a perpendicular to a given line from a given point are
shown in figure 14-9.
1. From point A above line XY, pick any two
points on line XY, such as C and B. Points C
and B can either be on opposite sides of A or
both on the same side of A.
2. With B as a center, and with a radius of BA, draw
short arcs above and below line XY.
3. With C as a center, and a radius of CA, draw
Figure 14-10.--Drawing a tangent to a circle at a given point.
short arcs intersecting the arcs drawn in step 2.
4. Draw line DA through the intersecting arcs.
1. Place a triangle against a straightedge, as shown
Line DA is perpendicular to line XY.
in view A. The hypotenuse of the triangle
should pass through the center of the circle and
BLENDING ARCS AND TANGENTS
the point where the line is to be tangent to the
circle.
Laying out circles or arcs, and straight lines tangent
2. Hold the straightedge firmly in place, and turn
to them, is difficult. It is difficult because there is an
the triangle over.
element of optical illusion involved. Drawing a straight
line to a curved one is easier than drawing a curved line
3. Move the triangle until the hypotenuse passes
to a straight one. Because of this, major circles or arcs
through the point of tangency, and then draw the
are drawn first on layouts.
tangent, as shown in view B.
Even when you draw a straight line to a curve, an
Drawing an Arc to a Line
optical illusion may make it difficult for you to blend
the curve and the line perfectly. This section discusses
You may find that when you draw an arc that ends
a few simple methods that should help you to blend lines
in a straight line, you have a tendency to overdraw the
and arcs.
arc (fig. 14-11). Use the following procedure and the
steps shown in figure 14-12 to avoid this.
The Draftsman's Method of Drawing a
Tangent to a Circle at a Given Point
1. Use any given radius as the distance to set your
dividers.
The draftsman's method of drawing a tangent to a
2. Set one point of the dividers at point A and strike
circle is described in the following steps. It is shown in
tangent points at B and C (view A).
figure 14-10, and you should refer to this figure as you
read this section. You will need your triangle and
3. Without changing the dividers, place one point
straightedge to draw a tangent to a circle.
at B and strike an arc at D.
Figure 14-9.--Drawing a perpendicular to a given line from a
given point.
Figure 14-11.--Errors in drawing a line to the end of an arc.
14-10
