What is the Electrical Discharge Machining (EDM) process?

By Jack Childerstone | 17/12/2020

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Learn more about electrical discharge machining (EDM) and the various different types of EDM available in our introductory guide.

EDM is a non-traditional manufacturing process that uses thermal energy to remove material from a workpiece as opposed to mechanical force. Engineers often turn to EDM when machining processes such as CNC milling and turning cannot produce the desired cut for example, when there are sharp internal corners or a particularly deep cavity.

Types of EDM

Electric discharge machining can be divided into three common types, Die sinking EDM, Wire EDM and hole drilling EDM.

Die sinking EDM

Die sinking EDM or Ram EDM is an attractive option when complex cavities need machining. Firstly, a graphite electrode is produced in the reverse shape to the required cavity to form the die. The process works by inducing a voltage between the die and the electrically conductive workpiece while being submerged in a dielectric fluid. The die is slowly lowered towards the workpiece until ‘electric breakdown’ occurs and a spark jumps the ‘spark gap’. This vaporises/melts the material, and the dielectric fluid subsequently carries the ejected particles away. Material is repeatedly removed from the workpiece by a series of high-frequency sparks, precisely cutting out the desired shape.

Die sinking EDM electrical discharge machining

Wire EDM

Wire EDM otherwise known as wire erosion, is commonly used for the production of extrusion dies. It cuts using the same mechanism as die-sinking, however, the die is replaced with a very fine electrically charged wire. This machining method is comparable to a cheese cutter, making a two-dimensional cut in a three-dimensional part. The cutting wire is burnt during the machining process so fresh wire is continuously replaced from an automated spool to ensure precise cutting. The diameter of the wire typically ranges from 0.05mm to 0.35mm. 

Hole drilling EDM

As the name implies Hole drilling EDM is used to machine holes. However, compared to traditional hole drilling methods, this technique can accurately machine extremely small and deep holes that don't require deburring. This method also uses the same fundamental principles as die-sinking EDM, though the cut is made with a pulsing cylindrical electrode that moves deeper into the workpiece as the material is removed. This method has been key to the advancement of high-temperature turbine blades, as it allows for very intricate cooling channels to be manufactured inside the turbine blades.

Advantages of EDM

Greater design freedom

One of the main advantages of electrical discharge machining is that it allows for shapes and depths to be cut which would be impossible using traditional machining methods. These include features such as undercuts and perfectly square internal corners, making it an ideal method for producing an inside corner during CNC machining. The machining process does not create a burr, which is an additional benefit.

Distortion-free machining

In contrast to conventional machining methods, the tool never comes into direct contact with the workpiece in this process. With no forces acting on the part, there is no distortion. This enables very thin features to be machined without the risk of breaking. Furthermore, the lack of distortion means very tight tolerances of +/- 0.012mm can be achieved.

High-quality surface finish

Conventional material removal processes such as CNC milling leave machining marks on the workpiece that require post-machining finishing to remove. EDM has zero-directionality to the surface finish, making uniformly smooth surfaces possible without the need for additional treatment. However quick EDM processing can leave behind a slight bead blasted-like texture.

Not affected by material hardness

A key feature of electric discharge machining is that it can machine through any material as long as it is conductive. This means that extremely hard materials such as Inconel and Tungsten carbide can be machined.

Greater design freedom

One of the main advantages of electrical discharge machining is that it allows for shapes and depths to be cut which would be impossible using traditional machining methods. These include features such as undercuts and perfectly square internal corners, making it an ideal method for producing an inside corner during CNC machining. The machining process does not create a burr, which is an additional benefit.

Square corner EDM

Disadvantages of EDM

Low material removal rate

The material removal rate is low when compared to conventional machining methods. As the manufacturing process is very power-intensive, the increase in manufacturing time has a big impact on the overall cost.

Not all materials can be machined

For a material to be processed using electrical discharge machining it needs to be electrically conductive. It should also be considered, although the process is technically stress-free machining there is still a thermal process that can alter the metallurgy of the workpiece.

Cost of the electrode

For Die sinking EDM a custom electrode that is reverse of the feature is required. At low manufacturing volumes, machining the electrode can seem expensive, however, at higher volumes, this additional cost can be absorbed across many components.

What surface finish can be achieved with EDM?

As with all machining processes, there is a balance between cutting speed and surface finish quality. It is common for the initial cut to be faster and rougher, then the subsequent cuts made at a slower speed to produce a clean surface finish. Further tool passes can be made at slower speeds to produce an excellent surface finish but this increases machining time and thus cost.

What is the accuracy of EDM?

EDM can work to very tight tolerances, +/- 0.012mm, this is why the aerospace and medical industry utilise the process.

What materials can be machined by EDM?

On the whole, all conductive materials can be machined with electrical discharge machining. Certain materials, such as aerospace grade high-nickel alloys can present some machining challenges. However, often the solution lies in changing the electrode material or machining speed. The main factors that influence the choice of electrode material are the electrode’s conductivity and resistance to erosion.

Summary

Electrical discharge machining is an excellent method to use in conjunction with a traditional machining method such as CNC machining when parts have specific geometrical requirements. Also, the ability of the process to machine hard materials make it an attractive option when working with materials like Inconel. However, the machining process is fairly slow, so high-volume tasks are not well suited for this method.

For more information on machining methods, industry insights and more why not check out our technical toolbox. Or if you have a part you would like to get made request a free quote from us now.

Jack Childerstone

Project Manager at Get It Made Ltd. Bike fanatic 🚴🏼

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