Design Drafting Lectures 0

Lecture 9

Pictorial Drawings: Axonometric Projection: Oblique Projection

So far we have discussed isometric drawing/projections. In this lecture we will discuss some

other, less popular types of axonometric projection. We will also be discussing oblique

projection, a relatively quick way of producing a pictorial drawing.

Dimetric Projection:

A dimetric projection is an axonometric projection of an object so placed that no two axes make

equal angles with the plane of projection. In other words each of the three axes and the lines

parallel to them, respectively, have different ratios of foreshortening when projected to the plane

of projection.

Trimetric Projection:

A trimetric projection is an axonometric projection of an object so placed that no two axes make

equal angles with the plane of projection. In other words each of the three axes and the lines

parallel to them, respectively, have different ratios of foreshortening when projected to the plane

of projection.

Diametric projection of a cube

Isometric projection

Two dimetric drawings with 45º rotation

In both the dimetric and trimetric methods of projection, there are scales, graph paper and

templates that may be purchased for the various angles chosen. Creation of the scales and

ellipses is a time consuming process and is beyond the scope of our discussion.

Oblique Projection:

If the observer is considered to be stationed at an infinite distance from the object, and looking

toward the objects so that the projectors are parallel to each other and oblique to the plane of

projection, the resulting drawing is an oblique projection. As a rule, the object is placed with one

of its principle faces parallel to the plan of projections.

Common approximate trimetric angles and scales

Receding Axis:

The receding lines may be drawn at any convenient angle or direction.

The angle that should be used in an oblique drawing depends upon the shape of the object and

the location of its significant features.

Length of Receding Lines:

Since the eye is accustomed to seeing objects with all receding parallel lines appearing to

converge, an oblique projection presents an unnatural appearance, with more or less serious

distortions depending upon the object shown. (See above)

A chief limitation of oblique projection is that objects characterized by great length should not be

drawn in oblique with the long dimension perpendicular to the plan of projection.

Foreshortening of Receding lines

Unnatural Appearance of Oblique Drawing

Types of Oblique Drawings:

Cavalier Oblique

45° with Full Scale receding lines

Cabinet Oblique

45° with Half Scale receding lines

General Oblique

Any angle and any scale for receding lines

1. Cavalier (45° with Full Scale receding lines)

2. Cabinet (45° with Half Scale receding lines)

3. General (any angle and any scale for receding lines)

Choice of Position

The face of an object showing the essential contours should generally be placed parallel to the

plane of projection. If this is done, distortion will be kept at a minimum and labor reduced.

Summary / Review Questions:

1. Describe diametric and trimetric drawings.

A diametric drawing has two of its axes at the same angle to the projection plane, while a

trimetric has three different angles.

2. What is the typical angle for the receding lines in an oblique drawing?

Typically 45°, but they can be drawn at any convenient angle.

3. Describe the different types of oblique drawings.

a. Cavalier (45° with Full Scale receding lines)

b. Cabinet (45° with Half Scale receding lines)

c. General (any angle and any scale for receding lines)

Assignment / Independent Practice:

Axonometric / Oblique Projection Worksheet 3.

Comparison of Cavalier and Cabinet Projections