Dimensioning a multi-view drawing is a critical step in technical communication, providing all the necessary size and location information for manufacturing or inspection. It involves a systematic application of lines, symbols, and text to clearly define an object's geometry without ambiguity.
Multi-view drawings use orthographic projection to present an object from different perspectives, typically showing the front, top, and right-side views. Each view offers specific information about the object's dimensions:
- The front view primarily illustrates the object's height and width.
- The top view reveals its depth and width.
- The side view (commonly the right-side view) displays its depth and height.
This intrinsic relationship between views is fundamental for proper dimension placement. For instance, width dimensions are typically aligned vertically between the top and front views, ensuring consistency. Similarly, height dimensions are often positioned horizontally between the front and side views, maintaining their respective alignment.
Core Principles of Effective Dimensioning
Effective dimensioning adheres to several universal principles that ensure clarity, completeness, and adherence to industry standards.
1. Clarity and Readability
Dimensions must be easy to read and understand. Avoid cluttering the drawing with excessive or redundant information. The goal is to convey information precisely and efficiently.
2. Completeness
Every feature necessary for manufacturing, assembling, or inspecting the part must be dimensioned. No critical dimension should be left to assumption or scaling.
3. Non-Redundancy
Avoid duplicating dimensions. Each dimension should appear only once, ideally on the view where the feature is most clearly depicted. Adding the same dimension multiple times can lead to confusion if design changes occur.
4. Placement and Arrangement
Dimensions should be placed on the view that best shows the feature being dimensioned. For example, a diameter should be dimensioned on the circular view of a hole, not its rectangular projection. Strive to place dimensions outside the object lines wherever possible.
5. Dimension Transfer
Dimensions must be properly transferred between views. For example, the width of a feature shown in the front view must correspond to the width shown in the top view. This alignment ensures consistency and helps in visualizing the 3D object from 2D projections.
6. Adherence to Standards
Always follow relevant dimensioning standards, such as ASME Y14.5, which specifies practices for dimensioning and tolerancing. These standards provide a common language for engineers and manufacturers worldwide.
Key Elements of Dimensioning
A dimension is composed of several elements that work together to convey specific measurements:
- Dimension Line: A thin, solid line with arrowheads at each end, indicating the extent of the dimension.
- Extension Lines: Thin, solid lines extending from the object's features to the dimension line. They should have a small gap between the object and the start of the extension line.
- Dimension Text: The numerical value indicating the size, often placed above the dimension line or in the middle.
- Arrowheads: Placed at the ends of dimension lines to point to the extension lines, indicating the measurement's boundaries.
- Leader Lines: Thin, solid lines leading from a note or a dimension to a specific feature on the drawing, often used for diameters, radii, or specific instructions.
Types of Dimensions
Dimensions can be broadly categorized into two main types:
-
Size Dimensions: These specify the actual dimensions of features, such as length, width, height, diameter, or radius.
- Length: The longest horizontal or vertical measurement.
- Width: The measurement from front to back (depth is also used interchangeably with width depending on context or standard).
- Height: The vertical measurement.
- Diameter (∅): Used for circular features like holes, often dimensioned on the view showing the circle.
- Radius (R): Used for arcs and rounded corners.
-
Location Dimensions: These specify the position of features relative to each other or to a datum (a reference point, line, or plane).
- Center-to-Center: The distance between the centers of two features.
- Edge-to-Center: The distance from an edge to the center of a feature.
- Angular: Used for specifying angles between surfaces.
Practical Steps for Dimensioning a Multi-View Drawing
To dimension your drawing effectively, follow these practical steps:
- Identify Critical Features: Begin by identifying the most important features of the part. These might include overall dimensions, critical hole locations, or specific geometries that define the part's function.
- Place Overall Dimensions First: Start with the overall length, width, and height of the object. These usually define the bounding box of the part.
- Dimension on the Most Descriptive View: Place each dimension on the view where the feature it describes is shown most clearly, avoiding hidden lines.
- For example, if you have a cylindrical hole, dimension its diameter on the view where the hole appears as a circle, and its depth on the view where it appears as a rectangle with hidden lines.
- Distribute Dimensions Evenly: Try to distribute dimensions across all views rather than crowding them onto a single view. This enhances readability.
- Chain or Datum Dimensioning: Choose a consistent method for locating features.
- Chain Dimensioning: Dimensions are placed end-to-end, from one feature to the next. This can accumulate tolerance errors.
- Datum Dimensioning (Baseline Dimensioning): All dimensions originate from a common reference point, line, or plane (datum). This helps control accumulated tolerances and is generally preferred for precision parts.
- Avoid Dimensioning to Hidden Lines: As a general rule, do not place dimension lines or extension lines that cross hidden lines.
- Dimension Holes and Rounds Properly:
- Holes: Dimension their diameter, depth (if blind), and location (from two adjacent edges or a datum).
- Rounds/Fillets: Dimension their radius, often with a leader line.
- Add Notes and Tolerances: Include general notes, specific notes for particular features, and any required tolerances (e.g., ±0.010, GD&T symbols) to ensure manufacturing accuracy.
- Review and Verify: After dimensioning, carefully review the entire drawing. Ask yourself:
- Is every necessary feature dimensioned?
- Are there any redundant dimensions?
- Is the drawing easy to read and interpret?
- Does it conform to standards?
Dimensioning Examples
The following table illustrates common dimensions and their typical placement:
| Dimension Type | Description | Typical View Placement (Most Descriptive) | Alignment Principle |
|---|---|---|---|
| Overall Width | Total left-to-right extent | Front View (also Top View) | Vertically aligns top and front views |
| Overall Height | Total top-to-bottom extent | Front View (also Side View) | Horizontally aligns front and side views |
| Overall Depth | Total front-to-back extent | Top View (also Side View) | Can be projected between top and side views |
| Hole Diameter | Size of a circular hole | View showing the circular shape | Leader line pointing to the circle |
| Hole Location | Position of a hole's center | Views that clearly show its offset from datums | From adjacent edges or established datum lines |
| Radius of Fillet | Curvature of rounded edges | View showing the arc | Leader line pointing to the arc |
| Slot Width | Width of a rectangular slot | View showing the slot's true width | On the view where the slot appears as a rectangle |
| Angular | Angle between two non-parallel surfaces | View showing the true angle | Between extension lines of the surfaces |
By following these guidelines and understanding the interplay between multi-views and dimensioning principles, you can create clear, unambiguous, and manufacturable drawings.
