Electrochemical Machining (ECM)

 

      ECM - Electro-Chemical Machining

How does the electro-chemical machining process work?

ECM stands for "Electro-Chemical Machining" and is - in contrast to EDM - a gentle electro-chemical process that removes the metal without sparking, with the workpiece and the tool being polarised positively (as anode) and negatively (as cathode), using a DC or a pulse source. The charge in the working gap between cathode and anode  runs through an electrolyte solution - usually sodium nitrate or sodium chloride - and detaches metal ions from the workpiece. This material can then be filtered out from the electrolyte solution in form of metal hydroxides. The contour of the tool cathode is customised to suit machining requirements. Electro-chemical machining thus removes metal only at those points where it is required; and it does so without causing mechanical or thermal stresses. This is where the main advantage of the process lies. The narrowly defined process allows for the reproducible precision machining of even the most delicate filigree components.

 

Michael Faraday’s early metallurgic researches, from 1818 to 1824, anticipated the developments which have led to widespread use today of alloy steels. Much effort has been expended to improve their performance for their service as cutting tools in machining. The aim has always been to yield higher rates of machining and to tackle recently developed harder materials on the principle that the tool material must be harder than the workpiece which is to be machined. Much progress has been made; however, in recent years some alloys, which are exceedingly difficult to machine by the conventional methods, have been produced to meet a demand for very high-strength, heat resistant materials. Moreover, these new materials often have to take a complex shape. A search has had to be made for alternative methods of machining since the evolutioof suitable tooling has not kept pace with these advances. 

     

Electrochemical machining (ECM) has been developed initially to machine these hard to machine alloys, although any metal can so be machined. ECM is an electrolytic process and its basis is the phenomenon of electrolysis, whose laws were established by Faraday in 1833. The first significant developments occurred in the 1950s, when ECM was investigated as a method for shaping high strength alloys. As of the 1990s, ECM is employed in many ways, for example, by automotive, offshore petroleum, and medical engineering industries, as well as by aerospace firms, which are its principal user.
Metal removal is achieved by electrochemical dissolution of an anodically polarized workpiece which is one part of an electrolytic cell in ECM. Hard metals can be shaped electrolytically by using ECM and the rate of machining does not depend on their hardness. The tool electrode used in the process does not wear, and therefore soft metals can be used as tools to form shapes on harder workpieces, unlike conventional machining methods. The process is used to smooth surfaces, drill holes, form complex shapes, and remove fatigue cracks in steel structures. Its combination with other techniques yields fresh applications in diverse industries. Recent advances lie in computer-aided tool design, and the use of pulsed power, which has led to greater accuracy for ECM-produced components.

                                                                                                                                                                                                                                                                                

Advantages and disadvantages

Because the tool does not contact the workpiece, its advantage over conventional machining is that there is no need to use expensive alloys to make the tool tougher than the workpiece. There is less tool wear in ECM, and less heat and stress are produced in processing that could damage the part. Fewer passes are typically needed, and the tool can be repeatedly used.
Disadvantages are the high tooling costs of ECM, and that up to 40,000 amps of current must be applied to the workpiece. The saline (or acidic) electrolyte also poses the risk of corrosion to tool, workpiece and equipment.

 

Setup and equipment

ECM machines come in both vertical and horizontal types. Depending on the work requirements, these machines are built in many different sizes as well. The vertical machine consists of a base, column, table, and spindle head. The spindle head has a servo-mechanism that automatically advances the tool and controls the gap between the cathode (tool) and the workpiece.
CNC machines of up to six axes are available.
Copper is often used as the electrode material. Brass, graphite, and copper-tungsten are also often used because they are easily machined, they are conductive materials, and they will not corrode

 

Applications

Some of the very basic applications of ECM include:

• Die-sinking operations
• Drilling jet engine turbine blades
• Multiple hole drilling
• Machining steam turbine blades within close limits

Similarities between EDM and ECM

• The tool and workpiece are separated by a very small gap, i.e. no contact in between them is made.
• The tool and material must both be conductors of electricity.
• Needs high capital investment.
• Systems consume lots of power.
• A fluid is used as a medium between the tool and the work piece (conductive for ECM and dielectric for EDM).
• The tool is fed continuously towards the workpiece to maintain a constant gap between them (EDM may incorporate intermittent or cyclic, typically partial, tool withdrawal).