ABRASIVES are either natural or artificial crystalline forms. Natural abrasives are Diamond, Corundum, Garnet, Emery, Quartz and other softer materials found in the earth. Artificial abrasives are manufactured, such as Diamond, Borazon CBN, Silicon Carbide, Aluminum Oxide, Boron Carbide and various aluminas, which are divided into two groups, fused and unfused. Fused abrasives are the result of tremendously high electric furnace temperatures, which produce hard crystals.
Unfused alumina abrasives are the result of lower temperatures and chemical additives. They do not have the hard crystalline structure of fused abrasives.
TYPES OF ABRASIVES
(Rating, MOHS 10) Both a natural and man-made synthetic abrasive. The hardest and sharpest abrasive known. Best suited for tungsten carbide and other very hard materials. Because it is so hard it should not be used on softer metals where embedding may be a factor. When a plate has been embedded with the Diamond abrasive, it cuts fast and produces fine finishes. In recent years, synthetic fine Diamond powders have been increasingly used in industrial applications.
CUBIC BORON NITRIDE (commonly known as Borazon CBN):
A man-made synthetic abrasive that is almost as hard as Diamond on the MOHS scale. This abrasive material is well suited to ferrous metals in a lapping operation, as it will not carbonize as Diamond will when interacting with Fe (Iron). Borazon CBN is especially well suited for lapping 52100 bearing steel, cast iron, die steel, tool steel, stellite, super alloys and in some cases ceramic materials.
(Rating, MOHS 9.7) A fused abrasive with high grain strength. It has a hexagonal structure and is not easily friable. Useful only for unusual or special lapping operations.
(Rating, MOHS 9.5) A fused, hard crystalline abrasive. Fast cutting with good crystal breakdown when used to lap either high or low tensile strength material. It is well suited for rough lapping operations, forged or hardened gears, valves, tool room work and general maintenance where polish is not essential. With Silicon Carbide, all lapped material will have a frosty or gray finish.
(Rating, MOHS 9, just under Silicon Carbide) A fused, crystalline abrasive. It has a very hard crystal structure that is slowly dulled and hard to fracture. It is best suited for use on high tensile strength materials, rough lapping operations, hardened hears, ball bearing grooves or lapping operations where pressure can be exerted to break down the crystals. It does not lend itself to fine finishes or precision lapping.
(Rating, MOHS 9) Another form of Fused Alumina is the 38 white Aluminum Oxide abrasive, which is white in color with friable crystals. The pressure on (friable) crystals, while lapping, causes them to keep breaking down into still smaller particles, which perform the finishing operation, to produce the low r.m.s. finishes or polish. 38 while Aluminum Oxide is valuable for lapping stainless, chrome plate, beryllium and ferrite whose hardness range is below the 62-63 Rockwell C Scale.
(Rating, MOHS 9) A natural abrasive found in the earth, with a softer crystalline structure than Silicon Carbide or Aluminum Oxide. It breaks down readily and is important for lapping a great variety of medium-hard metals (Rockwell C 35-45). It gives a medium polish or reflective finish.
(Rating, MOHS 8 to 9) A natural abrasive mined from the earth, with a blocky crystalline structure that does not readily embed itself in lapped parts. Its greatest asset is that it may be safely used for lapping cast iron gears, brass or bronze running seals and instrument gears where non-embedding qualities are desired. It has a medium polishing quality.
INFUSED ALUMINA (hydrate-calcined):
Aluminas are produced in a wide variety-Gamma and Alpha, hydra and calcined. Hydreate Alumina is relatively soft and is used for polishing. Calcined Aluminas are produced by heat treatment and the degree of calcination determines the characteristics of the product. The terms soft, medium and hard relate to them as mild, medium and high degree of calcination.
The calcined types are recommended for the lapping and polishing of harder metals (Rockwell C 45-63). Their shape unlike the blocky crystals, are composed of flat or "platey" crystals with their thickness about one-sixth their diameter. Unfused aluminas allow more equal pressure to be distributed over a larger surface area than with fused because of their "playey" shape. The disc shaped particles work with a shaving action rather than the rolling and gouging action of ordinary abrasives and are not apt to scratch the work being lapped.
MICRO GRADED FLOURS-OPTICAL FINISHING FLOURS:
A fused Aluminum Oxide type abrasive called flour because of its fine grit size, ranging from 500 to 1000. Originally produced to meet requirements of the glass and optical trade. It has a general blocky, crystalline structure that is effectively used when relatively soft materials are being lapped. The pressure exerted on the abrasive will cause it to enter the material and cut rather than fracture. For soft material pressure per inch need not be higher than two to five pounds. The softer the material the lower the pressure needed. Its greatest use is with special lapping machines where the abrasive is mixed with water, glycerin and various oils and is fed from an agitated tank onto the lapping area producing a matte finish. Because this material is hard and not friable enough for precision lapping, we have found it unsuitable for production into our paste type, ready mixed compounds.
MICRO GRADED FLOURS
These powders represent alumina purity of 99.98%. They are used for polishing purposes on hard materials (Rockwell C 45-65) and for extreme finishing operations. Excellent for steel balls or ball bearings, Linde powders produce a very high polish or luster. They can also be used with Pitch laps for producing mirror finishes.
OTHER ABRASIVE MATERIALS:
ABRASIVE GRIT - SIZES 8 TO 240:
These are called "screened" sizes. The U.S. Department of Commerce has specifications for each screen number.
ABRASIVE GRIT - SIZES 280 AND FINER:
There are no standardizations for the "subsieve" or finer grit numbers from 280 and finer. Considerable variation exists in both nomenclature and sizing practice between producers. Grit sizes differ from one producer to another though they try to remain competitive. Personal testing is the only sure way to determine if a particular grit size meets your needs.
AVERAGE PARTICLE SIZE OF ABRASIVE GRAIN