Electrical Discharge Machining: WHAT IT IS AND HOW IT WORKS

Frequently used in tooling and molding processes for a broad expanse of industries, such as aviation, aerospace and gas turbines, electric discharge machining (EDM) uses thermal energy to remove excess material from an object to create the shape needed for a certain task. Manufacturers often rely on EDM machining processes when traditional machining approaches reach their limit since EDM allows for high accuracy and is applicable for any conductive material used. 

Similar to manufacturing processes like laser cutting, EDM does not need or attempt to use mechanical force in the debris removal process, which is why it is considered non-traditional, compared to machining processes that rely on cutting tools that actually come into contact with the object.

At Hi-Tek Manufacturing, we work to help you find the best approach to working with hard metals, using unique methods where other machining processes are frequently, if not always, ineffective. 

WHAT IS ELECTRICAL DISCHARGE MACHINING?

Also known as spark machining, wire erosion, die sinking and spark eroding, EDM is a manufacturing process used to create desired shapes by using an electrical discharge of sparks. In this machining process, material is shaved, carved and otherwise removed from the workpiece through a series of rapidly repeating current discharges between two different electrodes. The electrodes are separated by a dielectric liquid, which is subjected to an electric voltage. An important point to remember with EDM machining is that it will only work with materials that are electrically conductive.

The electrode that is intended to change shape to fit a particular purpose is called the workpiece electrode, or sometimes simply the or "work piece" or "anode." The other electrode is known as the tool-electrode, or simply the "tool," "cathode" or "electrode." For the EDM process to work, changing the workpiece-electrode into the desired shape, the two electrodes must not make actual contact, but they must create a spark, which takes reaching about 8,000 to 12,0000º C to achieve. 

As the voltage between the two electrodes increases, the intensity of the electric field in the volume in between the electrodes surpasses the strength of the dielectric, it breaks down to allow the current to flow freely between the two electrodes. This particular phenomenon is identical to the breakdown of a capacitor, or condenser. This process is also known as breakdown voltage, which is the minimum voltage at which an insulator becomes electrically conductive.

For diodes, this is the minimum reverse voltage that causes the diode conduct discernibly in reverse. As a result of this breakdown, material is removed from the electrodes, and when the current stops—or is manually stopped, depending on the type of generator—a new liquid dielectric is generally conveyed into the inter-electrode volume to allow the solid particles, or debris, to be carried away while the insulating properties of the dielectric are restored. The process of adding new liquid dielectric in the inter-electrode volume is often called "flushing." Additionally, after a current flow, the difference of potential between the two electrodes becomes restored to what it had been before the breakdown, allowing for a new liquid dielectric breakdown to occur. 

WHAT HAPPENS IN THE EDM PROCESS? 

In EDM, a potential difference is applied across the tool and workpiece in pulse form, and they must be electrically conductive while a small gap is continuously maintained between them. The tool and workpiece are then immersed in a dielectric medium, which can be kerosene or deionized water and, as the potential difference is applied to the process, electrons from the tool start to move toward the workpiece. Here, the tool is negative and the workpiece is positive, with the electrons moving from the tool to the workpiece, colliding with the molecules of the dielectric medium.

Due to the collision of electrons with the molecule, it gets converted into ions, which increases the concentration of electrons and ions in the gap between the tool and work piece. The electron moves towards the workpiece while ions move toward the tool, and an electric current is set up in between the tool and workpiece and is referred to as plasma. As the electrons and ions strike the workpiece and the tool, the kinetic energy changes to heat energy, and the temperature of the heat produced is about 10,000º C. This high heat vaporizes and melts the material from the workpiece.

As voltage is broken down, the current stops to flow between the tool and workpiece, and the molten material in the work piece is flushed by circulating dielectric medium leaving behind a crater.

WHY IS THE EDM PROCESS IMPORTANT? 

EDM has become increasingly valued in the tool and die industry and is commonly used for mold-making processes over the past several years. Not only that, but it has also become an integral aspect of creating prototype and production parts. 

In certain industries like aviation, aerospace, electronics and turbine engines, it is vital to create precision instruments to work with blades and other hot gas path component parts that must withstand the most intense conditions and the quantities remain low. With good EDM machining equipment, it is possible to cut and manipulate complex shapes into detailed contours or cavities of hardened steel and exotic metals, such as inconel, carbide, titanium, kovar and hastelloy. 

THE MAIN PARTS AND EQUIPMENT REGULARLY USED IN EDM 

The primary parts of an EDM include the following: 

  • A DC pulse generator is a power source for the machining operation.
  • A voltmeter is a device used to measure voltage.
  • An ammeter is the tool that measures or checks the flow of the current. Without an ammeter, operators may not be able to see or check whether the current is flowing or not.
  • A tool is connected to negative power sources, as opposed to the workpiece, which is connected to positive sources. The fluid moves to the tool for the operation from the filter and, when the power supply increases between the tool and the workpiece, the spark generates and the machining begins. 
  • Dielectric fluid has a property similar to insulation, which is where no current flows from one to another. The dielectric fluid will become ionized in the form of ion, which serves as assistance between the tool and the workpiece. When the power supply stops, the fluid comes to its initial position. 
  • A pump sends fluid to the filter and works by allowing the fluid to flow from one source to another.
  • A filter is used to filtrate the different particles. If there are dust particles present, the filter then removes those particles before sending the object to the tool for operation.
  • A controlled feed is the constant feed that is supplied for the operation.
  • Fixtures hold the table in place.
  • A table holds and stabilizes the work piece.

THE THREE TYPES OF ELECTRICAL DISCHARGE MACHINING 

EDM has long been the answer for high accuracy, demanding machining applications where conventional metal debris removal is difficult or impossible. The dielectric fluid acts as an electrical insulator unless enough voltage is applied to bring it to its ionization point when it then becomes an electrical conductor. The resulting spark discharge erodes the workpiece to form its desired final shape. 

While the EDM process is unconventional and unique, it does not mean that there is only one way to approach it to get the desired results. Also, it helps to know that if one type of machining process does not fit, there are others available to use. There are three types of EDM we regularly use at Hi-Tek. 

WIRE EDM

Wire EDM, or sometimes WEDM, originally developed in the 1960s and 1970s to serve as a new method for making dies out of hardened steel. In this approach to EDM, a thin wire serves as the electrode and moves in a carefully controlled pattern, which causes a spark to occur between the wire and the workpiece. Also known as spark EDM, wire EDM is an electro thermal production process that uses a thin single-strand metal wire that is typically made of brass, along with deionized water to conduct electricity, allowing the wire to cut through metal by the use of heat from the generated electrical sparks. 

Since the electrical discharge erodes and becomes compromised by the wire and the workpiece, wire EDM machines use spool full of wire that is continuously moving to add a fresh discharge path in the cut. 

Also known as the "cheese cutter" EDM approach, WEDM works well but has one key limitation since the wire must pass completely through the workpiece, which makes it an essentially two-dimensional cut within a three-dimensional part. 

WEDM can easily machine precision components and complex parts associated with aviation and power generation and turbines that rely on hard conductive materials, making it a perfect fit for our clients at Hi-Tek.

Hi-Tek began its own wire EDM processing practices in 1984. Since then, Hi-Tek has continually grown and updated its WEDM capabilities. Our WEDM equipment handles parts as large as 51" x 39" x 20". The department is supported by a total of (8) 4-axis and 5-axis CNC controlled machines to generate a variety of shapes and geometries. We are instrumental in the production of electrode holders for other EDM work, as well as performing metallurgical cut ups and cross sections to evaluate brazing and other related disciplines.

PLUNGE EDM

Also called conventional EDM, sinker EDM and ram EDM, plunge EDM machines use spark erosion via electrical discharges to cut or drill through metal or graphite, using generated heat that can rise from eight to 20,000 degrees. As long as a material is conductive, it can be punched, drilled or cut, using a normal EDM. However, plunge EDMs use a dielectric fluid, and the project is fully submerged into that dielectric fluid for insulation while an electrode and the project become charged. 

The piece of material to be worked on is connected to a power supply. Then an electrode is used to create a conductive path and cut the material into the desired shape or pattern.

There is no actual contact between the electrode and the workpiece as the erosion takes place as a response to the electrical current being produced. The process takes place in a dielectric fluid, allowing electricity to be conducted. The fluid is always used to flush away the debris from the process allowing for clean and burr free edges. Plunge EDM is ideal for applications such as injection mold tooling, micro hole drilling, keyways, washers and scientific research apparatus.

Hi-Tek's vast assortment of plunge EDM Machines can handle parts of many sizes up to as large as 48" x 29"x 20". This EDM equipment ranges from 3 axis manual to 6 axis CNC. CNC electrode changers exist on many of these machines. Hi-Tek was founded in 1980 and since then, plunge EDM machines have vastly expanded and have been regularly updated ever since. Annually, millions of holes, slots, grooves, cavities, etc. are produced using our plunge EDM machines.

FAST HOLE EDM

Developed for speedy hole making, fast hole drilling is one of the latest EDM developments in EDM technology. Hi-Tek stays ahead of the industry by designing and building our own fast hole drilling EDM machines to outperform those that are commercially available. Hi-Tek has designed and built many sizes and configurations to best satisfy the specific needs of our customers. Regardless of metal type or hardness, precision holes can be EDM drilled up to 70% faster than any conventional method.

  • 6-axis machining capability
  • 008” to 0.25” hole size
  • Parts up to 1,000 lbs.
  • Speed drilling – long runs

At Hi-Tek, we are set up for short and long-term contract runs. We regularly drill holes .006″ to .200″ in diameter.

Key industries we serve with our plunge EDM services include aerospace, tool and die, medical, power engine turbines, automobile and military since this process allows us to achieve high tolerances on complex patterns and geometries. Compared to WEDM, which utilizes a pre-drilled hole to feed the wire through the process, plunge EDM does not require a designated or prefabricated hole. Some of the most common plunge EDM electrodes include machined copper, graphite, tungsten and brace.

EDM ADVANTAGES OVER TRADITIONAL MACHINING PROCESSES           

There are several distinct advantages to using EDM over conventional machining processes, especially in industries like aviation and marine and land based turbines. Here are just a few of those advantages: 

  • Can be used to work on any hard material—even in a heat-treated state. 
  • Possible to create or reproduce a variety of complicated shapes on a tool. 
  • High accuracy to about .005 mm is achievable in EDM.
  • High-quality surface finish achievable economically up to .2 microns. 
  • Can be applied to any type of electrically conductive materials. 
  • Features higher tool life due to proper cooling and lubrication processes. 
  • Takes less time than conventional machining process time. 
  • Easy to develop hard and resistant surface on the dies. 
  • No mechanical stresses develop in this process since there is no contact between the tool and the workpiece.

WHY USE ELECTRIC DISCHARGE MACHINING?

EDM is optimal for dealing with one major issue in manufacturing and dealing with objects. This key issue is hardness. In traditional practices and machining processes, metal workpieces need to be made from special grades of steel that were able to be hardened in an anneal treatment that changes physical and chemical properties of a material, which thereby increases its ductility to reduce its hardness, thus making it more malleable and workable in order to change shape and more. 

Once the desired shape has been machined by any of the three types of EDM, the parts are then hardened by one or more heat treatments. The advantage of EDM is that it can cut hardened materials and exotic alloys while also providing excellent surface finished, often resulting in a reduced need for post-processing or surface treatment.

Additionally, EDM machining processes offer the advantage of being highly predictable, accurate and repeatable. This benefit is true to the point that all EDM machining can be performed unattended, helping to reduce direct labor rates and manufacturing costs. 

It is rare to need to perform secondary processes in EDM to remove any type of friction machining, which creates a burr that must also be removed. The process to remove the burr becomes its own project, requiring workers to scratch the burrs off the workpiece or us some vibratory method that is time-consuming and redundant since the part has already been managed. 

Finally, one of the biggest and most attractive advantages of EDM machining is when it is used for parts that have small or complex features, which become harder, and as their geometry changes, they become even smaller and deeper. 

HI-TEK MANUFACTURING IS THE LEADER IN ELECTRIC DISCHARGE MACHINING TECHNOLOGIES AND INNOVATIONS YOUR BUSINESS NEEDS

Our Hi-Tek EDM machining processes are the perfect answer at times when conventional machining methods have reached their limit. We rely on the EDM machining process because it allows for high accuracy and is applicable for any conductive material to help us better serve our customers in high-stress industries like aviation and power generation turbines where precision is crucial. 

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