Overview
This lecture introduces orthographic projections, explaining how they are used to represent three-dimensional objects in engineering and the techniques for drawing and interpreting these views.
Orthographic Projections Basics
- Orthographic projection is a method to represent 3D objects on 2D surfaces using parallel projection lines.
- A projection plane is the paper/surface onto which the object's view is projected.
- Six principal views can be created: top (plan), front, left, right, bottom, and rear.
- Plan view shows the top, while elevation shows any vertical face.
Projection Systems
- Third angle projection (common in the United States) arranges views as if projected outward onto the outside of a box.
- First angle projection (used elsewhere) arranges views as if projected through the object onto the inside of a box.
- This lesson focuses on third angle projection.
Arrangement and Dimensions in Views
- The six views are arranged on paper according to the unfolded box method.
- Three primary dimensions: width (front, top, bottom, rear views), depth (top, bottom, right, left views), and height (front, rear, right, left views).
- Dimensions must be consistent and scaled the same in every view.
Transferring Dimensions and Points
- Align datum lines between adjacent views to transfer width and height directly.
- Depth is transferred either by measurement or by using a 45° auxiliary line between views.
- To transfer a point, project its depth and height from adjacent views; the intersection locates the point.
Drawing Guidelines
- Visible object outlines are drawn with thick, dark lines.
- Hidden features are shown with dashed lines.
- Hidden outlines are omitted if directly behind visible outlines.
Key Terms & Definitions
- Orthographic Projection — Drawing method where 3D objects are represented by projecting features onto a 2D plane with parallel lines.
- Projection Plane — Surface where object's view is projected (the drawing paper).
- Plan View — The top view of an object.
- Elevation — A vertical view showing height.
- Datum Line — Reference line aligned between views to ensure accuracy.
- Third Angle Projection — US method projecting views onto the outside of the box.
- First Angle Projection — International method projecting views onto the inside of the box.
- Auxiliary Line — A 45° line used to transfer depth between non-adjacent views.
Action Items / Next Steps
- Practice drawing third angle orthographic projections using the unfolded box method.
- Experiment with transferring dimensions and points between views using projection and auxiliary lines.