1. Pre-Engineering & Computer-Aided Design I
Segment 5
Projection Systems: Orthographic

2. Orthographic Projection
Lesson Objectives
Define Orthographic Projection and It’s Main Advantage
Explain View Selection
Explain the Glass Box Approach
Define First and Third Angle Projections
Clarify Line Precedence
Complete Two View and Three View Drawings
Instructional Objectives:
Represent a 3-D objects in 2-D using 3 principal views of an orthographic projection
Conclude that a dimension (Wd./Ht./Depth) is shared between two views of an orthographic projection
Discuss line precedence and first/third angle projections
Provide tips on making orthographic drawings
Questions that are not answered by pictorial presentation:
Does the hole go all the way through the object?
Are there any feature like slots or holes on the back sides of the object?
orthographic projection drawings will answer these and other questions. Multi View drawing is one of the most commonly used graphics tool by Engineers.

3. Orthographic Projection What is Orthographic Projection?
Ortho – Greek word meaning perpendicular
Shows the Views of an Object Projected in
2D, Usually the Top, Front, and Right Side
Views
What is the Advantage?
Instructional Objectives:
Represent a 3-D objects in 2-D using 3 principal views of an orthographic projection
Conclude that a dimension (Wd./Ht./Depth) is shared between two views of an orthographic projection
Develop orthographic sketches of simple isometric sketches on rectilinear grid sheet
Questions that are not answered by pictorial presentation:
Does the hole go all the way through the object?
Are there any feature like slots or holes on the back sides of the object?
Orthographic projection drawings will answer these and other questions. Multi View drawing is one of the most commonly used graphics tool by Engineers.
Represents Features of an
Object More Accurately

4. Defining the Six Principal Views or Orthographic Views
Orthographic Projection
Defining the Six Principal Views or Orthographic Views
Although any face could be chosen to be the front, once front and two other face are selected all are determined. There are really SIX PRINICPAL VIEWS as defined in the diagram.
Generally do not need all six to fully describe the object. A conventional Engineering Drawing will normally have 2 to 3 views unless it required more views to describe the geometry/ profile.
We know which ones they are on the drawing, because we always present them in the same relationship to each other. I.e. Top above front, right to right of front, etc. This convention is called as the Third angle method.. The other method in which the views can be placed is the First angle method in which the Top view is below front view, Right side view is on left side of front view. For this class we will be following the Third angle convention.
These are often called orthographic projections – because the line of sight is perpendicular to the principal view

5. Deciding Which Views to Present
Orthographic Projection
Deciding Which Views to Present
General Guidelines
Pick a Front View That is Most Descriptive of Object
Normally the Longest Dimension is Chosen
as the Width (or Depth)
Most Common Combination of Views to
Use: Front, Top, and Side View
Pick the views which will help in describing the object with highest clarity.
Explain what is an auxiliary view. Explain that they are drawn to show specific features that are not clear in the Principal views.
Any Other View Different From the Principal
Views is Called an Auxiliary View

6. Orthographic Projection
The Idea is to have them take an object from the table.
Declare front. FRONT View is the MOST DESCRIPTIVE VIEW OF THE OBJECT. The view that gives MORE INFORMATION ABOUT THE OBJECT.
Rotate 90 degrees “up” to get top view.
Rotate Back.
Rotate 90 degrees clockwise to get right side.
This give three principal views commonly used.

7. Orthographic Projection
Glass Box Approach
Most Powerful Technique to Understand Orthographic Projection
Suspend the Object With Transparent
Strings Inside a Glass Box
Freeze the View From Each Direction
(Each of the Six Sides of the Box) and
Unfold the Box
At this point, give an introduction to Glass-box approach for developing orthographic projection drawings.
Student slides contain snapshots of the animation
Animation illustrates glass-box approach

8. Orthographic Projection
Glass Box Approach
The object, whose orthographic projection needs to be drawn, is enclosed in a glass-box

9. Orthographic Projection
Glass Box Approach
Project points on the front view of the glass-box

10. Orthographic Projection
Glass Box Approach
Project points on the the top view of the glass-box, just as done for front

11. Orthographic Projection
Glass Box Approach
Project points on the right view of the glass-box, just as done for front and top

12. Orthographic Projection
Glass Box Approach
Unfold the glass box, see how the views align

13. Orthographic Projection
Glass Box Approach
Unfold the glass box, see how the views align

14. First and Third Angle Projections
Orthographic Projection
First and Third Angle Projections
Third-angle Projection
First-angle Projection
Instructor:
Third angle projection is normally used in the US while Europe uses the First Angle projection. Note the symbols at the bottom of each one which tell you which projection that you are viewing.
These can be confusing to students. We are only highlighting the fact that there are different ways to represent projections. It is not expected for students to fully understand the differences.
First Angle – International
Third Angle – U.S.

15. Conventional Orthographic Views
Orthographic Projection
Conventional Orthographic Views
Height
Depth
Width
Front View
Top View
Right Side View
Note that the views are placed and aligned in the manner shown in the diagram. Remind the students that they have to follow the above convention for all their home work problems and exam problems.
It is very important to maintain the alignment and correct placement relative to each other.
Means line for top (and bottom) is straight across for both front view and right side view for example.
Same thing between front and top for sides.
Note : The following can be seen from the slide:
Top View and front view have the same width
Front View and Right / Left side view have the same height.
The depth of Top view is same as the width of right/ left side view.

16. Are The Orthographic Views OK?
Orthographic Projection
Are The Orthographic Views OK?
At this point have the students decide what is wrong and compare notes.

17. Orthographic Views Must Be In Projection
Orthographic Projection
Orthographic Views Must Be In Projection
At this point have the students decide what is wrong and compare notes.

18. Hidden and Center Lines
Orthographic Projection
Hidden and Center Lines
Hidden Line – Used to Represent Features That Cannot be Seen in the Current View
Centerlines – Used to Represent Symmetry and to Mark the Center of Circles and the Axes of Cylinders, and the Axes of Symmetrical Parts, Such as Nuts & Bolts
Note the graphical convention for hidden and centerlines.

19. Orthographic Projection
Example:
1. Visible
In engineering and technical drawing, it is important that hidden features be represented, so that the reader of the drawing can clearly understand the object.
Thus we need hidden lines to emphasize that those features exist and are hidden in that particular view.
We also need center lines to understand how the features defined in the 2D views translate into 3D.
NOTE: It must be emphasized that hidden lines and center lines are used only on Orthographic projection drawings, never on isometric drawings
Q: Do we need a convention for what line to show if two lines fall on top of each other?
A: Yes! Otherwise features which are more important (eg: visible lines) would be overridden by less important features (eg: hidden lines) and the resulting drawing would be interpreted inaccurately. The next slide shows the convention followed.
2. Hidden
3. Center

20. Orthographic Projection
In engineering and technical drawing, it is important that hidden features be represented,
so that the reader of the drawing can clearly understand the object.
Thus we need hidden lines to emphasize that those features exist and are hidden in that particular view.
We also need center lines to understand how the features defined in the 2D views translate into 3D.
NOTE: It must be emphasized that hidden lines and center lines are used only on Orthographic projection drawings, never on isometric drawings
Q: Do we need a convention for what line to show if two lines fall on top of each other?
A: Yes! Otherwise features which are more important (eg: visible lines) would be overridden by less important features (eg: hidden lines) and the resulting drawing would be interpreted inaccurately. The next slide shows the convention followed.

21. Orthographic Projection
Precedence of Lines
Visible lines takes precedence over all other lines
Hidden lines and cutting plane lines take precedence over center lines
Center lines have lowest precedence
.35 mm Line Weight
Note the thickness specified for given three types of lines
The precedence of lines governs which lines are drawn when more than one line occupies the same position on a drawing.
For example the figure above shows that while a visible line takes precedence over all other lines, hidden line and cutting plane line take precedence over center lines.
Standard engineering drawing practices requires the use of standard linetypes, which are called the alphabet of lines. The sizes show the recommended line thicknesses.
0.05 mm Line Weight
0.05 mm Line Weight

22. Example: Application of Precedence
Orthographic Projection
Example: Application of Precedence
Slide rearranged.
This slide shows the precedence of
Visible over hidden lines
Hidden over center lines (Note observation D)
Visible over center lines
(D) Note that the center lines still show but are drawn away from the hidden line and visible line in the right side view. Remind students that when the draw center lines – The center lines should extend beyond the boundary of the object and also they should leave some space between the boundary and the extended center line --- Refer Label D in above slide.
(E) Note gap between body and start of center lines.

23. Intersecting Lines in Orthographic Projections
Solid Line Intersections
Hidden Line Special Case Intersections
Gap

24. Orthographic Projection
Two-View Drawings
Some Objects Can Be Fully Described By Two Views, Look For:
Symmetry or Bodies of Rotation
In this case the top and the right side views would be the same. Thus a two view sketch captures all the details.
The book shows how to draw the sketch, so emphasize having the students going through the explanation given in the text book.
Emphasize on the glass box approach and how the alignment of the views is essential.
Right Side
Front View
Right Side View
Front View

25. Other Two-View Examples
Orthographic Projection
Other Two-View Examples
This can be confusing topic to students. It may be difficult to definitively specify when two views are enough. For this course we should just introduce the concept.
These two-view examples show that some symmetrical objects can be fully represented in two views.
Things to emphasize
The center lines in the top view are drawn such that the small dashes cross at the center of the circle and they are separated by some space from the longer dashes.
The center lines in the front view shows the centers of the cylindrical bores.

26. Orthographic Projection
Review Questions
Based on the lines of sight, orthographic projection drawing is classified as a ___________ projection technique.
There are ____ standard principal views of orthographic projections.
Each view in an orthographic projection concentrates on ____ dimensions of the object
Answers:
Parallel
6 (However, only 3 are used for representation)
2 dimensions

27. Animation – Glass Box Theory.
Orthographic Projection
Animation – Glass Box Theory.
This slide summarizes about what we have talked about in this basics session.