Technical Drawing Annotation: Principles and Practices

Technical Drawing Annotation

Annotation is the process of recording dimensions and details on a technical drawing using lines, numbers, signs, and symbols, following established rules and conventions. Accurate annotation is crucial for manufacturing, as it requires knowledge of dimensioning standards and machining processes.

Principles of Annotation

A drawing is correctly annotated when it provides the minimum, sufficient, and adequate dimensions for production. Key principles include:

• Dimensions are indicated only once, unless repetition is necessary.
• All necessary dimensions must be included.
• Dimensions are placed on views that clearly represent the features.
• Consistent units are used throughout the drawing.
• Dimensions of manufacturing processes are not annotated.
• Dimensions are generally placed outside the piece outline.
• Hidden edges are generally not dimensioned.
• Dimensions are arranged for clarity and order.
• Related dimensions (e.g., diameter and depth) are on the same view.
• Avoid deriving dimensions from sums or differences.

Elements of Annotation

Dimension lines are parallel to the measured surface. Dimension figures indicate the magnitude and are placed on or near the dimension line. Dimension line terminators can be arrowheads, oblique strokes, or small circles.

Auxiliary dimension lines extend from the drawing perpendicular to the surface being dimensioned. They slightly exceed the dimension lines (approx. 2mm).

Leader lines indicate a dimensional value or note, connecting text to the feature. They can end with an arrowhead on the outline, a dot inside the piece, or no terminator when ending on another line. The text is parallel to the dimensioned element.

Symbols represent formal characteristics, simplifying annotation and reducing the need for multiple views.

Classification of Dimensions

By Importance

• Functional dimensions (F): Essential for the piece to fulfill its function.
• Non-functional dimensions (NF): Necessary for complete definition but not essential for function.
• Auxiliary dimensions (AUX): Overall measurements, often in parentheses, derived from other dimensions.

By Role

• Size dimensions (d): Indicate the size of elements (e.g., hole diameters, widths).
• Location dimensions (s): Define the position of elements on the piece.