Electro Chemical MaciningVratraj Kaladhar Joshi

Content

Need of Advance Machining ProcessHigh hardness and the strength of the material Work-piece too flexibleComplex shapeSurface finish and dimensional tolerancesUndesirable Temperature rise and dimensional tolerances

Electrochemical MachiningThis process is reversal of the electro plating

Electrolyte acts as current carrier

High rate of electrolyte movement in tool work piece gap washes metal ions away from the work piece ( ANODE)

This is washed just before they have a chance to plate on the tool ( cathode)

Shaped tool made of brass , copper , bronze , or stainless steel

Electrolyte is pumped at a high rate through the passages in the tool

Machines having current capacities as high as 40,000 A and as low as 5A are available

Design considerations for Electrochemical MachiningElectrolyte erodes sharp surfaces and profiles so not suited for sharp edgesIrregular cavities may not be produced to the desired shape with acceptable dimensional accuracyDesigns should make provisions for small taper for holes and cavities to be machined

Advantages of ECM:

Can machine metallic materials of very high hardness (as can grinding and electro discharge machining).Surfaces produced are free of stress and/or damage (in contrast to grinding on EDM).Can produce curved surfaces, complex and intricate shapes as quickly as simple shapesCan be combined with standard machining practices.Production rates on complex parts can be on an order of magnitude faster than direct machining Surface finishes as good as 0.1 pm can be obtained.There is no wear on the tooling if certain precautions are observed in its use.

Besides worrying about arcing, the tooling designer must contend with the spatial variations in gap thickness, fluid flow, velocity,electrolyte dilution and electrolyte temperatures, a l l of which are interconnected.Fluid cavitations must be avoided, which may require an exhaust backpressure,which i n turn can cause sealing problems and pumping capacity problems.Electrolyte splash onto adjacent part surfaces can cause unwanted etching.

Highamperage 100-1000 A/in2 (15-150 A/cm2) i s used to provide a suitable removalrate.bubbles and heat, which are kept a t tolerable levels by high flow velocityo f the electrolyte $100-200 f t / s e c ( 33-63cms/sec).substantial pressure drops 50-400 psi (3-28 MPa) and can involve h i g h flowrates on the order of hundreds o f gpm (M3/s).this anode-cathode gap can be as smal 1 as 0.004" (1000 mm) .These conditions lead t o rapid generation of hydroxide solids, gasThis i n turn requires

Disadvantages:1. A massive machine structure i s required f o r ordinary-sized parts, becauseof the h i g h pressure loads and because extreme r i g i d i t y i s needed t oavoid water-harmer i n s t a b i l i t y .The electrolyte handling system (pumps, centrifuges, s e t t l i n g tanks, e t c . )is also massive.gallon (7574/L) s e t t l i n g tanks, and i s limit?< t o the two electrolytesi n those tanks.The corrosive nature of the electrolyte presents a continued maintenanceproblem for equipment i n the ECM area.2.For example, the Lockheed Sunnyvale system has two million3 .

4. The complexity of the processes w i t h i n the machining gap make thetooling design a combination of science and black a r t .turn means:a. Tool designers must have a thorough engincxing understandingThis i n/ plus a l o t of experience.b. Tooling lead times of several months are typical, since severaliterations may be required.5. Part geometry i s limited (in commercially available machines) t o t h a tcompatible w i t h a one dimensional d i e - s i n k i n g form, e.g., re-entrantshapes cannot be handled.details on a semi-circular shape, may cause unbalanced pressure forcest h a t cause great d i f f i c u l t y .Also, asymmetric shapes, such as s i n k i n g6. Because of factors of force, temperature, distortion, and variationsi n electrolyte properties, the repeatability of the process i s limited.The d i f f i c u l t i e s o f tooling design limit the basic accuracy.on the order of 2 0.005" (0.1250 mn) are normal, except that k 0.001"(0.025 mm) can be held on specific features such as hole depth.Tolerances

Summary :ECM is most suitable i n the following types of applications:a ) Large production quantities are required i n a d i f f i c u l t t o machineb) The material hardness and/or surface requirements make the j o b i m -shape or material which i s suitable f o r ECM.possible o r a t l e a s t very unattractive t o do by any method except ECM. .

Diminishing the inter-electrode to below 0.1 [mm], what is one of thefundamental condition of increasing ECM accuracy,Reducing the inaccuracy of the machined profile caused by internaldisturbance of physical and chemical properties of electrolyte (more exact:medium in the gap) in inter-electrode gap,l Simplifying of tool design since the much more uniform distribution of thegap size,l Eliminate the macro - defects on the machined surface connected with thehydrodynamic flow disturbances,l Monitoring and control the gap sizes on line i.e. during the machining cycle.

1, voltage stabilizer; 2, gap control system; 3, tool feed rate controller; 4, displacement control system; 5, electrolyte parameter control system; 6, tool feed drive; 7, linear displacement sensor; 8, exhaust ventilation; 9, electrolyte transfer pump; 10, centrifuge; 11, electrolyte tank; 12, electrolyte supply pump; 13, machine table; 14, tool electrode; 15, rectifiers; 16, transformer; 17, voltage regulator.

Figure 3 shows the experimental system for micro wire ECM.A tool electrode and a workpiece were attached to a threeaxisstage with a 0.1 m resolution. A pulse generator wasused to produce a few tens of nanoseconds pulses and thevoltage and current were monitored by an oscilloscope. Apotentiostat was used to control the potential of the workpieceand the tool electrode. Platinum electrodes were used asa reference electrode (RE) and a counter electrode (CE).As an electrolyte, sulfuric acid was used. It is suitablefor the ECM of stainless steel with a good surface finish[6]. A voltage follower was attached between the mainmachining circuits and the measurement circuit to minimizethe effect of impedance. The workpiece was stainless steel,and the surface was mechanically polished before machining.Tungsten wire with a 10 m diameter (Nilaco, Corp.) wasused as a tool electrode. The tungsten wire was insulated withenamel, except the machining area. It is useful to concentratethe reaction current in the machining area and increase themachining rate [4].

Abstract. Pulse electrochemical machining (PECM) provides an economical and e.ective method for machining high strength, heat-resistantmaterials into complex shapes such as turbine blades, die, molds and micro cavities. Pulse Electrochemical Machining involves the applicationof a voltage pulse at high current density in the anodic dissolution process. Small interelectrode gap, low electrolyte .ow rate, gap state recoveryduring the pulse o.-times lead to improved machining accuracy and surface .nish when compared with ECM using continuous current. Thispaper presents a mathematical model for PECM and employs this model in a computer simulation of the PECM process for determination ofthe thermal limitation and energy consumption in PECM. The experimental results and discussion of the characteristics PECM are presented.

Parts Made by Electrochemical MachiningTurbine blade made of nickel alloy of 360 HB. Note the shape of the electrode on the right. Thin slots on a 4340-steel roller-bearing cage. Integral airfoils on a compressor disk.

Electrochemical Machining Reverse of electroplating An electrolyte acts as a current carrier and highelectrolyte movement in the tool-work-piece gap washesmetal ions away from the work piece (anode) before theyhave a chance to plate on to the tool (cathode). Tool generally made of bronze, copper, brass orstainless steel. Electrolyte salt solutions like sodium chloride orsodium nitrate mixed in water. Power DC supply of 5-25 V.

Recent study in the field of ECMMaterial removal rate in electrochemical machining is analyzed in context of over voltage and conductivity of the electrolyte solution.Material removal rate decreases due to increase in over voltage and decrease in current efficiency, which is directly related to the conductivity of the electrolyte solution.

the effect of several process parameters of the machining characteristics of high carbon high chromium die steelon material removal rate and surface roughness has been reported. Three different process parameters wereundertaken for this study; applied voltage, tool feed rate and electrolyte discharge rate.

ReferencesFEASIBILITY OF ELECTROCHEMICAL MACHINING-A Committee Report,Cornpi1ed by :William L. Randall October 1974