side of these lines, may be completed either by hand or
by a combination of circular arcs.
In drawings where utility is of more importance than
elegance or beauty of finish, the threads are most appro-
priately represented by a series of parallel and equidistant
straight lines. When the screws are very small, it suffices
to draw parallel lines simply between the two lines which
indicate the exterior diameter of the screw.
Elementary Examples of Mechanical Drawing.
Examples showing the Use of Sections in Drawing.
1. Footstep for an upright shaft. Figs. 1, 2, 3. This
object is composed of a number of pieces placed one within
another, which, though visible in the plan Fig. 1, are
concealed in the elevation Fig. 2. It is, then, necessary
to compose a vertical section of the object, made by a plane
passing through the centre, Fig. 3, to expose the internal
construction, and to exhibit the dimensions of the various
parts. This figure shows: 1st, the thickness of the ex-
terior piece A, and the opening a at the bottom. 2d, the
thickness and diameter of the interior piece B, with the
slots h, b. 3d, The form and adjustment of the bearing C,
which is prevented from turning with the shaft by the
key c, of which the ends rest in the slots b, b.
Figs. 1 and 3 alone would suffice to show the construc-
tion of the footstep. The elevation, Fig. 1, however, is
useful to show the depth and level of the rectangular flat
surfaces d, d. Although, had these surfaces had any pro-
jection beyond their tangential position on the cylindrical
surface, their position would have been sufficiently indi-
cated without the aid of Fig. 1.
2. Stuffngbox-gland. Figs. 4, 5, 6. The outline of
this object is completely shown by Figs. 4 and 5 ; and
the plan Fig. 4, shows that the openings are circular ;
but the sectional view, Fig. 6, is necessary to show the
thickness of metal, and the form of the interior, and also
the ferule, b, at the lower end. The parts a and b are of
the same diameter, that of the piston-rod; and c is a little
wider to clear the rod, and to form an oil-chamber. The
section shows, also, that the bolt-holes, e, e, are made quite
through the wings d, d. It shows also the form of the
space / to receive the charge of oil or grease.
3. Spherical Joint. Figs. 7, 8, 9. This joint consists,
1st, of a sphere A, of the same thickness as the tube B,
with which it is in one piece. 2d, of two spherical
inclosures C, D, embracing the piece A, and joined by
bolts through the wings a, a.
4. Equilibrium-valve. Figs. 10, 11, 12. This is com-
posed of the seat, A; and the valve B, fitted on the rod, C.
The valve rests on two conical surfaces a, b. The body of
the valve is hollow, and is formed with four fins c, c, to
unite the centre and the circumference. The valve-seat A
is similarly formed. Fig. 13 shows the construction for a
part of the outline elevation, Fig. 11.
The foregoing examples show the utility of sectional
views for the complete representation of hollow objects.
In some cases, it is true, the form of the interior may be
indicated by dot-lining upon the elevation. The separate
sectional view is, however, generally more intelligible;
and, as already remarked, it is often sufficient of itself, in
connection with the plan, without the exterior elevation.
Projections of a Pillow-Block.—Plate XII.
The pedestal, pillow-block, or, as it is frequently called,
plummer-block, is a fixed piece intended for supporting
a revolving shaft or axle. It consists of two distinct prin-
cipal parts: the lower part or body U of the pillow-block,
which is fixed immovably by bolts to the framework or
foundation of the machine; and the upper part Y, called
the cover, which is also attached by bolts to the body U,
but in such a way as to admit of its easy removal, and
ready adjustment to a greater or less degree of proximity
to the latter.
These pieces are usually formed of cast-iron; but in
order to reduce the friction, and at the same time to faci-
litate the renewal of the bearing surfaces as they become
worn by use, the pillow-block is provided with steps or
bushes, usually of gun-metal accurately fitted within the
cheeks of the lower part, and to the under surface of the
cover. These bushes are bored out to carry the shaft.
Fig. 1 represents the vertical projection or elevation of
a pillow-block for a 6-inch journal, of the form and pro-
portions usually adopted by the best mill-wrights. In this
view are exhibited the length and height of the pillow-
block, together with the length and thickness of the sole,
by which it is bolted to the framework or foundation plate.
Here we see also the height from sole to centre, the ex-
ternal form of the bushes, and of the cover, &c. But it is
obvious that the breadth of the pillow-block cannot be
indicated in this view, nor the form of the exterior pro-
jections through which the cover bolts penetrate. To
obtain these we are therefore obliged to have recourse to
the plan Fig. 2.
But further, since there are peculiarities of internal con-
struction which cannot be exhibited in either of these
external views, it is necessary to construct a longitudinal
section formed by a vertical plane passing through the
centre line S T. This section is represented at Fig. 3, in
which the brass bushes are seen to be formed with lateral
snugs, which prevent their turning round with the shaft.
The cover is also shown here to be very accurately fitted
within the cheeks of the pillow-block by means of project-
ing pieces, also fitted to the upper brass; this is for the
purpose of securing greater rigidity and firmness of all the
parts, and of relieving the cover bolts from lateral strain.
Again, in the second plan (Fig. 4), we have supposed the
cover and upper bush to have been removed, to show how
longitudinal displacement of the bushes is prevented by
the snugs. These two views also show distinctly the
dimensions and mode of fixing of the cover bolts; in Fiir. 4