Machine tool that uses a rotating abrasive grinding wheel to change the shape or dimensions of a hard, usually metallic, workpiece. Grinding is the most accurate of all the basic machining processes. All grinding machines use a wheel made from one of the manufactured abrasives, silicon carbide or aluminum oxide. To grind a cylindrical form, the workpiece rotates as it is fed against the grinding wheel. To grind an internal surface, a small wheel moves inside the hollow of the workpiece, which is gripped in a rotating chuck. On a surface grinder, the workpiece is held in place on a table that moves under the rotating abrasive wheel.
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The grinding of solid matters occurs under exposure of mechanical forces that trench the structure by overcoming of the interior bonding forces. After the grinding the state of the solid is changed: the grain size, the grain size disposition and the grain shape.
Grinding may serve the following purposes in engineering:
with W as grinding work in kJ/kg, c as grinding coefficient, dA as grain size of the source material and dE as grain size of the ground material.
A reliable value for the grain sizes dA and dE is d80. This value signifies that 80% (mass) of the solid matter has a smaller grain size. The BOND's grinding coefficient for different materials can be found in various literature. To calculate the KICK's and RITTINGER's coefficients following formulas can be used
with the limits of BOND's range: upper dBU = 50 mm and lower dBL = 0.05 mm.
A typical type of fine grinder is the ball mill. A slightly inclined or horizontal rotating cylinder is partially filled with balls, usually stone or metal, which grinds material to the necessary fineness by friction and impact with the tumbling balls. The feed is at one end of the cylinder and the discharge is at the other. Ball mills are commonly used in the manufacture of Portland cement.
A rotating drum throws large rocks and steel balls in a cataracting motion which causes impact breakage of larger rocks and compressive grinding of finer particles. Attrition in the charge causes grinding of finer particles. SAG mills are characterized by their large diameter and short length. The inside of the mill is lined with lifting plates to lift the material inside up and around the inside of the mill, where it then falls off the plates and falls back down.
SAG mills are primarily used in the gold, copper and platinum industries with applications also in the lead, zinc, silver, alumina and nickel industries.
It consists of a pair of vertical cylindrical rollers through which material is passed. The two rollers rotate in opposite directions, "nipping" and crushing material between them. A similar type of intermediate crusher is the edge runner, which consists of a circular pan with two or more heavy wheels known as mullers rotating within it; material to be crushed is shoved underneath the wheels using attached plow blades.