Descriptive Geometry
Tech 1521
Introduction
Orthographic projection is a great way to analyze objects that exist in three dimension.
Descriptive geometry is used to extract useful information about how 3-D objects (Points, Lines, Planes, Vectors, Cylinders, etc.) relate to each other.
Examples of DG Problems
Where does the cable intersect the bulkhead?
Do these two lines intersect?
How close are the two wires to each other.
What is the angle (dihedral angle) between these planes?
Principal Planes
Frontal
Horizontal
Profile
This is nothing new...
...just a little more abstract
Orthographic Views of a Line
Can you see what's going on here? I'm here to help!
Principal Planes
Frontal, Horizontal, Profile
what the...?
True Length Lines
ohhh, ok!
Think of F as the blackboard view.
H and P are Reaching in.
Point View of a Line
In which views is AB shown in TL?
Auxiliary View of a Line
Auxiliary
View of a Line
Each view "reaches in" to the next.
Point on a Line
Which points are on the line?
Points on a Line
Where could pt2 be in the horizontal plane?
Planes
Planes parallel to the principal planes:
Non-Principal Planes
These appear as line in the other two principal planes
<<< appear as a line in one principal plane
<<< never appear as a line
Planes
True Size
Foreshortened
Edge View
Principal Plane
Inclined Plane
Oblique Plane
Finding True Size of a Plane
Easiest case...
True Size of a Plane
Next easiest case...
True Size of a Plane
Not easy at all case...
First find the Edge View of the plane...
...Then project that edge
now all at once...
ooooh
Dihedral Angle
To see the dihedral angle in its true proportions, we need to look at the line formed by the intersection straight-on (so it appears as a point).
Example
The first step to seeing the "seam" as a line is to show it in true length.
We are just solving for point view of a line, but developing all the points.