PRIN. OF DRAFTING AND SHOP DRAWINGS - OD1641 - LESSON 1/TASK 2
that the 90, angle is not 90,, or that the edge of the triangle is curved, or that
the edge of the T-square is curved.
To use the T-square and the triangle as a guide for drawing a line parallel to a
given inclined line, align the long leg of the triangle with the given line and
then align the T-square to one of the other legs of the triangle. By holding the
T-square in place with your left hand, you can slide the triangle along the T-
square and the long leg will always be parallel to the originally given line.
c. Compass. A compass is used to draw circles and arcs. The three basic kinds of
compass are drop, bow, and beam. The bow is the most common.
To use a compass, set the compass opening equal to the radius of the desired circle
or arc by using a scale.
Then place the compass point directly on the circle
center point and, using only one hand, draw in the circle.
d. Protractors. A protractor is used to measure angles. The edge of a protractor
is calibrated into degrees and half-degrees.
Figure 19 (on the following page)
shows part of a typical protractor edge, together with some sample measurements.
Measurements more accurate than half a degree (0.5,) must be estimated.
To measure an angle, place the center point of the protractor on the origin of the
angle so that one leg of the angle aligns with the 0, mark on the protractor. Read
the angle value where the other leg of the angle intersects the calibrated edge of
the protractor.
e. Curves.
Curves are used to help draw noncircular curved shapes.
Draftsmen
refer to then as French curves or ship's curves, depending on their shapes (ship's
curves look like the keel of a ship).
Noncircular shapes are usually defined by a series of points and a curve is used to
help join the points with a smooth, continuous line. Using a curve to help create
a smooth line is difficult and requires much practice.
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