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All printing should be done by freehand.
_Border lines_ are seldom put on engineering drawings.
_Working Drawings._--A good working drawing should be prepared in the
following manner. It must first be carefully outlined in pencil and then
inked in. After this all parts cut by planes of section should be
coloured, the colours used indicating the materials of which the parts
are made. Parts which are round may also be lightly shaded with the
brush and colours to suit the materials. The centre lines are now inked
in with _red_ or _blue ink_. The red ink may be prepared by rubbing down
the cake of crimson lake, and the blue ink in like manner from the cake
of Prussian blue. Next come the _distance_ or _dimension_ lines, which
should be put in with _blue_ or _red ink_, depending on which colour was
used for the centre lines. Dimension lines and centre lines are best put
in of different colour. The arrow-heads at the ends of the dimension
lines are now put in with _black ink_, and so are the figures for the
dimensions. The arrow-heads and the figures should be made with a common
writing pen. The dimensions should be put on neatly. Many a good drawing
has its appearance spoiled through being slovenly dimensioned.
We may here point out the importance of putting the dimensions on a
working drawing. If the drawing is not dimensioned, the workman must get
his sizes from the drawing by applying his rule or a suitable scale. Now
this operation takes time, and is very liable to result in error. Time
is therefore saved, and the chance of error reduced, by marking the
sizes in figures.
In practice it is not usual to send original drawings from the drawing
office to the workshop, but copies only. The copies may be produced by
various 'processes,' or they may be tracings drawn by hand. Many
engineers do not ink in their original drawings, but leave them in
pencil; especially is this the case if the drawings are not likely to be
much used.
_Scales._--The best scales are made of ivory, and are twelve inches
long. Boxwood scales are much cheaper, although not so durable as those
made of ivory. If the student does not care to go to the expense of
ivory or boxwood scales, he can get paper ones very cheap, which will be
quite sufficient for his purpose. The divisions of the scale should be
marked down to its edge, so that measurements may be made by applying
the scale directly to the drawing. For working such exercises as are in
this book the student should be provided with the following scales:--
A scale of 1, or 12 inches to a foot.
" 1/2 " 6 "
" 1/3 " 4 "
" 1/4 " 3 "
" 1/6 " 2 "
A scale of 1 is spoken of as 'full size,' and a scale of 1/2 as 'half
size.'
Engineers in this country state dimensions of machines in feet, inches,
and fractions of an inch, the latter being the 1/2, 1/4, 1/8, 1/16, &c.
In making calculations it is generally more convenient to use decimal
fractions, and then substitute for the results the equivalent fractions
in eighths, sixteenths, &c. The following table will be found useful for
this purpose:--
_Decimal Equivalents of Fractions of an Inch._
+----------+--------------------+
| Fraction | Decimal Equivalent |
+----------+--------------------+
| 1/32 | .03125 |
| 1/16 | .0625 |
| 3/32 | .09375 |
| 1/8 | .125 |
| 5/32 | .15625 |
| 3/16 | .1875 |
| 7/32 | .21875 |
| 1/4 | .25 |
| 9/32 | .28125 |
| 5/16 | .3125 |
| 11/32 | .34375 |
| 3/8 | .375 |
| 13/32 | .40625 |
| 7/16 | .4375 |
| 15/32 | .46875 |
| 1/2 | .5 |
| 17/32 | .53125 |
| 9/16 | .5625 |
| 19/32 | .59375 |
| 5/8 | .625 |
| 21/32 | .65625 |
| 11/16 | .6875 |
| 23/32 | .71875 |
| 3/4 | .75 |
| 25/32 | .78125 |
| 13/16 | .8125 |
| 27/32 | .84375 |
| 7/8 | .875 |
| 29/32 | .90625 |
| 15/16 | .9375 |
| 31/32 | .96875 |
| 1 | 1.0 |
+----------+--------------------+
Engineers use a single accent (') to denote _feet_, and a double
accent (") to denote _inches_. Thus 2' 9" reads two feet nine inches.