MANUFACTURING THROUGH ELECTRO CHEMICAL MACHINING




ABSTRACT:

The machining of complex shaped designs was difficult earlier, but with the advent of the new machining processes incorporating in it chemical, electrical & mechanical processes manufacturing has redefined itself. This paper intends to deal with one of the revolutionary process called Electro Chemical Machining (ECM).

INTRODUCTION:

Electro chemical machining (ECM) is the controlled removal of metal by anodic dissolution in an electrolytic medium in which the work piece is the anode & the tool is the cathode.
Working: Two electrodes are placed at a distance of about 0.5mm & immersed in an electrolyte, which is a solution of sodium chloride. When an electrical potential of about 20V is applied between the electrodes, the ions existing in the electrodes migrate toward the electrodes.
Positively charged ions are attracted towards the cathode & negatively charged towards the anode. This initiates the flow of current in the electrolyte. The electrolysis process that takes place at the cathode liberates hydroxyl ions & free hydrogen. The hydroxyl ion combines with the metal ions of anode to form insoluble metal hydroxides &the material is thus removed from the anode. This process continues and the tool reproduces its shape in the work piece (anode). The high current densities promote rapid generation of metal hydroxides and gas bubble in the small spacing between the electrodes. These become a barrier to the electrolyzing current after a few seconds. To maintain a continuous high density current, these products have to be removed continuously. This is achieved by circulating the electrolyte at high velocity through the gap between the electrodes. It is also to be noted that the machining gap size increases. Therefore to maintain a constant gap the cathode should be advanced towards the anode at the same rate at which the material is removed.

ELECTROLYTES & ELECTROLYTE SYSTEMS:

The electrolyte has three main functions in ECM:
1. It carries the current between the tool and the work piece.
2. It removes the product of machining from the cutting region.
3. It dissipates heat produced in the operation.

ELECTROLYTE FLOW ARRANGEMENT

Correct electrolyte flow across the tool is essential for proper machining. Attention should be paid to the tool shape where cavitation of the electrolyte is likely to occur. Tool design must permit a uniform electrolyte flow in all machining areas. Excessive flows are not desirable as they cause tool erosion. Basically, two methods of flow are used, namely divergent flow and convergent flow. The convergent flow method provides a smooth flow of electrolyte. The electrolyte is admitted through a chamber called ‘dam’ to pressurize the area outside the work and the tool. The advantages of the system are:
1. More uniform and predictable side over cut and front machining gap.
2. Improved surface finish.
3. Reduced possibility of arcing.
4. Much cleaner operating conditions.
5. Elimination of undesirable machining due to stray currents.
It is, however, to be noted that cost of tooling with convergent flow is more than that with divergent flow system.


ECM PROCESS CHARACTERISTICS
Material removal rate:

It depends chiefly on feed rates. The feed rate determines the current passed between the work & the tool. As the tool approaches the work, the length of the conductive current path decreases & the magnitude of current increases. This continues until the current is just sufficient to remove the metal at a rate corresponding to the rate of tool advance. A stable cut is then made with a fixed spacing between the work and the tool, termed as the equilibrium-machining gap. If the tool feed rate is reduced, the tool advance will momentarily lag behind, increasing the gap and thus resulting in a reduction of current. This happens until a stable gap is once again established.

Accuracy:

Under ideal conditions & with properly designed tooling, ECM is capable of holding tolerance of the order of .02 mm & less. Repeatability of the ECM process is also very good. This is largely due to the fact that the tool wear is virtually non-existent on a good machine, tolerance can be maintained on a production basis in the region of .02-.04 mm. As a general rule, the more complex the shape of the work, the more difficult is to hold tight tolerances, and the greater is the attention required for developing a proper tooling and electrode shape.

Surface Finish:

ECM under certain conditions can produce surface finishes of the order of 0.4m. This can be obtained by the frontal cut or the rotation of the tool or the work. The important variables affecting the surface finish are feed rate, gap dimension, electrolyte composition, viscosity, temperature & flow. Any defect on the tool will cause machining defects on the surface of the work.
The operating parameters which are within the control of the operator and which influence ECM process capabilities can be described as follows:
1. Feed Rate:
A high feed rate results in higher metal removal rate. It decreases the equilibrium machining gap resulting in improvement of surface finish and tolerance control.

2. Voltage:
Low voltage decreases the equilibrium-machining gap and results in better surface finish and tolerance control.

3. Electrolytic Concentration:
Low concentration decreases the equilibrium machining gap and thus a better surface finish and tolerance control is achieved.

4. Electrolytic Temperature:
Low temperature is conducive for better surface finish and tolerances.
It should be noted that the metal removal rate is comparatively lower with low voltage, low electrolytic concentration and low temperature.

ECM TOOLING

Tooling design is the key to successful application of ECM. There are two aspects of the design of ECM tooling. The first is the determination of tool size together appropriate machining conditions necessary to produce the required shape. The second part of the tool design is concerned with making the tool of appropriate material, foxing it on the machine connecting it to the power supply, arranging an adequate supply of electrolyte between the tool and work piece and insulating a part of the tool to prevent over cutting and generating a taper.


APPLICATION OF ECM TECHNIQUE

ECM is to be applied only in specialized areas where conventional machining is not feasible. One of the main applications of ECM is in the aerospace industry to machine difficult-to-machine materials and complex shaped parts.

1. ELECTROCHEMICAL TURNING

The tool is made as wide as possible to cover the intended area. Holes are provided in the end of the tool to supply the electrolyte. The tool is fed into the work in the same way as in conventional machining. The electrical and other parameters are the same as in ECM.

2. DRILLING AND TREPANNING

It is a process of drilling performed in ECM. The tool is a tube with insulated sides and the electrolyte is fed down the center of the tube to the working gap. It is suitable for producing deep, small-diameter holes in tough materials. Electrochemical drilling is extensively used for drilling the cooling holes in gas turbine blades. The chief advantage of this process is that the holes are burr-free and can be made in thin work pieces.

CONCLUSION:

The basic advantages of ECM are found to be:
1. The tool does not wear. Once the tool is developed it can be used indefinitely
2. There are no thermal or mechanical stresses on the work piece.
3. Faster stock removal & better surface finish can be obtained.
4. The hardness of the work piece is not a factor.
5. Any shape that can be produced in a tool can be reproduced in the work piece.
However the cons of the machine are:
1. The cost of the equipment is very high.
2. Rigid fixturing is required to withstand the high electrolyte flow rates.
The tool is more difficult to make since it must be insulated to maintain correct conductive paths to the work piece.