Engineering Drawing
Basic drafting and lettering
Lettering, Sketching and Line Techniques
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A engineering drawing is a highly stylized graphic
representation of an idea. The idea might be of something
that we can see such a real or virtual object, space or
environment. In some cases, such as an electronic
schematic diagram for example, the drawing will bear no
visual resemblance to the physical object that will be built
from the information it provides.
In every case with the possible exception of "3D" and
rendered drawings, which communicate with a different
graphics language, we can understand engineering
drawings only because we can understand the basic
language of technical graphics.
Attachment of paper to the board
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The sheet should be placed close to
the left edge of the drafting board.
Working in this area makes the T
square easier to handle and reduces
the likelihood of error because of T
square “swing.”
The drafting sheet should be far
enough from the bottom of the board
about 100 mm. to ensure firm support
for the head of the T square when
you are drawing at the lower part of
the sheet.
After aligning the drawing sheet,
smooth out any wrinkles and fasten
the four corners with short strips of
drafting tape. If you are attaching
large sheets, you should place
additional strips of tape at the top and
bottom edges of the sheet. Avoid the
use of thumbtacks; they will
eventually ruin the drafting board
Horizontal Lines
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The draftsman’s horizontal line is constructed by drawing from
left to right along the working edge of a T square. This working
edge, when true, is perpendicular to the working edge of the
drafting board.
Horizontal Lines
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When you draw horizontal lines, keep the working edge of the T square
head in firm contact with the working edge of the drafting board. The
pencil should be inclined to the right at an angle of about 60 degrees, with
the point close to the junction of the working edge and the paper.
Horizontal Lines
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Hold the pencil lightly and, if it was sharpened with a conical point, rotate it
slowly while drawing the line to achieve a uniform line width and preserve the
shape of the point. Normally, when a series of horizontal lines is being drawn,
the sequence of drawing is from the top down
Vertical Lines
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Vertical lines are produced parallel to the working edge of the drafting board
by using triangles in combination with a T square.
Vertical Lines
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One leg of a triangle is placed against the working edge of the blade and the
other faces the working edge of the board to prevent the draftsman from
casting a shadow over his work.
Vertical Lines
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Lines are drawn from the bottom up. The pencil is inclined toward the top of the
working sheet at an angle of approximately 60 degrees, with the point as close
as possible to the junction of the triangle and the drafting paper.
Vertical Lines
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Sequence in drawing a series of vertical lines is from left to right. At no time
should the lower edge of the T square blade be used as a base for triangles
Inclined Lines
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Inclined lines at standard angles are
constructed with the T square as a
base for triangles used either singly, as
shown or in combination.
Used in combination with the T square
as a base, the triangles serve as guides
for producing lines at intervals of 15
degrees.
Used singly, the 45-degree triangle will
divide a circle into 8 equal parts; the
30°/60° triangle will divide a circle into
12 equal parts. For drawing lines at
angles other than those, you should
use a protractor.
It is understood that the triangles in
each case are resting on the blade of
the T-Square
Inclined Lines
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It is possible to
divide 360
degrees angle
into twenty-four
15 degrees
sectors with the
triangles used
singly or in
combination.
Protraction of Angles
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To measure an angle, place the
center mark of the protractor at
the vertex of the angle, with the
0-degree line along one side.
Then note the degree mark that
falls on the side.
To lay off an angle, position the
protractor as above and use a
needlepoint or a sharp-pointed
pencil to mark the desired
values. Then project lines from
the vertex to these marks.
Using only the three points on
the protractor as described
before may result in
considerable inaccuracy,
particularly if the lines of an
angle are to be extended for
some distance beyond the
protractor.
Protraction of Angles
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A refinement of the procedure is as
follows: Suppose angle BOA is to
be measured. Extend line AO on to
C; extend line BO on to D. When
you set the center of the protractor
at O, make sure that both points c
and a are on line AC. Take your
reading at point d as well as at
point b when you measure the
angle.
If you are laying off the angle BOA,
protract and mark point d as well
as point b; this gives you three
points (d, O, and b) for
establishing line DB. If you are
using a semicircular protractor, you
can’t, of course, locate point d; but
your accuracy will be improved by
lining up c, O, and a before you
measure or lay off the single angle
BOA.
Parallel Lines
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To draw a line parallel to a given line, adjust the hypotenuse of a triangle in
combination with a straightedge (T square or triangle) to the given line;
then, holding the straightedge firmly in position, slip the triangle to the
desired position and draw the parallel line along the hypotenuse
Perpendicular Lines
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To construct a line perpendicular to an existing line, use the triangle and
straightedge in combination, with the hypotenuse of the triangle resting
against the upper edge of the straightedge. Adjust one leg of the triangle to a
given line. Then slide the triangle along the supporting straightedge to the
desired position and draw the line along the leg, perpendicular to the leg that
was adjusted to the given line.
Circles and Arcs
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When you are drawing
circles and arcs, it is
important that the lines
produced with the
compass are the same
weight as
corresponding pencil
lines.
Since you cannot exert
as much pressure on
the compass as you
can with pencils, you
should use a compass
lead that is about one
grade softer than the
pencil used for
corresponding line
work.
For dim construction
lines, use 4H to 6H
leads. Avoid using
leads that are too
short.
Circles and Arcs
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To draw a circle with a
compass, lightly press
the needlepoint into
the drawing paper and
rotate the marking leg
around it. As you
rotate, lean the
compass slightly
forward. With a little
practice, you will find
that you can easily
draw smooth circles
using only the thumb
and forefinger of one
hand.
It is important that you
use an even pressure
as you rotate the
compass. You may find
it necessary to rotate
the compass several
times to produce a
circle with a uniform
dense black line
Drawing a circle of a given diameter with a compass
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Draw a horizontal line and a
vertical line intersecting the
horizontal line.
Measure the radius of the circle
with a scale, and draw a second
vertical line from this point.
Set the needlepoint at the
intersection of the first vertical
line and the horizontal line. This is
the center of the circle.
Set the marking leg to fall on the
intersection of the second vertical
line and the horizontal line, and
draw a half circle with the
compass.
Check your work by measuring
the diameter established by this
half circle with a scale.
Once You have set the compass
to the exact radius of the circle,
set the needlepoint at the center
of the circle and carefully rotate
the compass to draw a line
describing the circumference of
the circle.
Use of the French Curve
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The french curve is
used to draw a smooth
line through
predetermined points.
After the points are
plotted, a light pencil
line should be sketched
to connect the points
in a smooth flowing
line.
To draw the finished
line over the freehand
line, match the various
parts of the french
curve to various
segments of the
freehand curve. Avoid
abrupt changes in
curvature by placing
the short radius of the
french curve toward
the short radius
portion of the line to
be drawn.
Use of the French Curve
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Change your position
around the drawing
board when necessary
so that you can work
on the side of the
french curve that is
away from you. You
should avoid working
on the “under” side of
the french curve.
Place the french curve
so that it intersects at
least two points of the
line. When drawing the
line along the edge of
the french curve, stop
short of the last point
intersected. Then
move the french curve
along to intersect two
or three more points
and make sure that the
edge of the curve
connects smoothly
with the line already
drawn.
Use of Drafting Templates
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Circles or arcs can be drawn
more quickly with a
template than with a
compass. Templates must
be used properly to be
effective.
To draw a circle with the
circle template, lay out
center lines on the drawing
where the circle is to be
drawn, then place the
correct circle opening over
the center line so that the
quadrant lines on the
template coincide with the
center lines on the paper.
Draw the circle, using a
sharp, conical point on the
pencil.
Allowance must always be
made for the width of the
pencil line in placing the
template opening in the
right position on the
drawing.
Use of Drafting Templates
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To draw an arc, lay
out tangent lines on
the drawing, then
place the correct size
circle of the template
on the paper so that
the template
quadrant lines
coincide with the
tangent lines, and
draw the arc.
When using a
template, you must
hold it down firmly to
keep it from slipping
out of position.
Figures or circles
from the template
must be drawn with
the correct line
weight on the first
setting as it is
difficult to reset the
template in the exact
position.
Use of The Dividers
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Dividers are used to transfer
measurements, to step off a
series of equal distances, and
to divide lines into a number of
equal parts.
To transfer measurements on a
drawing, set the dividers to the
correct distance, then transfer
the measurements to the
drawing by pricking the
drawing surface very lightly
with the points of the dividers.
To measure off a series of
equal distances on the line, set
the dividers to the given
distance. Then step off this
distance as many times as
desired by swinging the
dividers from one leg to the
other along the line, first
swinging clockwise 180
degrees, then counterclockwise
180 degrees, and so on.
Use of The Dividers
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In dividing either a straight line or a
curved line into a given number of equal
parts (for example, four) by trial, open
the dividers to a rough approximation of
the first division (in this case, one
quarter of the line length) and step off
the distance lightly, holding the dividers
by the handle and pivoting the
instrument on alternate sides of the line
at each step.
If the dividers fall short of the end of the
line after the fourth step, hold the back
leg in place and advance the forward
leg, by guess, one quarter of the
remaining distance.
Repeat the procedure until the last step
falls at the end of the line. Be careful
during this process not to punch holes in
the paper, but just barely mark the
surface for future reference.
To identify prick marks made with small
dividers for future reference, circle the
marks lightly with a pencil
Text in Engineering Drawings
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Text is an important part of
a technical drawing. Not all
information required on
technical drawings can be
communicated graphically.
Examples are dimension,
and material types.
Several different ways are
used to create text. The
traditional method is
freehand lettering. Other
methods include mechanical
lettering as scriber
template.
Neat lettering is important
so that the information
being conveyed can be
easily read.
Text in Engineering Drawings
Text in Engineering Drawings
Lettering Guidelines
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For uniformity, all letters should
be the same height, proportion
and inclination. A necessary tactic
for maintaining uniformity is the
use of guidelines.
Guidelines are a critical part of
freehand lettering. Uniformity,
neatness and stability can not be
achieved without using guidelines.
Guidelines ensure consistency in
the size of the letter characters. If
your lettering consists of capitals,
draw only the cap line and base
line. If lowercase letters are
included as well, draw the waist
line and drop line.
Method of using lettering guidelines
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The waist line indicates the upper limit of
the lowercase letters. The ascender is the
part of the lowercase letter that extends
above the body of the letter; for example,
the dot portion of the character i in the
figure in view A. All ascenders are as high
as the caps.
The drop line indicates the lower limit of the
lowercase letters. The descender is the part
of the lowercase letter that extends below
the body of the letter, an example being
the tail of the character g in figure 3-42,
view A. The vertical distance from the drop
line to the base line is the same as the
vertical distance from the waist line to the
cap line. It is about one third of the vertical
distance between the base line and the cap
line, or about one half of the vertical
distance between the base line and the
waist line.
Method of using lettering guidelines for Capital Letters
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Lay off letter heights, H,
and draw light guidelines
with a 2H pencil.
Space lines no closer than
H/2 apart.
Draw vertical guidelines as
light, thin, randomly spaced
lines.
Draw letters with single
strokes using a mediumgrade pencil.
Method of using lettering guidelines
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To lay out guidelines for
caps and lowercase, let the
height of a capital be 1 1/2
times the distance "a.“
Set a compass or dividers to
distance "a," and lay off
distance "a" above and
below the midline selected
for the guidelines, this
locates the cap line and the
drop line.
Then set the compass or
dividers to one half of ’’a,"
and lay off this distance
above and below the
midline. This method locates
the waist line and the base
line.
Method of using lettering guidelines