The term EDM stands for Electrical Discharge Machining.
You all know about conventional machines like lathe, milling, grinding, or drilling where the cutting tool is always harder than the work-piece; also the cutting tool applies the force to remove the material from the work-piece during machining.
The Electrical Discharge Machining (EDM) is an unconventional or non-traditional method of machining, and in this process, the hardness of the tool does not matter at all. In EDM, you have to ensure only one basic necessity and that is both the tool and the work material are electrically conductive.
You can use the tool made from a soft material like copper to machine a hard material like carbide.
Also, there is no physical contact between the tool and the workpiece in your EDM process and hence no mechanical cutting forces.
In the EDM process, the tool which is called Electrode and the work-piece are separated by a small gap and both the electrode (tool) and the work-piece are submerged in a dielectric fluid; the electrode and the work-piece are connected to a DC power source and material is removed from the work-piece surface by repeated sparks in the gap between the electrode (tool) and the work-piece surface.
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What is a Dielectric Fluid?
You can define the dielectric fluid as an insulator that normally insulates the electrode (tool) from the workpiece in an EDM process; however, the dielectric fluid, under certain specific conditions (like a high voltage exceeding its dielectric strength) breaks down and acts as an electric conductor.
You use dielectric fluid in your EDM machine as an insulator between the electrode and the work-piece; however, the dielectric fluid in the area of the gap between the electrode and the work-piece breaks down due to ionization when the applied voltage exceeds the strength of the dielectric fluid and allows the current to pass in the form of a spark.
Some of the desired qualities of the dielectric fluid for an EDM machine are:
- Sufficient and stable dielectric strength.
- Low viscosity for better flow.
- Chemically neutral to work-piece and electrode materials, fixture, etc.
- Sufficiently high flash point.
- Not harmful to the human skin and no emission of toxic vapor during the process.
The dielectric fluid in the EDM machine cools the workpiece and the electrode (tool) and flushes away the tiny chips formed during the EDM process.
Popularly used dielectric fluids are hydrocarbon oils (kerosene etc.) and deionized water.
Electrical Discharge Machining Working Principle
What is the basic principle of EDM and how the material is removed in this process?
The electrode (tool (Cathode)) in the machine is connected to the negative terminal and the work-piece (anode) to the positive terminal of the DC power supply (DC Pulse Generator); an electrical potential is generated between the electrode (tool) and the work-piece.
Both electrode and the workpiece are completely submerged in the dielectric fluid and the surfaces of the tool and the workpiece are separated by the spark gap.
As the electrode approaches the work-piece, at a certain point, the intensity of the electric field in the spark-gap area exceeds the strength of the dielectric fluid; the dielectric fluid breaks down due to ionization and allows the current to pass from the electrode (tool) to the work-piece in the form of a spark.
Sparking results in the generation of extreme electro-thermal heat in the spark-gap zone and leads to melting and vaporizing a portion of material from the work-piece surface (spark erosion).
DC pulse generator of the machine momentarily pauses the flow of current, and the continuously flowing dielectric fluid flushes away the minute EDM chips and fills the spark-gap zone with fresh clean dielectric fluid (with original properties). The dielectric fluid quenches the spark and cools the electrode and the workpiece which gets heated up during spark discharge.
The dielectric fluid helps to concentrate the spark energy to a very small area (where the gap between the electrode and the workpiece is the least).
The electric voltage and current in the EDM machines are not applied continuously but in pulses; the pulse cycle has an off-time (no sparking) and on-time when the spark happens.
This cycle is repeated continuously and the number of sparks between the electrode and the workpiece will be more than a thousand sparks per second.
Sparking in an EDM machine takes place between the surfaces of tool and work-piece at the point of minimum gap and this point of a minimum gap shifts continuously over the work-piece surface; hence sparing takes place all over the work-piece surface in a continuous manner and in the end, the replica (negative) of the tool profile is reproduced on the work-piece surface.
A tiny portion of the tool is also eroded due to the spark discharge.
What is EDM Machine?
Your EDM Machine facilitates controlled sparking for regulated erosion of material from the work-piece surface; the electrode and the work-piece are submerged in a dielectric fluid and precisely controlled cyclic recurring spark discharges are initiated between the electrode and the work-piece.
The machine has a bed with guide-ways for mounting the saddle and for its cross-movement; the bed is mounted on a base (please see the diagram above).
The table of the machine is mounted on the guideways of the saddle; the table has movement in the X and Y-axis to help you locate the workpiece with reference to the electrode (tool).
The dielectric tank of the machine is mounted on the machine table and moves along with it; the front of the tank (door) can be opened during the setting of the electrode (tool) and the workpiece. The dielectric tank of your machine has a level control to ensure no overflow of dielectric fluid.
Some EDM machines may have a dielectric tank that goes down and disappear during work-piece settling and comes up during the EDM process.
The vertical column of your machine is directly mounted on the bed and Electrode-head is mounted on the column; the electrode-head has vertical up and down (Z-axis) movement and in some designs, it has rotary movement also around Z-axis.
Your machine may have a manually controlled X and Y-axis with the help of a DRO (Digital Read Out scales) and Z-axis under the control of a CNC or Servo motor.
Alternately you can have a machine where the movements of X, Y, and Z-axes are under the control of CNC.
The electrode-head has a platen (insulated) with T-slots for the mounting of the electrode and you can mount the work-piece on the fixture (insulated) clamped to the table using T-slots; the platen and the fixture are insulated since the electrode and the work-piece are connected to the DC electric source.
A small gap (approximately 0.25 millimeters, exact value depends on the work-piece material and other parameters) is maintained between the electrode and the work-piece, and this gap is called spark-gap.
The electrical panel box of your machine has a voltmeter, ammeter, DC pulse generator, and power supply to the dielectric pump, servo motor, etc. Output current is an important factor for the EDM machine, higher current enables a higher material removal rate per unit of time.
The DC pulse generator of the machine has to (i) ensure an adequate supply of voltage/current to initiate the spark discharge and later to maintain it and (ii) to vary the intensity of the discharge current and duration of the discharge as per needs.
The DC pulse generator produces a square waveform of current pulses and the energy in each pulse is maintained constant; the pulse generator has accurate control over the duration of On-time and Off-time and the current.
The dielectric fluid system of an EDM machine consists of a tank, pump, and a filter to feed clean dielectric fluid into the machine dielectric tank and also flooding the spark-gap area through the passage given in the tool; used dielectric fluid from the machine dielectric tank is collected continuously (for recirculation) either by gravity flow or through suction.
Z-axis movement of the Electrode-head in your machine is very important since the spark-gap is varied continuously (based on the signals (regarding voltage) received from sensors in the spark gap area) to maintain a spark-gap favorable for spark.
Most machines have a remote or pendant type of Control Box for the convenience of the operator.
What materials can be machined by EDM?
You can machine any electrically conductive material in your EDM machine; however, since the material removal rate of EDM is slow when compared to conventional machines, EDM process is best suitable and economical for machining hardened tool steel, titanium, carbide, super alloys like Hastelloy and Inconel, which are difficult to machine by conventional techniques.
Your EDM process is recommended for machining die cavities with intricate and complicated shapes in materials listed above and the cavities can be blind or through.
Types of EDM Machines
Basically, you have two types of EDM machines
- Sinker EDM or Ram EDM
- Wire EDM or Wire cut Machine
Sinker EDM
The Sinker EDM Machine is used for machining of cavities with complicated and intricate profiles; the electrode (machined to the profile required) and the workpiece are separated with a spark-gap and are submerged in a dielectric fluid. Material is removed from the work-piece surface by continuous spark discharges between the electrode and work-piece surface.
Wire EDM
Your Wire EDM machine uses a thin electrically conductive wire (generally brass) as an electrode to cut the workpiece to create the desired profile or shape. Both the workpiece and the wire are submerged in the dielectric fluid. Your machine has the arrangement to provide fresh wire continuously for effective cutting and the path of the wire is controlled and guided by the CNC.
There is another type of EDM machine which is used for drilling accurate and micro-holes on hard material.
EDM Drilling
The construction of the EDM drilling machine is somewhat similar to your Sinker-EDM machine; the electrode used is of the tubular form (allows dielectric fluid to pass through) and has rotary as well as up and down movement. The rotating electrode moves toward the work-piece of your machine and the hole-cutting takes place by continuous spark discharge at the spark-gap; the work-piece and the electrode are flooded with the dielectric fluid.
What are the advantages and disadvantages of EDM?
Advantages of EDM
- You can machine any electrically conductive material, irrespective of its toughness, hardness, and microstructure.
- You can use EDM for cutting extremely hard electrically conductive materials, which cannot be machined by conventional methods.
- There is no physical contact between the tool and the workpiece in your EDM machine, hence no mechanical cutting forces on the workpiece.
- You can duplicate the exact profile of the tool (however intricate or critical) onto the work-piece surface.
- Production of the electrode (tool) is easy since you can use soft materials like copper, graphite, or brass as tool material.
- The EDM machining gives a fairly good surface finish, accuracy, tolerance, and repeatability.
- EDM process does not leave any burr or tool mark on the work-piece surface.
- Constant immersion of the electrode and the workpiece in a dielectric fluid ensure minimum after-effects of heating on the workpiece material.
Disadvantages and Limitations
- The MRR (material removal rate) of the Electrical Discharge Machining (EDM) process is very low (compared to conventional machines) making this process limited to hard materials which cannot be machined by other conventional process and critical and micromachining required in aerospace and hi-tech industries.
- You cannot use the EDM process for electrically nonconductive materials.
- Recast layer is an in-built part of the EDM process. You need to use a secondary operation like polishing to remove the recast layer.
What is the Recast Layer in EDM?
The EDM process generates a very high or intensive electro-thermal heat in the area of spark discharge and the workpiece is to be protected from this heat. The dielectric system in your EDM machine cools the workpiece and the electrode, by the continuous flow of dielectric fluid.
The high electro-thermal heat generated by your EDM machine in the area of spark discharge melts and vaporizes a portion of material from the work-piece surface and the continuously flowing dielectric fluid flushes away this material and cools the work-piece. The dielectric fluid leaves behind a small portion of the partially melted material which could not detach from the surface of the work-piece and this layer of material on the work-piece surface is called recast layer or white layer.
The recast layer may also happen if the dielectric fluid system of your machine is not effective and there is a delay in flushing out the melted material from the spark discharge area; the melted material may form a recast layer on the work-piece surface if not flushed out.
The property of the recast layer in your EDM is different from your work-piece material; the recast layer is harder than the work-piece material, porous, and may have micro cracks.
For a better surface finish, you must remove the recast layer by a secondary process like polishing.
Polarity
Polarity in Direct Current (DC) indicates the direction of flow of current in a circuit and there are two types- straight (positive) polarity and reverse polarity.
In EDM, in straight (positive) polarity the work-piece is connected to the positive terminal of the DC supply and the electrode to the negative terminal.
In reverse polarity, the work-piece is connected to the negative terminal of the DC supply and the electrode to the positive terminal.
Your EDM process adopts straight (positive) polarity as a standard practice; however, polarity may be changed for some specific applications.
The DC power supply may have a provision for changing the polarity.
The Safety factor in EDM
A Sinker-EDM uses hydrocarbon oils like kerosene as a dielectric fluid and this can be a fire hazard under extreme conditions.
Also, EDM Machines use DC electric source for the spark discharge and there can be a possibility of shock. The electrical circuit of your EDM Machines is designed to keep the safety of the operator in view.
There is an emergency button that the operator can use in case of an emergency to stop the machine completely.
In case of exigencies like a low level of dielectric fluid in the dielectric tank of your EDM, the machine automatically stops the operation; many such safety factors are built into the electrical circuit.
The room of your EDM Machine has good ventilation for removing any harmful fumes generated in the EDM process.