Technical Drawing Principles and Projection Systems

Technical Drawing Features

Core Characteristics

• Graphic: Lines, symbols, and figures replace words to convey information.
• Universal: It is understood globally, regardless of the language spoken.
• Accurate: A set of drawings for a project must correspond exactly during assembly.

Spatial Dimensions

Space is categorized as two-dimensional (length x width) or three-dimensional (length x width x height).

Projection Systems and Definitions

• Project: To draw a line from the center of projection through an object, intersecting the projection surface to produce an image.
• Projection of a Point: The intersection where the projecting line passes through a specific point.
• Projecting a Picture: Drawing straight lines starting from a point that pass through all points in a figure.
• Projection Cone (Central): When rays leave a projection center (P) and pass through all points, impacting the projection plane to create an image larger than the object.
• Orthogonal: Projecting lines are always perpendicular to the plane of projection.
• Oblique Cylindrical Projection: When projecting lines are oblique to the projection plane.
• Orthogonal Cylindrical Projection: When projecting lines are perpendicular to the plane of projection.

Presentation Principles and Standards

General principles for technical drawing presentation follow UNE 1 032-82, which is equivalent to ISO 128-82.

Standard Views in Technical Drawing

A view is the general name given to projections of a piece according to different viewing directions. The six standard views include:

• Elevation
• Plan
• Left Profile
• Right Profile
• Lower Floor
• Rear Elevation

Methods of View Arrangement

• Normal Method: A fixed viewing system where the piece is observed directly and rotated in 90-degree increments.
• Cube Method: Includes the American System (3rd dihedron) and the European System (1st dihedron).

Criteria for Selecting Views

1. Choose the view that provides the most information.
2. The part should be drawn in its functional position.
3. If used in multiple positions, it is drawn in its machining position.
4. Use only the views needed to define the part; avoid excessive views, hidden lines, and repetition of details.

Technical Sketching and Line Types

Sketching Fundamentals

A sketch is a set of projections or views made in pencil and freehand. These interrelated views allow for the full interpretation of a piece. It can be described as a quick drawing or draft that serves to make the final drawing clear and definite. It represents a material part, a perspective, an idea, or a specific need.

Rules for Effective Sketching

• Views must correspond: widths must be the same in plan and elevation, and heights must be the same in elevation and profile.
• Rigid Norm: Chosen views must represent the piece with complete accuracy and without ambiguity.
• Use a reduced number of views.
• The elevation should be the main view.
• Select views with the fewest hidden lines.

Standard Line Types

Common lines used in technical drafting include:

• Thick solid lines
• Dashed lines
• Chain lines (dash-dot)
• Continuous fine lines