Engineering Dimensioning Practices

While orthographic projections show the exact shape of a part, dimensioning is what provides the exact numerical size and location information required for manufacturing. The ASME Y14.5 standard dictates how these dimensions are applied to ensure uniform interpretation.

Note: This article focuses on basic coordinate dimensioning. Advanced Geometric Dimensioning and Tolerancing (GD&T) principles are covered in separate fundamental units.

Basic Rules of Dimensioning

To prevent manufacturing errors and ensure clarity, several basic rules govern how dimensions are placed on engineering drawings:

1. Clarity and Legibility: Dimensions must be placed where they are easiest to read and interpret. Ideally, dimensions should be grouped together rather than scattered.
2. No Redundancy: Each feature of a part must be dimensioned once and only once. Over-dimensioning (providing more dimensions than necessary to define the geometry) leads to conflicting requirements and confusion.
3. Place on the Most Descriptive View: Features should be dimensioned in the view that best shows their true shape (e.g., the diameter of a hole should be dimensioned in the view where it appears as a circle, not where it appears as hidden lines).
4. Outside the Part Outline: Whenever possible, dimension lines and text should be placed outside the visible outline of the part to keep the drawing uncluttered.
5. Extension Line Gaps: Extension lines should start with a small, visible gap (about 1-2 mm) from the edge of the part, ensuring they are not mistaken for the geometric outline of the part.
6. Avoid Crossing Lines: Dimension lines should not cross each other, nor should they cross extension lines if it can be avoided. If extension lines must cross each other or other lines, they should generally not be broken, but dimension lines must never be broken.

Dimensioning Methods

Different manufacturing processes and functional requirements dictate how dimensions should be arranged relative to each other.

1. Chain / Series Dimensioning

Dimensions are placed end-to-end in a continuous chain.

• Advantage: Easy to read and saves space on the drawing.
• Disadvantage (Tolerance Accumulation): The tolerance of each dimension stacks up. If a part has 5 chained features, the overall length has the accumulated tolerance of all 5 features combined, which can cause assembly issues.

2. Baseline / Parallel Dimensioning

All dimensions for a given direction originate from a single common baseline or reference edge.

• Advantage: Prevents tolerance accumulation. The location of each feature is independent of the others.
• Disadvantage: Can take up more space on the drawing sheet.

3. Coordinate Dimensioning

Features (such as a pattern of holes) are located by X and Y distances from a common origin (often the bottom-left corner of the part or a central hole). This method pairs extremely well with modern CNC (Computer Numerical Control) machining, which operates on coordinate systems.

Applying Tolerances to Dimensions

Because no manufacturing process is perfect, every dimension must have an allowable variation, known as a Tolerance.

• Limit Dimensions: The maximum and minimum acceptable sizes are stated directly (e.g., 25.10 over 24.90).

• Plus-Minus Dimensions: States the nominal size followed by the acceptable positive and negative variation (e.g., $25.00 \pm 0.10$).

• General Tolerances: As mentioned in drawing practices, if a dimension does not have a specific tolerance applied to it, the general tolerance from the drawing's Title Block must be used, based on the number of decimal places (e.g., X.XX = $\pm 0.05$).