Orthogonal Projections and Principal Views in Technical Drawing

Orthogonal projections and principal views

Usually called the principal views of an object, these are the orthogonal projections of the object onto six planes arranged as faces of a cube. They can also be described as the orthogonal projections of an object according to the different directions from which you view it. The rules that govern the representation of the views of an object are included in UNE 1-032-82, "Technical drawings: General principles of representation," which is equivalent to ISO 128-82.

Name of views

How the six views are obtained

If we place an observer according to the six directions indicated by the arrows, we obtain six possible views of an object.

Name and common terms for each view

These views have the following denominations:

• View A: front view or elevation
• View B: top view or plan
• View C: right view or right lateral
• View D: left view or left lateral
• View E: bottom view
• View F: back view

Relative positions of views

European and American projection methods

For placing the different views on paper, two orthogonal projection variants of equal importance may be used:

• First dihedral method — also called the European system (formerly Method E).
• Third dihedral method — also called the American system (formerly Method A).

In both methods, the object is enclosed in a box whose six faces receive the corresponding orthogonal projections. The essential difference is that, in the European system, the object is between the observer and the projection plane; in the American system the projection plane lies between the observer and the object.

EUROPEAN SYSTEM   AMERICAN SYSTEM

Unfolding the projection box

After obtaining the six orthogonal projections on the faces of the cube, and keeping the face corresponding to the elevation (A) fixed, we proceed to unfold the box. The unfolded arrangement differs depending on which system is used.

> EUROPEAN SYSTEM   AMERICAN SYSTEM

The unfolding of the projection box provides, on a single drawing sheet, the six principal views of an object in their relative positions. To identify which system has been used to represent the object, add the appropriate symbol shown in the figures. The symbol represents the elevation and left lateral view of a truncated cone in each system: AMERICAN SYSTEM   EUROPEAN SYSTEM.

Correspondence between the views

Correspondence between the views

As shown in the previous figures, there is a forced correspondence between different views. This correspondence concerns:

1. The elevation (front view), the plan (top view), the bottom view and the back view: these agree in widths.
2. The elevation, the right side view, the left side view and the back view: these agree in heights.
3. The plan, the left side view, the right side view and the bottom view: these agree in depth.

Usually, with only three views — the elevation, the plan and one side view — a piece is perfectly defined. Considering the previous correlations, it follows that given any two views we can obtain the third, as shown in the figure:

Also, from the above it follows that different views cannot be placed arbitrarily. A single view may be correct by itself, but if not properly located relative to the others it will not fully define the piece.