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Dr. Salmon prefers to specify concentration by the number of grains along a linear line of the arc of cut. Ideally, he likes to see 4 to 10 grains along any linear line. Concentration numbers may practically range from 30 to 175.
The concentration number represents grain density - 100 concentration means that there are 4.4 carats weight of abrasive per cubic centimeter of the grinding wheel (72 carats per cubic inch.) The relationship is linear, so that a 150 concentration would be 6.6 carats per cubic centimeter (108 carats per cubic inch.) It is essential to have the proper concentration of diamond or cBN in a vitrified or resin bonded superabrasive wheel as the concentration not only impacts the efficiency and effectiveness of the grinding process, but also the wheel life. The concentration determines the number of active grains in the grinding zone. It is important therefore, to calculate the length of the arc of cut and the area of the grinding zone.
Dr. Metzger likes to specify concentration based upon the area of the arc of cut. Ideally, he likes to see a 30 to 50 concentration used for an area larger than 50mm2 (0.075in2,) a 75 concentration for an area between 15mm2 and 50mm2 (0.025in2 and 0.075in2) and a 100 to 150 concentration for areas up to 15mm2 (0.025in2.)
Two equal volumes with equal carat weight, but a different size of abrasive grain. On one face (left) there are about 50 grains whereas on the right there are approximately 3.
Grains per carat for some typical mesh sizes for diamond and cBN.
Mesh
grains/ct 2530 6950 19600 32800 55600 268000 1270000 2140000
46 60 80 100 120 200 325 400
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