# Projection Systems: Orthographic and Isometric

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Projection Systems: Orthographic and Isometric
Engineering Tech GT Engineering Principals & Applications Introduction to Engineering & Technology Concepts Projection Systems: Orthographic and Isometric Objectives:

Objectives Orthographic Projections View Selection Glass Box Approach
Line Precedence Two View Drawings Tips 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 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 Projections
Advantage – Represent features of an object more accurately Example Problem 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.

Orthographic Projections
Orthographic Projections are a collection of 2-D drawings that work together to give an accurate overall representation of an object. By definition for each element of a orthographic projection drawing you only present 2 of the three dimensions. Think of it as an observer look at one face, what do they see. Any orthographic projection drawing normal has three views… Front view, Top view and side view (Right or left side view)

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

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 is to use: Front, Top, and Side View Any other view different from the Principal Views is called an Auxiliary 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.

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.

Glass Box Approach Most powerful technique to understand orthographic projections 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 Animation illustrates glass-box approach At this point, give an introduction to Glass-box approach for developing orthographic projection drawings. Student slides contain snapshots of the animation

Glass Box Approach Available on the FEH website:
Lecture Notes Engineering H191 – Autumn 2005 Glass Box Animation Upon demonstrating the glass-box theory using the software, return to following slide Following slides provide snapshots of the demo software

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

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

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

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

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

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

First and Third Angle Projections
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. From Fundamentals of Graphic Communications by Bertoline, McGraw-Hill First Angle – International Third Angle – U.S.

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.

Is The Orthographic View OK?
At this point have the students decide what is wrong and compare notes.

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

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.

For Example: 1. Visible 2. Hidden 3. Center
From Bertoline: Figure 2.38 / Pg 42 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

Precedence of Lines Visible lines take precedence over all other lines
Hidden lines and cutting plane lines take precedence over center lines Center lines have lowest precedence From: Bertoline, Figure 2.40/ Pg 43 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.

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 Intersections Gap This slide is from the Technical Graphics notebook, Chapter 3.

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. Front View Right Side View Right Side Front View

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.

Summary Introduced to orthographic projections
We recommend the software animation exercise introduced in class. Animation can be found on my website. This slide summarizes about what we have talked about in this basics session.

Review Questions Rectilinear grids are used for sketching isometric pictorials … True/False Based on the lines of sight, orthographic projection drawings 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: False – rectilinear are for orthographic projection Parallel or Orthographic projection 6 (However, only 3 are used for representation) 2 dimensions

Hints for Orthographic Projection Sketching
Identify the major features and overall dimensions of the object Do not use any straight-edge devices as a pencil guide when sketching by hand Start by drawing bounding boxes with light construction lines. Keep views aligned while sketching Please emphasize these to the students. They are very useful!!

Hints for Orthographic Projection Sketching
Title Information is required – follow conventions Usage of construction lines is encouraged. Mandatory for circle or ellipse Orthographic projection: Alignment of the views is important! You will lose points, if not aligned

Hints for Orthographic Projection Sketching
Map inclined and oblique faces to all three views Follow the precedence of lines Darken all visible, hidden, and center lines

Sketching a Circle Draw a square whose sides are the diameter of the circle. At the center of each side define the point of tangency for the circle. Draw the diagonals of the square. Orient the paper so you can draw equal arcs to construct the circle