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Level 41

25 - GRINDING AND OTHER ABRASIVE PROCESSES


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Importance of abrasive processes
Important reasons include (1) applications on all types of materials, (2) very fine finishes, and (3) close tolerances.
Grinding wheel parameters
The parameters are (1) abrasive material, (2) grit size, (3) bonding material, (4) wheel structure, which refers to the relative spacing of grains, and (5) wheel grade, which refers to the bond strength of the wheel in retaining abrasive grains.
Abrasive materials
The principal abrasive grit materials include (1) aluminum oxide, (2) silicon carbide, (3) cubic boron nitride, and (4) diamond.
Wheel bonding materials
The bonding materials in grinding wheels are (1) vitrified bond - clay and ceramics, (2) silicate, (3) rubber, (4) resinoid, (5) shellac, and (6) metallic.
Wheel structure
Wheel structure indicates the relative spacing of the abrasive grains in the wheel. An open structure is one in which the grains are far apart, and a dense structure indicates that the grains are close together.
Wheel grade
Wheel grade refers to the wheel's ability to retain abrasive grains during cutting. It indicates the bond strength of the bonding material used to shape the wheel. A soft grade indicates that the grains are released easily from the bonding material. A hard wheel is one which retains the abrasive grains.
High energy in grinding
Reasons for higher specific energy in grinding include: (1) size effect - smaller chip size means higher specific energy; (2) extremely negative rake angles on the abrasive particles in a grinding wheel; and (3) not all of the grains in the wheel surface are engaged in cutting; some are plowing or deforming the surface while others are simply rubbing and creating friction at the surface of the work.
Temperature damage in grinding
High temperatures in grinding create surface burns and cracks. High temperatures can also soften the surfaces of workparts that have been heat treated for high hardness.
Mechanisms of grinding wheel wear
The mechanisms are (1) grain fracture, in which a portion of the grain breaks off during cutting; (2) attritious wear, in which the grains become dull during cutting; and (3) bond fracture, in which the grains are pulled out of the bonding material.
Dressing
Dressing is a procedure applied to worn grinding wheels to break off dull grits and expose fresh grits, and to remove chips of work material that have become clogged in the wheel. It uses a rotating disk or abrasive stick held against the wheel while it rotates.
Truing
Truing is similar to dressing, but it also restores the ideal cylindrical shape to the wheel. It uses a diamond-pointed tool fed slowly and precisely across the wheel while it rotates.
Grinding cemented carbides
Choose a diamond wheel for grinding cemented carbides.
Grinding steel and cast irons
For grinding steel and most cast irons select aluminum oxide.
Grinding nonferous metals
For grinding nonferous metals select silicon carbide
Grinding hardened steels
For grinding hardened tool steels and aerospace alloys select cubic boron nitride.
Grinding very hard materials
For grinding hard abrasive materials such as ceramics, cemented carbides, and glass select diamond as the abrasive.
Grinding soft metals.
Select a large grit size and harder grade wheel.
Grinding hard metals.
Select a small grit size and softer grade wheel.
How to optimize surface finish?
Select a small grit size and dense wheel structure. Use high wheel speeds and lower work speeds.
How to maximize material removal rate.
Select a large grit size, more open wheel structure, and vitrified bond.
How to minimize heat damage, cracking, and warping of the work surface?
Maintain sharpness of the wheel. Dress the wheel frequently. Use lighter depths of cut, lower wheel speeds, and faster work speeds.
What to do if the grinding wheel glazes and burns?
Select wheel with a soft grade and open structure.
What to do if the grinding wheel glazes and burns?
Select wheel with a soft grade and open structure.
What to do if the grinding wheel breaks down too rapidly
Select wheel with a hard grade and dense structure.
Grinding fluid
Functions of a grinding fluid include (1) reducing friction, (2) removing heat, (3) washing away chips, and (4) reducing workpiece temperature.
Centerless grinding
Centerless grinding is a grinding operation in which cylindrical workparts (e.g., rods) are fed between two rotating wheels: (1) a high speed grinding wheel and (2) a low speed regulating wheel which is tilted at a slight angle to control the feed-through rate.
Creep feed grinding
In creep feed grinding, the depth of cut is very high - several thousand times higher than conventional grinding - and the feed rates are lower by about the same proportion.
Abrasive belt grinding
Instead of a grinding wheel, abrasive belt grinding uses abrasive particles bonded to a flexible cloth belt loop which is moved around a pulley system to obtain the speed motion. Parts are pressed against the belt to accomplish grinding.
Wheel ring test
A wheel ring test is performed by suspending the wheel and lightly striking it with a solid, non-metal object, similar to striking a bell. The wheel should ring a clear long tone. If it has cracks, it will not ring properly.
Dressing grinding wheel
Two purposes of dressing a grinding wheel are (1) to renew the wheel surface by fracturing abrasive particles and (2) to remove tiny pieces of embedded workpiece material.
Coolant in grinding
The purpose of using coolant in the grinding process are threefold: (1) to reduce grinding power required, (2) to maintain work quality, and (3) to stabilize part dimensions over long production runs.
What machining process is closest to grinding?
Milling is the closest process to grinding, since the multiple sharp particles can be likened to a mills teeth, each tooth removing its portion of work material.
High surface finish abrasive operations
High finish abrasive processes include honing, lapping, superfinishing, buffing, and polishing.
Honing
Honing is an abrasive process performed by a set of bonded abrasive sticks. A common application is to finish the bores of internal combustion engines. Other applications include bearings, hydraulic cylinders, and gun barrels. Surface finishes of around 0.12 micro meters. n addition, honing produces a characteristic cross-hatched surface that tends to retain lubrication during operation of the component, thus contributing to its function and service life.
Lapping
Lapping is an abrasive process used to produce surface finishes of extreme accuracy and smoothness. It is used in the production of optical lenses, metallic bearing surfaces, gages, and other parts requiring very good finishes. Metal parts that are subject to fatigue loading or surfaces that must be used to establish a seal with a mating part are often lapped. Instead of a bonded abrasive tool, lapping uses a fluid suspension of very small abrasive particles between the workpiece and the lapping tool.
Superfinishing
Superfinishing is an abrasive process similar to honing. Both processes use a bonded abrasive stick moved with a reciprocating motion and pressed against the surface to be finished.
Polishing
Polishing is used to remove scratches and burrs and to smooth rough surfaces by means of abrasive grains attached to a polishing wheel rotating at high speed—around 2300 meters per minute. The wheels are made of canvas,leather,felt, and even paper; thus, the wheels are somewhat flexible. The abrasive grains are glued to the out side periphery of the wheel.
Buffing
Buffing is similar to polishing in appearance, but its function is different. Buffing is used to provide attractive surfaces with high luster. Buffing wheels are made of materials similar to those used for polishing wheels—leather, felt, cotton, etc.—but buffing wheels are generally softer. The abrasives are very fine and are contained in a buffing compound that is pressed into the outside surface of the wheel while it rotates.