06-12-2012, 02:49 PM
Abrasive Machining
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Abrasive Machining Processes
Abrasive machining is one of the oldest forms of metal removal. It is also one of the most important. Abrasive machining can produce surface finishes ranging from rough to extremely fine.
Abrasive machining is a process where chips are formed by small cutting edges on abrasive particles.
Applications
Manufactured products which are candidates for abrasive machining include:
Hard to machine metals such as super alloys or heat treated tool steels
Parts which have closes tolerances
Size (+/- .0005)
Parallelism
Flatness
Sheet type products such as:
Plywood, thin stainless steels, and decorative plaque material
Process Characteristics
Abrasive machining has two unique characteristics:
“Small” cutting edges cut simultaneously.
The abrasives used are very hard and capable of cutting materials in the range of Rc45 and above
Abrasive Grain Size
Grains are separated by mechanical sieving machines. The number of openings per linear inch in the sieve(or screen) through which the particles can pass determines the grain size.
Typical classifications:
Course, medium, and fine
Silicon Carbides range from 2-240 in size
Aluminum Oxides range from 4-240 in size
600 range are generally used in honing or lapping operations
The Grinding Process
One process, which utilizes abrasive machining, is the grinding process.
A machining process where the abrasives used are bonded together into a wheel. The grinding wheel is the cutting tool in this process. In grinding, high precision and tight tolerances are possible. Typical tolerances associated with this process are +/- .0001.” Product examples include bearings, machining fixtures, shafts, and precision measurement gauges.
Chip Formation
Chips in this process are formed by the same mechanism of compression and shear as other machining processes.
As the grains or abrasives become dull, the cutting forces increase. The increase in the cutting force causes the grains to plow and rub rather than cut. As the plowing and rubbing increases, the grains fracture at the cutting edge to revile a new cutting edge.
The Tool: Grinding Wheels
The spacing of particles with respect to one another is called the structure of the wheel. Wheel structure can be open, medium, or dense.
“Dense structure is used for hard materials, for high speed grinding operations, and also for producing fine finishes and tight tolerances.”
“Open structure is used for high contact area, grinding of tough materials, and high stock removal rates.”
Hardness - resistance to penetration
Toughness - ability of a metal to absorb energy without failure.
Wheel Bonding Agents
Vitrified - composed of clay and other ceramic substances. Abrasive particles are mixed with wheel material then pressed together and fired in a kiln.
Resiniod or phonolic - Plastic compound wheels designed for a wide variety of applications. This is a bit more flexible than other wheels.
Silicate - This bond uses silicate of soda as the bonding agent. Wheels are formed then baked at 500F for a day or longer. They are not as strong as vitrified-bonded wheels. Because they are not as strong, abrasive grains fracture more easily. This results in lower operating temperatures which can be a positive factor depending on the application of the wheel.
Grinding Machines
Grinding operation on all machines can be accomplished in three ways:
In-feed - moving the wheel into the work
Cross feed - traversing the wheel across the work
Plunge feed - wheel is forced in on the radius of the work. This is similar to form turning on a lathe.