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b. T-Square and Triangles. A T-square is used as a guide for drawing horizontal lines and as a support for the triangles which, in turn, are used as guides for drawing vertical and inclined lines. To use a T-square or triangle as a guide for drawing lines, pull the pencil along the edge of the straight edge from left to right. (These directions are for right-handed people. Left-handed people should reverse the directions.) Rotate the pencil as you draw so that a flat spot will not form on the lead. Flat spots cause wide, fuzzy lines of uneven width. Always remember to keep your drawing lead sharp.

When using a T-square, hold the head (top of the T) firmly and flat against the left edge of the drawing board. Use your left hand to hold the T-square still and in place while you draw. When you move the T-square, always check to see that the head is snug against the edge of the drawing board before you start to draw again.

When a T-square and a triangle are used together to create a guide for drawing, the left hand must not only hold the T-square firmly in place; it must also hold the edge of the triangle firmly and flat against the edge of the T-square. To accomplish this, use the the heel of your hand to hold the T-square in place and your fingers to keep the triangle against the T-square.

It is important that all your tools be accurate. A T-square, for example, must have a perfectly straight edge. If it does not, you will draw wavy lines and inaccurate angles with the triangles. To check a T-square for accuracy, draw a long line by using the T-square as a guide. Then flip the T-square over and, using the same edge just used as a guide, see if the T-square edge (now upside down) matches the line. If it does not, the T-square is not accurate.

Triangles should be checked for straightness in the same manner used to check a T-square, but, in addition, they must be checked for "squareness." To check a triangle for squareness, align the triangle against the T-square and draw a line by using the edge of the triangle which forms a 90° angle to the T-square as a guide. Holding the T-square in place, flip the triangle over, and see if the triangle edge matches the line. If it does not, the triangle is not square, meaning either 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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