one leg between the thumb and the first and
second fingers, and hold the other leg between the
third and fourth fingers. Place the second and
third fingers on the inside of the legs; the dividers
are opened by spreading these fingers apart.
Dividers are closed by squeezing the thumb and
first finger toward the fourth finger while
gradually slipping out the other two fingers.
To transfer measurements on a drawing, set
the dividers to the correct distance, then transfer
the measurements to the drawing by pricking the
drawing surface very lightly with the points of the
dividers.
To measure off a series of equal distances on
the line, set the dividers to the given distance.
Then step off this distance as many times as
desired by swinging the dividers from one leg to
the other along the line, first swinging clockwise
180 degrees, then counterclockwise 180 degrees,
and so on.
In dividing either a straight line (fig. 3-12,
view B) or a curved line (fig. 3-12, view C)
into a given number of equal parts (for example,
four) by trial, open the dividers to a rough
approximation of the first division (in this case,
one quarter of the line length) and step off the
distance lightly, holding the dividers by the
handle and pivoting the instrument on alternate
sides of the line at each step. If the dividers
fall short of the end of the line after the
fourth step, hold the back leg in place and
advance the forward leg, by guess, one quarter
of the remaining distance. Repeat the procedure
until the last step falls at the end of the line. Be
careful during this process not to punch holes in
the paper, but just barely mark the surface for
future reference. To identify prick marks made
with small dividers for future reference, circle the
marks lightly with a pencil.
USE OF THE DRAFTING SCALE
Accuracy in drawing depends to a great
extent upon correct use of the scale in marking
off distances. You should place the edge of the
scale parallel to the line being measured (fig. 3-13).
To eliminate shadows cast by your body or hands,
point the desired scale face away from you for
horizontal measurements and toward your left for
vertical measurements. With a sharp pencil,
mark off short dashes at right angles to the
scale at the correct distances, aligning the mark
carefully with the scale graduation. Have your
eye approximately over the point being measured,
Figure 3-13.-Use of the drafting scale.
and make light marks to denote the point of
measurement.
When setting the compass to a given radius
or when setting divider points, never place the
sharp points of these instruments on the scale. Lay
out the desired radius or distance on a straight
pencil line by using the scale in the manner
described above. Then adjust the compass or
dividers to the indicated length by using
the measured line. A scale surface marred by
pinpricks is difficult to read and is unsuitable for
accurate work.
In making successive measurements along
the same line, make as many measurements as
possible without moving the scale. If a number
of distances are to be laid out end to end, hold
the scale in one position and add each successive
measurement to the preceding one. If the scale
is moved to a new position each time, slight
errors in measurement may accumulate. For
example, four successive measurements of 1 5/8
in. each should give an overall length of 6 1/2 in.,
not 6 9/16 in. Therefore, make as many
measurements as you can without changing the
reference point. This will avoid cumulative errors
in the use of the scale.
Note that your pencil touches the scale only
for the purpose of marking a point on the paper.
Never use a scale as a straightedge for drawing
lines. A typical office ruler has a metal edge;
it is a scale and straightedge combined. But a
draftsman’s measuring scale is for measuring
only; it is not a ruler. A scale properly
used will last for decades, but a scale used as a
straightedge will soon have the graduations worn
away.
3-10
