The Technological World
Chapter 11:Communicating with Symbols
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Everything we use has symbols and
legends for us to follow. But we would
never know what the symbols meant
unless we had a Legend.
Drawings use graphics to communicate.
Writing is a form of graphic
communication.
Technical drawing is also graphic
communication specific to technology.
• Most common types: engineering
drawings and diagrams.
Diagram of
pliers
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Engineering drawing of
pliers
Technology: is a set of techniques used
by humans to design, build and maintain
objects and systems that we want or
need.
2 Lines and Geometry in Technical
Drawings
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Before a technical object can be
manufactured, its shape and
dimensions must be determined. A
technical drawing must be created.
Technical drawings can be made by
hand or on a computer.
Technical Drawings: are used in
technology to communicate
information about an object or a
system.
Basic and Geometric Lines
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Basic Lines: used in drafting are
lines whose appearance and meaning
are determined by international
agreements.
• Thick lines
• Medium lines
• Fine lines
Types of Lines in Orthographic Projections
Construction lines - Basic faint lines which are drawn to provide a
framework for the completion of the projection.
Dimension Lines display precise information about the size of the object.
10cm
Object/Visible Lines – represent visible features for an object. They are
the darkest and most present.
Hidden Lines – represent features that cannot be seen in the current view.
They are as dark as the object lines.
Center lines – represent symmetry and mark the center of circles, the
axes of cylinders, and the axes of symmetrical parts, such as bolts
Section/Cutting Plane Lines Are used to represent where the object is to
be “cut” or sectioned to reveal interior details. This is used when
other views may not adequately reveal information about the object.
A
A
Construction Lines
Construction lines When a drawing is rendered, basic faint lines are drawn to provide
a framework for the completion of the projection.
Although not part of
the object, these lines
provide the framework
to construct the
drawing of the object.
Dimension Lines
Dimension Lines display precise information about the size of the object.
Width
Top View
Depth
Front View
Right
Side
View
Height
Object/Visible Lines
Object
Lines Represent features that are only visible in the current view.
Extension
Line
Dimension
Line
Width
Top View
Top View
Depth
Front View
Right
Side
View
Height
Hidden lines
• represent features that cannot be seen in the current view.
We show these unseen details, with dashed lines.
ALL Hidden or internal
details must be included.
Top View
Front View
What does the inside of
this block look like?
Right
Side
View
Center Lines
• represent symmetry and mark the center of circles, the axes of
cylinders, and the axes of symmetrical parts, such as bolts.
Top View
Front View
Right
Side
View
Again, though not part
of the object, these
lines are important
elements of the
objects dimension and
construction.
Sectioning of a view
A
The section
line labeled
A:A shows the
arrows
A pointing in the
direction to
which you are
observing the
“cut” or
section.
A
A
Section Lines Are used to represent
where the object is to be “cut” or
sectioned to reveal interior details.
This is used when other views may
not adequately reveal information
about the object.
Sectional view A:A
Geometric Lines
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Are figures that are composed
according to the rules of geometry,
the art of drafting lines and curves
with a ruler and a compass.
Geometric rules and tools allow
for:
• Horizontal, straight, parallel lines.
• Vertical, straight, parallel lines that
are perpendicular to the horizontal
lines.
• Oblique lines.
• Circles and Ellipses with defined radii.
2.3 Three Ways of Producing
Technical Drawings
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Freehand: Sketch
Drawing with
drafting tools
(manual drafting
instruments)
Using drafting
software: computer-
Sketch, Drafting, & CADs
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Sketch: Quick illustration of an object
that is drawn freehand, respecting, as
much as possible, the conventions of
drafting.
1st drawings of an object are sketches.
2nd would be the manual drafting
instruments’ drawings or CADs.
Drawings with Manual Drafting
Instruments: used to create very
precise technical drawings prior to
CADs.
3 Projections
A projection is the representation of a
three-dimensional object on a twodimensional surface.
To understand these drawings, we must
be able to recognize and analyze
different projections.
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Space tech. terminology pg. 343
Projections differ by two aspects:
• Position of object with respect to paper.
• Angle between the visual rays and paper.
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Three of the most commonly used
projections are:
1. Multiview
2. Isometric
3. Oblique
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An Orthogonal Projection is a projection
in which all of the visual rays from the
object are perpendicular to the surface of
a sheet of paper.
Projections
Orthogonal
projections
Multiview
projections
1
Oblique
projections
3
Isometric
projections
2
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3.1 Multiview Projections
In a multiview
projection, each face
of the object is
drawn separately
looking at it from
straight on. 2D
6 views:
•
•
•
•
•
Front
Rear
left-side
right-side
Top
• Bottom.
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Multiview Projections
Usually only the top,
front and right side of
the object are
illustrated; presented in
an L-shape.
One side of object is
always parallel to the
paper. All
measurements and
angles present.
Conventional
Orthographic
Views
Each view is constructed so that information
(dimensions, and object edges, etc ..), are clearly related to the other views.
Width
Top View
Top View
Depth
Front View
Right
Side
View
Height
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3.2 Isometric Projections
Is a form of perspective
drawing in 3D.
Perspective Drawing
represents the three
dimensions of an object in
the same view.
An isometric projection is
a perspective drawing of an
object where the principal
edges are arranged on three
isometric axes (120° each).
No surface of the object is parallel to the
paper, measurements parallel to isometric
axes are to scale, angles are not.
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Isometric Drafting
3.3 Oblique Projections
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Is a perspective drawing in 3D.
An oblique projection is a
perspective drawing in which one of
the object’s sides is parallel to the
sheet of paper, but its depth is
represented by parallel straight lines
drawn at an oblique angle.
Produce drawings with precise
measurements for two dimensions:
height and length.
Oblique Projections
The Use of Projections in
Engineering Drawings
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Multiple projections are combined in
the engineering drawings for a
project.
Most commonly used are:
• General drawings
• Axonometric Projections: Exploded view
drawings
• Detail drawings
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General Drawing: an engineering drawing,
shows overall design. Multiview or isometric.
Axonometric Projection: parallel projection
used to create a 3D drawing, where the object
is rotated along one or more of its axes relative
to the plane of projection.
• Exploded View Drawing: is an engineering
drawing that shows the different parts, or features,
of the object separately. Usually multiview and
isometric projections. Bill of materials: includes
name of each part, quantity, and the materials
needed for construction.
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Detail Drawing: is an engineering drawing,
specifies the details needed to make a part of
an object. Usually a multiview projection.
Includes size, position and diameter of holes.
General Drawing
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Exploded View
Detail Drawing
4. Engineering Drawing
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Scale: is used to reduce or enlarge the
representation of an object on a sheet of paper.
This is called drawing to scale.
Scale reduction: reduce all measurements of
an object by the same factor. 1:50 make
measurements 50 times smaller.
Scale increase: increase all measurements of
an object by the same factor. 20:1 make
measurements 20 times bigger.
Full-size: object represented with its real
measurements. 1:1 make the measurements
exactly the same size.
Drawing’s title block includes: name of
draftsperson, title of drawing, date when made,
and scale that was used.
Scale of 1:2
Dimensional Tolerances
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Since machines, tools, instruments,
and the operators of machines are
not perfect, the manufactured parts
may be slightly different from the
dimensions indicated on the drawing.
A dimensional tolerance is an
indicator of the maximum acceptable
difference between a specified
measurement and the actual
measurement on the finished object.
Tolerance
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If the tolerance value applies to the
entire drawing then its written in the
title block.
Dimensioning
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Refers to the process of indicating
the real dimensions of an object as
well as the position of various
elements of the object.
• Use dimension lines and extension lines
with symbols. Ex: Ø – diameter of hole
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R – radius of circle or curve
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- degrees of an angle
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Cross Sections and Sections
Cross section: reveals the interior of an
object, exposing its hidden details to
view.
1st step: select dimension to imaginarily
cut to produce cross-sectional view.
• For multiview projection use cutting plane line
2nd step: draw cross-sectional view
indicated by the arrows of the cutting
plane line. Surface “cut” represented by
hatched lines and hidden lines not used.
Section: represents a surface in a crosssectional view.
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Aligned/Revolved
Section: is drawn
directly on the
object represented
Offset/Removed Section: is drawn
outside of the object represented. It is used
when adding a revolved section might
overload the drawing and make it harder to
understand.
Diagrams
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A simplified representation of an
object, a part of an object or a
system.
Symbols Used in Diagrams
Common Types of Diagrams
Refer to diagrams of
pliers
Using appropriate symbols
Circuit Diagram