Technical Drawing
Designing things on paper
Conceptual Sketches
• When you first get
an idea for
something you want
to build you may
draw it roughly,
without using
instruments or
accurate scales.
This is called a
conceptual sketch.
Types of Drawings
All Drawings
Technical
Artistic
Sketches
Diagrams
Drawings
(conceptual)
(design & technical)
(technical/engineering)
simulated
perspective
Diagram
Diagram
Oblique
Isometric
Multi view
(design plan)
(technical)
projection
projection
orthographic
Less technical
More technical
• A conceptual sketch…
– Allows an idea to be
expressed quickly in graphic
form
– Is prepared free-hand (without
drawing instruments)
– Is not done to scale, but it
respects the rules of technical
drawing as much as possible,
and is made roughly
proportional to the object
represented.
Technical Drawings
• Serve as a reference
to workers, architects
or machinists.
• When you are ready
to design the details,
you make a technical
drawing, done with
more detail and more
accuracy.
Difference:
• Sketch: made without drafting tools...
Just pencil and paper. Scale is
approximate, not accurate, but the sketch
should still look like the object.
• Drawing: made with drafting tools...
Ruler, set squares, protractor and
compass. Scale should be accurate and
the drawing carefully made.
A technical drawing…
Presents all the information necessary for the object’s construction.
Is made with great precision,
Requires the use of rulers, compass and protractor or drafting software.
Is done to scale, and respects the proportions of the object represented.
Respects conventions in the mode of the representation.
Basic Lines (part 1)
Represents the
outline
Thick
Draw the visible
object edges
with these lines
Shows hidden
details
Medium
dashed
Draw hidden
edges with
these
Construction Used during
Line
drafting
Fine
Faint lines,
sometimes
coloured
Dimension
Line
Used to indicate
a dimension
Fine,
Dimension &
extension lines
go together!
Extension
Line
Used with a
dimension line
Fine
Center
(Axial) Line
Shows center or
symmetry
Fine
Visible
(Object) Line
Hidden Line
with arrows
Near arrows
With dash in
middle
8cm
8 cm
Basic Lines (part 1)
Ghost Line
Sometimes in
other color
Shows possible
movement
Fine
Cutting
plane line
Position of a
cross-section
Thick
Hatched
Lines
Surface of a
cross-section
Fine
Show where it
is solid (section
view only!!)
Long Break
Shows that a
line is shortened
Fine
Used only for
large objects
Short break
(cutaway)
Object shortened
or cut away
Medium
Leader Line
Points to
something
Fine
dashed
With arrows
With zig-zag
Used only if you
plan to draw a
cross section
With curve
With bent
arrow
Attaches a comment
or angle or dimension
to an object.
Technical Drawing of Fighter Plane
(Isometric Projection, exploded view)
Technical drawings can represent things…
as simple as a spoon,
or…
Technical Drawing of a spoon
Multi-view, Orthographic projection
• as complex as a space ship
Photograph of Mercury Spacecraft
Technical Drawing of Mercury Spacecraft, Top View, Orthographic
 Technical Drawing of Mercury Spacecraft, Isometric Projection
Projections
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•
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Perspective projections
Multi-view projections
Isometric projection
Oblique projection
Vanishing Point
True Perspective
• Objects drawn in true perspective
look realistic.
• They have “vanishing points”
where straight lines seem to
Vanishing Points
converge
• They can have one, two or three vanishing
points, depending on how much the artist
wants to work.
• But in true perspective, objects far
away will be drawn smaller than
nearby objects… not a good idea
in technical drawing!
Vanishing Points
More True Perspective Drawings
• Now that you have seen how nice
perspective drawings can be...
• FORGET ABOUT THEM!
– We hardly ever use perspective projections
when doing technical drawing.
– They are too much work, and they don’t show
all the details we may need to show.
– Also, they distort both angles and dimensions.
• Leave them for ART class...
Isometric Projection
(a simulated perspective drawing style)
• Isometric (or simulated
perspective) drawings
look at first like
perspective drawings…
• But the lines don’t
converge. There are no
vanishing points and
distant objects are the
same size as nearby
ones.
• Right angles in isometric
projections are usually
represented by 60° or
120° angles.
60˚
Represents 90˚
Represents 90°
More isometric drawings
Warning: Your workbook classifies isometric drawings as “perspective”
drawings, but they are not true perspective. They resemble perspective
drawings but in a true perspective drawing, distant objects are drawn smaller.
In isometric drawing, distant objects are not smaller..
Oblique Projection
another “simulated” perspective
• Similar to isometric projection, it
is also a “simulated perspective”
• In oblique projections, the side
of the object facing you is drawn
“square” and accurate (that is
with right angles at 90° and its
measurements proportional)
• The sides not facing you are
distorted
•
Warning: Your workbook calls this oblique
perspective, but it is not a true perspective.
This side
is not!
This side is
accurate
90°
=60°
90°
Orthographic Projections
• Orthographic projections “flatten” one view
of the object onto a sheet of paper, while
retaining the correct proportions (angles
and dimensions)
• Maps are an example of orthographic
projection (a top view)
• The trouble with orthographic projections
is that one view usually isn’t enough.
• Maps and floor-plans are exceptions, where one
top-view is often enough.
Orthographic Projection
(Multi-view)
• Draws an object as it
would be seen from
several different
directions
• The views are “flat”,
with all angles shown
correctly and all
measurements to
scale.
Comparing Projections
• Perspective drawings look nicer
when used by an artist, but…
• Isometric, oblique and multi-view
drawings give more accurate
information when used in
technical drawing.
• Isometric drawings show accurate
dimensions, but distorted angles.
• Oblique drawings give accurate
dimensions for one side only.
• Orthographic (multi-view) are the
best choice for most technical
drawing.
Oblique
Orthographic
(multi-view)
Comparison of Projections
Projection
Used for
Tested on
Orthographic
Drafting, maps, floor plans
Yes
Technical drawings,
drafting, conceptual
sketches (sometimes)
Artistic drawing, conceptual
sketches
Conceptual sketches,
technical drawings (sometimes)
Conceptual sketches
Yes
(top view)
Orthographic
(multi-view)
True Perspective
(one, two or three point)
Isometric
(simulated perspective)
Oblique
(simulated perspective)
No
Yes
No
An Isometric Drawing
Scale
• SCALE is the relationship between the
measurement of an object drawn on a sheet
of paper, and the measurements of the actual
object.
• Often technical drawings are made a different
size from the objects they represent.
• Scale-reduction is when the drawing is smaller
than the actual object
• Scale-increase is when the drawing is larger
than the actual object.
Dimensioning
• When a drawing is done to scale, you should
label it with its dimensions.
• Use dimension lines to label each dimension, with
extensions where necessary
• You should label just enough edges to show all the
dimensions, but you don’t need to repeat.
• In Canada (at least for science) dimensions are
usually given in millimetres (mm). If you use different
units (cm, in, feet etc.) you need to write the unit. You
do not need to write mm if all of your dimensions are
in millimetres.
• Angles can be shown in degrees (°), diameter by Ø,
and radius by R
300 means 300mm
unless another unit is
specified
Dimensioning
A Toy Truck
How to label
an angle.
Ø means diameter
(R would be radius)
How to label small
dimensions.
Dimensioning
Dimensions can also be
shown on isometric
drawings, but you have to
be a bit more careful.
Also, you should only
show the most important
dimensions on an
isometric drawing.
Diagrams
• Diagrams are simplified versions of a
drawing. Diagrams show how an object
works, not necessarily how it looks.
• Diagrams often use abstract symbols
rather than actual pictures to represent
things.
A Circuit Diagram
– Doesn’t show what the circuit looks like, but
tells an electrician how its all connected.
Diagram of Energy Conversion Unit
– It doesn’t show what the unit looks like, but rather,
what it does or how it works.
Exploded Views
• A diagram that shows an object “taken
apart” is sometimes called an exploded
view.
Define:
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Technology
Technical Drawing
Basic Lines
Geometric Lines
Sketch
Projection
Isometric (projection) Oblique (projection)
Orthagonal (proj.) Multiview (projection)
General drawing Exploded drawing
Detail drawing
Scale
Dimensioning
Tolerance
Section
Cross section
Diagrams: design, technical, and circuit
Basic Lines to Know
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Visible(or object)
Hidden
Construction
Centre
Dimension and extension
Cutting Plane
Hatching
Leader (or reference)
Geometric Lines to know
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Horizontal (straight)
Vertical (straight)
Oblique
Ellipse (and circle)
Curve
Methods of Drawing
• Sketching
• Drafting
• Computer Assisted Design (C.A.D.)
Projections
• Multiview (orthagonal)
• Isometric
• Oblique
Scale
• Dimensioning
• Calculating Scale