Symbols, Revisions, and Drawing Numbers

An engineering drawing is more than just lines and dimensions; it is a controlled legal document that dictates manufacturing requirements. Proper document control—including symbols, numbering, and revision tracking—ensures that the correct parts are manufactured without errors.

Common Engineering Symbols

To keep drawings uncluttered and language-independent, engineers use standardized symbols instead of writing out long text descriptions.

Common symbol concepts include:

• Diameter (Ø) and Radius (R): Used to indicate circular features.
• Thread Symbols: Abstract representations of threaded holes or fasteners, eliminating the need to draw every individual thread.
• Surface Finish Symbols: A checkmark-like symbol that indicates the required smoothness or roughness of a machined surface.
• Weld Symbols: Complex symbols placed on a reference line that tell a welder exactly what type of weld to apply, how large it should be, and where to place it.

Note: The specific geometries of these symbols are defined by international standards (like ISO or ASME).

Drawing Identification and Numbers

Every engineering drawing must have a unique identifier, known as the Drawing Number.

• Purpose: To definitively identify a specific component or assembly across an entire organization, supply chain, or project.
• Structure: Drawing numbers are often part of a larger numbering system that might encode the project code, the assembly subsystem, and the specific part sequence.

Revision Concept and History

Engineering is an iterative process. Parts are frequently modified to improve performance, fix design flaws, or reduce manufacturing costs.

A Revision (Rev) represents a specific, controlled version of a drawing.

• Revision Block: A table (usually in the top right corner or adjacent to the title block) that logs every change made to the drawing over its lifetime.
• Document Control Awareness: If a machinist uses "Rev A" to manufacture a part, but the engineering team has already released "Rev B" with a different hole size, the manufactured part will be defective.

Strict revision control guarantees that everyone is working from the single, correct source of truth. When a drawing is revised, the changes are logged in the revision block with a date, a brief description of the change, and the initials of the approver.