Have you ever seen metal blocks that fit perfectly like a jig-saw puzzle and wondered how these precise cuts are made in metal?
Wire Electrical Discharge Machining or Wire EDM technology is what's used to make extremely precise cuts that create such perfect fits.
It is a non-conventional zero-tolerance machining process that requires an experienced operator to attain perfect results.
This article discusses Wire EDM and provides a complete overview of this machining technique.
What is Electrical Discharge Machining (EDM)?
Wire Electrical Discharge Machining (WEDM) is an electro-thermal machining process that uses a single strand of electrically conductive wire to perform through cuts in metal workpieces under the action of a dielectric fluid. It is a non-contact cutting technique that uses electric discharge (sparks) to melt and vaporize the material.
Electrical discharge machining (EDM) uses electric discharge (sparks) to cut a piece of metal in the desired shape.
This process is also known as electro-discharge erosion or spark erosion.
It is a non-conventional machining process in which there is no physical contact between the workpiece and the cutting tool.
In this process, a series of electric discharges occur rapidly in the gap between the tool and the workpiece (electrodes).
These sparks generate intense heat with a temperature of around 8000 - 12000 ℃ to melt and vaporize the material and produce the desired cut.
An adaptive control system is used to maintain a constant gap between the workpiece and the electrode. This gap is known as the spark gap or discharge gap.
The workpiece and the tool are submerged in a dielectric fluid that acts as a resistor and prevents the overheating of the workpiece.
Furthermore, the dielectric fluid also flushes the eroded metal particles from the kerf and helps to produce clean cuts with a high surface finish.
Introduction to Wire EDM
Wire Electrical Discharge Machining (WEDM) is a type of EDM in which a thin strand of conductive wire with a diameter of around 0.004" - 0.012" is used as an electrode.
The non-contact nature of Wire EDM enables the thin wire to perform through cuts in blocks of metals without any deflection in the wire.
It is a highly precise cutting technique that offers a part accuracy of around 2 microns (0.002mm) and can be used to cut any conductive material.
Wire EDM can be used to perform taper cuts with a maximum cutting angle of 45 degrees.
The ability to control the upper and lower end of the wire independently enables to make cuts with different profiles on the top and bottom surface of the workpiece.
Furthermore, the thickness and hardness of the workpiece does not have a significant affect on the cutting speed of the process.
This makes Wire EDM ideal for cutting intricate patterns on thick blocks of metals.
How Does Wire EDM work?
Wire EDM works on the electro-thermal principle, where the electric energy is converted into heat energy to melt and vaporize the material.
It consists of a wire, generally made of brass, copper, or tungsten, that acts as the cathode (negative terminal) and a conductive workpiece as the anode (positive terminal).
The potential difference between the workpiece and the wire generates thousands of sparks per second.
These sparks increase the temperature of the workpiece and thereby melt and vaporize the material.
However, wire EDM can only be used to perform through cuts in conductive materials.
To begin the cutting process, a hole is drilled in the workpiece to thread the wire, or cutting is started from the edge of the workpiece.
The wire is surrounded by a ring of sparks that enable the wire to perform the cut without coming in contact with the workpiece.
Each spark that hits the surface of the material creates a crater and removes a small amount of material in an erosive manner.
These sparks also impact the cutting wire and weaken it.
Therefore, a spool of continuously-fed wire is used in Wire EDM.
Wire EDM is performed on a CNC machine, so the path followed by the cutting wire to attain a precise cut is controlled by a computer.
Types of Wire EDM
Depending on the way the dielectric fluid is supplied, there are generally two types of Wire EDM: Submerged Wire EDM and Flush Wire EDM.
Submerged Wire EDM
In submerged Wire EDM, the workpiece and the wire are entirely submerged under the dielectric fluid throughout the cutting process.
This enables uniform heat dissipation from the cutting area and facilitates easy removal of the chips from the kerf.
Submerged Wire EDM is less prone to wire breakage as the dielectric fluid surrounds the wire and acts as a shield that prevents short-circuiting.
Furthermore, submerged Wire EDM does not cause any deflection in the cutting wire and therefore produces cuts with a better surface finish than flush Wire EDM.
Flush Wire EDM
In flush Wire EDM, a jet of dielectric fluid flows through a nozzle towards the cutting area.
This jet enables easy clearance of the chips from the kerf but causes a deflection in the cutting wire, resulting in the poor surface finish of the cut.
The surface finish of the cut can be improved by reducing the speed of the jet, but the quality of the cut produced by submerged Wire EDM is far superior to flush Wire EDM.
A Flush Wire EDM machine is comparatively cheaper than the submerged Wire EDM, making it ideal for beginners who want to step into Wired EDM with a budget-friendly machine.
However, the deflection of wire and lack of constant fluid flow in the kerf can result in a short-circuit due to contact between the wire and the workpiece.
This short-circuit results in the breaking of the wire and reduces the productivity of the process by increasing downtime.
Process Parameters Involved in Wire EDM
Apart from the type of dielectric fluid supply used, there are various parameters that affect the material removal rate and quality of the cut produced by the Wire EDM.
Pulse On Time
The material removal during Wire EDM occurs due to the impact of sparks on the surface of the workpiece.
These sparks are not a continuous flow of electricity but are short bursts of electricity occurring millions of times in a second.
The pulse on-time is the duration for which the electric discharge occurs between the workpiece and the wire.
It is the actual machining time during which material removal takes place.
Generally, the material removal rate (MRR) increases with an increase in pulse on time.
Gap current is the current flowing in the gap between the electrode and the workpiece.
Up to a certain limit, an increase in gap current increases the material removal rate.
Further increase in the current will result in a decrease in the material removal rate.
Therefore, it is essential to find and maintain the appropriate gap current for the process.
Gap voltage is the potential difference between the workpiece and the electrode (wire).
This voltage determines the total energy carried by the spark, and the higher the gap voltage, the higher the energy carried by the spark will be.
Furthermore, gap voltage also affects the optimum distance (gap) between the workpiece and the wire.
Increasing the voltage increases the optimum gap, which provides better clearance for the eroded particles and thereby increases the material removal rate.
Advantages of Wire EDM over other Methods
Wire EDM offers various advantages over traditional cutting techniques, making it highly popular in the manufacturing industries.
Quality of Cuts
Wire EDM is a non-contact machining process in which the material removal occurs due to the electric discharge generated between the workpiece and the wire.
This eliminates the risk of developing any unwanted mechanical stresses in the workpiece due to the abrasion between the tool and the workpiece.
Furthermore, as there is no contact between the workpiece and the wire, the cuts produced by wire EDM are burr-free and have a high surface finish.
Wire EDM is one of the most precise machining processes used to cut metal workpieces.
Some industrial level Wire EDM machines, such as UON-01 by Makino can have wire diameter as small as 0.00006" and showcase a positional accuracy of around +/-0.5μm (~0.00002").
This makes it ideal for manufacturing parts that require very high precision, such as aerospace and medical equipment.
Furthermore, the high precision enables Wire EDM to produce intricate cuts in thick metal workpieces, which was otherwise not possible by traditional machining processes.
Minimum Heat Distortion
The dielectric fluid used during the EDM process facilitates heat dissipation, preventing the workpiece from overheating.
This limits the heat-affected zone (HAZ) to the surface of the workpiece and minimizes the heat distortion of the material.
Moreover, the high precision of wire EDM can be used to produce cuts with a high surface finish that does not require any secondary finishing processes.
This eliminates the unwanted heat generated by secondary processes and further minimizes the chances of heat distortion.
Although Wire EDM can only be used for electrically conductive materials (metals), it provides flexibility in the thickness and types of metals that it can cut.
Wire EDM can be used to cut almost all the hard and soft metals with a thickness of around 0.012" - 16".
The thickness of the material does not affect the surface finish of the cut, making it ideal for cutting thick workpieces with high precision.
Drawbacks of Wire EDM over other techniques
The various advantages of wire EDM over traditional cutting techniques are followed by some drawbacks that limit Wire EDM from being an all-rounder for all machining scenarios.
Slow Cutting Speed
One of the major drawbacks of wire EDM is the slow cutting speed.
Generally, the wire EDM process has a maximum cutting speed of a few inches per minute (around 4-6 ipm) during the roughing cuts, and this speed further decreases when performing skim cuts for a high surface finish.
This makes wire EDM unsuitable for mass production and can only be used for applications where less quantity with high precision is required.
It is generally used in aerospace and medical industries, where precision is valued over production speed.
Wire EDM uses electric discharge to remove the material and produce the desired cut.
Materials such as wood, plastics, etc., have poor electric conductivity, which means they do not facilitate the flow of electricity through them and therefore fail to generate the electric discharge.
Due to this reason, wire EDM can only be used for cutting materials that offer good electrical conductivity to facilitate the generation of the spark.
Not Ideal for Tubing Cutoff
Wire EDM produces the best results when cutting solid metal objects and is not recommended for hollow tubular workpieces.
When using wire EDM, it is essential to hold the workpiece firmly to maintain a constant gap between the workpiece and the wire.
Using a high clamping force for holding the tubular workpiece can risk the deformation of the workpiece.
Furthermore, using weak clamping force can result in deflection of the workpiece, which can lead to contact between the wire and the workpiece.
The contact between the cathode (wire) and anode (workpiece) results in a short circuit and breaks the wire.
Although a rotary axis offers an alternative by performing the cut by rotating the tube and holding the wire stationary, the cut produced by this method has a poor surface finish.
Wire EDM machines have high initial and operational costs compared to traditional cutting techniques.
The wire used in the EDM process is impacted by the electric discharge (sparks), and a constant feed of new wire is required.
Furthermore, the use of dielectric fluid further adds to the operational cost of wire EDM and makes it unaffordable for individuals and small-scale businesses.
Wire EDM is a high-precision metal cutting technique that can be used for making intricate parts with tight tolerance.
Using wire EDM requires a skilled operator with prior experience in EDM processes to produce high-quality cuts efficiently.
Cutting hard metals by Wire EDM produces better quality cuts when compared to soft metals like aluminum.
Although wire EDM is a slow process, the tolerance and quality of the cut produced by this process are far superior to any traditional cutting methods.
Frequently Asked Questions (FAQ)
Is Wire EDM more precise than laser cutting?
Yes, wire EDM is more precise than laser cutting.
Laser cutting has an accuracy of around 0.002" which further decreases as the thickness of the material increases. Whereas wire EDM has an accuracy of around 0.0002", and the thickness of the material does not affect the accuracy of the cut.
What is Small Hole EDM?
Small hole EDM is a type of EDM that is used for making extremely small holes with diameters of around 0.010″ – 0.118". A hollow electrode rotates along its axis and produces holes in the workpiece. The hollow electrode carries an electric charge that generates the spark and produces the desired holes. It is similar to the drilling process and is often referred to as EDM Drill.
Can wire EDM cut thin sheets of metal?
Yes, wire EDM can be used to cut thin sheets of metal. However, clamping extremely thin sheets of metal without deforming them can be challenging. Therefore wire EDM is generally not recommended for cutting thin and soft metals.
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