_SHADOW LINES AND LINE SHADING._
SECTION LINING OR CROSS-HATCHING.
When the interior of a piece is to be shown as a piece cut in half, or
when a piece is broken away, as is done to make more of the parts show,
or show more clearly, the surface so broken away or cut off is
section-lined or cross-hatched; that is to say, diagonal lines are drawn
across it, and to distinguish one piece from another these lines are
drawn at varying angles and of varying widths apart. In Figure 87 is
given a view of three cylindrical pieces. It may be known to be a
sectional view by the cross-hatching or section lines. It would be a
difficult matter to represent the three pieces put together without
showing them in section, because, in an outline view, the collars and
recesses would not appear. Each piece could of course be drawn
separately, but this would not show how they were placed when put
together. They could be shown in one view if they were shaded by lines
and a piece shown broken out where the collars and, recesses are, but
line shading is too tedious for detail drawings, beside involving too
much labor in their production.
[Illustration: Fig. 87.]
Figure 88 represents a case in which there are three cylindrical pieces
one within the other, the two inner ones being fastened together by a
screw which is shown dotted in in the end view, and whose position along
the pieces is shown in the side view. The edges of the fracture in the
outer piece are in this case cross-hatched, to show the line of
fracture.
[Illustration: Fig. 88.]
[Illustration: Fig. 89.]
In cross-hatching it is better that the diagonal lines do not quite meet
the edges of the piece, than that they should in the least overrun, as
is shown in Figure 89, where in the top half the diagonals slightly
overrun, while in the lower half they do not quite meet the outlines of
the piece.
In Figure 90 are shown in section a number of pieces one within the
other, the central bore being filled with short plugs. All the
cross-hatching was done with the triangle of 60 degrees and that of 90
degrees. It is here shown that with these two triangles only, and a
judicious arrangement of the diagonals, an almost infinite number of
pieces may be shown in cross section without any liability of mistaking
one for the other, or any doubt as to the form and arrangement of the
pieces; for, beside the difference in spacing in the cross-hatching,
there are no two adjoining pieces with the diagonals running in the same
direction. It will be seen that the narrow pieces are most clearly
defined by a close spacing of the cross-hatching.
[Illustration: Fig. 90.]
In Figure 91 are shown three pieces put together and having slots or
keyways through them. The outer shell is shown to be in one piece from
end to end, because the cross-hatching is not only equally spaced, but
the diagonals are in the same direction; hence it would be known that D,
F, H, and E were slots or recesses through the piece. The same remarks
apply to piece B, wherein G, J, K are recesses or slots. Piece C is
shown to have in its bore a recess at L. In the case of B, as of A,
there would be no question as to the piece being all one from end to
end, notwithstanding that the two ends are completely severed where the
slots G, I, come, because the spacing and direction of the
cross-hatching are equal on each side of the slots, which they would not
be if they were separate pieces.
[Illustration: Fig. 91.]
[Illustration: Fig. 92.]
Section shading or cross-hatching may sometimes cause the lines of the
drawing to appear crooked to the eye. Thus, in Figure 92, the key edge
on the right appears curved inwards, while on the left the key edge
appears curved outwards, although such is not actually the case. The
same effect is produced in Figure 93 on the right-hand edge of the key,
but not on the left-hand edge.
[Illustration: Fig. 93]
[Illustration: Fig. 94.]
A remarkable instance of this kind is shown in Figure 94, when the
vertical lines appear to the eye to be at a considerable angle one to
the other, although they are parallel.
The lines in sectional shading or cross-hatching may be made to denote
the material of which the piece is to be composed. Thus Professor Unwin
has proposed the system shown in the Figures 95 and 96. This may be of
service in some cases, but it would involve very much more labor than it
is worth in ordinary machine shop drawings, except in the case of cast
iron and wood, these two being shown in the simplest and the usual
manner. It is much better to write the name of the material beneath the
piece in a detail drawing.
[Illustration: Fig. 95.]
[Illustration: Fig. 96.]
LINE SHADING.
Mechanical drawings are made to look better and to show more distinctly
by being line shaded or shaded by lines. The simplest form of line
shading is by the use of the shade or shadow line.
In a mechanical drawing the light is supposed, for the purposes of line
shading or of coloring, to come in from the upper left-hand corner of
the drawing paper; hence it falls directly upon the upper and left-hand
lines of each piece, which are therefore represented by fine lines,
while the right hand and lower edges of the piece being on the shadow
side may therefore, with propriety, be represented by broader lines,
which are called shadow or shade lines. These lines will often serve to
indicate the shape of some part of the piece represented, as will be
seen from the following examples. In Figure 97 is a piece that contains
a hole, the fact being shown by the circle being thickened at A. If the
circle were thickened on the other side as at B, in Figure 98, it would
show that it represented a cylindrical stem instead of a hole.
[Illustration: Fig. 97.]
[Illustration: Fig. 98.]
[Illustration: Fig. 99.]
In Figure 99 is represented a washer, the surfaces that are in the
shadow side being shown in a shade line or shadow line, as it is often
called.
In Figure 100 is a key drawn with a shade line, while in Figure 101 the
shade line is shown applied to a nut. The shade line may be produced in
straight lines by drawing the line twice over, and slightly inclining
the pen, or by opening the pen points a little. For circles, however, it
may be produced either by slightly moving the centre from which the
circle is drawn, or by going over the shade part twice, and slightly
pressing the instrument as it moves, so as to gradually spring the legs
farther apart, the latter plan being generally preferable.
[Illustration: Fig. 100.]
[Illustration: Fig. 101.]
[Illustration: Fig. 102.]
Figure 102 shows a German pen, that can be regulated to draw lines of
various breadths. The head of the adjusting screw is made rather larger
than usual, and is divided at the under side into twenty divisional
notches, each alternate notch being marked by a figure on the face. By
this arrangement a uniform thickness of line may be maintained after
filling or clearing the pen, and any desired thickness may be repeated,
without any loss of time in trial of thickness on the paper. A small
spring automatically holds the divided screw-head in any place. With
very little practice the click of the spring in the notches becomes a
sufficient guide for adjustment, without reference to the figures on the
screw-head. Another meritorious feature of this pen is that it is armed
with sapphire points, which retain their sharpness very long, and thus
save the time and labor required to keep ordinary instruments in order
for the performance of fine work.
An example of line shading in perspective drawing is shown in the
drawing of a pipe threading stock and die in Figure 103.
[Illustration: Fig. 103.]
Shading by means of lines may be used with excellent effect in
mechanical drawing, not only to distinguish round from flat surfaces,
but also to denote to the eye the relative distances of surfaces. Figure
104 represents a cylindrical pin line shaded. As the light is supposed
to come in from the upper left-hand corner, it will evidently fall more
upon the left-hand half of the stem, and of the collar or bead, hence
those parts are shaded with lighter or finer lines than the right-hand
sides are.
[Illustration: Fig. 104.]
[Illustration: Fig. 105.]
Two cylindrical pieces that join each other may be line shaded at
whatever angle they may join. Figure 105 represents two such pieces, one
at a right angle to the other, both being of equal diameter.
[Illustration: Fig. 106.]
Figure 106 represents a drawing of a lathe centre shaded by lines, the
lines on the taper parts meeting those on the parallel part A, and
becoming more nearly parallel to the axis of the piece as the centre of
the piece is approached. The same is the case where a piece having a
curved outline is drawn, which is shown in Figure 107, where the set of
the bow-pen is gradually increased for drawing the shade lines of the
curves. The centres of the shade curves fall in each case upon a line at
a right angle to the axis of the piece, as upon the lines A, B, C, the
dotted lines showing the radius for each curve.
[Illustration: Fig. 107.]
The lines are made finer by closing the pen points by means of the screw
provided for that purpose. The pen requires for this purpose to be
cleaned of the ink that is apt to dry in it.
In Figure 108 line shading is shown applied to a ball or sphere, while
in Figure 109 it is shown applied to a pin in a socket which is shown in
section. By showing the hollow in connection with the round piece, the
difference between the two is quite clearly seen, the light falling
most upon the upper half of the pin and the lower half of the hole. This
perhaps is more clearly shown in the piece of tube in Figure 110, where
the thickness of the tube showing is a great aid to the eye. So,
likewise, the hollow or hole is more clearly seen where the piece is
shown in section, as in Figure 111, which is the case even though the
piece be taper as in the figure. If the body be bell-mouthed, as in
Figure 112, the hollow curve is readily shown by the shading; but to
line shade a hollow curve without any of these aids to the eye, as say,
to show a half of a tin tube, is a very difficult matter if the piece
is to look natural; and all that can be done is to shade the top darkly
and let the light fall mostly at and near the bottom. An example of line
shading to denote the relative distances from the eye of various
surfaces is given in Figure 113, where the surfaces most distant are the
most shaded. The flat surfaces are lined with lines of equal breadth,
the degrees of shading being governed by the width apart of the lines.
[Illustration: Fig. 108.]
[Illustration: Fig. 109.]
[Illustration: Fig. 110.]
[Illustration: Fig. 111.]
[Illustration: Fig. 112.]
[Illustration: Fig. 113.]
Line shading is often used to denote that the piece represented is to be
of wood, the shade lines being in some cases regular in combination with
regular ones, or entirely irregular, as in Figure 114.
[Illustration: Fig. 114.]