TM 1-1500-204-23-11
Figure 3-19. Countersink for Aramid Laminates
f. Abrasives. Abrasives are miniature cutting sur-
they bind in the cut, fracturing of the disc can occur.
faces combined to form a tool. They are derived from
Diamond cutoff discs are priced much higher than
abrasive grains. Abrasive grains are hard crystals
iber reinforced abrasive but if used properly can last
either found in nature or manufactured. The most com-
for years.
monly used abrasive materials are aluminum oxide,
Silicon Carbide, cubic boron nitride, and diamond.
(2) Coated Abrasive Discs. Coated abrasive
Other materials such as garnet, zirconia, glass, and
discs (see igure 3-20) have an abrasive mineral
even walnut shells are used for some applications.
bonded to a paper or iber backing. Aluminum oxide
and silicon carbide are the two abrasives most com-
monly used for aerospace applications. Silicon carbide
is the preferred type of abrasive media because it
stays sharp longer than the aluminum oxide. Coated
Fiber reinforced abrasive cutoff wheels that
abrasive discs are the most frequently used discs for
have been fractured can explode when run
machining composite repair joints. These discs are
up to a high rpm.
usually mounted on a 90 degree die grinder motor
for use but other tools may be used when conditions
(1) Cutoff Wheels. Cutoff wheels for compos-
merit. Various grit sizes are required (80 to 150
ites are of two main types: iber reinforced abrasive
grit for rough sanding and 150 to 240 grit for inish
wheels and steel wheels with the edge coated with
sanding).
diamond grit. The iber discs can wear rapidly and if
