[Illustration: Fig. 259.]
Figs. 251 to 258, inclusive, show the section lining and cross-hatching
by which it is customary to represent the various materials entering
into a construction.
In fig. 259 is outlined a representation of a section of a cog-wheel;
section 1 being the wood cogs; 2, the iron wheel, and 3 the wedges at
the root of the gear. It would be impossible to convey the same ideas by
ordinary plan or elevation drawing; all the objects on the same page are
more clearly represented by the use of section lines or cross-hatching.
Sectioning is executed by drawing a series of parallel lines about ³⁄₃₂
inches apart. Lay the 45° triangle on the upper edge of the T-square and
draw the top-most line of the sectioning. Then slide the triangle along
the T-square for each successive line. The sectioning should be inked in
without previous penciling and the lines should be finer than the lines
of the general drawing.
Various devices are in use for mechanically equalizing the distances in
section lining, but the trained eye is the most practical method. When
two abutting pieces are sectioned, the section lining on one piece
slants in an opposite direction to that on the other.
To draw an object to be sectioned on both sides of its center line, only
one side is sectioned, while the other side is drawn in full.
Sections are necessary in nearly all machine drawings; they are usually
taken horizontally or vertically, but they may be taken in any
direction; the position of a section should be shown by a line upon the
object; this line is called the cutting plane.
In fig. 261 is shown the hub of a wheel, it is also a sample of work for
practice.
[Illustration: Fig. 260.]
[Illustration: Fig. 261.]
Fig. 260 shows the mode of representing two different materials in one
plane, or a section may be represented by the darker portion, and the
lighter shaded portion being a surface resting on the section.
Fig. 261 shows the section of a shaft surrounded by the surface of a
wheel.