_EXAMPLES IN LINE-SHADING AND DRAWINGS FOR LINE-SHADED ENGRAVINGS._
Although in workshop drawings, line-shading is rarely employed, yet
where a design rather than the particular details of construction is to
be shown, line-shading is a valuable accessory. Figure 295, for example,
is intended to show an arrangement of idle pulleys to guide belts from
one pulley to another; the principle being that so long as the belt
passes to a pulley moving in line with the line of rotation of the
pulley, the belt will run correctly, although it may leave the pulley at
considerable angle. When a belt envelops two pulleys that are at a right
angle to each other, two guide pulleys are needed in order that the belt
may, in passing to each pulley, move in the same plane as the pulley
rotates in, and the belt is in this case given what is termed a quarter
twist.
It will be observed that by the line-shading even the twist of the belt
is much more clearly shown than it would be if left unshaded.
An excellent example of shading is given in Figure 296, which is
extracted from the _American Machinist_, and represents a cutting tool
for a planing machine. The figure is from a wood engraving, but the
effect may be produced by lines, the black parts being considered as
simply broad black lines.
[Illustration: Fig. 295.]
The drawings from which engravings are made are drawn to conform to the
process by which the engraving is to be produced. Drawings that are
shaded by plain lines may be engraved by three methods. First, the
drawing may be photo-engraved, in which process the drawing is
photographed on the metal, and every line appears in the engraving
precisely as it appears in the drawing.
[Illustration: Fig. 296.]
For this kind of engraving the drawing may be made of any convenient
size that is larger than the size of engraving to be produced, the
reduction of size being produced in the photographing process. Drawings
for photo-engraving require to have the lines jet black, and it is to
be remembered that if red centrelines are marked on the drawing, they
will be produced as ordinary black lines in the engraving.
The shading on a drawing to be photo-engraved must be produced by lines,
and not by tints, for tints, whether of black or of colors, will not
photo-engrave properly.
It is generally preferred to make the drawing for a photo-engraving
larger than the engraving that is to be made from it, a good proportion
being to make the drawing twice the length the engraving is to be. This
serves to reduce the magnitude of any roughness in the lines of the
drawing, and, therefore, to make the engraving better than the drawing.
The thickness of the lines in the drawing should be made to suit the
amount of reduction to be made, because the lines are reduced in
thickness in the same proportion as the engraving is reduced from the
drawing. Thus the lines on an engraving reduced to one-half the
dimensions of the drawing would be one-half as thick as the lines on the
drawing.
Drawings for photo-engraving should be made on smooth-faced paper; as,
for example, on Bristol board; and to make the lines clean and clear,
the drawing instruments should be in the best of condition, and the
paper or Bristol board quite dry. The India rubber should be used as
little as possible on drawings to be photo-engraved, because, if used
before the lines are inked in, it roughens the surface of the paper, and
the inking lines will be less smooth and even at their edges; and for
this reason it is better not to rub out any lines until all the lines
have been inked in. If used to excess after the lines have been inked in
it serves to reduce the blackness of the lines, and may so pale them
that they will not properly photo-engrave.
To make a drawing for an engraver in wood it would be drawn directly on
the face of the box-wood block, on which it is to be engraved. The
surface of the block is first whitened by a white water color, as
Chinese white. If the drawing that is to be used as a copy is on
sufficiently thin paper, its outline may be traced over by pencil lines,
and the copy may then be laid face down on the wood block and its edges
held to the block by wax, the pencilled lines being face to the block.
The outline may then be again traced over with a pencil or pointed
instrument, causing the imprint of the lead pencil lines to be left on
the whitened surface of the block. If the copy is on paper too thick to
be thus employed, a tracing may be made and used as above; it being
borne in mind that the tracing must be laid with the pencilled lines on
the block, because what is the right hand of the drawing on the block is
the left hand in the print it gives. The shading on wood blocks is given
by tints of India ink aided by pencilled lines, or of course pencilled
lines only may for less artistic work be used. Another method is to
photograph the drawing direct upon the surface of the wood block; it is
unnecessary, however, to enter into this part of the subject.
The third method of producing an engraving from a drawing is by means of
what is known as the wax process. Drawings for this process should be
made on thin paper, for the following reasons: The process consists,
briefly stated, in coating a copper plate with a layer of wax about 1/32
inch deep, and in drawing upon the wax the lines to compose the
engraving, which lines are produced by means of tools that remove the
wax down to the surface of the copper.
The plate and wax are then placed in a battery and a deposit of copper
fills in the lines and surface of the wax, thus forming the engraving.
Now if the drawing is made on thin paper, the engraver coats the surface
of the drawing with a dry red pigment, and with a pointed instrument
traces over the lines of the drawing, which causes them to leave a red
imprint on the surface of the wax, and after the drawing is removed the
engraver cuts these imprinted lines in the wax. If the drawing is on
thick paper, this method of transferring the drawing to the wax cannot
be used, and the engraver may take a tracing from the drawing and
transfer from the tracing to the wax. It is obvious, also, that for wax
engravings the drawing should be made of the same size that the
engraving is required to be, or otherwise the tracing process described
cannot be used. Figure 297 represents an engraving made by the wax
process from a print from a wood engraving, and it is obvious that since
all the lines drawn on the wax sink down to the surface of the copper
plate, the shading is virtually composed of lines, the black surfaces
being where the lines have been sufficiently close together and broad to
remove all the wax enclosed within those surfaces.
[Illustration: Fig. 297.]
[Illustration: Fig. 298.]
The wax process is, however, more suitable for engravings in plain
outline only, and is especially excellent when the parts are small and
the lines fall close together; as, for example, in Figures 298 and 299,
which are engravings of a boiler drilling machine, and were produced
for the _American Machinist_ by tracing over a wood engraving from
London, "Engineering" in the manner already described. The fineness and
cleanness of the lines in the wax process is here well illustrated, the
disposition of the parts being easily seen from the engraving, and
easily followed in connection with the following description:
The machine consists of two horizontal bed-plates A 1 and A 2, made with
$V$ slides on top, and placed at right angles to each other. Upon each
of the bed-plates is fitted a vertical arm B 1 and B 2, each of which
carries two saddles, C 1 and C 2, these being each adjustable vertically
on its respective arm by means of rack and pinion and hand wheels D 1
and D 2. The saddles are balanced so that the least possible exertion is
sufficient to adjust them. The vertical arms, B 1 and B 2, are cast each
with a round foot by which the arms are attached to the square boxes E 1
and E 2, which are fitted to the $V$ slides on the horizontal beds A 1
and A 2, and are adjustable thereon by means of screw and ratchet motion
F 1 and F 2. Each of the square boxes has cast on it a small arm G 1 and
G 2, carrying studs upon which run pinions gearing into the circular
racks at the foot of the vertical arms. The square boxes have each a
circular groove turned in the top to receive the bolts by which the
vertical arms are connected to them, and thus the vertical arms, and
with them the drill spindles N 1 and N 2, are adjustable radially with
the boiler--the adjustment being effected by means of the pinions and
circular racks. The pinions are arranged so that they can be worked with
the same screw key that is used for the bolts in the circular grooves.
The shell to be drilled is placed upon the circular table H, which is
carried by suitable framework adjustable by means of screw on a $V$
slide I, placed at an angle of 45 deg. with the horizontal bed-plates. By
this arrangement, when the table is moved along I, it will approach to
or recede from all the drills equally. J 1 and J 2 are girders forming
additional bearings for the framework of the table. The bed-plates and
slides for the table are bolted and braced together, making the whole
machine very firm and rigid. Power is applied to the machine through the
cones K 1 and K 2, working the horizontal and vertical shafts L 1 and L
2, etc. On the vertical shafts are fitted coarse pitch worms sliding on
feather keys, and carried with the saddles C 1 and C 2, etc. The worms
gearing with the worm wheels M 1 and M 2 are fitted on the sleeves of
the steel spindles N 1 and N 2. The spindles are fitted with self-acting
motions O 1 and O 2, which are easily thrown in and out of gear.
The machine is also used for turning the edge of the flanges which some
makers prefer to have on the end plates of marine boilers. The plates
are very readily fixed to the circular table H, and the edge of the
flange trued up much quicker than by the ordinary means of chipping.
When the machine is used for this purpose, the cross beam P, which is
removable, is fastened to the two upright brackets R 1 and R 2. The
cross beam is cast with $V$ slides at one side for a little more than
half its length from one end, and on the opposite side for the same
length, but from the opposite end. The $V$ slides are each fitted with a
tool box S 1 and S 2, having a screw adjustment for setting the tool to
the depth of cut, and adjustable on the $V$ slides of the cross beam to
the diameter of the plate to be turned. This arrangement of the machine
is also used for cutting out the furnace mouths in the boiler ends. The
plate is fastened to the circular table, the centre of the hole to be
cut out being placed over the centre of table; one or both of the tool
boxes may be used. There is sufficient space between the upright
brackets R 1 and R 2, to allow that section of a boiler end which
contains the furnace mouths to revolve while the holes are being cut
out; the plate belonging to the end of a boiler of the largest diameter
that the machine will take in for drilling. The holes cut out will be
from 2 feet 3 inches in diameter and upwards. Power for using the
turntable is applied through the cone T. The bevel wheels, worms, worm
wheels, and pinions for driving the tables are of cast steel, which is
necessary for the rough work of turning the flanges.
[Illustration: Fig. 299. (Page 275.)]
As to the practical results of using the machine, the drills are driven
at a speed of 340 feet per minute at the cutting edges. A jet of
soapsuds plays on each drill from an orifice 1/32 in. in diameter, and
at a pressure of 60 lbs. per square inch. A joint composed of two 1-inch
plates, and having holes 1 and one-eighth in. in diameter, can be
drilled in about 2-1/2 minutes, and allowing about half a minute for
adjusting the drill, each drill will do about 20 holes per hour. The
machine is designed to stand any amount of work that the drills will
bear. The time required for putting on the end of a boiler and turning
the flange thereon (say 14 feet diameter) is about 2-1/2 hours;
much, however, depends on the state of the flanges, as sometimes
they are very rough, while at others very little is necessary to true
them up. The time required for putting on the plate containing the
furnace mouths and cutting out three holes 2 feet 6 in. in diameter, the
plate being 1 and one-eighth in. thick, is three hours. Of course, if
several boilers of one size are being made at the same time, the holes
in two or more of these plates can be cut out at once. The machine is of
such design that it can be placed with one of the horizontal bed-plates
(say A 1), parallel and close up to a wall of the boiler shop; and when
the turning apparatus is being used, the vertical arm B 2 can be
swiveled half way round on its square box E 2, and used for drilling and
tapping the stay holes in marine boiler ends after they are put
together; of course sufficient room must be left between bed-plate A 2,
and the wall of boiler shop parallel with it, to allow for reception of
the boiler to be operated upon.
It would obviously be quite difficult to draw such drawings as in
Figures 298 and 299 on thin paper, so as to enable the drawing to be
traced on the wax direct by the process before described, unless indeed
the draftsman had considerable experience in fine work; hence, it is not
uncommon to make the drawing large, and on ordinary drawing paper. The
engraver then has the drawing photographed on the surface of the wax,
and works to the photograph. The letters of reference in wax engravings
are put in by impressing type in the wax, and in this connection it may
be remarked that the letters I and O should not be used on drawings to
be engraved by the wax process, unless they are situated outside the
outlines of the drawing, because the I looks so much like part of a
dotted line that it is often indistinguishable therefrom, while the O
looks like a circle or an ellipse.