Electroforming: Electrochemical and Electrical Discharge Machining

Over the period, industrial materials have been developed into more advanced forms than ever before in the long history of human scientific endeavors. This landmark achievement is not limited to the materials alone. There is a parallel scientific venture that goes on to develop means for machining those materials.

Today, the materials have taken over their conventional forms. They are developed into advanced engineering materials such as complex alloys and composites, among many others, that require concomitant progress within the environs of machine tools and machining to benefit in terms of improved cutting quality yet at affordable economics.

To machine such engineered industrial materials, primitive means of machining such as milling and drilling using mills and drill presses have lost their resplendent history, if not wholly than partly only because of the recent invention of more subtle and nuanced methods of machining and control that are commonly known as electroforming.

The present article is sincerely dedicated to discussing two such electroforming techniques as follows.

  1. Electrochemical Machining (ECM)
  2. Electric Discharge Machining (EDM)

Electrochemical Machining Process (ECM)

To drill, trepanned, and shape very hard materials, electrochemical machines are used that range from a small drill press of a bench type to a large-size automatic production machine.

electrochemical machining process

Basic Principle

So, what is the basic principle that rules the operation of an ECM machine? In simple words, in an ECM machine, a low-voltage electric current is allowed to pass through a conductive fluid between the tool and a workpart electrode. This conductive fluid acts simultaneously to flush away the residue obtained as a result of machining.

How does an ECM machine work?

In literature, the electrochemical process is deemed to be the opposite of the electroplating process: in electrochemical machining, electrolytic action is used to dissolve the material from the workpart. Therefore, it is sometimes referred to as the electrolytic grinding process, which in turn is a modified form of electrochemical machining.

working of ecm machine transformed

At its simplest, the following electrical diagram relates to an ECM machine. The workpart which should be an electric conductor, is placed in a chemical tank where the electrochemical process takes place and is connected to the positive terminal (anode) of a DC power supply.

The cutting tool that is used to perform the cutting action by cutting cavities into the worpart is made electrode and is connected to the negative terminal (cathode) of the same DC source. Through the gap between the tool and the workpart, an electrolyte fluid flows that keep circulating by getting pumped back into the working zone.

A low-voltage electric current passes through the electrolyte that dissolves the metal of the workpiece, which acts as an anode. Electrical resistance is low, and therefore current is maximum where there is a tool positioned closest to the workpart.

Due to being close of a tool and the work, machining takes place at a rapid rate and as a result, the shape of the tool electrode is reproduced on the workpiece. That is, identical geometry is machined on the work.

There is no physico-mechanical contact between the tool and the workpart in ECM. There is a sharp likelihood that the removed particles of the workpart would get deposited onto the tool (cathode). Still, such an act is counteracted by the free flow of the electrolyte that keeps removing the dissolved metal particles from the working zone continuously. So, there is a rare chance that there would be a tool wear any sooner or the deposition of the work particles on the tool until a large number of workparts are produced using it.

Construction

When mass production of the parts is not required, the ECM machine equipped with only push buttons or controls for manual cycling can be used. However, in case of higher and yet faster machining rates, the ECM machine with a semi-automatic or fully automatic cycling arrangement is to be selected for use.

In the figure cited below, a vertical ram is shown to have supported a platen for holding the ECP cutting tool electrode. It (the ram) is completely insulated from the tool and is connected to the DC power source using a bus bar and electric cables. The power source, as discussed above, ensures the delivery of low-voltage electric current to the work zone.

vertical ram in ecm

Made of corrosion-resistant materials such as plastics or stainless steel,  the components of the electrolyte system (section) include a reservoir or tank, pump, filter, and drive motor, as well as work compartment connections and a return flow line. Before admission into the work area, the electrolyte is cleaned in filters.

What should be the nature of the electrolyte solution is governed by the type of material of the work as well as the complexity of the cutting operation. It should be noted that the electrolytic solution can be neutral, acidic, or alkaline.

ECM Operations

  • Electrochemical machining operation is advantageous in terms of almost negligible tool wear. The same tool electrode can pierce several holes in the work before its life ends, as shown in the figure below.
ecm operation one
  • Internal gear shapes can also be machined using the ECM process, as shown below. There are no machining defects on the part: no tool marks and burrs.
ecm operation two
  • The die made in the ECM machine is illustrated below, along with the electrode tool that was used to machine the die.
image of die

In short, a long list of machining operations can be done with the ECM machine, such as drilling, trepanning, shaping, broaching, and cavity sinking on any electric conductor work metal.

Electrical Discharge Machining Process (EDM)

Using the electrical discharge machining process, harder and tougher materials can be machined to any geometry that is machineable on the electrode. The term ‘tool’ would now be abstained as the word electrode is used frequently in the related literature throughout in its place.

Basic Principle

EDM process is performed with the work and the electrode submerged in the dielectric fluid. Both are kept close to each other. DC pulses are discharged from the electrode at a high frequency that causes metal removal or, more precisely, particle erosion in the workpart.

How does an EDM machine work?

It is sometimes known as electroerosion or electrospark process. It works on the basic principle as stated above: the metal is removed from the workpiece as a result of the repetitive short-lived pulses between the workpart and the electrode (tool).

The circuit used in the EDM machine is a DC relaxation circuit that is fed from the mercury arc or the selenium-type rectifier. Modern machines make use of the diodes as well. Due to advanced research, less expensive semiconductors are now being used in the latest machines to cause electric pulses to be generated.

To achieve particle erosion from the surface of the work, a minimal gap between the work and the electrode ranges from 0.025 to 0.05mm, and it is achieved using a servo drive which, however, does not prove to provide a very efficient circuit.

working of edm machine

The figure above shows the circuit for an EDM machine. Here, the placement of the capacitor across the work and the electrode is noteworthy. Once the power is turned on, the voltage begins to rise gradually, and the capacitor starts getting charged.

At this point, the spark-gap acts as an open circuit due to which no current flows. But, when the voltage across the capacitor builds up much enough, it takes over the breakdown voltage of the gap (between the work and the electrode).

The breakdown voltage of the gap is governed by two factors: the distance between the work and the electrode and the nature or the composition of the dielectric fluid. Thus, again, a spark is produced only when the voltage across the gap reaches a critical point.

At this point, the dielectric fluid gets ionized with the capacitor going discharged. Once the capacitor is discharged, the dielectric fluid starts to deionize again, and it would now act as an effective insulator. It ends the previous cycle with the beginning of the next.

All attempts are sustained to cause the consistent production of electric sparks with a production frequency as high as 1000 per second.

During peak production times, the DC circuit does not favor high-intensity, low-frequency sparks. Rather, it does require the production of pulses at a much higher frequency to meet production targets. To have better control of the frequency and thus higher metal erosion rates, the circuit is provided with a rotary impulse generator.

Construction

edm machine

An EDM machine, as shown in the figure above, consists of an electrohydraulic servo that is used to maintain a constant gap between the work and the electrode. It constantly feeds the tool to keep that gap the same as the machining advances.

A DC power source supplies current to the servo that moves the electrode toward work. It stops the supply of current once the electrode has attained the pre-set depth or gap or until the pre-defined amount of stock (metal) is eroded.

Thus the power supply is aimed at serving two purposes: to control the movement of the servo and, at the same time, the metering of the high-frequency DC pulses to the electrode. For instance, the servo retracts the tool if the gap between the two gets too thin or if the forming operation causes the bridge to develop using residue.

In short, EDM is a mechanical yet sophisticated precision device that holds the workpiece while an electrode is fed to it at a measured distance or gap. The gap or the depth of cut is maintained using a combo of micrometer and limit switches. It results in a highly repeatable and uniform process, which gives accurate predictions over the final machining results.

EDM Operations

Today, EDM is a popular way of machining jobs. It is typically used for general-purpose machining or for die and mold work.

  • In the figure shown below, the EDM machine is used to drill 18 holes in an aircraft fuel nozzle using multiple electrode setups in a single machining cycle.
edm operation one

The figure below shows the small hole cutting using EDM in the workpiece.

small hole using edm
  • As EDM can be used to machine hardened steels, carbines, and other tougher alloys, the figure illustrated below shows some of the parts and components prepared on an EDM machine.
edm operation two
  • EDM processes have superlative significance over machining die (and mold parts) after they are hardened. These parts are difficult to machine using conventional forms of machining such as milling and drilling.
edm operation three
  • EDM is also mainly used for cutting irregular shapes, such as shown below in the stainless shroud, using an electrode made of brass. The cutting tool electrode is a soft tool and does not cause stress or heat due to elevated cutting pressures observed during conventional cutting.
edm operation four
  • The figure shown below illustrates other parts that are fabricated using EDM, for instance, pressure relief valves.
edm operation five
  • Conventional tools can also be used in EDM once they are heat-treated. For instance, the figure below shows the EDM profile milling in which the electrode acts as a milling cutter.
edm operation

Likewise, the figure below shows an EDM grinding in which the grinding wheel is the electrode.

grinding electrode in edm

Frequently Asked Questions

How cutting speed and surface finish are achieved in an EDM machine?

The cutting speed is controlled by the cutting current; whereas, the surface finish of the workpart is governed by the rate or frequency at which the cutting current pulses occur.

Is it possible to predict the results of a workpart machined in an EDM tool?

Yes! And the yes-ness takes refuge in the repeatability of an EDM process. Sizes, tolerances, and the degree of surface finishes, all can be accurately predicted within very close limits.

How is the level of dielectric fluid maintained in the machine tank where metal discharge or erosion takes place?

Dielectric fluid quantity is maintained using an adjustable level control and an adjustable safety float in the workpan which provides a setup for the work.

What are the merits of using an ECM machine?

The merits of using an ECM machine include Increased cutting precision, cutting hardened materials such as hard steels and carbides, machining without causing burrs, very slow tool wear, reduced heat affected zones, no mechanical contact between the tools and the work, and so forth.

What are the demerits of using an ECM machine?

The ECM machine cannot cut a job that is not a conductor of electricity. Another minus point is its high initial cost.

What are the merits of using an EDM machine?

The merits of using an EDM machine include machine versatility over cutting a range of conductive materials including exotic composites, machining intricate shapes, geometries and profiles, top-notch surface finish, and almost negligible tool wear among many others.

What are the demerits of using an EDM machine?

One of its major drawbacks is the slow machining rate due to which it cannot be used where mass production is required.