This article will take an in-depth look at EDM Machining.
The article will bring more understanding on topics such as:
This chapter delves into EDM machining and its core principles of operation.
Electrical Discharge Machining (EDM) is a manufacturing process utilized to eliminate material from a workpiece by employing a succession of rapid electrical discharges between electrodes within a dielectric fluid. This method is particularly effective for producing parts that are challenging or impractical to machine with conventional techniques, as it depends on electrical forces instead of mechanical ones.
The precise nature of EDM makes it ideal for creating complex and detailed shapes, even from hard metals like titanium. To work properly, the materials involved in EDM must conduct electricity.
In an EDM machine, the workpiece electrode (anode) is linked to the positive terminal of a DC power source, while the tool electrode (cathode) attaches to the negative terminal. Both electrodes exist submerged in a dielectric fluid with a spark gap between them. Upon electrical discharge, intense electrothermal heat occurs at the spark gap, melting and vaporizing segments of the workpiece surface through spark erosion.
Although the foundational principles of EDM machining are consistent, wired EDM and sinker EDM methods each have unique distinctions. Both methods employ anodes and cathodes to shape the workpiece according to specific parameters. However, the application of electrical current in shaping the workpiece varies distinctly between these techniques.
Sinker EDM machining involves establishing an electrical potential difference between the conductive tool and workpiece, submerged in a dielectric fluid like hydrocarbon oil or deionized water. The dielectric fluid satisfies the spark gap between the tool and workpiece, with the electric field relying on the potential difference and spark gap width.
In sinker EDM, the tool connects to the negative terminal, and the work material connects to the positive terminal. The application of the electric field results in electrostatic forces on the tool's free electrons. If the tool exhibits a lower work function or bonding energy, electron emission, or cold emission, occurs from the tool due to its negative terminal connection.
Within the dielectric medium, these cold-emitted electrons accelerate toward the work material. As their velocity and energy increase, they collide with dielectric molecules, ionizing them, based on the dielectric’s work function or ionization energy and the electron's energy. The accelerated electrons continuously generate positive ions and additional electrons through these interactions.
This ongoing process intensifies electron and ion concentration within the dielectric fluid between the tool and work material at the spark gap, forming a “plasma.” The plasma channel’s minimal electrical resistance enables significant electron flux from the tool to the work material and rapid ion movement from the work material to the tool, known as an avalanche.
This swift movement of electrons and ions generates spark heat within the range of 8,000°C to 12,000°C. The fast-traveling electrons impact the work material, while ions strike the tool. These collisions on the workpiece's surface convert into thermal energy or heat flux.
The EDM wire machining process, an alternative to sinker EDM, functions similarly to a band saw, utilizing a wire for cutting. This wire, made from copper or brass, conducts a high voltage electrical discharge, enabling it to cut through the workpiece's thickness.
In wire EDM, the wire generates sparks in deionized water where conductivity is meticulously controlled. This water simultaneously cools the material and removes excess material. The process ensures clean dielectric fluid is constantly introduced to flush away waste.
During the EDM process, extreme temperatures rapidly remove surplus material from the workpiece through methods such as vaporization, melting, or spark erosion. Some molten metal is displaced, and when the electric potential ceases, the plasma channel collapses, generating pressure waves that expel molten material and form a crater at the spark site.
Material removal occurs via shock waves formed from the collapsing plasma channels when the electric potential ceases, switching the work material to positive and the tool to negative. Electrons impacting the workpiece create craters through heating, melting, and material removal, while positive ions impact the tool, causing wear.
Electrical discharge machining requires significant power, necessitating generators capable of providing the necessary power for effective and successful operations. These generators are selected based on their capability to meet the specific power demands 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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