What Are Auxiliary Views in CAD Documentation?
This article explores the concept of auxiliary views in Computer Aided Design (CAD), detailing what they are, how they function, and precisely when they are required in engineering drawings. Standard orthographic projections—such as front, top, and side views—often distort non-orthogonal geometry. Auxiliary views solve this limitation by projecting inclined and oblique features onto custom planes, ensuring accurate dimensions, clear design intent, and error-free manufacturing communication.
Understanding Auxiliary Views
In engineering graphics and CAD documentation, standard orthographic projection relies on six primary planes: front, top, bottom, right, left, and rear. When a component contains a surface that is tilted or angled relative to these primary planes, standard projections cause the surface to appear foreshortened. Foreshortening distorts the surface's true shape, size, and angular relationships.
An auxiliary view is an orthographic projection taken on any plane other than the standard primary planes. It is created by positioning a line of sight perpendicular to an inclined or angled face. This allows the feature to be projected in its true shape and size (true geometry), eliminating the visual and dimensional distortion inherent in standard views.
Types of Auxiliary Views
CAD software typically categorizes auxiliary views based on the reference plane from which they are projected:
- Primary Auxiliary Views: Projected from one of the primary views. They are perpendicular to one primary plane and inclined to the other two. These are commonly used for single-angle inclined surfaces.
- Secondary Auxiliary Views: Projected from a primary auxiliary view. These are necessary when an object contains an "oblique" surface—one that is tilted relative to all three principal planes.
- Partial Auxiliary Views: Because displaying the entire object from an angled perspective can create visual clutter with foreshortened backgrounds, CAD users frequently generate partial views. These display only the inclined surface of interest, omitting unnecessary background details.
When Are Auxiliary Views Necessary?
Auxiliary views are not merely decorative; they are functionally essential under specific geometric and manufacturing conditions:
1. Dimensioning Inclined Features
Standard drawing practices dictate that dimensions must only be placed on features showing their true size and shape. If a hole, slot, or perimeter sits on an inclined plane, dimensioning it in a standard front or top view violates drafting standards and leads to manufacturing ambiguities. An auxiliary view provides the true surface plane where these dimensions can be clearly annotated.
2. Accurately Representing True Shapes
Complex profiles—such as circular holes, curved cutouts, or non-rectilinear shapes located on an angled face—appear as distorted ellipses or skewed curves in standard orthogonal projections. An auxiliary view projects these geometries perpendicularly, revealing the actual circular or defined form.
3. Communicating Angular Relationships
When components interface with mating parts at specific compound angles, standard views cannot adequately show the true angle of the bend or slope. An auxiliary view aligns with the edge view of the plane, revealing the true dihedral angle between two intersecting surfaces.
4. Preventing Tooling and CNC Errors
In modern digital workflows, CAD models feed directly into Computer Aided Manufacturing (CAM) tools. However, 2D documentation remains the legal and contractual standard for quality control and inspection. Quality control teams rely on auxiliary views to program Coordinate Measuring Machines (CMM) and measure tolerances without converting foreshortened values mathematically.
Summary
Auxiliary views are indispensable in CAD whenever an object's functional features sit at an angle to the principal planes of projection. By defining an alternate viewing angle perpendicular to the feature, designers preserve dimensional integrity, eliminate interpretation errors on the shop floor, and provide full geometric clarification for manufacturing and inspection.