3. A sketch should be intelligible to any one, even if they are
unacquainted with drawing.
4. Horizontal and vertical lines and a few curves will enable one to
make almost any simple sketch.
5. It must be also remembered in making drawings from actual measurement
that the instruments are not in the first place employed; the rough
sketch is first made and then it is converted into a drawing. The
draughtsman makes a rough sketch entirely by the hand and eye, measures
the various parts, and jots down the measurements in his sketch; after
this he reduces the whole to the desired scale, and proceeds to make his
mechanical drawing.
6. Let the sketch book be the constant companion of the student; it may
be advantageously filled with outlines of machine or other work suitable
for preservation, to be made into finished drawings, or for reference.
Sketches are often valuable for reference as aids in originating new
designs.
7. A sketch, when possible, should have all the dimensions written upon
it, but--
8. Sketches in shop practice should not take the place of
working-drawings; the latter have a check upon them in being drawn to a
scale--hence the figures written upon them and dimensions by scale must
agree.
9. Place title and date on each sketch--no matter how seemingly
unimportant--for future reference.
10. Practice sketching at every favorable opportunity. There is no
necessity for detail at first--simply the outlines of the article and
its parts.
11. Sketch-books, with paper bound in cloth covers, are utilized for
bold, off-hand sketches by experienced draughtsmen, but a single sheet
of paper, used on both sides, is not unworthy of service in an
emergency--or even the blank side of a letter may be available.
Sketching-blocks, or paper “pads,” 4 × 6, or more, in size, and
containing 48 sheets, are sold by stationers, and are found to be most
convenient to have in hand and for practical use. Portfolio-envelopes,
made of extra length paper (manila) are useful in filing away sketches
and drawings. The size 10¹⁄₂ × 15 is used for United States Patent
Office drawings.
The function of the pencil--in mechanical drawing--is to make a path for
the pen to follow. If it were possible to make a drawing with all its
lines ending at the proper place, at the first time, there would be no
necessity for using the pencil. One is obliged, however, so to use the
pencil that all lines pass beyond the actual ending place, thus making a
distinct point for the drawing pen to stop at.
The pencil should be pressed to the paper just enough to make a clean,
fine line, and no more; once over the path is sufficient, if the line is
visible and true.
To sharpen drawing pencils, 1, use a fine file, after taking off enough
of the wood with a knife; 2, make a conical point for the free-hand
drawing pencils and a chisel point for ruling and marking distances.
Pencil compasses are instruments where one leg is provided with a pencil
point. Fig. 209 shows the mode of manipulation of those shown in fig.
180 and fig. 181.
The pencil compass is held by the projection above the joint between the
thumb and first finger, which enables it to be rotated by a movement of
the finger without causing any undue pressure on the points. Should much
pressure be applied, there is a tendency to force the point or center
through the paper, making an ugly center mark; at the same time the
pressure tends to break off the pencil point.
These few illustrations from fig. 212 to fig. 219 are made designedly
simple, so that they may be utilized in “free-hand” work, for which they
are good practice, as well as serving for examples in mechanical
drawing.
[Illustration: Figs. 212.]
Figs. 212, showing a spool or bobbin, exhibit three views, viz.: front
elevation or plan, and section; both are drawn with simple lines, the
end elevation by circles.
[Illustration: Fig. 213.--Fig. 214.]
Figs. 213 and 214 are two side views of a hexagon head bolt. Figs. 216
and 217 are a square head bolt--two side views and end view. Figs. 218
and 219 are a front and edge view of a forked or double joint.
[Illustration: Figs. 215.]
Figs. 215 show three views of a file handle; the front view and section
are practice for compound curves and curved lines meeting straight ones;
all these are capable of being produced by instruments.
Moreover, many of the views and illustrations used to instruct and
explain machine tools and other devices in other parts of the volume,
are drawn so that they may be used also as examples in advanced
instrumental practice. This is a “hint” to the diligent and painstaking
student worthy of remembering.
[Illustration: Fig. 216.--Fig. 217.--Fig. 218.--Fig. 219.]
The designing and drawing of arcs and whole circles occupy a large
proportion of space in nearly all mechanical drawings. The making of a
complete circle is a matter of no great difficulty, but the beginning
and termination of parts of circles require both judgment and
considerable practice.
To aid the student these two illustrations of circles are introduced. To
draw fig. 220 with a pencil, using the upper edge of the blade of the
T-square as a guide, draw a center line, _A B_, mark on it a distance of
4 inches, space this into half inches, using the dividers and making the
points with it; then with the pencil compasses or bow pencil, which must
be held as shown on page 209, and rotated from left to right, or
clockwise, draw a series of circles through these points, tangent to one
another or all touching at _A_, care being taken that the pencil lines
exactly meet at _A_, and also cut the divided points as shown in the
illustration. For fig. 221 divide the center line as before, and draw
the semi-circles on it _A B_, _B C_, meeting at _B_, and _C D_, _D E_,
etc.
[Illustration: Fig. 220.]
[Illustration: Fig. 221.]
Now from center _B_ draw circles _A C_, _C E_, meeting in _C_, and so on
with the circles, arcs or segments; success in drawing this figure
depends on the correct spacing of the center line in the first instance
into equal parts.
The T-square should be used for drawing horizontal lines only. Its head
should always be placed upon the left edge of the board. Vertical lines
should be drawn by the use of a triangle placed upon the T-square and
not by means of the T-square only; because the edges of a board are
seldom at right angles to each other, and the blade of the T-square is
often not at right angles to the head, so that lines at right angles to
each other will not result from the use of the T-square upon all edges
of the board. Only the upper edge of the T-square should be used, as the
edges are often not quite straight or parallel.
The 45° triangle has two angles of 45° and one of 90°. The 30° and 60°
triangle has an angle of 30°, one of 60°, and one of 90°. By placing
these triangles upon the T-square, lines at any of these angles with a
vertical or horizontal line may be drawn.
Drawings finished in ink are much more effective and desirable than
pencil drawings; but as a good inked drawing cannot be made except upon
an accurate pencil drawing, students should begin with the pencil, and
should not use ink until they are able to produce satisfactory results
in pencil.
Projection.
The word projection means to throw forward, and in ordinary machine
drawing it is the projecting or throwing forward of one view from
another view.
In drawings the lines in one view or plan may be availed of to find
those of others of the same object, and also to find their shape or
curvature as they would appear in the other representations; this is
called projection-drawing.
Fig. 222 is the illustration as shown in fig. 212 on page 143, with the
addition of dotted projection line, which illustrates the method of
throwing forward the section and the end view of the object; these two
views are procured from the plan or first figure, as shown in fig. 222.
Fig. 224 represents the square bolt and nut shown in fig. 216, and the
mode of projecting is similarly shown by dotted lines.
Fig. 223 shows file handle shown in fig. 215 and the mode of projecting.
[Illustration: Fig. 222.]
[Illustration: Fig. 223.]
[Illustration: Fig. 224.]
The principles upon which “projection” in drawing is based, are
illustrated in the following examples and text: As a real object can be
scaled with a foot rule, so a drawing must permit of scaling and
measuring. This measuring may take place as with the real object in full
size or the drawing may, for the sake of convenience, be reproduced and
measured in a reduced scale, as half, or in still smaller sizes.
Sometimes it may prove convenient to enlarge the drawing to twice the
natural size of the object, as a means of making it stand out more
clearly than the real size would accomplish.
For practical purposes, it is productive of economy of time to mark the
dimensions of height and width or depth on the drawing in figures, to
avoid the scaling. This marking of the dimensions is best done at the
time of making the drawing, while the conception of the object is clear.
To convey a correct impression of the object, all lines that are marked
to be of equal length should appear equally long on the drawing and be
capable of being scaled to such equal length; for this, it must be
assumed that the eye of the observer is equally distant from all points
of a plane through the nearest point, or one of the axes of the object,
and that the lines of sight are all parallel to each other and square to
this plane.
In fig. 225 these lines of sight are seen as directed toward one side of
a cube or block; it will be readily understood, however, that in this
way nothing is visible and accessible for scaling and dimensioning
except this front face of the block, thus, a determination of the
dimensions of only height and width would be possible, while the
dimension of depth is entirely undetermined.
[Illustration: Fig. 225.]
It is thus necessary to get a view of the block from another side; the
direction in which it will be most instructive to obtain additional
views is in the direction of the breadth and of the length and square to
the lines of sight of the first view.
Fig. 226 shows how the lines of sight would strike the object in the
three directions. If these lines should be rays of light, some of them
would pass by the body until they squarely strike the large plane
surfaces, _I_, _II_, _III_; naturally the rays of light on the faces of
the object will be retained, and cannot strike the plane surfaces, thus
leaving dark shadows of exactly the same outlines as the block; these
would be exact drawings of the faces, and if by some means they can be
fixed and retained on the plane they can be completely measured and
dimensioned.
This throwing forward of the outline of the object in different views on
the planes is called projection of the object, and furnishes a highly
important means of fixing the outlines and dimensions in the three main
directions of height, width and depth; evidently, the light rays passing
by the front face may not all reach the plane of projection, but they
may be retained by protruding parts of the object behind the front face.
These protruding parts naturally would also be projected on the plane in
the same manner as the main body of the block.
These projections of the protruding part are plainly visible in plane
_II_ and plane _III_, while the part would not be drawn in outline in
plane _I_. It may be imagined, however, that the greater thickness of
the body in the direction of the protruding part would intensify also
the shadow, thus outlining the face of the protruding part in plane _I_.
It is apparent that these three projections are all needed, but as
drawing is all done in one single plane, the three projections will for
the sake of convenience have to be brought into a single plane. This can
be realized if plane _II_ is swung around axis _O Z_ and plane _III_
around axis _O Y_, until all three surfaces are in one single plane,
which would then appear as shown in fig. 227.
It is also possible to assume transparent planes in front of the body
and extend the parallel lines of sight forward instead of backward. Thus
an outline picture will be created on each of the three planes _I_,
_II_, _III_ in fig. 228, in a manner similar to fig. 226. For drawing
purposes, all three views again have to be brought into a single plane,
which is done by swinging _II_ around _O Z_, and _III_ around _O Y_ in
the same manner as fig. 227 was evolved from fig. 226.
It will be noted that in fig. 229 plane _III_ is now above _I_ and plane
_II_ on the left-hand side of _I_, while in fig. 227 they were below and
at the right-hand side of plane _I_. As the swinging of the top and side
planes takes place around the edges of the front plane _I_ two systems
may thus be distinguished, according to the position of plane _I_ in
regard to the object. Fig. 226 thus represents the system of backward
projection, while fig. 228 represents the system of forward projection.
Either system can have, however, the plane _II_ at the right- or
left-hand side edge, while plane _III_ may be attached to the top or
bottom edge of plane _I_; it is readily understood that a number of
combinations are possible for each system, as it is not necessary to
adhere absolutely to one rule. The system of forward projection is the
one generally practiced and further examples are all executed by this
system, meaning that the planes are always between the observer and the
object.
[Illustration: Fig. 226.]
[Illustration: Fig. 227.]
[Illustration: Fig. 228.]
For the clearness of the drawing, it is desirable to have all corners,
edges and outlines appear in such solid lines as they appear to the eye.
If, therefore, certain sharply defined outlines occur on one side more
than on the opposite one, it is most desirable to take the view against
the side that has the most definitely marked outlines.
If the opposite side should show a number of wholly different features,
it may prove even desirable to show this side also, thus gaining four
views instead of the usual three, and so obtain a more complete
understanding of the shape of the object, besides giving increased
facilities for dimensioning each part of the body.
This additional view may be taken from the sides, as well as up or down,
thus making a maximum of five views possible by which the outside of the
object may be delineated.
Fig. 230 shows why it may be desirable to take five views of a block
that has a receding space of different outlines in each of the side and
top and bottom faces.
It is not necessary, however, to resort to five views in such a case as
is represented in fig. 230, as the only differing feature, the circular
space in _III_ bottom, might be shown in _III_ top by dotted lines, and
the difference of _II_ right might be shown in _II_ left, also by dotted
lines. It is desirable to show four or five views only where great
complication and consequent lack of clearness through numerous dotted
lines would result from having a less number of views.
[Illustration: Fig. 229.]
So far the projections and views have only represented the outlines of
the object; it may often be desirable, however, to show central holes or
other perforations or variations of sections; in this case it is
possible to imagine the object cut in slices, by planes, through certain
well defined axes, or other lines of distinctive importance, and then
take a view of this sectional plane with its newly created intersections
or sharply marked outlines; thus, a section may often take the place of
the third, fourth or fifth view to great advantage.
[Illustration: Fig. 230.]
It is not always possible to get a view against a face or side of the
object, but, with irregularly shaped bodies or under special conditions,
it may be necessary to take a view of corners, sloping planes, curved or
irregularly shaped surfaces.
Fig. 231 shows a hexagonal nut in the three normal projections; from the
top view it is readily seen that the front view shows the side of the
hexagon in a contracted scale, and that therefore the scaling and
dimensioning for the horizontal direction have all to be done in the top
or plan view; the front and side views convey, however, the dimension of
height correctly, and these are therefore the right places for scaling
and dimensioning in the vertical direction.
Fig. 231 shows how a cylindrical outline appears in the three views; the
hole in the nut presents itself as circular in the top view, while it
appears in the front and side views as a rectangle. For simple objects,
it is unnecessary to show the edges of the planes, and the three views
are grouped, as regards distances and positions, as most convenient for
the execution of the drawing.
Where sloping surfaces are of irregular form, it may be necessary to
employ help lines for their full determination in the three views. Fig.
232 shows how the surface that is produced by a slanting cut through a
cylinder would appear in the three views. The help lines are placed in
the top view in eight equal divisions around the circumference of the
cylinder. These division lines are shown by dotted lines on the cylinder
in the front and side views. Their intersections, with the sloping cut
in the front view, furnish also the height of the corresponding points
for the side view.
[Illustration: Fig. 231.]
By progressive determination of points, lines and surfaces, even the
most complicated bodies can be completely represented for their
reproduction in any application to mechanical or industrial purposes.
[Illustration: Fig. 232.]
The spur wheel shown on page 163 is an example of projection drawing.
The wheel is illustrated in three views: fig. 235 is a side view, or
elevation; fig. 234 is a front elevation; fig. 233 is a section view on
_C D_. The section is projected from the front elevation by drawing
parallel lines from the points in front elevation where the lines are
intersected by the center line _C D_, cutting the plane of view and
showing the interior shape at _C D_.
The side elevation, fig. 235, is projected from the front elevation,
fig. 234, by drawing parallel lines from the edges in the front view
across its face.
In actual drawing practice the figures should be made about three times
larger than the example, and as follows:
For the front elevation draw the center lines _A B_, _C D_, and from
their point of intersection as a center, with the compasses draw the
inner circle or hole, also the pitch line _E E_. With the dividers space
this line into nineteen equal divisions; each point on this line will be
the center of a tooth, and the distance from one point to the next one
is the pitch of the tooth.
Now, with the dividers mark off the thickness of tooth at each side of
these points on pitch line; with the compasses draw the outer circle for
points of teeth, the inner circle for the root of teeth, and circles for
thickness of rim and hub; also circles representing the fillets at rim
and hub. For clearness and to prevent confusion these lines are shown on
one-half the wheel only, terminating in center line _C D_; all the lines
of the front elevation are now complete excepting the teeth.
In the drawing office it is unusual to delineate all the teeth in a gear
wheel, the lines without the teeth as now completed being deemed
sufficient, giving all particulars; however, to prevent the error of
mistaking the circles it is well to represent one or two teeth on the
drawing.
[Illustration: Fig. 233.--Fig. 234.--Fig. 235.]
In the example, proceed and complete the entire wheel, taking on the
compasses a radius equal to the pitch of the tooth; set them on the
pitch line at the point _G_, already spaced for thickness of tooth, draw
line _H_ from pitch line to root of tooth; proceed similarly all around
the circle, completing one side. Next, reverse the operation and draw
the corresponding side of the root of tooth. Now take a radius equal to
half the space between teeth and the thickness of tooth on pitch line,
and, with center in pitch line, as shown at _I_, draw the outside or
addendum of tooth, _J_; it will be apparent that the reverse addendum of
the tooth next adjoining can be formed at the same time, with one
setting of the compasses; finish all the teeth similarly; the front
elevation will thus be completed.
Fig. 233 is an excellent example of sectional drawing, to be executed as
follows:
First draw the center line _L M_ of fig. 233, lay off at each side of it
half the breadth of face and of the hub; now project, from center line
_C D_, in the front elevation, fig. 234, with the T-square the upper and
lower teeth, the fillets, the chamfers, the hub and center hole, also
dot in the pitch line _F F_, take the radii and draw the fillets and
chamfers; draw section lines and the view will appear as shown in fig.
233.
Fig. 235 is a side elevation of fig. 234, and is a fine sample of
projection, to be executed as follows:
Proceed similarly as for fig. 233, projecting the lines from the outside
edges of the front elevation, instead of from the center line _C D_ as
in last figure, and the end elevation will be as shown in fig. 235.
The student will be assisted in understanding this working drawing by
consulting the pages under the heading of “Gearing.”
[Illustration: LETTERING, INKING