Two types of abrasives available as natural and artificial form. Some examples for natural abrasives are sandstone, flint, corundum, quartz, pumice, emery, garnet and diamond. Artificial abrasives are silicon carbide and calcium carbide. These products produced from electro-thermal industry. Each abrasive product have a unique feature and used for special tool works.
Natural types of abrasives:
- Diamonds: It is the best hardest material occurs in nature. It is stand on top in abrasive power. It is used in making drill points. To drill a rock or to cut, polish these diamonds are used in the drill tips and polishing wheels. Grinding wheels are made with diamond tips are used for cutting the rock into slices.
- Corundum: It is made of aluminium oxide with chemical formula Al2O3 that exist in crystalline form. Heavy abrasive wheel used in metal industry are made from corundum.
- Emery: It has high scratch hardness. It has a character of consistent break down of it crystal structure under pressure. It is perfect mixture of black magnetite (Fe2O3) and corundum.
- Granet: It occupy at 6.5 to 7.5 reading on Mohr’s scale of hardness. It is vastly applied in making of sandpaper used for surface smoothing and wood polishing. It composition is mainly of silicate minerals.
- Pumice: In lithography, it is widely used in polishing and cleaning stones. It is a composition made of silicates of aluminium, sodium and potassium. It has pale grey texture. Porous blocks are the main source for pumice.
- Quartz: It stands seventh position in Mohr’s hardness scale. It is applied for sand paper making. It is quite brittle in nature due to the shell like structure of silica crystal.
- Sand stone: It is used in making of sharpening stones, pulp stones and grindstones.
- Flint: It is widely used as mill grinding stones. Due to its conchoidal fracture, it appears in light colour. It is a modified from silica.
- Diatomaceous earth and Tripoli: Diatomite is obtained from diatoms a unicellular organism, which have siliceous skeleton. This siliceous skeleton is deposited over time and form diatomaceous earth. Similar to this is a Tripoli but difference in the appearance. It has property that it sinks in water at once but diatomite floats for some time and then sinks. These two products are starting materials for make polishing cream used for metals.
Artificial or synthetic types of abrasives:
- Silicon carbide: It is manufacture in an electrical furnace with raw materials of coke, sand and sawdust.
- Fused alumina or alundum (Al2O3): It is made by arc resistance furnace from bauxite ore.
- Calcium carbide (CaC2): It is manufacture in an electric furnace at temperature ranging from 2000-2200 degree centigrade. Carbon and quick lime are used as raw materials.
- Boron carbide (B4C): It is also called as norbide. It is manufactured in resistance furnace at 2600 degree centigrade. Boric oxide and coke are used as raw materials.
Types of Abrasives Nature and Physical Qualities:
Abrasive power is the properties of a material that can with stand and do the function like rubbing the surface of other objects. It should get damage during such kind of operation. An abrasive must be able to cut down an object to a designed shape and size. Abrasive power is a function of three main factors of refractoriness, toughness and hardness.
In metalwork shops, tasks like cutting a metal with other harder substance are conducted. Abrasive used as that hard substance for scratch works. Abrasive substances are selected based on the use and measured with a comparing scale depending on the property of hardness. Talc, Gypsum, Calcite, Fluorite, Apatite, Feldspar, Quartz, Topaz, Corundum and finally Diamond are used as comparison of abrasive power. This is called as Mohr Scale. The material like diamond has capacity to scratch corundum and similarly corundum has capacity to scratch topaz and so on.
Two other properties of abrasives are brittleness and refractoriness.
- Refractoriness property of an abrasive gives the withstanding capacity of it grain when operation temperature is increased so that it does not get scattered.
- Sharp nature of an abrasive is known by its brittle and friable property. Abrasive substance can do alteration or remove material from other objects when it is quite sharp.
Fluidized abrasive application in finishing process
Centrifugal disk finishing is one of the advances finishing technology, where a fluidized abrasive is used to complete the task. Dimensions of the subjects are well obtained by this type of finishing and due to size of the abrasive granulates high accuracy is achieved.
Important cutting methods:
- Saw cut method is regular and common to cut with less hardness
- Milling is complex it is motor driven
- Submerged plasma, Plasma, Laser are high energy precision metal trimming
- Wire electric discharge machining heavy and thick metal finishing
- Abrasive water jet used for all smooth and hard materials
- Ultrasonic machining
- Electrochemical machining
- Chemical machining is famous for chip and electrochemical finishing
- Electro beam machining
The cutting material or abrasive selection depends on its chemical microstructure and hardness of the tool. Cutting process mainly concerns about the rough of the material surface.