Engineering Drawing Basics: Dimensions, Scale and Projection Methods

Simply put, engineering drawings use lines and text to precisely express the internal and external structure of an object. Since drafting methods are similar worldwide, engineering drawings are often called the "Universal Language of Engineers." They typically contain plans, sections, and use tolerances to indicate critical areas.

Standard Engineering Drawing Concepts:

*Units and Scale
Engineering drawings typically use mm as the unit of length. If other units are used, they must be noted (e.g., the US uses inches).
*Frame Settings
According to National Standards (CNS), A4 (210*297) / A3 (297*420) borders are typically 10mm. For drawings requiring binding, the left border is 25mm.
*Scale Selection
Scale depends on paper size and part size. Usually drawn at 1:1, but scaled down or up as necessary. Common reductions: 1:2, 1:5, 1:10, 1:20... Common enlargements: 2:1, 5:1, 10:1, 20:1...
*Title Block
The content of the title block varies by organization. Basic items include: Title, Drawing Number, Company Name, Names (Designer, Drafter, Checker, Approver), Date, Projection Method (1st/3rd Angle), Scale, etc.
*Mechanical Projection Methods
First-angle and Third-angle projection are the two main methods, varying by country standards.
First-angle Projection: Most European countries.
Third-angle Projection: USA.
Both used: China, Taiwan (mostly Third-angle), Japan, International Standards (ISO), UK.
First-angle:

Projection plane is behind the object.
Object is in the first quadrant: "Observer" → "Object" → "Projection Plane". First-angle Symbol:
Third-angle:

Projection plane is in front of the object.
Object is in the third quadrant: "Observer" → "Projection Plane" → "Object". Third-angle Symbol:
*Let's look at an actual example:

Nowadays, 3D software is convenient and often includes 3D schematics for easy identification. The dimensions on the drawing allow the machinist to fabricate accordingly. Tolerances are added where parts must mate or fit together, ensuring the machinist pays extra attention to those critical dimensions. For example, if a 15mm thickness has a ±0.05 tolerance, the manufacturing process might differ to achieve that precision.
Bottom right sequence: 1. General Tolerance 2. Scale 3. Unit 4. Material 5. Surface Treatment 6. Projection Angle.