tdmidget said:
The "A" in 32A a means aluminum oxide.
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A quick history of aluminum oxide abrasives:
The white aluminum oxide abrasives are pure alumina and do not contain titanium or iron oxide impurities. The grey, brown, blue, and red aluminum oxide abrasives contain iron, titanium, and for the red wheels chromium additions. The abrasives are made by flash cooling molten alumina on a chilled conveyor belt. The flash cooling is required to produce small alumina crystals. The cooled chunks of alumina are crushed to the desired grit size. This process originated in the early 's. It was a major improvement over the original process developed in . The earlier process produced 10 ton blocks of alumina by resistance heating the raw starting materials. The blocks were slow cooled over several days and then broken apart and crushed to the desired grit size. The slow cooling produced large crystals.
A 46 grit abrasive grain will have on average three crystals of alumina. The white alumina grain is not durable and breaks apart with relatively little pressure. Many years ago it was the first choice for dry grinding due to its rapid breakdown and the resulting sharp fractured remaining grain. The pressure that is exerted on the grain increases as the grain surface becomes dull. The
grinding wheel tends to be self sharpening assuming that it also has a low grain bonding strength. This would be roughly a J grade. The grey, brown or blue grinding wheels containing the iron and titanium oxide impurities have a much higher grain strength and require much higher pressures for the grain to fracture. The fractured grain does not have the sharp edges of the white aluminum oxide. The red wheels contain chromium oxide which increases the grain strength slightly while still maintaining the self sharpening characteristic.
There are also alumina-zirconia abrasives which are used in cutoff and snagging wheels. These are very sharp abrasives which erode rapidly.
In the 's Norton developed the 32 A single crystal per grain abrasive. This abrasive is produced by pre selecting the grit size by sorting through the
bauxite starting material by size. A batch of 46 grit size alumina crystals will be selected, for example, and then run through a furnace to fuse the internal fractures within the crystal. The abrasive produced does contain the original iron and titanium impurities of the raw alumina.
The 32A abrasive grain when it becomes dull will fracture along one of the many crystal lattice planes. The grain is self sharpening and there will be a large number of grain fractures before the grain is consumed. The fracturing process requires higher pressures than the white alumina oxide but the grinding wheel will have a much longer life. This abrasive would be the first choice for a general purpose wheel.
The SG or the roughly equivalent 3M cubiton abrasives introduced in the early 's are microcrystalline alumina grains produced by a chemical process. Each grain contains thousands of crystals. The process involves producing a gel which contains the micro crystals. The low strength gel is crushed to produce the desired grain size.
The gel grain is then run through a flash furnace process to sinter the grain. The waste dust from the crushing process can be recycled back into the next batch of gel. It is a energy and material efficient process.
Each microcrystalline grain can be fractured thousands of times before it is consumed. The only difficulty is that it requires a very large pressure to break off a micro crystal.
The solution to the problem is to mix the seeded gel abrasive with the easily fractured 's abrasive. The mixes from Norton are SG at 10%, 3SG at 30% and 5SG at a 50% mix. Grinding application with large wheel contact areas will use the lower concentrations of the seeded gel abrasive.
There are a number of grain geometry variations possible with the seeded gel abrasives. The Norton Targa wheels, for example, use a extruded gel grain that is 10 times as long as it is wide. This produces a very porous light weight wheel that is stronger than a conventional grinding wheel. The wheels can be used at surface speeds significantly higher than the ft/minute of the conventional abrasives.
The seeded gel wheels will still require high wheel pressures to be self sharpening and would not be suitable for some grinding applications.
When the grinding operation is less than ideal the abrasives will not be self sharpening and you will need to use the diamond dresser to pull out the dull grain and expose the next layer of abrasive. This will occur if you are working with a grinding wheel pressure that is too low for the abrasive being used.
A quick history of aluminum oxide abrasives:The white aluminum oxide abrasives are pure alumina and do not contain titanium or iron oxide impurities. The grey, brown, blue, and red aluminum oxide abrasives contain iron, titanium, and for the red wheels chromium additions. The abrasives are made by flash cooling molten alumina on a chilled conveyor belt. The flash cooling is required to produce small alumina crystals. The cooled chunks of alumina are crushed to the desired grit size. This process originated in the early 's. It was a major improvement over the original process developed in . The earlier process produced 10 ton blocks of alumina by resistance heating the raw starting materials. The blocks were slow cooled over several days and then broken apart and crushed to the desired grit size. The slow cooling produced large crystals.A 46 grit abrasive grain will have on average three crystals of alumina. The white alumina grain is not durable and breaks apart with relatively little pressure. Many years ago it was the first choice for dry grinding due to its rapid breakdown and the resulting sharp fractured remaining grain. The pressure that is exerted on the grain increases as the grain surface becomes dull. The grinding wheel tends to be self sharpening assuming that it also has a low grain bonding strength. This would be roughly a J grade. The grey, brown or blue grinding wheels containing the iron and titanium oxide impurities have a much higher grain strength and require much higher pressures for the grain to fracture. The fractured grain does not have the sharp edges of the white aluminum oxide. The red wheels contain chromium oxide which increases the grain strength slightly while still maintaining the self sharpening characteristic.There are also alumina-zirconia abrasives which are used in cutoff and snagging wheels. These are very sharp abrasives which erode rapidly.In the 's Norton developed the 32 A single crystal per grain abrasive. This abrasive is produced by pre selecting the grit size by sorting through thebauxite starting material by size. A batch of 46 grit size alumina crystals will be selected, for example, and then run through a furnace to fuse the internal fractures within the crystal. The abrasive produced does contain the original iron and titanium impurities of the raw alumina.The 32A abrasive grain when it becomes dull will fracture along one of the many crystal lattice planes. The grain is self sharpening and there will be a large number of grain fractures before the grain is consumed. The fracturing process requires higher pressures than the white alumina oxide but the grinding wheel will have a much longer life. This abrasive would be the first choice for a general purpose wheel.The SG or the roughly equivalent 3M cubiton abrasives introduced in the early 's are microcrystalline alumina grains produced by a chemical process. Each grain contains thousands of crystals. The process involves producing a gel which contains the micro crystals. The low strength gel is crushed to produce the desired grain size.The gel grain is then run through a flash furnace process to sinter the grain. The waste dust from the crushing process can be recycled back into the next batch of gel. It is a energy and material efficient process.Each microcrystalline grain can be fractured thousands of times before it is consumed. The only difficulty is that it requires a very large pressure to break off a micro crystal.The solution to the problem is to mix the seeded gel abrasive with the easily fractured 's abrasive. The mixes from Norton are SG at 10%, 3SG at 30% and 5SG at a 50% mix. Grinding application with large wheel contact areas will use the lower concentrations of the seeded gel abrasive.There are a number of grain geometry variations possible with the seeded gel abrasives. The Norton Targa wheels, for example, use a extruded gel grain that is 10 times as long as it is wide. This produces a very porous light weight wheel that is stronger than a conventional grinding wheel. The wheels can be used at surface speeds significantly higher than the ft/minute of the conventional abrasives.The seeded gel wheels will still require high wheel pressures to be self sharpening and would not be suitable for some grinding applications.When the grinding operation is less than ideal the abrasives will not be self sharpening and you will need to use the diamond dresser to pull out the dull grain and expose the next layer of abrasive. This will occur if you are working with a grinding wheel pressure that is too low for the abrasive being used.
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