In engineering drawing, oblique projection is a method used to represent a three-dimensional object on a two-dimensional plane. It provides a pictorial view that combines some advantages of both multiview orthographic and isometric projections.
Understanding Oblique Projection
Based on the reference provided, the core definition of oblique projection is:
An oblique projection is a parallel projection in which the lines of sight are not perpendicular to the projection plane.
This differentiates it from orthographic projection (like the standard front, top, and side views used in engineering) where the lines of sight are always perpendicular to the projection plane. In oblique projection, these lines are parallel to each other but strike the projection plane at an angle other than 90 degrees.
Key Characteristics
- Parallel Projection: Like orthographic and isometric projection, the lines projecting from the object to the plane are parallel to each other. This ensures that parallel lines on the object remain parallel in the projection.
- Non-Perpendicular Lines of Sight: The defining feature is that the projection lines hit the projection plane at an oblique (non-90 degree) angle.
- One Face in True Size: Typically, one face of the object (often the front face) is placed parallel to the projection plane, meaning it is shown in its true shape and size. This can make certain details on that face easier to draw and understand compared to isometric views.
Common Practices in Oblique Projection
The reference highlights a common approach:
Commonly used oblique projections orient the projection plane to be perpendicular to a coordinate axis, while moving the lines of sight to intersect two additional sides of the object.
This means that while one face (parallel to the projection plane) shows true dimensions, the lines representing depth (receding from this face) are drawn at an angle and may not be shown in their true length.
Types of Oblique Projection
While the reference doesn't detail specific types, oblique projections are commonly categorized based on how the depth (receding) lines are represented:
- Cavalier Projection: The receding lines are drawn at a convenient angle (often 30°, 45°, or 60°) and are shown in their true length.
- Cabinet Projection: The receding lines are also drawn at an angle, but their length is foreshortened, typically to one-half or two-thirds of their true length. This often results in a view that appears more realistic by reducing distortion.
| Feature | Orthographic Projection | Oblique Projection |
|---|---|---|
| Lines of Sight | Perpendicular to plane | Not perpendicular to plane |
| Projection Plane | Multiple, perpendicular | Single, often parallel to face |
| Showing True Shape | Multiple views needed | One face in true shape/size |
| Depth Representation | Shown via adjacent view | Shown via angled receding lines |
Practical Applications
Oblique projection is useful in engineering drawing when:
- You need to show one face of the object in true size, making it easy to dimension or understand the features on that specific face.
- A quick pictorial representation is needed that is simpler to draw than an isometric view, especially for objects with many details on one face.
- Less formal or assembly illustrations are required where accurate spatial relationships of all parts might not be critical.
Examples:
- Drawing a box with a complex pattern on the front face. Oblique allows the pattern to be drawn accurately while still showing the 3D form.
- Illustrating a simple part where the primary features are on one plane.
While less common for formal component drawings than orthographic or isometric projection, oblique projection remains a valuable tool for specific illustration needs in engineering and technical documentation.
