This article will take an in-depth look at EDM Machining.
The article will bring more understanding on topics such as:
This chapter will discuss EDM machining together with its working principle.
Electrical Discharge Machining (EDM) is a manufacturing technique that removes material from a workpiece through a series of rapid electrical discharges between electrodes submerged in a dielectric fluid. This process is ideal for creating parts that are difficult or impossible to machine using traditional methods, as it relies on electrical rather than mechanical forces.
EDM is a highly precise technique, well-suited for crafting complex and intricate shapes, including those made from hard metals like titanium. For EDM to be effective, the materials being machined must be electrically conductive.
In an EDM machine, the workpiece electrode (anode) is connected to the positive terminal of a DC power supply, while the tool electrode (cathode) is connected to the negative terminal. Both electrodes are submerged in dielectric fluid and separated by a spark gap. When an electrical discharge occurs, it creates intense electrothermal heat in the spark gap, causing parts of the workpiece surface to melt and vaporize through a process known as spark erosion.
While the fundamental principles of EDM machining remain consistent, there are notable differences between wired EDM and sinker EDM processes. Both methods use anodes and cathodes to shape the workpiece according to the desired specifications. However, the way each process employs electrical current to achieve this shaping differs significantly.
In sinker EDM machining, an electrical potential difference is established between the tool and the workpiece, both of which are electrically conductive and immersed in a dielectric fluid like hydrocarbon oil or deionized water. The dielectric fluid fills the spark gap between the tool and the workpiece. The electric field generated depends on the potential difference and the width of the spark gap.
In sinker EDM machining, the tool is connected to the negative terminal, while the work material is connected to the positive terminal of the power supply. When the electric field is applied, free electrons on the tool experience electrostatic forces. If the tool has a lower work function or lower electron bonding energy, electrons will be emitted from it. This electron emission from the tool, due to its connection to the negative terminal, is known as cold emission.
In the dielectric medium, cold-emitted electrons are accelerated toward the work material. As these electrons gain velocity and energy, they collide with the dielectric molecules. These collisions ionize the dielectric molecules, which depends on the work function or ionization energy of the dielectric and the energy of the electrons. As the electrons continue to accelerate, they generate positive ions and additional electrons through these collisions.
This cyclic process enhances the concentration of electrons and ions in the dielectric fluid between the tool and the work material at the spark gap. The concentration becomes so intense that the material in this region forms a “plasma.” The plasma channel has very low electrical resistance, allowing a significant flux of electrons to move from the tool to the work material, while ions move rapidly from the work material to the tool. This movement of electrons is referred to as an avalanche.
The sudden movement of electrons and ions creates the thermal energy of the spark with a heat range of 8,000°C up to 12,000°C. The rapid motion of the electrons hits the work material and the ions on the tool. The impact of the electrons and ions on the surface of the workpiece is converted into thermal energy or heat flux.
The EDM wire machining process, an alternative to sinker EDM machining, works much like a wood band saw using a wire for the cutting process. The wire, made of copper or brass, has a high voltage electrical discharge passed through it that makes it possible for the wire to cut through the thickness of the workpiece.
In EDM wire machining, a wire creates sparks in deionized water, where conductivity is precisely controlled. The water not only cools the material but also washes away the removed material. Clean dielectric fluid is continuously pumped into the process to flush away excess waste.
In the EDM process, extreme temperatures quickly remove excess material from the workpiece through vaporization, melting, or spark erosion. The molten metal is partially displaced. When the electric potential is removed, the plasma channel collapses, creating pressure or shock waves that expel the molten material and form a crater around the spark site.
Material is removed through the formation of shock waves when the plasma channel collapses due to the discontinuation of the electric potential, with the work material becoming positive and the tool negative. As electrons strike the workpiece, craters are formed from the heating, melting, and removal of material. Positive ions then strike the tool, leading to tool wear.
Electrical discharge machining demands substantial power. Generators used in this process must be capable of delivering the required power to ensure efficient and successful operation. They are chosen based on their ability to meet the specific power requirements of the process.
There are three categories of EDM machines are sinker, wire, and hole, each of which uses the same
A DC power generator is the power supply for the EDM machining process. The negative terminal is connected to the tool while the positive terminal is connected to the part being machined (i.e. the workpiece). Different types of power generators are used such as:
The workpiece is the component being machined. It is secured in the dielectric container with a fixture and is connected to the positive terminal of the power supply.
The fixture is used for holding the workpiece properly in the dielectric container.
Aspects of dielectric fluid are discussed below.
The dielectric medium is crucial to the EDM machining process. Typically, the dielectric fluid used is low-viscosity hydrocarbon oil. It acts as a separator between the workpiece and the electrode. During machining, as sparks occur, the dielectric fluid ionizes to create a conductive path between the tool and the workpiece. When the gap between the tool and workpiece is approximately 0.03 mm and the voltage reaches about 7V, the dielectric medium ruptures.
The spark discharge will be at around 10,000°C and thousands of atmospheric pressure in a micro small area. It takes less than a microsecond for this to take place for each spark. As the column of ionized dielectric vapor collapses, a small tiny part of the workpiece vaporizes due to the arc being forced out. The small tiny metal particles are then cooled down into small spheres and the flux of the dielectric fluid sweeps the particles from the area.
The following are essential functions of a dielectric fluid used in the EDM machining process:
The dielectric fluid must have the following properties to effectively serve as a dielectric medium and fulfill its functional requirements:
The most commonly used dielectric fluids are hydrocarbon and mineral oils, known for their low viscosity. Other types include paraffin oil, lubricating oil, transformer oil, and deionized water. Distilled water is preferred when higher material removal rates are needed.
Different dielectric fluids offer varying performance in EDM machining. Distilled water provides a medium material removal rate with a low wear ratio, while tetraethylene glycol offers a high material removal rate but also a high wear ratio. To maintain efficiency, the dielectric fluid must be filtered before reuse to separate the removed metal from the workpiece and tool electrode. This ensures optimal performance throughout the process.
Flushing refers to the effective circulation of dielectric fluid between the workpiece and the electrode tool in EDM machining. The efficiency of the cutting process heavily relies on proper flushing of the dielectric fluid. To ensure optimal machining conditions, effective flushing is crucial in EDM operations.
During the machining process, the dielectric fluid becomes contaminated with eroded metal particles and carbon from the breakdown of the fluid due to heat. This contamination decreases the insulation strength of the dielectric fluid, potentially causing premature spark discharge. If contamination levels exceed permissible limits, it can lead to the formation of bridges between the tool and workpiece, resulting in short circuits and damage to both. Effective flushing is essential to remove these contaminants and prevent such issues.
The following are different methods of flushing in EDM machining:
Pressure flow systems are commonly used to circulate dielectric fluid in EDM machining. The fluid is directed through the gap between the tool and the workpiece by forcing it through holes in the electrode. This pressurized flow helps to flush out solid metal particles, cooling both the workpiece and the tool electrode. A needle-like residue often forms in the electrode hole, which is subsequently removed to achieve a clean machined surface.
In this flushing method, the dielectric fluid flows upward from the bottom of the electrode tool. Both reverse and pressure dielectric flow processes create a tapered shape at the mouth of the cavity.
This method creates straight holes in the workpiece by using a vacuum pump to draw the dielectric fluid around the tool electrode through a central hole. This process leaves a central needle-like core of work material, which is removed afterward to achieve a clean, machined surface.
In this method, flushing is achieved by vibrating the tool, which is ideal for small tools that cannot accommodate a fluid passage. This technique is particularly effective for deep hole drilling with small diameters.
A pump is used to channel the dielectric fluid from the base of the container to the tool and workpiece, facilitating increased material removal rate (MRR).
A filter, positioned just above the pump, removes any impurities or dust particles present in the dielectric medium.
The tool holder serves to hold the tool properly.
Between the tool and the part to be machined, a spark is generated, in the presence of a dielectric medium. Therefore, the removal of material occurs from the surface of the workpiece.
In EDM machining, any electrically conductive material can serve as the electrode tool. The tool's shape is replicated in the cavity created during machining, making the shape and precision of the machined surface directly dependent on the electrode's form and accuracy. Both the electrode and the workpiece experience erosion during the process, with a typical wear ratio ranging from 5:1 to 100:1.
The wear ratio is the comparison between the wear of the tool and the amount of material removed from the workpiece. This ratio is influenced by the physical and chemical properties of both the tool and work material, the type of dielectric fluid used, and the operating conditions of the machining process. Additionally, the melting points of the tool and workpiece can affect the wear ratio.
Wear ratio is given by: Wr=2.25Mt-2.3
Where, Wr is the work/tool wear ratio; and Mt is the Work/tool melting point ratio
As the cross-sectional area of the workpiece and tool increases, the wear ratio decreases. However, higher cutting rates, especially when machining sintered hard metals such as vanadium and molybdenum steels, can increase the wear ratio. This results in narrower, deeper sections with sharper corners. To reduce tool wear, reversing the polarity and using copper tools are effective strategies. EDM electrode tools can be made through casting, machining, or powder metallurgy, with successful use of tools as small as 0.1 mm in diameter.
EDM tools need to have high melting and vaporization temperatures, or high thermal conductivity, such as those found in graphite or copper. In addition to these thermal properties, tool materials should also be easy to fabricate or shape and resistant to wear. Cost is also a crucial factor when selecting materials for EDM machining tools.
Key factors that determine the suitability of a material for use as an electrode tool in EDM machining include:
While any electrically conductive material can be used as an electrode in EDM, graphite is the most commonly chosen due to its favorable properties. It offers a low wear rate, high electrical efficiency, and is both cost-effective and easy to fabricate.
High-quality isotropic grain graphite, characterized by its fine grains and uniform current-carrying and wear properties in all directions, is an excellent choice for EDM tool electrodes. Copper, copper-tungsten, and brass are also used as tool materials in EDM machining. To achieve minimal tool wear, graphite electrodes are typically operated with a frequency around 3000 kHz and a power supply of 30A.
To manage the wear of graphite tools, they are often coated with steel. However, machining under conditions that minimize wear can result in a very rough surface, requiring frequent finishing cuts. The power supply significantly impacts key operating factors such as material removal rate, wear ratio, and machining stability. Additionally, the choice of tool electrode material is influenced by the type of machine used, as well as the material being machined.
To achieve the desired dimensions of the finished workpiece, a specific amount of clearance must be allowed on the tool side, even though the EDM electrode is designed to be a mirror image of the workpiece's size and shape. This clearance is a crucial factor to consider when designing the tool electrode.
The required clearance primarily depends on the tool material, the material removal rate, and the workpiece material. It also varies with the nature of the cut, such as roughing or finishing. For example, when machining hardened steel with brass electrodes, a side clearance of 0.25 mm is typical; with duralumin, it’s 0.3 mm; and for graphite electrodes, it’s 0.35 mm. For different removal rates, the clearances vary as follows: 0.5 mm for high removal rates (rough cuts), 0.3 mm for medium rates, and 0.05 mm for slow rates (fine surface finishes).
During the EDM machining process, material is removed from both the workpiece and the electrode tool, which increases the gap between them. To maintain a constant gap and arcing voltage and prevent short circuits, the tool must be properly fed using a device that controls the feed rate.
The system must respond quickly with low inertia to prevent overshooting, which could close the arc gap and cause short circuits. Therefore, a rapid reversing feed motion is crucial. A signal from an electrical sensor, which monitors gap voltage or working current, actuates the control to maintain proper operation.
A voltmeter is a device used to measure voltage. In an EDM system, voltmeters are employed to monitor and measure the voltage levels.
An ammeter is used to measure or check the flow of current. The ammeter must be connected in order for us to check whether the current is flowing or not.
A servo control mechanism automatically maintains a gap roughly the thickness of a human hair between the workpiece and the tool. This system is used in both wire and vertical EDM machines. It is crucial to avoid physical contact between the electrode and the workpiece, as this can cause arcing, leading to damage to the workpiece and potential breakage of the wire. As the operation progresses, the servomechanism adjusts the electrode’s position relative to the workpiece, ensuring the correct arc gap by continuously sensing the work-wire. Maintaining this proper arc gap is essential for the successful operation of EDM.
The table serves to hold the material to be machined (i.e. the workpiece).
This chapter will discuss two main types of EDM machines.
This type of EDM machine is also known as sinker EDM, die sinking, volume EDM, ram EDM, and cavity-type EDM. This type of EDM is popular because of being suitable for creating complex shapes.
Conventional EDM involves machining an electrode to form a specific shape, which is then sunk into the material being processed. This electrode creates a negative impression or inverse copy of its shape in the material.
Conventional EDM utilizes shaped electrodes, making it particularly valuable for creating dies and molds. It's well-suited for small-batch production and prototype manufacturing. This method is widely used in industries like automotive and aerospace due to its ability to precisely produce complex engine components. Additionally, it's extensively employed in various industries for injection molding processes.
These machines, also referred to as wire burning, spark EDM, or wire erosion systems, utilize a thin, electrically charged wire as the electrode. A hard diamond guide maintains the wire’s stability. The wire is moved through the workpiece to create a specific shape, but only the electrical discharges from the wire come into contact with the workpiece; the wire itself does not touch it. In this EDM method, the wire moves at a slow pace.
In wire EDM, The wire remains continuously available for cutting a smooth, uninterrupted shape due to its constant feed from an automated spool. For shapes that require cutting through the middle rather than around the perimeter, wire EDM can be combined with hole-drilling EDM. This technique involves drilling a small hole through the center of the workpiece, allowing the wire to be threaded through the hole for precise shaping. In such cases, the electrodes are tube-shaped, and dielectric fluid is circulated through them to the hole.
This type of EDM provides several distinct advantages. It combines robust and reliable performance with cutting-edge technology while remaining user-friendly. Here are some benefits of wire EDM compared to conventional EDM:
In traditional EDM, electrodes are subject to erosion and require frequent replacement once they become worn. Additionally, traditional EDM involves the time-consuming process of machining electrodes to specific shapes. In contrast, wire EDM eliminates the need for pre-machining, as it starts immediately once the wire is set in place. This reduces both time and material costs associated with electrode preparation. Wire EDM is ideal for time-sensitive applications and complex shapes that would be challenging to match with custom electrodes. It is also widely used in the production of extrusion dies.
A variety of EDM (Electrical Discharge Machining) machines are available today, crucial for their ability to precisely and intricately machine hard materials and complex shapes. These machines are essential in industries such as aerospace, automotive, and tooling. Here, we examine several prominent EDM machine brands in the United States and Canada, highlighting specific models and their unique features and capabilities:
Features: The Mitsubishi Electric MV2400-R Advance Plus M800 is a high-precision wire EDM machine designed for superior accuracy and speed. It boasts advanced features including non-contact cylindrical drive technology, automatic wire threading, and intelligent power supply technology. The machine is equipped with sophisticated corner control and a user-friendly touchscreen interface that offers intuitive programming and monitoring. Additionally, the MV2400-R Advance Plus M800 supports unattended operation, enhancing productivity and efficiency.
Features: The Sodick AG60L is a high-performance sinker EDM machine renowned for its linear motor technology, which delivers exceptional accuracy, surface finish, and productivity. It features a robust construction and intelligent control systems that ensure stable and efficient machining. The AG60L includes advanced automation features such as tool changers and electrode wear compensation. It also supports complex 3D machining and is equipped with a user-friendly interface for easy operation.
Features: The Makino EDAF2 is a precision wire EDM machine designed for high-speed machining. It boasts advanced wire threading and automatic rethreading capabilities, along with intelligent control systems that enhance accuracy and reduce cycle times. The machine is equipped with Makino's Hyper-i control system for efficient programming and monitoring, supports fine surface finishes, and includes reliable flushing and filtration systems.
Features: The GF Machining Solutions FORM 20 is a compact and versatile EDM machine.
Features: The GF Machining Solutions FORM 20 excels in high precision and superior surface finishes across various applications. It features an intelligent spark generator for optimized machining performance and incorporates advanced control systems to boost productivity and reliability. The FORM 20 also supports automation and offers customization options to meet specific machining requirements.
Features: The Fanuc Robocut α-CiC series consists of wire EDM machines engineered for high-speed and precise machining. These machines are equipped with reliable wire threading, efficient power supply, and advanced servo control systems. They boast a compact design and high rigidity for stable and accurate performance. Featuring Fanuc’s intuitive CNC system with intelligent programming and operational capabilities, the Robocut α-CiC series supports unattended machining and offers versatile automation options.
Please note that specific model availability and features may vary over time, so it is advisable to contact the manufacturers or their authorized distributors for the most up-to-date information on the models that suit your requirements.
This chapter will explore the various applications, benefits, and drawbacks of EDM machining. It will also cover key factors to consider when selecting the most suitable EDM machining option.
However, it must be noted that the disadvantages of EDM machining are far outweighed by the advantages.
Electrical discharge machining (EDM) is a high-power process, so the generators used must provide substantial power to ensure efficient operation. Choosing the right generator for EDM machining is crucial. Additionally, the type of dielectric fluid used plays a significant role, as different fluids have varying wear ratios and material removal rates. An optimal dielectric fluid should offer a high material removal rate while minimizing wear. While EDM machining offers advantages like high accuracy and excellent surface finish, it also has its drawbacks that should be considered.
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