CHAPTER 4
DRAFTING: GEOMETRIC CONSTRUCTION
Knowledge of the principles of geometric
construction and its applications are essential
to an Engineering Aid, As a draftsman, you must
be able to “construct” or draw any of the
various types of lines. In a line drawing, a line
may be a straight line, a circle, an arc of a
circle or a fillet, a circular curve, a noncircular
curve, or a combination of these basic types of
lines.
You must also be able to construct line
drawings at specified angles to each other,
various plane figures, and other graphic repre-
sentations consisting exclusively of lines. This
chapter provides information that will aid
you in drawing different types of geometric
constructions.
STRAIGHT LINES
One method of drawing horizontal and
vertical lines, perpendicular and parallel lines, and
inclined lines is by using a straightedge (or a
T square) with a triangle. Another practical
method of constructing straight lines is by using
a drafting compass.
Figure 4-1 shows a method of drawing a line
parallel to another line. Here, the line is to be
drawn through given point C. To draw a line
through C parallel to AB, place the needlepoint
of the compass on any point D on AB,
and strike arc CE. Shift the needlepoint
to E, maintaining the same radius, and strike
arc DF. Set a compass to a chord of arc CE,
and lay off the chord DF from D, thus locating
point F. A line drawn through F and C is parallel
to AB.
Figure 4-2 shows another method of drawing
one line parallel to another, this one being used
Figure 4-1.-Drawing a line through a given point, parallel
to another line.
Figure 4-2.-Drawing a parallel line at a given distance from
another line.
when the second line is to be drawn at a given
distance from the first. To draw a line parallel
to AB at a distance from AB equal to CD, set a
compass to the length of CD, and, from
any points E and F on AB, strike two arcs. A
line A’B’ drawn tangent to (barely touching) the
arcs is parallel to AB, and located CD distance
from AB.
In the preceding chapter, you learned how to
draw a line perpendicular to another by the use
of a straightedge and a triangle. Two other
methods of solving this problem are explained
below.
4-1
