Page:Sheet Metal Drafting.djvu/18

4 board may be tested for straightness by holding them as shown in Fig. 5. They should be in contact along their entire length. If they are not, one or the other is not straight.

The working position of the drawing board and T-square is illustrated in Fig. 6. In this position the blade of the T-square can be moved up and down over the surface of the board with the left hand while holding the head firmly against the working edge

of the board. For a left-handed man, the working edge of the board will be at the right, with the head of the T-square held firmly in place with the right hand.

All horizontal lines should be drawn with the T-square, drawing from left to right, meanwhile holding the head of the T-square firmly against the working edge of the hoard with the left hand.

The Triangles.—The triangles are used for drawing lines other than horizontal. They are made of hard rubber, celluloid,

or steel. There are two common shapes, called the 45° triangle and the 30°×60° triangle. These are illustrated in Figs. 7 and 8. The 45° triangle, shown in Fig 7, has one angle (the one marked 90°), a right angle. There are 90° in a right angle. The other angles are 45° each (just half of a right angle). One angle of the 30°×60° triangle is a right angle; another angle is 60° (just ⅔ of a right angle) ; and the other is 30° (just ⅓ of a right angle).

For drawing vertical hues (lines at right angles to the horizontal lines which have already been explained), the T-square should