_Drawing Instruments._--For working the exercises in this book the
student should be provided with the following:--A well-seasoned yellow
pine _drawing-board_, 24 inches long, 17 inches wide, and 3/8 inch or
1/2 inch thick, provided with cross-bars on the back to give it strength
and to prevent warping. A =T= _square_, with a blade 24 inches long
attached permanently to the stock, _but not sunk into it_. One 45 deg. and
one 60 deg. _set square_. The short edges of the former may be about 6
inches and the short edge of the latter about 5 inches long. A _pair of
compasses_ with pen and pencil attachments, and having legs from 5
inches to 6 inches long. A _pair of dividers_, with screw adjustment if
possible. A _pair of small steel spring pencil bows_ for drawing small
circles, and a _pair of small steel spring pen bows_ for inking in the
same. A _drawing pen_ for inking in straight lines. All compasses should
have _round points_, and if possible _needle_ points. A piece of
india-rubber will also be required, besides two pencils, one marked H or
HH and one marked HB or F; the latter to be used for lining in a drawing
which is not to be inked in, or for freehand work.
Pencils for mechanical drawing should be sharpened with a _chisel
point_, and those for freehand work with a _round point_. _Do not wet
the pencil_, as the lines afterwards made with it are very difficult to
rub out.
Drawing-paper for working drawings may be secured to the board by
_drawing-pins_, but the paper for finished drawings or drawings upon
which there is to be a large amount of colouring should be _stretched_
upon the board.
The student should get the best instruments he can afford to buy, and he
should rather have a few good instruments than a large box of inferior
ones.
_Drawing-paper._--The names and sizes of the sheets of drawing paper are
given in the following table:--
Inches
Demy 20 x 15
Medium 22 x 17
Royal 24 x 19
Imperial 30 x 22
Atlas 34 x 26
Double Elephant 40 x 27
Antiquarian 52 x 31
The above sizes must not be taken as exact. In practice they will be
found to vary in some cases as much as an inch.
Cartridge-paper is made in sheets of various sizes, and also in rolls.
Hand-made paper is the best, but it is expensive. Good cartridge-paper
is quite suitable for ordinary drawings.
_Centre Lines._--Drawings of most parts of machines will be found to
be symmetrical about certain lines called _centre lines_. These lines
should be drawn first with great care. On a pencil drawing centre
lines should be thin continuous lines; in this book they are shown
thus -- - -- - --.
After drawing the centre line of any part the dimensions of that part
must be marked off from the centre line, so as to insure that it really
is the centre line of that part: thus in making a drawing of a rivet,
such as is shown at (_a_) fig. 1, after drawing the centre line, half
the diameter of the rivet would be marked off on each side of that line,
in order to determine the lines for the sides of the rivet.
_Inking._--For inking in drawings the best Indian ink should be used,
and not common writing ink. Common ink does not dry quick enough, and
rapidly corrodes the drawing pens. The pen should be filled by means of
a brush or a narrow strip of paper, and not by dipping the pen into the
ink.
In cases where there are straight lines and arcs of circles touching one
another _ink in the arcs first_, then the straight lines; in this way it
is easier to hide the joints.
_Colouring._--Camel's-hair or sable brushes should be used; the latter
are the best, but are much more expensive than the former. The colour
should be rubbed down in a dish, and the tint should be light. The
mistake which a beginner invariably makes is in having the colour of too
dark a tint.
First go over the part to be coloured with the brush and _clean_ water
for the purpose of damping it. Next dry with clean blotting-paper to
take off any superfluous water. Then take another brush with the colour,
and beginning at the top, work from left to right and downwards. If it
is necessary to recolour any part let the first coating dry before
beginning.
Engineers have adopted certain colours to represent particular
materials; these are given in the following table:--
_Table showing Colours used to represent Different Materials._