The rules for patent drawings in England are practically the same as in
the United States; the paper sizes are, however, different. They must be
on sheets of one of the two following sizes (the smaller being
preferable), 13 inches at the sides by 8 inches at the top and bottom,
or 13 inches at the sides by 16 inches at the top and bottom, including
margin, which must be one-half an inch wide.
If there are more figures than can be shown on one of the smaller-sized
sheets, two or more of these sheets should be used in preference to
employing the large size. When an exceptionally large drawing is
required, it should be “_continued_” on subsequent sheets. There is no
limit to the number of sheets that may be sent in.
[Illustration: PRACTICAL POINTS]
[Illustration: Fig. 293. See page 244.]
Useful Hints and “Points.”
Many of these “points” are repetitions, with but little variation from
the way they have been previously stated; they are thus repeated to
emphasize their practical worth.
A good draughtsman leaves his work in such a state that any competent
person can without difficulty ink in what he has drawn.
The criterion of a good set of drawings is that with a properly prepared
specification they are complete in themselves and require no
explanation.
A “break” in a figure or object in a drawing is shown in rough irregular
lines, as in fig. 134, on page 131; this is useful when the paper is not
large enough to show the whole.
Never use a sloping line in writing fractions on a drawing. The
objection arises from the fact that such a dimension as 1³⁄₁₆, if
written with the inclined line, unless very distinctly executed, may be
read as ¹³⁄₁₆.
In inking do not draw the lines further than you wish them to go, but in
penciling it is well to extend the lines, free up.
Never use a scale for a ruler.
Do not overload the pen with ink.
Having filled the pen, nearly close the nibs and try the width of the
line on a piece of paper or the margin of the drawing.
Never refill or lay the pen aside without first cleaning it.
The application of the science of geometry to the drawing-board is
absolutely necessary to success, for the reason that the whole fabric of
mechanical drawing rests on the principles of geometry, which is well
termed the science of measurements.
Section lines should be the last inked and always without previous
penciling.
Center lines are necessary in working drawings.
In choosing T-squares, care should be exercised to see that the head
slides up and down the _left_-hand side of the board easily, and that
when pressed against the board with the left hand there is no “slogging”
of the blade up or down, or in other words, that the head is bearing
firmly for its whole length against the board.
The best place for the title of a drawing is said to be the upper
left-hand corner; this facilitates the filing of the sheet.
Never use a soft pencil except for finishing in shadow lines.
The rubber should always be kept clean.
Great care should be taken to keep drawing boards out of the way of heat
or damp, as these cause the wood to warp.
Circles and curves are to be “inked in” before straight lines. First ink
the smallest and afterwards the larger curves.
Do not press heavily on the pencil so as to cut the paper, but draw
lightly, so that the mark can be erased and leave no trace, especially
if the drawing is to be inked.
The draughtsman should commence his work at the top of the paper,
keeping the lower part covered over until he needs to use it.
Shade lines should be avoided in all working drawings, as their use
interferes with accurate measurements.
To make ink stick to the tracing cloth, with a woolen cloth rub some
powdered chalk or pounce over the surface on which the ink lines are to
be drawn, then wipe the surface clean and use a good quality of ink.
For striking small circles a small bow pen should be used.
To fix lead pencil marks on sketches so that they cannot be readily
erased, sponge them with milk carefully skimmed, then lay blotting paper
over them and iron with a hot flat-iron.
To have the ink preserve its fluidity and to keep out all dirt and dust,
keep the cover on the ink slab; the mistake is often made of putting too
liberal a supply of water in ink well, which causes a waste of both time
and ink; no more should be prepared than to meet immediate requirements.
Always draw on the right side of the sheet, which can be found by
holding the sheet up to the light and looking across its surface with
the eye nearly in the same plane as the paper; note which side is the
smoothest and has the least number of blemishes on it; this is the right
side to draw on.
As to sharpening pencils, it is always best to cut a chisel point on the
pencil used for drawing, and put a circular point on the pencils in the
bow pencil and pencil leg. The chisel point makes a finer line and lasts
much longer than a round point.
The varnish used in many large drawing-rooms is simply white shellac
dissolved in alcohol; it requires a little experience to mix these to a
proper consistency, but this is soon acquired.
Never sharpen your pencil over the drawing.
A center line of a drawing is the line upon which the figure is to be
constructed; the center line is the first line to be drawn.
The T-square belongs to the left side of the drawing-board, and is
operated by the left hand. The right hand should be kept free for the
purpose of picking up pencil, pen and bows, adjusting and marking off.
The left hand controls the T-square and the triangle that slides along
the upper edge of the square; the right hand is for the instruments.
The advantage of a paper rule or scale is that the paper will expand and
contract under varying degrees of atmospheric moisture the same as the
drawing does.
Avoid rubbing out and constantly cleaning the drawing with India rubber;
if wrong lines are made or it is desired to make alterations, the part
to be changed should be rubbed out and completely re-drawn.
When using the bows see to it that the steel-pointed leg that is put
down first on the paper, to secure a center for a curve or a circle, is
a trifle longer than the pencil or pen leg.
To clearly indicate the position of a center which is to be used again,
lightly pencil a small circle about it; never put the point of a pencil
in the center hole to enlarge or blacken it; the prick point made by the
dividers and needle points should be no more than can be just seen,
hence the circle to be made as advised above.
[Illustration: Fig. 294.]
Be particular in having the legs of the dividers exactly the same
length, and sharp, so that in pricking off distances, and dimensions,
and centers, the indent or hole made in the paper is as small as
possible.
The term “plane” means a perfectly flat surface; that is, something
which has length and breadth but no thickness.
The best way to indicate on the drawing the surfaces which are to be
finished is to write on the lines which represent the finished surfaces
“finished,” tool-finish, or “faced,” according to the degree of finish
required. The single letter _f_ is frequently used.
Avoid fingering the drawing sheet as much as possible; in pointing to
any part of the drawing use a pencil and not the finger.
Remember that a drawing is made to be read.
The skill in inking does not depend on the fineness of the line, but on
its clearness.
A soft pencil should never be used on a mechanical drawing unless in
rare cases when it is used for pencil shading; the hardness or softness
of pencils is denoted by letters.
Never ink any portion of a drawing until the penciling is entirely
finished.
Stretching or pasting the paper to the board is very seldom resorted to,
for the reason that the mechanical drawings are _to scale_ and the paper
is natural when pinned to the board and more correct than if under a
strain. Mechanical drawings are always required in practice _right
away_, and time would be wasted and lost in damping and pasting and
drying again.
A working drawing, whether made to a scale or not, must have all the
dimensions plainly written upon it, for a workman should never be
compelled to measure a drawing.
In marking off distances, centers, etc., a fine needle point is useful;
the hole should not be punctured through the paper, merely a prick
point, so that it will leave an impression, which will not be
obliterated by the use of rubber; drawing-pens are often equipped with
such a needle point in the end of the handle, that is visible only when
the pen is unscrewed from the handle; but in the absence of one of this
kind the point of the divider leg will be of use.
Mechanical construction drawings represent a large amount of mental and
manual work, as well as a considerable cost in money; hence, they are of
value quite as much as property which has been acquired by the
expenditure of either labor or capital. It is wise to keep copies of
original designs and sketches, as well as data and formulæ, for record
and comparison.
The best system for keeping drawings is to make them of certain standard
sizes, and to keep them flat, unrolled, in drawers, numbered, lettered
and labeled.
In an office where space is limited and drawings have to be rolled it is
well to use a number of pasteboard cases about three feet long and three
inches in diameter. These are shown in fig. 294.
A puncture can be made near the top and, when a new drawing or
blue-print is inserted in this cylindrical case, a cardboard tag can be
looped through the puncture. This label will give the title and number
of drawings in that case.
A manuscript book methodically and neatly kept should tell immediately
the number of the drawing and the case.
Fig. 293 is good for practice in line drawing and also as an optical
illusion. “You look and are deceived. At first glance you say, ‘Of
course, those two lines are curved.’ You are mistaken. They are exactly
parallel. In order to prove this hold them up edgewise to the eye. It
is, of course, the subsidiary lines which lead the vision astray. It is
a case of first impressions being quite wrong.”
[Illustration: LINEAR PERSPECTIVE]
[Illustration: Fig. 295.]
Linear Perspective.
It should be mentioned that this subject is outside the limits of
mechanical drawing, which only deals with objects that can be measured,
projected or dimensioned to an accurate scale.
But, in rounding out the more formal subjects it is well to look a
little outside the rigid lines of mechanics into the methods of nature,
for no system of teaching drawing is complete that does not include some
explanations for sketching from nature--the objects being always around
the student, the eye always clear to see and the hand only needing the
training to make permanent the impressions received.
The word perspective means to _see through_; the word perspective being
derived from the Latin word _perspicere_, to look through, hence,
perspective is a science which teaches us to _see_ correctly and enables
us to represent the _appearance_ of anything we may wish to draw; care
should be taken in perspective drawing, to select objects interesting in
themselves, and the best specimens of their class, so as to cultivate
taste, while they at the same time afford useful and instructive drawing
lessons.
The meaning of the term linear perspective is a line view; the previous
examples have been composed of surfaces placed fronting the eye;
perspective is the science which treats of the changes of form produced
by viewing them in various oblique positions.
The slightest alteration of _position_ will change the _appearance_ of
an object; this can be easily shown--for illustration take a coin, the
actual shape of which is a perfect round; or, strictly speaking, a
circle. If we take the coin between the thumb and the first finger,
holding it in an upright position, and exactly facing the eyes, as in
Fig. 296, it appears of its true form, viz., a circle. If we alter its
position, balancing it upon the thumb, in a level position, with its
edge directly opposite the eye, as in Fig. 297, its appearance is
changed, and what we know to be really a circle, appears to us as a
straight line.
[Illustration: Fig. 296.--Fig. 297.--Fig. 298.]
Now, still balancing the coin upon the thumb, but changing its position
with regard to the eye, by holding it a little lower than in the last
position, that is slightly beneath the level of the eye, as in fig. 298,
we see both the edge and the surface, the coin now appearing neither a
circle nor a straight line, but a curved figure of an elliptical form.
Thus the same coin held in three different positions has assumed three
different shapes.
Let us take two coins of the same size, holding (in the position shown
at fig. 296) one in each hand. Now, closing one eye, (which will make
the experiment more clear), hold one coin out at arm’s length, and the
other at about the distance of a foot from the eye. On comparing them,
we find that the coin which is further from the eye appears less than
the nearer one. We know that the coins are really equal in size, yet one
appears smaller than the other.
We thus see that when we change the position of an object, we have as a
consequence a change of appearance; also that the change of appearance
may affect both the shape and the size of the object.
These diversities of appearance may be remarked in everything around us.
We can observe them in the street by looking at a building from
different points of view, or by comparing the apparent sizes of the
street lamps; in the railway station, by watching the arriving or
departing train; and at sea, by noticing the vessels as they approach,
or as they retire, ultimately vanishing from our sight in that line
where the sea and sky appear to meet.
All these interesting variations of appearance are in strict accordance
with the laws of =GEOMETRY= and =OPTICS=. The former subject has been
enlarged upon beginning with page 81 of this work, where a line, a
point, an angle, etc., are defined; other terms are explained at page 41
and the following pages; to these we add a few definitions essential to
the subject.
=A PLANE= is a surface which is perfectly even and flat; to use a
familiar illustration, a plane is like the surface of a sheet of plate
glass; recollect particularly, that a surface which is at all curved, is
not a plane.
The =GROUND-PLANE= is the plane on which we stand; the _base-line_ is an
imaginary line passing through the middle of the feet as we stand square
and erect; and the _vertical plane_ is supposed to stand on the
base-line and perpendicular to it.
[Illustration: Fig. 299.--See page 255.]
Planes are parallel to each other when they are throughout their entire
surfaces the same distance apart.
[Illustration: Fig. 300.]
=THE PERSPECTIVE PLANE= is an upright square of glass, usually framed
like a picture, with a base, so that it can stand up alone. This is
placed between the eye of the spectator and the subject to be drawn, and
as the drawing is sometimes made directly upon it, it is sometimes
called the _Picture_ or the _Plane of the Picture_.
[Illustration: Fig. 301.]
=HORIZONTAL= means perfectly level, like the surface of still water. We
must be careful to understand perfectly the difference between the terms
“level” and “even” or “flat.” A surface may be even or flat, without
being level. Thus the wall is even and flat, but it is upright, not
level; level means a fixed, constant position.
In fig. 301 a house is shown in perspective in which the line _H L_ is
the line of the horizon and _V P_ is the,--
=VANISHING POINT.=--The vanishing point is familiarly represented by the
rails on a trolley track on a straight road, which seem to approach each
other in the distance, as shown in fig. 302 at _V P_.
[Illustration: Fig. 302.]
_All parallel lines seen in perspective appear to meet in the same
vanishing point._
The value of the vanishing point may be seen in the view of a wooden
house, fig. 303, where it (_V P_) gives direction to the retiring lines
of the roof, side planks and door.
[Illustration: Fig. 303.]
=POINT OF SIGHT.=--This is that point in the eye where the lines or rays
from the object cross each other, as shown at _P_ in fig. 305, also in
fig. 299 at _S_.
=VERTICAL= means perfectly upright. If we attach a piece of thread to a
weight, a small piece of lead for example, and hold the thread with the
lead hanging downwards, the thread will fall in an upright or vertical
position.
=PARALLEL= lines are said to be parallel to each other when they are
throughout their whole lengths the same distance apart.
=PERPENDICULAR.= When one straight line, meeting another, makes the
angles at the point of contact equal, each of the angles is called a
right angle, and the lines are said to be perpendicular to each other.
Remember especially that perpendicular and vertical have not the same
meaning. Vertical means an unvarying upright position. Perpendicular
means that one line or plane meets another line or plane at right
angles.
The fig. 295 on page 246 is a study in perspective, showing a water
reflection. As rays from every visible part of the object are reflected,
all following the same law, the reflection will appear to the eye
_inverted_, and of the _same size as the object_. The arch itself forms
the upper half of a hollow cylinder, and the reflection forms the lower
half. The reflection shows much more of the interior of the arch than
can be seen directly. The leaning tree, the boy fishing, and the
receding banks, all are seen in accordance with the laws of reflection
and perspective.
=THE HORIZONTAL LINE, THE POINT OF SIGHT AND THE VANISHING POINTS= are
the principal items. These should be studied in every room and during
every walk, and the more pleasing accidents of form stored in the mind
or committed to paper for future use.
[Illustration: Fig. 304.]
=OPTICS=, the science of sight, gives us the following laws: