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#### Presentation Transcript

**1. **ENDG 105 Class 6.2
DESCRIPTIVE GEOMETRY

**2. **Introduction Descriptive Geometry is the foundation of engineering graphics
Gaspard Monge is considered the “father of descriptive geometry”. He developed techniques to solve geometric problems. These techniques are the basis of the engineering graphics techniques we use today.
Descriptive Geometry used to find true length lines, point view of a line, edge view of a plane, and true size of a plane.
We will look at 5 basic principles of Descriptive Geometry today.

**3. **Basic Principle #1 If a line is positioned parallel to a projection plane and the line of sight is perpendicular to that projection plane, the line will appear true length

**4. **True length lines In orthographic projection the true length line will be
parallel to the folding lines in the adjacent views, and will be foreshortened in the adjacent views.

**5. **If the line is not parallel to any of the projection planes, how do we find the true length? There are two methods that can be used.
Auxiliary view method (aka - folding line method)
And Revolution method

**6. **Auxiliary method Remember when we created Partial Auxiliary views before (class 3.2).

**7. **Revolution method In the revolution method we want to revolve one of the lines in one view so it becomes parallel to the folding line.
Then we project the new revolved point to the adjacent view.(perpendicular projector)
Next we project from the original point in the adjacent view perpendicular to the projection line from the revolved view. That point becomes our new point for the true-length line.

**8. **Revolution method (usually easier for finding true-length lines)

**9. **Basic Principle #2 If your line of sight is parallel to a true-length line, the line will appear as a point in the adjacent view.

**10. **Point view of a line Again we can use both methods to find the point view of a line, but first we have to find the true length of the line.

**11. **Point view of a line Auxiliary method

**12. **Point view of a line Revolution method

**13. **Basic Principle #3 Planar surfaces of any shape always appear either as edges or as surfaces of similar configuration.

**14. **Basic Principle #4 If a line in a plane appears as a point, the plane appears as an edge.
The edge view of a plane occurs when the line of sight is parallel to the plane.

**15. **Edge view of a plane Auxiliary method To find the edge view of a plane we have to establish a true length line on the plane, then take a line of sight parallel to the true-length line.

**16. **Edge view of the plane Revolution method Revolve the true-length line and the rest of the plane until the true-length line is perpendicular to a folding line.
Then project new points to adjacent view.

**17. **Basic Principle #5 A true-size plane must be perpendicular to the line of sight and must appear as an edge in all adjacent views.
Must find the edge view of the plane first.

**18. **True size of plane Auxiliary method This example is an oblique plane.

**19. **True size of a plane Revolution method This example is also an oblique plane that uses an auxiliary view to find the edge view of the plane, then revolves the edge view and projects new points to the adjacent view to find the true size plane.

**20. **Individual in-class assignment 6.2 Using AutoCAD, complete the following templates: (105 Library – Raborn folder- Descriptive Geometry folder)
DescGeo1.dwt
DescGeo2.dwt
DescGeo3.dwt
Staple and turn in before leaving today.

**21. **Homework Assignment 6.2 Using AutoCAD, complete the following templates: (105 Library – Raborn folder- Descriptive Geometry folder)
DescGeo4.dwt
DescGeo5.dwt
Due one week from today