Why Engineering Drawings? Engineering drawing is a formal and precise way of communicating information about the shape, size, features and precision of physical objects. Drawing is the universal language of engineering.
Jun 19, 2015
Why Engineering Drawings?Engineering drawing is a formal and precise way of communicating information about the shape, size, features and precision of physical objects.Drawing is the universal language of engineering.
Elements of Engineering Drawing
Engineering drawing are made up of graphics languagegraphics language
and word languageword language.
Graphicslanguage
Describe a shape(mainly).
Wordlanguage
Describe size, location andspecification of the object.
Basic Knowledge for Drafting
Graphicslanguage
Wordlanguage
Linetypes
Geometricconstruction Lettering
Projectionmethod
PROJECTION METHOD
Perspective
Oblique Orthographic
Axonometric Multiview
Parallel
PROJECTION THEORY
The projection theory is based on two variables:
1) Line of sight
2) Plane of projection (image plane or picture plane)
The projection theory is used to graphically represent
3-D objects on 2-D media (paper, computer screen).
Line of sightLine of sight is an imaginary ray of light between an
observer’s eye and an object.
Line of sight
Parallel projectionParallel projection
Line of sight
Perspective projectionPerspective projection
There are 2 types of LOS : parallel convergeand
Plane of projectionPlane of projection is an imaginary flat plane which
the image is created.
The image is produced by connecting the points where
the LOS pierce the projection plane.
Parallel projectionParallel projection Perspective projectionPerspective projection
Plane of projection Plane of projection
Disadvantage ofPerspective Projection
Perspective projection is notnot
used by engineer for manu-
facturing of parts, because
1) It is difficult to create.
2) It does not reveal exact
shape and size.Width is distorted
Orthographic Projection
Orthographic Projection
5
Orthographic projectionOrthographic projection is a parallel projection technique
in which the parallel lines of sight are perpendicular to the
projection plane
MEANING
Object views from top
Projection plane
1
2
3
4
51 2 3 4
Orthographic projection technique can produce either
1. 1. Multiview drawingMultiview drawing that each view show an object in two dimensions.
2. 2. Axonometric drawingAxonometric drawing that show all three dimensions of an object in one view.
Both drawing types are used in technical drawing for
communication.
ORTHOGRAPHIC VIEW
Axonometric (Isometric) Drawing
Easy to understand
Right angle becomes obtuse angle.
Circular hole becomes ellipse.
Distortions of shape and size in isometric drawing
AdvantageAdvantage
DisadvantageDisadvantage Shape and angle distortion
Example
Multiview Drawing
It represents accurate shape and size.AdvantageAdvantage
DisadvantageDisadvantage Require practice.
Multiviews drawing (2-view drawing)Example
Reading Isometric Sketches
Reading Isometric Sketches
Orthographic Projection
• An orthographic projection is a 2 dimensional representation of a 3 dimensional object.
Six Principle Views
• Any object can be viewed from six perpendicular views
The Glass Box
• One way to understand the standard arrangement of views on a sheet of paper is to envision the object in a glass box
• The outside observer would see six standard views of the object through the sides of this imaginary glass box
Glass Box Method
• Glass Box Method: – The object is placed in a glass box. – The image of the object is projected on the sides
of the box. – The box is unfolded.– The sides of the box are the principle views.
Glass Box Method
• The object is placed in a glass box.
• The side of the box represent the 6 principle planes.
Six Principle Views
OBSERVER MOVE AROUND
Front view Right side view
Top view
THE GLASS BOX CONCEPT
Bottom view
Left side view
Rear view
HeightWidth
De
pth
History
Example: Create the orthographic projections for the following object.
The height and depth of is equal to 3 grid squares.
Example: Create the orthographic projections for the following object.
Number of Required Views• When drawing an orthographic projection you
must include the appropriate number of views to fully describe the true shape of the part.
• You may use a fewer number of views if you can fully describe the part in the given views.– How many views are required to fully describe a
rectangular box?– How many views are required to fully describe a
sphere?
Number of Required Views
• You may use additional views, such as the left, back, or bottom views to describe an object if the object cannot be fully described in the three standard views.
– We will also learn about additional views this quarter such as section views, detail views and auxiliary views.
Each View Provides Two MeasurementsEach View Provides Two Measurements
• Top and Bottom views show DEPTH and WIDTH.
• Left and Right views show HEIGHT and DEPTH
• Front and Rear views show HEIGHT and WIDTH.
Measurement Transfer Between ViewsMeasurement Transfer Between Views
• The mitre line (drawn at a 45° angle) is used to transfer depth measurements between the top and right side (or left side) views.
Normal PlaneNormal Plane
• Normal Planes will appear as an edge in two views and a true sized plan in the remaining view when using three views such as a top, front and right side.
Inclined PlaneInclined Plane
• Inclined Planes will appear as an edge view in only one of the three views.
• The inclined plane will appear as a rectangular surface in the other two views.
Oblique PlanesOblique Planes• Oblique Planes will
not appear as an edge view in any of the six views since they are not parallel or perpendicular to the projection planes.
• They always appear as a “plane” and have the same number of corners in each of the six views.
Foreshortened Lines
Identify the foreshortened lines in the orthographic projection below.
Curved SurfacesCurved Surfaces
• A cylinder will appear as a circle in one view and a rectangular shape the other two views.
Axis and Center LinesAxis and Center Lines
• The axis appears where the cylinder appears rectangular.
• Center marks are used to identify the center of the cylinder where it appears circular.
Choosing the Front View
• Considerations when choosing the front view of an object.– Chose the view that shows the most features or
characteristics of the object– Choose the view that contains the least number of
hidden lines. – Choose the view so the part is oriented with its
longest length parallel to the bottom of the drawing.
First- and Third-Angle Projection
• There are two main systems used for projecting and unfolding the views:– Third-angle projection which is used in the United
States, Canada and some other countries– First-angle projection which is primarily used in
Europe and Asia
• You should understand both methods
Third-angle Projection
First-angle Projection
Object
We consider here the development of the orthographic views of a simple object.
Viewing
Direction
Picture Plane
For the Top view we view from the top!
Viewing
Direction
Point of intersection with picture plane
Projectors
Perpendicular to picture plane
Viewing
Direction
Intersections of allextreme points
Top View
Front View
Similarly, viewing from the front with parallel projectors
Top & Front Viewson opening up the page
Notice the interrelation
Similarly, the
Right Side View
Again notice the interrelation
Mitre
The third view can also be obtained by taking projections from the two views, using the mitre line, a line at 450
Two types of projections commonly used: I & III angle
In third angle, picture planes in between the viewer & object
In first angle, picture plane behind the object
Opening up of the box with the various views in III angle
The relationship on plane paper of the various views in III angle
TopView
FrontView
RightView
Left View
The relationship on plane paper of the various views in I angle
TopView
FrontView
Left View
RightView
Principles of Multi-View Drawings
1. The Front and the Top views are aligned vertically.
2. The Front and the Side views are aligned horizontally.
3. The front of the object in the top view faces the front view.
4. The front of the object in the side view faces the front view.
5. The depth of top view is the same as the depth of side view.
6. The width of top view is same as the width of front view.
7. The height of side view is same as height of front view.
Front View
Top View
Mitre
Front
Front View
Top View
Front
Drawing three views in III angle
Front View
Top ViewMitre
Front