Types of Abrasives
-Micro Graded Flours-Optical Finishing Flours
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.
DIAMOND: (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.
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.
NORBIDE ABRASIVE: (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.
SILICON CARBIDE: (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.
ALUMINUM OXIDE: (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
FUSED ALUMINA: (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.
CORUNDUM: (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.
GARNET: (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.
of Norton Company, Worcester, Mass.
LINDE POWDERS: 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:
Red Rouge (Ferric Oxide) , jeweler's rouge for polishing
Green Rouge (Chromium Oxide), for polishing hard material
such as chrome plate, stainless, etc.
Natural Emery is one of the oldest abrasives but due to
its impurities it has little place in modern lapping
Cerium Oxide, best suited for glass polishing
Titanium Oxide and a host of other abrasive materials have
usage for unusual or special applications only
- 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
millionth of an inch 0.000001 inch
microinch should be qualified by adding r.m.s. which signifies
the root-mean square.