The typical ECM machine consists of a table for mounting the workpiece. The tool is mounted on a quill fixed on a ram. Vertical box frame structure has excellent rigidity required and is best suited for precision work.
Vertical C-frame structure has less rigidity but offers greater accessibility to the workpiece and tooling. The machine base, worktable and all other parts are constructed of corrosion-resistant, materials to avoid corrosion.
Tool Feed Rate:
In ECM process, a gap of about 0.01 mm to 0.7 mm is maintained between the tool face and the work. For smaller gap, the electrical resistance between the tool and work is least and the current is maximum, and accordingly maximum metal is removed. The tool is fed into the work depending upon how fast the metal is to be removed, which is turn depends upon the material of workpiece and the current density.
Considering the defined relationship between the above parameters, it is possible to predict accurately the feed rate. The movement of the tool slide is controlled by a hydraulic cylinder giving an infinite range of feed rates.
The electrolyte is essential for the electrolytic process to work. In addition to removing the heat generated in the cutting zone due to the flow of high current, it also carries the high current and removes the products of machining. The electrolyte is pumped at about 14 kg/cm2 and at speed of at least 30 m/s in order to constantly replenish the solution, which must never be allowed to reach boiling point as it would disturb the current flow.
The electrolyte should have high electrical conductivity and be chemically active enough to cause efficient metal removal, and not very corrosive. The electrolyte must have a good chemical stability. Also it must be inexpensive and safe. Generally an aqueous solution of the inorganic compounds is used.
Stainless steel parts are used for pipes, etc., which come into contact with the liquid. Salt solutions such as brine, or a 10% solution of sodium chloride, solutions of sodium nitrate or acids have been used as electrolytes. While common salt is best suited for most of the metals, it is corrosive and produces a large amount of sludge.
It can’t machine metals like tungsten carbide and pure titanium. Sodium nitrate gives better surface finish on alloys of aluminium and copper. Strong alkali solutions are used for WC based alloys, and HCl co mixture of brine and H2SO4 for nickel based alloys.
The current density must be uniform over the whole cutting surface in order to give exact workpiece reproduction. However, current density may vary due to a variation in electrolyte conductivity con account of changes in temperature across the area of the surface being machined. The recommended values of current density are 170 A per cm2 for steel, 184 for Al, 186 for Ni and 230 for titanium.
Since the conductivity of electrolyte varies with change in temperature, it must be held reasonably constant, otherwise the equilibrium of the machining gap will change. It may be noted that low electrolyte temperature results in low metal removal rate and high temperature leads to vaporisation of the electrolyte. It is maintained around 25° — 60°C.
As no wear takes place on tool, any good conductor is satisfactory as a tool material, but it must be designed strong enough to withstand the hydrostatic forces, caused by the electrolyte being forced at high speed through the gap between tool and work.
The tool is made hollow for drilling holes so that electrolyte can pass along the bore in tool. Cavitation, stagnation and vortex formation in electrolyte flow must be avoided because these result in poor surface finish.
It should be given such a shape that the desired shape of the jobs is achieved for the given machining conditions.
To prevent electro-chemical action between the sides of the tool and hole it is usual to relieve the tool and cover the relieved portion with an insulating coating. It must have adequate strength and be properly fixed and supported.
Fig. 10.24 shows the shape of tool to produce holes with minimum taper and excellent finish. Some features may be noted- Insulation of sides of tool permits machining at bottom and no ECM effect on sides and thus no tapering effect.
Provision of radii at bottom avoids turbulence and breaks of flow and encourages uniform electrolyte flow around the corner. A tip is bronzed on the shank of tool to allow easy replacement. The electrolyte is made to flow from the inside of the tool cut, around the cutting edges and up through the hole.
As the work temperature variation is kept within control by the electrolyte, no thermal or mechanical damage occurs to the machined surface. Surface finish upto 0.4 µm CLA and accuracy of ± 0.02 mm can be obtained.
Find finishes can be obtained in one cut. The high current density gives both the highest machining rate and the best finish. The depth of work hardened surface layer is around 1 µm. Residual stress in surface is nil.
Metal can be removed at a rate of 550 mm3/sec. It depends only upon the chemical composition and not on the hardness and toughness of the work material. As the current consumption per unit volume of metal removed per second is very high, it is an expensive process and is found to be economical for machining metals harder than 400 BHN. It is found to be best suited for machining complex shapes in very hard materials, especially those to be used in high temperature applications such as jet engine parts.
It is important to note that the gap produced in ECM process is inversely proportional to the feed rate normal to the surface. Thus when producing curved surface as in Fig. 10.25, OA is tool feed rate (f) downwards; but along direction OB, tool feed rate is f cos α. If the radius is large, then allowances can be made for this effect (i.e., variation of gap), but small form features or sharp corners cannot be accurately produced.