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Electrochemical grinding is a process that removes electrically conductive material by grinding with a negatively charged abrasive grinding wheel, an electrolyte fluid, and a positively charged workpiece. [1] Materials removed from the workpiece stay in the electrolyte fluid. Electrochemical grinding is similar to electrochemical machining but uses a wheel instead of a tool shaped like the contour of the workpiece.
The electrochemical grinding process combines traditional electrochemical machining and grinding processes to remove material from a workpiece. A grinding wheel is used as a cutting tool as a cathode and the workpiece is an anode. During the process, electrolytic fluid, typically sodium nitrate, [5] is pumped into the space between the workpiece and the grinding wheel. Other electrolytes used include sodium hydroxide, sodium carbonate, and sodium chloride. [6] This electrolytic fluid will cause electrochemical reactions to occur at the workpiece surface which oxidize the surface, thereby removing material. As a consequence of the oxidation which occurs, layers of oxide films will form on the workpiece surface, and these need to be removed by the grinding wheel. A couple schematics of the process are provided below.
Abrasive materials, either diamond or aluminum oxide, [5] are bonded to the grinding wheel, which allows the wheel to remove the oxide layers on the workpiece surface by abrasive action. Appropriate materials used for electrolyte fluid and the grinding wheel abrasives are summarized in the table below.
| Component | Material |
|---|---|
| Grinding Wheel Abrasives | Diamond, Aluminum Oxide [5] |
| Electrolytic Fluid | Sodium Nitrate, Sodium Hydroxide, |
Most material removal is by the electrochemical reactions which occur at the workpiece surface. Five percent or less of the material removal is carried out by the abrasive action of the grinding wheel. [5] The fact that most material is not removed by abrasive action helps increase the life of the grinding wheel; that is, the tool will take a long time to wear down. The electrolytic fluid serves another useful purpose - it flushes out leftover material in between the grinding wheel and workpiece. The abrasive particles bonded to the grinding wheel will help to electrically insulate the space between the grinding wheel and workpiece. [6] [7] An equation giving the material removal rate for an electrochemical grinding process is provided in [5] and is stated here as:
MRR = GI/ρF [5]
where ρ is the workpiece density, G is the total mass of the workpiece, I is the current supplied, MRR is the material removal rate, and F is Faraday's constant. [5]
Some of the main factors which govern the performance of an electrochemical grinding process include current supplied, rotation speed of the grinding wheel, the workpiece feed rate, the type of electrolyte used, electrolyte feed rate, and the workpiece's chemical properties. [6] [8] By changing these parameters, one can alter the material removal rate. Increasing the supplied current, rotation speed of the wheel, electrolyte feed rate, or the workpiece feed rate will lead to an increase in material removal rate (MRR), while decreasing these properties will do the opposite. If the workpiece is more reactive to the electrolyte used, then the material removal rate will increase. The grinding wheel is usually rotated with a surface speed of 1200–2000 m/min [5] and supplied currents are around 1000A. [6]
The accuracy of parts made by electrochemical grinding is strongly dictated by the chemical properties of the workpiece and electrolytic fluid used. If the workpiece is very reactive to the electrolyte, and if too much electrolyte is pumped into the space between the grinding wheel and workpiece, it may be difficult to control the material removal, which can lead to loss of accuracy. [6] Also, accuracy may be reduced if the workpiece feed rate is too high.
The wheels are metal disks with abrasive particles embedded. Copper, [4] brass, and nickel are the most commonly used materials; aluminum oxide is typically used as an abrasive when grinding steel. A thin layer of diamond particles will be used when grinding carbides or steels harder than 65 Rc.
An electrolytic spindle with carbon brushes, acting as a commutator, holds the wheel. The spindle receives a negative charge from the DC power supply, which gives the workpiece a positive charge. The electrolytic fluid is applied where the work contacts the tool by a nozzle similar to that which supplies coolant in conventional grinding. The fluid works with the wheel to form electrochemical cells that oxidize the surface of the workpiece. As the wheel carries away the oxide, fresh metal is exposed. Removing the oxidized fluid may only require a pressure of 20 psi or less, causing much less distortion than mechanical grinding. The wheel is subject to little wear, reducing the need for truing and dressing. [4]
Electrochemical grinding is often used for hard materials where conventional machining is difficult and time-consuming, such as stainless steel and some exotic metals. For materials with hardness greater than 65 HRC, ECG can have a material removal rate 10 times that of conventional machining. Because ECG involves little abrasion, it is often used for processes where the surface of the part is needs to be free of burrs, scratches, and residual stresses. Because of these properties, electrochemical grinding has a number of useful applications.
One of the key advantages of electrochemical grinding is the minimal wear that the grinding wheel tool experiences. This is because the majority of the material is removed by the electrochemical reaction that occurs between the cathode and anode. The only time that abrasive grinding actually occurs is in removing the film that develops on the surface of the workpiece. Another advantage of electrochemical grinding is that it can be used to machine hard materials. Hard materials pose a difficulty to other types of machining due to the tool wear that is associated with machining hard materials. [5] It may come as a bit of a surprise that electrochemical grinding can remove material from a hard surface and experience minimal wear. Because most material is removed through electrochemical reactions, the workpiece does not experience heat damage like it would in a conventional grinding process. [10]
Electrochemical grinding also has a few disadvantages as well. The system consists of the anode workpiece and the cathode grinding wheel. In order to create those conditions both the workpiece and the grinding wheel must be conductive. This limits the types of workpiece materials that are suitable for electrochemical grinding. Another disadvantage of electrochemical grinding is that it is only applicable to surface grinding. It is not possible to apply electrochemical grinding to workpieces that have cavities, due to the grinding wheels inability to remove the film deposit with in the cavity. [5] One other disadvantage is the electrolytic fluid can cause corrosion at the workpiece and grinding wheel surfaces. [10] Lastly, electrochemical grinding is more complicated than traditional machining methods. This will require more experienced personnel to operate the machinery, which will lead to higher production cost. Another disadvantage is that chemical used during grinding process need to be properly disposed of depending on the environmental regulation.
Electrical discharge machining (EDM), also known as spark machining, spark eroding, die sinking, wire burning or wire erosion, is a metal fabrication process whereby a desired shape is obtained by using electrical discharges (sparks). Material is removed from the work piece by a series of rapidly recurring current discharges between two electrodes, separated by a dielectric liquid and subject to an electric voltage. One of the electrodes is called the tool-electrode, or simply the tool or electrode, while the other is called the workpiece-electrode, or work piece. The process depends upon the tool and work piece not making physical contact.
Metalworking is the process of shaping and reshaping metals to create useful objects, parts, assemblies, and large scale structures. As a term it covers a wide and diverse range of processes, skills, and tools for producing objects on every scale: from huge ships, buildings, and bridges down to precise engine parts and delicate jewelry.
An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away by friction. While finishing a material often means polishing it to gain a smooth, reflective surface, the process can also involve roughening as in satin, matte or beaded finishes. In short, the ceramics which are used to cut, grind and polish other softer materials are known as abrasives.
Machining is a process in which a material is cut to a desired final shape and size by a controlled material-removal process. The processes that have this common theme are collectively called subtractive manufacturing, which utilizes machine tools, in contrast to additive manufacturing, which uses controlled addition of material.
A grinding machine, often shortened to grinder, is a power tool used for grinding. It is a type of machining using an abrasive wheel as the cutting tool. Each grain of abrasive on the wheel's surface cuts a small chip from the workpiece via shear deformation.
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 aluminides, Inconel, Waspaloy, and high nickel, cobalt, and rhenium alloys. Both external and internal geometries can be machined.
Grinding wheels contains abrasive compounds for grinding and abrasive machining operations. Such wheels are also used in grinding machines.
A grinding dresser or wheel dresser is a tool to dress the surface of a grinding wheel. Grinding dressers are used to return a wheel to its original round shape, to expose fresh grains for renewed cutting action, or to make a different profile on the wheel's edge. Utilizing pre-determined dressing parameters will allow the wheel to be conditioned for optimum grinding performance while truing and restoring the form simultaneously.
Surface finishing is a broad range of industrial processes that alter the surface of a manufactured item to achieve a certain property. Finishing processes may be employed to: improve appearance, adhesion or wettability, solderability, corrosion resistance, tarnish resistance, chemical resistance, wear resistance, hardness, modify electrical conductivity, remove burrs and other surface flaws, and control the surface friction. In limited cases some of these techniques can be used to restore original dimensions to salvage or repair an item. An unfinished surface is often called mill finish.
A burr is a raised edge or small piece of material that remains attached to a workpiece after a modification process.
Superfinishing, also known as micromachining, microfinishing, and short-stroke honing, is a metalworking process that improves surface finish and workpiece geometry. This is achieved by removing just the thin amorphous surface layer left by the last process with an abrasive stone or tape; this layer is usually about 1 μm in magnitude. Superfinishing, unlike polishing which produces a mirror finish, creates a cross-hatch pattern on the workpiece.
Polishing and buffing are finishing processes for smoothing a workpiece's surface using an abrasive and a work wheel or a leather strop. Technically, polishing refers to processes that uses an abrasive that is glued to the work wheel, while buffing uses a loose abrasive applied to the work wheel. Polishing is a more aggressive process, while buffing is less harsh, which leads to a smoother, brighter finish. A common misconception is that a polished surface has a mirror-bright finish, however, most mirror-bright finishes are actually buffed.
A diamond tool is a cutting tool with diamond grains fixed on the functional parts of the tool via a bonding material or another method. As diamond is a superhard material, diamond tools have many advantages as compared with tools made with common abrasives such as corundum and silicon carbide.
Abrasive flow machining (AFM), also known as abrasive flow deburring or extrude honing, is an interior surface finishing process characterized by flowing an abrasive-laden fluid through a workpiece. This fluid is typically very viscous, having the consistency of putty, or dough. AFM smooths and finishes rough surfaces, and is specifically used to remove burrs, polish surfaces, form radii, and even remove material. The nature of AFM makes it ideal for interior surfaces, slots, holes, cavities, and other areas that may be difficult to reach with other polishing or grinding processes. Due to its low material removal rate, AFM is not typically used for large stock-removal operations, although it can be.
Ultrasonic machining is a subtractive manufacturing process that removes material from the surface of a part through high frequency, low amplitude vibrations of a tool against the material surface in the presence of fine abrasive particles. The tool travels vertically or orthogonal to the surface of the part at amplitudes of 0.05 to 0.125 mm. The fine abrasive grains are mixed with water to form a slurry that is distributed across the part and the tip of the tool. Typical grain sizes of the abrasive material range from 100 to 1000, where smaller grains produce smoother surface finishes.
Grinding is a type of abrasive machining process which uses a grinding wheel as cutting tool.
Honing is an abrasive machining process that produces a precision surface on a metal workpiece by scrubbing an abrasive grinding stone or grinding wheel against it along a controlled path. Honing is primarily used to improve the geometric form of a surface, but can also improve the surface finish.
Surface grinding is done on flat surfaces to produce a smooth finish.
Flat honing is a metalworking grinding process used to provide high quality flat surfaces. It combines the speed of grinding or honing with the precision of lapping. It has also been known under the terms high speed lapping and high precision grinding.
Grinding wheel wear is an important measured factor of grinding in the manufacturing process of engineered parts and tools. Grinding involves the removal process of material and modifying the surface of a workpiece to some desired finish which might otherwise be unachievable through conventional machining processes. The grinding process itself has been compared to machining operations which employ multipoint cutting tools. The abrasive grains which make up the entire geometry of wheel act as independent small cutting tools. The quality, characteristics, and rate of grinding wheel wear can be affected by contributions of the characteristics of the material of the workpiece, the temperature increase of the workpiece, and the rate of wear of the grinding wheel itself. Moderate wear rate allows for more consistent material size. Maintaining stable grinding forces is preferred rather than high wheel wear rate which can decrease the effectiveness of material removal from the workpiece.