Computer Aided Design I Projection Systems: Orthographic

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 Understand the 3 Planes of Projection 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.

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

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

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

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.

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

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

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

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

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

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

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

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.

Conventional Orthographic Views Orthographic Projection Conventional Orthographic Views F Height Depth Width Front View Top View Right Side View P F 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. H

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.

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.

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 Cylinders and Bolts Note the graphical convention for hidden and centerlines.

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

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.

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 0.50 mm 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. 0.15 mm 0.15 mm

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.

Intersecting Lines in Orthographic Projections Solid Line Intersections Dashed Line Special Case Intersections Gap

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

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.

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

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