|This is the image diagram of the electrical discharge machining process.|
In this article, you will read about EDM, like EDM working principles, types, process parameters, equipment, advantages, disadvantages, and applications.
Electrical discharge machining (EDM), also known as spark machining, is a unique manufacturing process that removes metal by creating controlled electrical sparks between an electrode and the workpiece. The process can accurately produce intricate shapes and geometries in hard metals that would otherwise be impossible to machine with traditional techniques.
EDM has become an indispensable part of the manufacturing industry due to its ability to work with difficult-to-machine materials, and produce complex shapes while maintaining high levels of accuracy and with no tool force applied. This guide will provide a comprehensive overview of electrical discharge machining, its working principle, various types, advantages, disadvantages, applications, and more.
What is Electrical Discharge Machining?
Electrical discharge machining is a non-traditional machining process where material is removed from the workpiece by a series of rapidly recurring electrical discharges between the tool, called the electrode, and the workpiece in the presence of a dielectric fluid.
The process utilizes the heat from electrical sparks to erode material in the form of tiny craters on the workpiece. As the electrode approaches the workpiece, an electrical potential is applied, which ionizes the dielectric fluid and generates a plasma channel.
As soon as the gap distance becomes small enough, the potential difference causes a strong electrical discharge, which generates an arc that melts and vaporizes a small amount of material from both the electrode and workpiece. The removed material is washed away by the continuously flushing dielectric fluid.
These rapid, repetitive discharges occur in an area as small as 0.025mm, enabling EDM to remove material with extreme accuracy and detail. The absence of direct contact between the tool and the workpiece eliminates mechanical stresses, chatter, and vibration during machining.
Working Principle of EDM
The basic working principle of electrical discharge machining involves two metal components submerged in an insulating dielectric medium, usually hydrocarbons oil or deionized water. One component is the tool or electrode, which is connected to a power supply generating electrical voltage, while the other is the workpiece.
As the electrode approaches the workpiece, dielectric breakdown occurs in the fluid, allowing a high current to pass between the two parts. This results in super heating, melting, and vaporizing tiny portions of the workpiece surface. The removed material particles are washed away by the continuously flushing dielectric fluid.
This process repeats at a high frequency, enabling controlled erosion of the workpiece into the desired shape. The electrode is guided along the programmed path very close to the workpiece but without making contact. Accurate machining is achieved by precisely controlling the gap voltage, current and duration of the electrical discharges.
Types of EDM
There are several varieties of electrical discharge machining processes, with different methods of creating the electrical discharges and setups. The most commonly used types include:
Also called conventional EDM, cavity type EDM or volume EDM, this process involves an electrode with complex 3D shapes machining the negative shape into the workpiece. The electrode keeps moving vertically lower into the workpiece, creating a cavity identical to its shape.
It is used for creating 3D forms, slots, holes with high accuracy and is ideal for mold making, tool making, aerospace and automotive components.
In wire EDM, also known as wire cutting, a thin conductive wire serves as the electrode to cut a programmed contour in the workpiece. The thin wire allows this process to create complex 2D profiles with good surface finishes. The wire is continuously fed from a spool and the sparks erode a slot through the workpiece.
Wire EDM is commonly used in toolmaking, prototype production, and the manufacturing of precision parts like turbine blades.
EDM drilling or hole drilling utilizes a rotating tubular electrode made of brass, copper or graphite to machine holes in the workpiece. It is used for producing holes in hard materials at high accuracies, even ranging to 0.02mm.
EDM drilling is widely used to create spray holes in gas turbine components, fuel injector holes and other holes required in aerospace applications.
Key Advantages of EDM
Electrical discharge machining provides certain benefits that make it an indispensable manufacturing process, especially for difficult-to-machine materials.
- Extremely hard materials like carbides, ceramics, titanium alloys and heat treated tool steels that are very difficult-to-machine using conventional machining can be precisely machined by EDM.
- There is no direct contact between tool and workpiece, eliminating mechanical stresses, chatter and vibration. This makes EDM suitable for delicate or fragile parts.
- EDM can easily machine complex 3D shapes, contours and cavities that would otherwise be impossible to produce with traditional machining.
- It can machine parts with very fine details, sharp internal corners and very small holes with high accuracy and surface finishes.
- As only electrically conductive materials can be machined, EDM is suitable for materials like super alloys, MMCs, cermets etc.
- There are low equipment forces, no cutting forces, no tool chatter and no mechanical stresses on the workpiece.
- Automation of the EDM process is easy since machining is controlled by CNC programs.
Limitations of EDM
While EDM is highly advantageous, it does have some limitations:
- Only electrically conductive materials like metals can be machined. Non-conductive materials like wood, plastic, glass etc. cannot be processed with EDM.
- Specific power consumption is very high, making the process expensive.
- Material removal rate is low because material is removed by tiny bits.
- Electrode manufacturing can be quite expensive.
- There can be unstable spark generation under certain machining conditions.
- Highly skilled operators are required for programming, setup and monitoring the EDM process.
- Not suitable for removing large volumes of material as MRR is low. Initial rough machining by other process may be required.
EDM Process Parameters
The EDM process has certain controlled parameters that can be set to achieve optimal machining results. The key parameters are:
- Polarity - Straight polarity (electrode negative) is commonly used as it improves MRR and electrode wear ratio. Reverse polarity (electrode positive) helps improve surface finish.
- Voltage or gap voltage - Typically ranges from 50-300 Volts. Affects the energy of the sparks. Higher voltage gives higher MRR but may cause unstable arcing.
- Pulse on time - The duration of the discharge, normally ranging from 1-1000 microseconds. Longer on time increases MRR and roughness.
- Pulse off time - The time between two consecutive sparks. Allows flushing to remove debris. Affects process stability.
- Current - The peak amperage during the on time. Current can range from 1-400 Amps depending on electrode size. Higher amperage yields higher MRR.
- Gap or inter-electrode distance - The distance between electrode and workpiece. Maintained between 0.01-0.5mm. Affects spark consistency.
- Dielectric flushing - Helps create optimum sparking conditions and remove eroded debris. Dielectric fluid flowrate is controlled.
- Electrode feed rate - The velocity at which the electrode travels along the programmed path during machining.
EDM Process Flow
The standard EDM process can be broadly explained in the following steps:
Tool Design and Preparation
- For die-sinking EDM, the electrode is crafted into the desired 3D shape, usually out of copper or graphite.
- For wire EDM, a thin wire electrode made of brass or coated wires is selected. For EDM drilling, tubular electrode tubes are used.
- The tool electrode material should have high melting point, good machinability, low wear rate and good conductivity.
- The workpiece is cut to required dimensions. For soft or non-conductive materials, conductive cladding can be added.
- The workpiece is mounted and fixed on the EDM machine table. For sinker EDM, it must be submerged in the dielectric tank.
Selection of Machining Parameters
- Based on the material, thickness, surface finish and accuracy needs, the optimal machining parameters like current, voltage, pulse timing etc. are selected.
- The power supply sends controlled current between the electrode & workpiece separated by a small gap distance in the dielectric fluid.
- Thousands of electrical sparks melt and vaporize material from both parts, which is flushed away by the dielectric fluid.
- The electrode feeds lower into the workpiece, continuing the erosion and material removal.
- Once machining is complete, the workpiece is cleaned to remove any remaining debris.
- Any recast layers are removed by grinding, polishing, wire brushing or etching.
- The workpiece can undergo heat treatment if required. It is inspected to ensure accuracy of the final shape.
Standard EDM setups consist of the following components:
- Power supply unit - Generates the high amperage current and voltage between tool & workpiece to create sparks.
- Dielectric system - Provides filtration and circulation of dielectric fluid at controlled flow rates.
- Servo control mechanism - Controls electrode servo motions in X, Y and Z axes.
- Tool electrode - The conductive graphite, copper or brass electrode that is shaped to erode cavity in workpiece.
- Workpiece - The conductive part that will be machined by the sparks removing material.
- CNC controller - Automates electrode path motion and machining parameters per programmed job cycles.
- Dielectric fluid tank - Holds the dielectric oil/water that submerges tool & workpiece.
Applications of EDM
Thanks to its unique capabilities, EDM is applied across several industrial sectors:
- Mold and die making - For machining complex mold cavities and dies from hard metals.
- Automotive - Machining fuel injection nozzles, valves, engine parts.
- Aerospace - Machining turbine blades, combustion chambers, fir-tree root fixings.
- Medical - Creating intricate shapes in surgical instruments and prosthetics.
- Tool and part making - For making press tools, extrusion dies, jigs, fixtures.
- Jewelry and watches - Intricate jewelry wax model making.
- Coinage - Making dies for minting coins.
- Electronics - PCB drills, electrodes for capacitors, semiconductors.
- Nuclear - Uranium isotope enrichment plant components.
EDM vs Traditional Machining
EDM differs considerably from traditional machining processes like milling, turning, drilling and grinding. Here are the main differences:
- Uses electrical sparks and thermal energy to remove material.
- No direct contact between tool and workpiece.
- Only electrically conductive materials can be machined.
- Produces no mechanical stresses or chatter on workpiece.
- Enables machining of very hard materials beyond HRC 65.
- Requires specific dielectric fluids.
- Uses mechanical cutting forces and abrasion to cut material.
- Direct physical contact between cutting tool and workpiece.
- Can machine almost all material types like plastics, wood etc.
- Cutting forces produce vibration. Materials under HRC 45-50 are machinable.
- Uses coolant fluids only for cooling and lubrication.
Overall, EDM provides unique machining capabilities unmatched by traditional machining processes. At the same time, traditional machining offers faster material removal rates and machining flexibility.
Frequently Asked Questions
Here are some common questions about electrical discharge machining:
What materials can EDM machine?
EDM can machine any electrically conductive material, including all metals and metallic alloys. The most common materials are tool steels, alloy steels, titanium alloys, super alloys, carbides, and conductive ceramics. Non-conductive materials cannot be machined by EDM.
Does EDM produce heat affected zones?
No, EDM does not create a heat affected zone (HAZ) since material removal is achieved by localized melting and vaporization rather than traditional cutting forces. The workpiece retains its original metallurgical properties.
How accurate is EDM?
Modern CNC EDM machines can easily achieve positional accuracy within 0.005mm and dimensional accuracy within 0.01mm. Advanced EDM systems provide even closer accuracies.
How fine of a surface finish can EDM produce?
Typical EDM surface roughness values range between 1-15 microns Ra. However, certain finishing operations can further improve this to under 0.5 microns Ra.
What are the main components of an EDM setup?
The key components are the power supply, dielectric system, servo controls, tool electrode, workpiece, dielectric tank and CNC controller.
Is EDM environmentally friendly?
EDM produces no chips, swarf or hazardous wastes. However, the dielectric fluid requires proper handling and disposal. Wire EDM is considered more environmentally friendly than die-sinking EDM.
How deep can EDM drill?
EDM drilling can produce extremely deep holes with higher accuracy and aspect ratios than traditional drilling. Holes of over 500mm depth with less than 0.02mm deviation are possible, with depth mainly limited by electrode rigidity.