ENGR1025 - Lecture 12

Understanding Dimensioning

  • Interchangeability of Parts:

    • Fundamental principle for modern dimensioning.
    • Design engineers and detail drafters are responsible for size control.

  • Role of CAD:

    • Facilitates proper dimensioning techniques.
    • Requires intelligent selection and placement of dimensions to convey design clearly.

Techniques of Good Dimensioning

  • Standard Appearance of Lines:

    • Consistent sizing of lines, spacing, and arrowhead styles is crucial.

  • Placement of Dimensions:

    • Aim for readability and interpretation.
    • Dimensions should illustrate the relationship between features.

  • Choice of Dimensions:

    • Consider function and ease of manufacturing in dimensioning choices.

Tolerancing

  • Tolerance Definition:
    • The permitted variation on the dimension of a feature.
    • Specified often with a general note, such as "ALL TOLERANCES ±0.02 INCH UNLESS OTHERWISE NOTED".

Geometric Structure Breakdown

  • Engineering structures can be broken down into basic shapes.
    • Size Dimensions: Specify the size of geometric shapes.
    • Location Dimensions: Indicate the positions of elements relative to one another.
    • Always check for full dimensioning to ensure clarity.

Dimension and Extension Lines

  • Dimension Lines:

    • Thin, solid lines ending in arrowheads indicating dimension limits.

  • Extension Lines:

    • Thin lines extending from the object to indicate where dimensions apply.

  • Centerlines:

    • Alternating long and short dashes used for locating symmetrical features.

  • Best Practices:

    • Avoid crossing lines or shortening extension lines.
    • Maintain proper grouping to reduce clutter in the drawing.

Arrowheads and Leaders

  • Arrowheads:

    • Indicate dimension extents with a preferred uniform style.
    • Various styles available in CAD systems.

  • Leaders:

    • Thin lines directing attention to notes or dimensions.
    • Should not be parallel to nearby lines, crossed, or unnecessarily long.

Drawing Scale

  • Scale Notation:
    • Clearly marked in the title block and dimensions should not be scaled without an appropriate note.
    • For reduced prints, increase dimensions and lettering by 50% for clarity.

Dimension Units

  • Measurements can be in millimeters or decimal inches.
    • Clear general notes should be provided regarding measurement units.

Rounding of Decimal Values

  • Rounding Rules:
    • If digit after the rounding position > 5, round up.
    • If < 5, drop it.
    • If = 5, round to the nearest even number.

Symbol Usage in Dimensioning

  • Symbols:
    • Help save space and clearly communicate dimensions.
    • Include symbols for holes, arcs, countersinks, and depths on technical drawings.

Dimensioning Standards and Best Practices

  • Avoiding Superfluous Dimensions:

    • Ensure that dimensions do not repeat or provide unnecessary information.

  • Aligning Dimensions:

    • Dimensions should be appropriately placed without crowding and for maximum clarity.

Final Considerations in Dimensioning

  • Clarity and Precision:
    • Dimensions must precisely describe size, location, and processes.
    • Ensure standard practices are followed, and applicable standards are reviewed to avoid errors.

Summary

  • Dimensions and notes play vital roles in engineering design to precisely convey size and production processes on technical drawings.
  • Use prescribed standards and best practices to ensure clarity and avoid cluttering the drawing for ease of understanding and manufacturing.