45.132E
Figure 2-19.-Divider maintenance: (A) Evening the legs of
dividers; (B) Sharpening divider needlepoints.
pinhole. Since the same center is often used for
both the compasses and dividers, it is best that
needles on both be the same size. If the compass
needle is noticeably larger, grind it until it is the
correct size.
To make a compass needle smaller, wet one
side of the whetstone and place the needle with
its shoulder against this edge. Then grind it against
the whetstone, twirling it between your thumb and
forefinger (fig. 2-20). Test it for size by inserting
it in a hole made by another needle of the correct
size. When it is pushed as far as the shoulder, it
should not enlarge the hole.
The screw threads on bow instruments are
delicate. Because of this, you should take care
never to force the adjusting nut. Threads must
be kept free from rust or dirt.
If possible, it is best to keep drawing
instruments in a case, since the case protects them
from damage from falls or unnecessary pressures.
Then, too, the lining of the case is usually treated
with a chemical that helps prevent the instruments
from tarnishing or corroding.
To protect instruments from rusting when they
are not in use, clean them frequently with a soft
cloth and apply a light film of oil to their surface
142.35
Figure 2-20.-Shaping a compass needle.
with a rag. Joints on compasses and dividers
should not be oiled. When the surface finish of
instruments becomes worn or scarred, it is
subject to corrosion; therefore, a knife edge or
an abrasive should never be used to clean
drafting instruments.
Beam Compass
The BEAM COMPASS (fig. 2-21) is used for
drawing circles with radii larger than can be set
on a pivot joint or bow compass. Both the needle-
point attachment and the pen or pencil attachment
on a beam compass are slide-mounted on a metal
45.134X
Figure 2-21.-Beam compass.
2-15
