Electrochemical grinding
Encyclopedia
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. Materials removed from the workpiece stay in the electrolyte fluid. Electrochemical grinding and electrochemical machining are similar but a wheel is used instead of a tool shaped like the contour of the workpiece.
, 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 carbide
s or steels harder than Rockwell C65.
An electrolytic spindle with carbon brushes, acting as a commutator, hold 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.
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
abrasive grinding wheel, an 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....
fluid, and a positively charged workpiece. Materials removed from the workpiece stay in the electrolyte fluid. Electrochemical grinding and electrochemical machining are similar but a wheel is used instead of a tool shaped like the contour of the workpiece.
Process characteristics
- The wheels and workpiece are electrically conductive.
- Wheels used last for many grindings - typically 90% of the metal is removed by electrolysisElectrolysisIn chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...
and 10% from the abrasive grinding wheel. - Capable of producing smooth edges without the burrs caused by mechanical grinding.
- Does not produce appreciable heat that would distort workpiece.
- Decomposes the workpiece and deposits them into the electrolyte solution. The most common electrolytes are sodium chlorideSodium chlorideSodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...
and sodium nitrateSodium nitrateSodium nitrate is the chemical compound with the formula NaNO3. This salt, also known as Chile saltpeter or Peru saltpeter to distinguish it from ordinary saltpeter, potassium nitrate, is a white solid which is very soluble in water...
at concentrations of 2 lbs per gallon.
Process
The wheels are metal disks embedded with abrasive particles. CopperCopper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
, brass, and nickel are the most commonly used materials; aluminum oxide
Aluminium oxide
Aluminium oxide is an amphoteric oxide with the chemical formula 23. It is commonly referred to as alumina, or corundum in its crystalline form, as well as many other names, reflecting its widespread occurrence in nature and industry...
is typically used as an abrasive when grinding steel. A thin layer of diamond particles will be used when grinding carbide
Carbide
In chemistry, a carbide is a compound composed of carbon and a less electronegative element. Carbides can be generally classified by chemical bonding type as follows: salt-like, covalent compounds, interstitial compounds, and "intermediate" transition metal carbides...
s or steels harder than Rockwell C65.
An electrolytic spindle with carbon brushes, acting as a commutator, hold 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.
Tolerance
- This kind of grinding is mostly used because it can shape very hard metals and also because it is a chemical reducing process, the wheel lasts a longer time than normal grinding wheel can.
- This type of grinding has different types of wheels so it can shape metals to whatever they need to be shaped to.
- Produces a smoother, burr-free surface and causes less surface stress than other grinding methods.
Uses
- Production of tungstenTungstenTungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
carbide cutting tools. - Burr-free sharpening of hypodermic needleHypodermic needleA hypodermic needle is a hollow needle commonly used with a syringe to inject substances into the body or extract fluids from it...
s - Grinding of superalloySuperalloyA superalloy, or high-performance alloy, is an alloy that exhibits excellent mechanical strength and creep resistance at high temperatures, good surface stability, and corrosion and oxidation resistance. Superalloys typically have a matrix with an austenitic face-centered cubic crystal structure. ...
turbine blades - Form grinding of aerospace honeycomb metals
- Removal of fatigue cracksFatigue (material)'In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.Fatigue occurs...
from underwater steel structures. In this case, seawaterSeawaterSeawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% . This means that every kilogram of seawater has approximately of dissolved salts . The average density of seawater at the ocean surface is 1.025 g/ml...
itself acts as the electrolyte. Diamond particles in the grinding wheel remove any non-conducting organic matter, such as algaeAlgaeAlgae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
, before electrochemical grinding begins.