_EXAMPLES FOR PRACTICE._
Figure 217 represents a simple example for practice, which the student
may draw the size of the engraving, or he may draw it twice the size. It
is a locomotive spring, composed of leaves or plates, held together by a
central band.
[Illustration: Fig. 217.]
Figure 218 is an example of a stuffing box and gland, supposed to stand
vertical, hence the gland has an oil cup or receptacle.
In Figure 219 are working drawings of a coupling rod, with the
dimensions and directions marked in.
It may be remarked, however, that the drawings of a workshop are, where
large quantities of the same kind of work is done, varied in character
to suit some special departments--that is to say, special extra drawings
are made for these departments. In Figures 220 and 221 is a drawing of a
connecting rod drawn, put together as it would be for the lathe, vise or
erecting shop.
[Illustration: Fig. 218.]
[Illustration: Fig. 219.]
[Illustration: Fig. 220.]
[Illustration: Fig. 221.]
To the two views shown there would be necessary detail sketches of the
set screws, gibbs, and keys, all the rest being shown; the necessary
dimensions being, of course, marked on the general drawing and on the
details.
In so simple a thing as a connecting rod, however, there would be no
question as to how the parts go together; hence detail drawings of each
separate piece would answer for the lathe or vise bands.
But in many cases this would not be the case, and the drawing would
require to show the parts put together, and be accompanied with such
detail sketches as might be necessary to show parts that could not be
clearly defined in the general views.
The blacksmith, for example, is only concerned with the making of the
separate pieces, and has no concern as to how the parts go together.
Furthermore, there are parts and dimensions in the general drawing with
which the blacksmith has nothing to do.
Thus the location and dimensions of the keyways, the dimensions of the
brasses, and the location of the bolt holes, are matters that have no
reference to the blacksmith's work, because the keyways, bolt holes, and
set-screw holes would be cut out of the solid in the machine shop. It is
customary, therefore, to send to the blacksmith shop drawings containing
separate views of each piece drawn to the shape it is to be forged; and
drawn full size, or else on a scale sufficiently large to make each part
show clearly without close inspection, marking thereon the full sizes,
and stating beneath the number of pieces of each detail. (As in Figure
222, which represents the iron work of the connecting rod in Figure
220). In some cases the finished sizes are marked, and it is left to the
blacksmith's judgment how much to leave for the finishing. This is
undesirable, because either the blacksmith is left to judge what parts
are to be finished, or else there must be on the drawing instructions on
this point, or else signs or symbols that are understood to convey the
information. It is better, therefore, to make for the blacksmith a
special sketch, and mark thereon the full-forged sizes, stating on the
drawing that such is the case.
[Illustration: Fig. 222]
As to the material of which the pieces are to be made, the greater part
of blacksmith work is made of wrought iron, and it is, therefore,
unnecessary to write "wrought iron" beneath each piece. When the pieces
are to be of steel, however, it should be marked on the drawing and
beneath the piece. In special cases, as where the greater part of the
work of the shop is of steel, the rule may, of course, be reversed, and
the parts made of iron may be the ones marked, whereas when parts are
sometimes of iron, and at others of steel, each piece should be marked.
As a general rule the blacksmith knows, from the custom of the shop or
the nature of the work, what the quality or kind of iron is to be, and
it is, therefore, only in exceptional cases that they need to be
mentioned on the drawing. Thus in a carriage manufactory, Norway or
Swede iron will be found, as well as the better grades of refined iron,
but the blacksmith will know what iron to use, for certain parts, or the
shop may be so regulated that the selection of the iron is not left to
him. In marking the number of pieces required, it is better to use the
word "thus" than the words "of this," or "off this," because it is
shorter and more correct, for the forging is not taken off the drawing,
nor is it of the same; the drawing gives the shape and the size, and the
word "thus" conveys that idea better than "of," "off," or "like this."
In shops where there are many of the same pieces forged, the blacksmith
is furnished with sheet-iron templates that he can lay directly upon the
forging and test its dimensions at once, which is an excellent plan in
large work. Such templates are, of course, made from the drawings, and
it becomes a question as to whether their dimensions should be the
forged or the finished ones. If they are the forged, they may cause
trouble, because a forging may have a scant place that it is difficult
for the blacksmith to bring up to the size of the template, and he is in
doubt whether there is enough metal in the scant place to allow the job
to clean up. It is better, therefore, to make them to finished sizes, so
that he can see at once if the work will clean up, notwithstanding the
scant place. This will lead to no errors in large work, because such
work is marked out by lines, and the scant part will therefore be
discovered by the machinist, who will line out the piece accordingly.
Figure 223 is a drawing of a locomotive frame, which the student may as
well draw three or four times as large as the engraving, which brings us
to the subject of enlarging or reducing scales.
REDUCING SCALES.
[Illustration: Fig. 223.]
[Illustration: Fig. 224.]
[Illustration: Fig. 225.]
It is sometimes necessary to reduce a drawing to a smaller scale, or to
find a minute fraction of a given dimension, such fraction not being
marked on the lineal measuring rules at hand. Figure 224 represents a
scale for finding minute fractions. Draw seven lines parallel to each
other, and equidistant draw vertical lines dividing the scale into
half-inches, as at _a_, _b_, _c_, etc. Divide the first space _e d_ into
equal halves, draw diagonal lines, and number them as in the figure. The
distance of point 1, which is at the intersection of diagonal with the
second horizontal line, will be 1/24 inch from vertical line _e_. Point
2 will be 2/24 inch from line _e_, and so on. For tenths of inches there
would require to be but six horizontal lines, the diagonals being drawn
as before. A similar scale is shown in Figure 225. Draw the lines A B, B
D, D C, C A, enclosing a square inch. Divide each of these lines into
ten equal divisions, and number and letter them as shown. Draw also the
diagonal lines A 1, _a_ 2, B 3, and so on; then the distances from the
line A C to the points of intersection of the diagonals with the
horizontal lines represent hundredths of an inch.
Suppose, for example, we trace one diagonal line in its path across the
figure, taking that which starts from A and ends at 1 on the top
horizontal line; then where the diagonal intersects _horizontal_ line 1,
is 99/100 from the line B D, and 1/100 from the line A C, while where it
intersects _horizontal_ line 2, is 98/100 from line B D, and 2/100 from
line A C, and so on. If we require to set the compasses to 67/100 inch,
we set them to the radius of _n_, and the figure 3 on line B D, because
from that 3 to the vertical line _d_ 4 is 6/10 or 60/100 inch, and from
that vertical line to the diagonal at _n_ is seven divisions from the
line C D of the figure.
In making a drawing to scale, however, it is an excellent plan to draw a
line and divide it off to suit the required scale. Suppose, for example,
that the given scale is one-quarter size, or three inches per foot; then
a line three inches long may be divided into twelve equal divisions,
representing twelve inches, and these may be subdivided into half or
quarter inches and so on. It is recommended to the beginner, however, to
spend all his time making simple drawings, without making them to scale,
in order to become so familiar with the use of the instruments as to
feel at home with them, avoiding the complication of early studies that
would accompany drawing to scale.