Moreover, many of the views and illustrations used to instruct and explain machine tools and other devices in other parts of the volume, are drawn so that they may be used also as examples in advanced instrumental practice. This is a “hint” to the diligent and painstaking student worthy of remembering.
Fig. 216.—Fig. 217.—Fig. 218.—Fig. 219.
The designing and drawing of arcs and whole circles occupy a large proportion of space in nearly all mechanical drawings. The making of a complete circle is a matter of no great difficulty, but the beginning and termination of parts of circles require both judgment and considerable practice.
To aid the student these two illustrations of circles are introduced. To draw [fig. 220] with a pencil, using the upper edge of the blade of the T-square as a guide, draw a center line, A B, mark on it a distance of 4 inches, space this into half inches, using the dividers and making the points with it; then with the pencil compasses or bow pencil, which must be held as shown on page 209, and rotated from left to right, or clockwise, draw a series of circles through these points, tangent to one another or all touching at A, care being taken that the pencil lines exactly meet at A, and also cut the divided points as shown in the illustration. For [fig. 221] divide the center line as before, and draw the semi-circles on it A B, B C, meeting at B, and C D, D E, etc.
Fig. 220.
Fig. 221.
Now from center B draw circles A C, C E, meeting in C, and so on with the circles, arcs or segments; success in drawing this figure depends on the correct spacing of the center line in the first instance into equal parts.