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grinding [grahynd]

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grinding

[grahynd]

grinding, process by which surface material is removed from an object, usually metal, by the abrasive action of a rotating wheel or a moving belt that contains abrasive grains. A grinding wheel can be made by mixing a bonding material, usually clay, with abrasive grains of such substances as silicon carbide or aluminum oxide. The mixture is then shaped into a wheel and hardened. A grindstone is a grinding wheel made by shaping naturally occurring sandstone, which contains abrasive quartz grains. Grinding is used in many manufacturing processes to produce a fine surface finish on an object and to bring the size of an object to within very fine tolerances. A grinding machine has devices that hold an unfinished object and move it past the machine's abrasive wheel or belt, which is driven by a motor. For less exacting work, such as sharpening cutting tools, objects can be hand held and ground by a machine consisting mainly of an abrasive wheel or belt. For many products grinding is only one step in a finishing process that involves additional similar operations such as honing, lapping, polishing, and buffing.

The Columbia Electronic Encyclopedia Copyright © 2004.
Licensed from Columbia University Press

grinding machine

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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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

Mill (grinding)

A grinding mill is a unit operation designed to break a solid material into smaller pieces. There are many different types of grinding mills and many types of materials processed in them. Historically mills were powered by hand (mortar and pestle), working animal, wind (windmill) or water (watermill). Today they are also powered by electricity.

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:

magnification of the surface area of a solid
manufacturing of a solid with a desired grain size
pulping of resources

Grinding laws

In spite of a great number of studies in the field of fracture schemes there is no formula known which connects the technical grinding work with grinding results. To calculate the needed grinding work against the grain size changing three half-empirical models are used:

KICK for d > 50 mm

W_K=c_k*(ln (d_A) - ln (d_E)),

BOND for 50 mm > d > 0.05 mm

W_B= c_B*left(sqrtfrac{1}{d_E} - sqrtfrac{1}{d_A}right),

RITTINGER for d < 0.05 mm

W_R=c_R*left(frac{1}{d_E} - frac{1}{d_A}right),

with W as grinding work in kJ/kg, c as grinding coefficient, d_A as grain size of the source material and d_E as grain size of the ground material.

A reliable value for the grain sizes d_A and d_E 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

$c_K=1.151*c_B*(d_{BU})^{-0.5},$

$c_R=0.5*c_B*(d_{BL})^{0.5},$

with the limits of BOND's range: upper d_BU = 50 mm and lower d_BL = 0.05 mm.

Grinding degree

To evaluate the grinding results the grain size disposition of the source material (1) and of the ground material (2) is needed. Grinding degree is the ratio of the sizes from the grain disposition. There are several definitions for this characteristic value:

Grinding degree referring to grain size d₈₀

Z_d=frac{d_{80,1}}{d_{80,2}},

Instead of the value of d₈₀ also d₅₀ or other grain diameter can be used.

Grinding degree referring to specific surface

Z_S=frac{S_{v,2}}{S_{v,1}}=frac{S_{m,2}}{S_{m,1}},

The specific surface area referring to volume S_v and the specific surface area referring to mass S_m can be found out through experiments.

Pretended grinding degree

Z_a=frac{d_1}{a},

The discharge die gap a of the grinding machine is used for the ground solid matter in this formula.

Grinding machines

In materials processing a grinder is a machine for producing fine particle size reduction through attrition and compressive forces at the grain size level. See also crusher for mechanisms producing larger particles.

Ball mill

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.

Rod mill

A rotating drum causes friction and attrition between steel rods and ore particles. But note that the term 'rod mill' is also used as a synonym for a slitting mill, which makes rods of iron or other metal.

SAG mill

SAG is an acronym for Semi-Autogenous Grinding, and applies to mills that utilize steel balls in addition to large rocks for grinding. The SAG mills use a minimal ball charge of 6 to 15%.

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.

Autogenous mill

A rotating drum throws large rocks in a cataracting motion which causes impact breakage of larger rocks and compressive grinding of finer particles. It is similar in operation to a SAG mill as described above but does not use steel balls in the mill. Attrition in the charge causes grinding of finer particles. Also known as ROM or "Run Of Mine" grinding.

Pebble mill

A rotating drum causes friction and attrition between rock pebbles and ore particles. May be used where product contamination by iron from steel balls must be avoided.

High pressure grinding rolls

The ore is fed between two rollers which are pushed firmly together while their rotating motion pushes the ore through a small gap between them. Extreme pressure causes the rocks to fracture into finer particles and also causes microfracturing at the grain size level.

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.