Example 2: 15:20 = 3:4

points on two or more lines or plants.

Example 3:

plane.

As shown in the examples, a proportion is nothing

more than an equation of common fractions. The value

of proportion is that if any three of the terms are given,

the fourth or unknown term can be found. This is done

no dimensions.

by solving a simple problem of common fractions.

sides.

Patterns are geometric shapes that conform to a

the circumferece of that circle (one-half of the

draftsman's plan (blueprint) and contain allowances for

diameter).

draft, shrinkage, and machine finish. To construct a

pattern, you must solve graphics problems of geometric

construction. Graphics problems can be solved by trial

does not cross another curved or straight line at a point

and error or by measuring with a scale. However,

other than its ends.

neither method is accurate; therefore, they cannot be

used in patternmaking. Because of its accuracy, only

forming an angle.

the geometric method of construction should be used in

patternmaking. No other method is acceptable.

As you read this section, refer to figure 14-2. You

geometric construction. Sonic of these tools include a

arc going to learn how to bisect a line by using the

pencil divider, a 12-inch scale, a flexible straightedge, a

following steps:

30- to 60-degree triangle, and a T-square.

1. USC your dividers and adjust them with a spread

As you read about geometric construction, the

that is visually greater than one-half the length

discussion will contain some geometric terms that you

of the line.

should know. These terms arc defined as follows:

2. Insert one point of the dividers at one end of the

line (point A, view A) and draw an arc. Be

a common point .

careful not to change the adjustment on the

dividers.

3. Insert one point at the other end of the line (point

B, view B) and draw an arc from that end

intersecting the first arc.

4. Draw a straight line connecting the two

ellipse, or closed curve.

intersection points of the arcs to bisect the line

(view C).

straight, contains no angles, and does not form a closed

figure.

circumference of a circle, through the center, to the

opposite point on a circle.

opposite the right angle.

points. Two lines intersect at one point. Two plants

intersect at a series of adjacent points (which is the

definition of a line).

14-7

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