ABRASIVES,Mild abrasives
Materials used for surfacing and finishing metals, stone, wood, glass, and other materials by abrasive action. The natural abrasives include the diamond, emery, corundum, sand, crushed garnet and quartz, tripoli, and pumice. Artificial abrasives, or manufactured abrasives, are generally superior in uniformity to natural abrasives, and are mostly silicon carbide, aluminum oxide, boron carbide, or boron nitride, marketed under trade names. Artificial diamonds are also now being produced. The massive natural abrasives, such as sandstone, are cut into grinding wheels from the natural block, but most abrasive material is used as grains or built into artificial shapes.
For industrial grinding, artificial abrasives are preferred to natural abrasives because of their greater uniformity. Grading is important because uniform grinding requires grains of the same size. The abrasive grains are used as a grinding powder; are made into wheels, blocks, or stones; or are bonded to paper or cloth. Abrasive cloth is made of cotton jean or drills to close tolerances of yarns and weaves, and the grains are attached with glue or resin. But the Fabricut cloth of 3M is an open-weave fabric with alumina or silicon-carbide grains of 100 to 400 mesh. The open weave permits easy cleaning of the cloth in an air blast. Abrasive paper has the grains, usually aluminum oxide or silicon carbide, glued to one side of 40- to 130-lb kraft paper. The usual grain sizes are No. 16 to No. 500.

Abrasive powder is usually graded in sizes from 8 to 240 mesh. Coarse grain is to 24 mesh; fine grain is 150 to 240. Blasting abrasive for blast cleaning of metal castings is usually coarse grain. Arrowblast, of Norton Co., is aluminum oxide with grain sizes from 16 to 80 mesh. Grinding flour consists of extremely fine grains separated by flotation, usually in grain sizes from 280 to 600 mesh, used for grinding glass and fine polishing. Levigated abrasives are fine powders for final burnishing of metals or for metallographic polishing, usually processed to make them chemically neutral. Green rouge is levigated chromic oxide, and mild polish may be levigated tin oxide; both are used for burnishing soft metals. Polishing powder may be aluminum oxide or metal oxide powders of ultrafine particle size down to 600 mesh. Micria AD, of Monsanto Co., is alumina; Micria ZR is zirconia; and Micria TIS is titania. Gamal, of Fisher Scientific Co., is a fine aluminum oxide powder, the smaller cubes being 59 (xin (1.5 (xm), with smaller particles 20 (xin (0.5 (xm). Cerox is cerium oxide used to polish optical lenses and automobile windshields. It cuts fast and gives a smooth surface. Grinding compounds for valve grinding are usually aluminum oxide in oil.

Mild abrasives, used in silver polishes and window-cleaning compounds, such as chalk and talc, have a Mohs hardness of 1 to 2. The milder abrasives for dental pastes and powders may be precipitated calcium carbonate, tricalcium phosphate, or combinations of sodium metaphosphate and tricalcium phosphate. Abrasives for metal polishes may also be pumice, diatomite, silica flour, tripoli, whiting, kaolin, tin oxide, or fuller’s earth. This type of fine abrasive must be of very uniform grain in order to prevent scratching. Cuttle bone, or cuttlefish bone, is a calcareous powder made from the internal shell of a Mediterranean marine mollusk of the genus Sepia, and it is used as a fine polishing material for jewelry and in tooth powders. Ground glass is regularly marketed as an abrasive for use in scouring compounds and in match-head compositions. Lapping abrasives, for finish grinding of hard materials, are diamond dust or boron carbide powder.
Aluminum oxide wheels are used for grinding materials of high tensile strength. Silicon carbide is harder but is not as strong as aluminum oxide. It is used for grinding metals that have dense grain structure and for stone. Vitrified wheels are made by molding under heat and pressure. They are used for general and precision grinding where the wheel does not exceed a speed of 6,500 surface ft/min (33 m/s). The rigidity gives high precision, and the porosity and strength of bond permit high stock removal. Silicate wheels have a silicate binder and are baked. The silicate bond releases the grains more easily than the vitrified, and is used for grinding edge tools to reduce burning of the tool. Synthetic resins are used for bonding where greater strength is required than is obtained with the silicate, but less openness than with the vitrified. Resinoid bonds are used up to 16,000 surface ft/min (81 m/s), and are used especially for thread grinding and cutoff wheels. Shellac binder is used for light work and for high finishing. Rubber is used for precision grinding and for centerless-feed machines. 3M’s Trizact abrasives are microreplicating aluminum oxide or silicon carbide pyramid-like grains on flexible polyester cloth or film. Continued use keeps exposing fresh cutting grains.
Grading of abrasive wheels is by grit size number from No. 10 to No. 600, which is 600 mesh; by grade of wheel, or strength of the bond, which is by letter designation, increasing in hardness from A to Z; and by grain spacing or structure number. The ideal condition is with a bond strong enough to hold the grains to accomplish the desired result and then to release them before they become too dull. Essential qualities in the abrasive grain are penetration hardness, body strength sufficient to resist fracture until the points dull and then break to present a new edge, and an attrition resistance suitable to the work. Some wheels are made with a porous honeycombed structure to give free cutting and cooler operation on some types of metal grinding. Some diamond wheels are made with aluminum powder mixed with a thermosetting resin, and the diamond abrasive mix is hot-pressed around this core wheel. Norton diamond wheels are of three types: metal bonded by powder metallurgy, resinoid bonded, and vitrified bonded.

Additional comments :