Hall.
 
Wilson, D. (May 1999). Inside EDM. Design Engineering, Article 1959003. Retrieved on October 22, 2003 from http://web15.epnet.com
 
Wire and conventional EDM. Retrieved on November 2, 2003 from http://www.bgpeck.com/edm.html
 
Wire and conventional EDM. Retrieved on November 2, 2003 from http://www.bgpeck.com/edm_details.html