Electro chemical machining
Encyclopedia
Electrochemical machining (ECM) is a method of removing metal by an electrochemical process. It is normally used for mass production and is used for working extremely hard materials or materials that are difficult to machine using conventional methods. Its use is limited to electrically conductive materials. ECM can cut small or odd-shaped angles, intricate contours or cavities in hard and exotic metals, such as titanium aluminide
s, Inconel
, Waspaloy
, and high nickel
, cobalt
, and rhenium
alloys. Both external and internal geometries can be machined.
ECM is often characterized as "reverse electroplating
," in that it removes material instead of adding it. It is similar in concept to electrical discharge machining
(EDM) in that a high current is passed between an electrode and the part, through an electrolytic
material removal process having a negatively charged electrode (cathode
), a conductive fluid (electrolyte
), and a conductive workpiece (anode
); however, in ECM there is no tool wear. The ECM cutting tool is guided along the desired path close to the work but without touching the piece. Unlike EDM, however, no sparks are created. High metal removal rates are possible with ECM, with no thermal or mechanical stresses being transferred to the part, and mirror surface finishes can be achieved.
In the ECM process, a cathode (tool) is advanced into an anode (workpiece). The pressurized electrolyte is injected at a set temperature to the area being cut. The feed rate is the same as the rate of "liquefication" of the material. The gap between the tool and the workpiece varies within 80-800 micrometers (.003 in. and .030 in.) As electrons cross the gap, material from the workpiece is dissolved, as the tool forms the desired shape in the workpiece. The electrolytic fluid carries away the metal hydroxide formed in the process.
As far back as 1929, an experimental ECM process was developed by W.Gussef, although it was 1959 before a commercial process was established by the Anocut Engineering Company. B.R. and J.I. Lazarenko are also credited with proposing the use of electrolysis for metal removal.
Much research was done in the 1960s and 1970s, particularly in the gas turbine industry. The rise of EDM in the same period slowed ECM research in the west, although work continued behind the Iron Curtain
. The original problems of poor dimensional accuracy and environmentally polluting waste have largely been overcome, although the process remains a niche technique.
The ECM process is most widely used to produce complicated shapes such as turbine
blades with good surface finish in difficult to machine materials. It is also widely and effectively used as a deburring process.
In deburring, ECM removes metal projections left from the machining process, and so dulls sharp edges. This process is fast and often more convenient than the conventional methods of deburring by hand or nontraditional machining processes.
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.
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.
2. The tool must be a conductor of electricity.
3. Needs high capital investment.
4. These systems consume lots of power.
5. Only electrically-conductive material can be machined.
6. A fluid is used as a medium between the tool and the work piece (conductive for ECM and dielectric for EDM).
7. 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).
Titanium aluminide
Titanium aluminide, TiAl, is an intermetallic chemical compound. It is lightweight and resistant to oxidation and heat, however it suffers from low ductility. The density of gamma TiAl is about 4.0 g/cm³. It finds use in several applications including automobiles and aircraft...
s, Inconel
Inconel
Inconel is a registered trademark of Special Metals Corporation that refers to a family of austenitic nickel-chromium-based superalloys. Inconel alloys are typically used in high temperature applications. It is often referred to in English as "Inco"...
, Waspaloy
Waspaloy
Waspaloy is a registered trademark of United Technologies Corp that refers to an age hardening austenitic nickel-based superalloy. Waspaloy alloy is typically used in high temperature applications, particularly in gas turbines.-Nominal composition:...
, and high nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
, cobalt
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
, and rhenium
Rhenium
Rhenium is a chemical element with the symbol Re and atomic number 75. It is a silvery-white, heavy, third-row transition metal in group 7 of the periodic table. With an average concentration of 1 part per billion , rhenium is one of the rarest elements in the Earth's crust. The free element has...
alloys. Both external and internal geometries can be machined.
ECM is often characterized as "reverse electroplating
Electroplating
Electroplating is a plating process in which metal ions in a solution are moved by an electric field to coat an electrode. The process uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal...
," in that it removes material instead of adding it. It is similar in concept to electrical discharge machining
Electrical discharge machining
Electric discharge machining , sometimes colloquially also referred to as spark machining, spark eroding, burning, die sinking or wire erosion, is a manufacturing process whereby a desired shape is obtained using electrical discharges...
(EDM) in that a high current is passed between an electrode and the part, through an electrolytic
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
material removal process having a negatively charged electrode (cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
), a conductive fluid (electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
), and a conductive workpiece (anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
); however, in ECM there is no tool wear. The ECM cutting tool is guided along the desired path close to the work but without touching the piece. Unlike EDM, however, no sparks are created. High metal removal rates are possible with ECM, with no thermal or mechanical stresses being transferred to the part, and mirror surface finishes can be achieved.
In the ECM process, a cathode (tool) is advanced into an anode (workpiece). The pressurized electrolyte is injected at a set temperature to the area being cut. The feed rate is the same as the rate of "liquefication" of the material. The gap between the tool and the workpiece varies within 80-800 micrometers (.003 in. and .030 in.) As electrons cross the gap, material from the workpiece is dissolved, as the tool forms the desired shape in the workpiece. The electrolytic fluid carries away the metal hydroxide formed in the process.
As far back as 1929, an experimental ECM process was developed by W.Gussef, although it was 1959 before a commercial process was established by the Anocut Engineering Company. B.R. and J.I. Lazarenko are also credited with proposing the use of electrolysis for metal removal.
Much research was done in the 1960s and 1970s, particularly in the gas turbine industry. The rise of EDM in the same period slowed ECM research in the west, although work continued behind the Iron Curtain
Iron Curtain
The concept of the Iron Curtain symbolized the ideological fighting and physical boundary dividing Europe into two separate areas from the end of World War II in 1945 until the end of the Cold War in 1989...
. The original problems of poor dimensional accuracy and environmentally polluting waste have largely been overcome, although the process remains a niche technique.
The ECM process is most widely used to produce complicated shapes such as turbine
Turbine
A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and...
blades with good surface finish in difficult to machine materials. It is also widely and effectively used as a deburring process.
In deburring, ECM removes metal projections left from the machining process, and so dulls sharp edges. This process is fast and often more convenient than the conventional methods of deburring by hand or nontraditional machining processes.
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 ET 3000(Russia) http://commons.wikimedia.org/wiki/File:ET_3000_PECM_IND.LLC_(Russia,_Ufa).jpg http://commons.wikimedia.org/wiki/File:ET3000.png.jpg){kind=link}
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 are listed below:- It can be used for Die-Sinking operations.
- Drilling a jet engine turbine blade.
- Multiple Hole drilling.
- Steam turbine blades can be machined within close limits.
Similarities between EDM and ECM
1. The tool and Workpiece are separated by a very small gap, i.e. no contact in between them is made.2. The tool must be a conductor of electricity.
3. Needs high capital investment.
4. These systems consume lots of power.
5. Only electrically-conductive material can be machined.
6. A fluid is used as a medium between the tool and the work piece (conductive for ECM and dielectric for EDM).
7. 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).