It is not within the scope of the present work to explain and to
illustrate the principles according to which mechanical drawings are
executed. These must be studied in special treatises on Projection. The
several methods of giving expression and embellishment to this class of
drawings have, however, been fully described, and the principles upon
which these methods are founded carefully explained. It now remains for
us to add a few general remarks and some detailed instructions on the
practical application of these principles and methods.
Before commencing the delineation of any machine, the draughtsman should
make himself thoroughly acquainted with its character; that is, he
should ascertain the nature of the work it is designed to perform, the
means by which it performs that work, and the manner of its
construction. This preliminary study is necessary to enable him to
obtain a good general idea of the more important parts, which he will
have to give prominence to in the drawing, and to understand the nature
of the various connections between the numerous pieces of which the
machine is composed. The dimensions of the several parts must be
carefully taken, and when drawing from actual machinery, rough sketches
should be made to serve as a guide in getting out the complete drawing.
The dimensions should be clearly marked upon such sketches. As a general
rule, it is best to begin with the ground line and position of main
driving shafts, from which dimensions may be taken in every direction.
The manner of writing the dimensions, whether upon the rough sketch or
upon the complete drawing, should always be thus ← 2′ 6″ → for lateral,
and thus
↑
5″³⁄₈
↓
for vertical dimensions. To enable the draughtsman to take these with
accuracy, he should be provided with a pair of callipers for measuring
the diameters of shafts, a plumb-line for obtaining lateral distances
when the objects are not in the same horizontal plane, and a two-foot
rule.
The chief point to be attended to in commencing the drawing of a machine
is to obtain the correct positions of the centre lines of its principal
component parts, especial regard being had to the centres of motion.
These centre lines have been explained in a former Section. Having laid
down these lines accurately in their relative positions, separate
sketches may be made on a large scale of each part of the machine, and
the details of each part constructed upon each corresponding centre line
in succession, until the whole machine is built up. The centre lines
should be drawn in red, and the dimensions should be laid off on each
side of them. It will frequently be necessary to take a careful section,
to obtain sufficient information from which to draw the plan and the
elevation.
With respect to the written dimensions on a drawing, it may be remarked
that they cannot be too full or too numerous. Indeed, without complete
written dimensions a drawing is almost useless; for though a scale may
and should in all cases be attached, great labour would be required to
make use of the drawing by means of the scale only. Every dimension
which an engineer is likely to require to know should, therefore, be
plainly written. Nor is it sufficient to give a dimension once only, as
on the plan, for example, and to omit it on the elevation or on the
section. It should never be necessary to refer to another drawing to
find a dimension. The lettering should be clearly executed, and the
direction of the lettering should be the same as that of the figuring,
an example of which has been given; that is, it should read from the
front or from the right-hand side of the drawing.
If a drawing is to be coloured, the lettering, and all dark lines, such
as shade lines, must be left till after the colour has been applied. On
all coloured drawings the draughtsman should endeavour to obtain a
bright, clear tint by repeating the washes a sufficient number of times.
In preparing a flat-wash the tint should be mixed up slightly darker
than is required, and the solid colouring matter allowed to settle
before using. The solution, being poured off without disturbing the
sediment, will give a perfectly clear and pure tint. Tints for colouring
perspective drawings should always be prepared in this manner. The
methods of laying on flat-washes and of shading by colours have been
described in former Sections. The following additional remarks on colour
shading are taken from Worthen’s ‘Cyclopædia of Drawing.’
A means of adding considerably to the definiteness of a coloured
mechanical drawing, and of promoting, in a remarkable degree, its
effective appearance, is obtained by leaving a very narrow margin of
light on the edges of all surfaces, no matter what may be the angles
they form with the surfaces that join them. This should be done
invariably; we do not even except those edges which happen to have
shadows falling upon them. In such cases, however, this margin, instead
of being left quite white, should be slightly subdued. The difficulty of
achieving this effect of imparting a clear, regular, unbroken appearance
to these lines of light seems very formidable, and, indeed, almost
insuperable. The hand of the colourist may be as steady and confident as
a hand can be, and yet fail to guide the brush, at an almost
inappreciable distance from a straight or a circular line, with that
precision and sharpness so requisite for the production of this
beautiful effect. We shall, however, explain a novel and an effective
method of arriving at this most desirable result.
Suppose the object about to receive the colour to be the elevation of a
long flat rod or lever, on the edge of which a line of light is to be
left. Fill the drawing pen, as full as it will conveniently hold, with
tint, and draw a broad line just within, but not touching, the edge of
the lever exposed to the light. As it is essential to the successful
accomplishment of the operation that this line of colour should not dry,
even partially, before the tint on the whole side of the lever has been
laid on, it will be well to draw the pen a second time very lightly
along the line, so as to deposit as much tint as possible. Immediately
this has been done, the brush, filled with the same tint, should be
passed along so as to join the inner edge of this line of colour and the
whole surface of the lever to be filled in. By this means a distinct and
regular line of light is obtained without sacrifice of time. A still
more expeditious way of colouring such surfaces is to draw a second
line of colour along and in contact with the opposite edge of the lever
or other object, and to fill in the intermediate space between the two
wet lines with the brush. In this way a clear, uniform outline to the
tint is obtained. The blades of the drawing pen must not be sharp, and
care must be taken not to press heavily upon it, as otherwise the blades
will leave their course visible--an unsightly betrayal of mechanical
means to obtain such regularity in the colouring. Flat circular surfaces
may be treated in the same way, by using the pen compass instead of the
drawing pen. When such surfaces are large it will be judicious to colour
them in halves or in quadrantal spaces, but great care must be taken to
join the parts neatly. The lines of junction may be obliterated by
slightly washing them, or by laying a very light tint over the whole
surface, taking care in crossing the lines of junction to rub them
lightly with the brush.
The line of light upon cylindrical objects may be beautifully produced
by the same means. To indicate this line with perfect regularity is
highly important, for if strict uniformity be not maintained throughout
its length, the object will appear crooked or distorted. Having marked
in pencil the position of the light, and filled the drawing pen with a
just perceptible tint, draw a line of colour on one side of the line of
light. Then, with the brush filled with the same tint, fill up the space
unoccupied by the shade tint, within which the very light colour in the
brush will disappear. The portion of the surface on the other side of
the line of light being treated in the same way, the desired effect, of
a stream of light, clear and mathematically regular, will be obtained.
The effectiveness and expeditiousness of this method will be most
noticeable on long circular rods of small diameter, where a want of
accuracy is more immediately perceptible. The extreme depth of shade, as
well as the line of light, may, on such rods, be marked by filling the
pen with dark shade tint, and drawing it exactly over the line
representing the deepest part of the shade. On either side, and joining
this strip of dark colour, another, composed of lighter tint, is to be
drawn. Others successively lighter should follow, until, on one side,
the line of the rod is joined, and, on the other, the lightest part of
the rod is nearly reached. The line of light is then to be shown, and
the faint tint used at this part of the operation spread with the brush
lightly over the whole of that portion of the rod situate on either side
of this line, thus blending into smooth rotundity the graduated strips
of tint drawn with the pen.
For the correct representation of a building, plans, sections, and
elevations are required. The plan is usually a horizontal section of the
building close above the ground floor. The position and the dimensions
of the walls and the rooms of a house are shown by this means. As the
walls are shown in section in the plan, sections of the various walls
must, of course, be supplied before the plan can be drawn. It is usual
to colour the section of the walls in a ground plan; but not
unfrequently a dark wash of Indian ink is preferred to colour. The
number of sections required will depend upon the regularity of the
building; but generally it will be found that two half-sections are
sufficient. These two half-sections are usually placed side by side,
separated by a single line. The lines on which they are constructed must
be drawn distinctly on the plan, and lettered. The section is then
described as “Section” or “Half-section” on A B, &c. Usually the line of
section is broken in plan, and the section is then said to be on A B,
C D, one half being on A B and the other half on C D. Separate sections
to larger scales are required for the details of construction, such as
joints of rafters, mouldings to windows, and other parts needing
distinct representation. Elevations generally represent the whole of one
side of the building, and every side that differs from the rest must
have its own elevation. Such elevations are termed Front, Back, and End
Elevations, or North, South, East, and West Elevations. In order to show
the foundations, a section of the ground is sometimes given with an
elevation; in such a case the level of the ground should be shown by a
distinct line. Sometimes the portions of the structure below the ground
are shown by dotted lines. Such portions should not be coloured. In
getting out the drawings the plan should first be drawn, then the
sections, and finally the elevations. The colouring of elevations will
afford the student an opportunity of applying the knowledge he may have
acquired from a former Section of this work, and of displaying his
artistic taste.
In the accompanying Plates will be found examples of colouring
mechanical and architectural drawings. These should be studied in
conjunction with the Section on colouring in the first part of this
work. Plate 22 shows a piece of marine engine carefully coloured to
indicate the material of which the several parts are made, and Plate 23
contains a piece of permanent way, consisting of wrought-iron rail and
bolt, cast-iron chair and wooden sleeper and block, and an elevation of
a skew bridge, accurately coloured and shaded in accordance with the
principles already explained. It is not within the scope of this work to
treat the subject of projection, whether orthographic, isometrical, or
perspective; but we have given examples of each of these for the purpose
of illustrating the remarks and instructions on colouring given in the
Section referred to above. Thus Plate 24 is a perspective drawing, such
as are frequently made by architects, requiring a high degree of skill
and taste on the part of the colourist. And Plate 27 contains two
isometrical views of a building. These examples are intended to serve as
models of finished colouring.
The Plates relating to this Section are Nos. 22, 23, 24, and 27.