The most accurate way to get dimensions from a technical drawing is to read the explicit dimensions clearly written on the drawing itself, usually indicated between dimension lines.
Technical drawings, whether for manufacturing, architecture, or engineering, are designed to provide precise information. Instead of measuring the lines on the paper with a ruler, you should look for the numerical values that represent the actual size of the object or feature being depicted.
Here's a breakdown of how to locate and interpret dimensions:
Where to Find Dimensions
Dimensions are typically shown using specific graphical conventions:
- Dimension Lines: These are thin lines with arrows or slashes at each end, extending between the points or lines being measured.
- Extension Lines: These lines extend from the object's edges or features to where the dimension line is placed.
- Dimension Value: The numerical value (e.g., 25, 150, 3'-6") is written near the center of the dimension line. It represents the actual size in the specified units.
- Units: Drawings usually specify the units being used (e.g., mm, inches, feet and inches). This information is often found in the title block or notes.
Example:
Imagine a line segment on a drawing. You might see:
<------------------->
25
Here, the horizontal line with arrows is the dimension line, and '25' is the dimension value. If the drawing is in millimeters, this segment is 25 mm long.
The Best Approach: Trust the Written Numbers
As the reference states, the best way to get exact dimensions from drawings is to use the explicit dimensions (in millimeters or in feet and inches) written between the dimension lines. These numbers are the definitive source for the object's size.
- Why not measure the drawing directly? Drawings are drawn to a specific scale (e.g., 1:10, 1/4"=1'), but printing or copying can sometimes distort the scale slightly. Relying on the printed dimensions ensures accuracy regardless of minor scale variations.
- All Needed Dimensions Should Be There: Any measurements that you need should be somewhere on the drawings. Designers and drafters aim to include all necessary dimensions for manufacturing, construction, or assembly.
- Dimensions Appear Once: Drawings normally only give each dimension once. Avoid searching for the same measurement multiple times; find where it's explicitly listed for the relevant feature.
Practical Steps
- Identify the Feature: Determine which part or feature of the drawing you need the dimension for (e.g., the length of a wall, the diameter of a hole, the distance between two edges).
- Look for Dimension Lines: Find the dimension line(s) associated with that feature.
- Read the Value: Identify the number written along or near the dimension line.
- Note the Units: Confirm the units the drawing is using (e.g., mm, inches, feet and inches). This is crucial for correct interpretation.
Here’s a simple table summarizing the key elements:
| Element | Description | What to Look For |
|---|---|---|
| Dimension Line | Indicates the extent of the measurement. | Thin line with arrows/slashes at ends. |
| Extension Lines | Extend from the object to the dimension line. | Lines extending from the feature being measured. |
| Dimension Value | The numerical size of the feature. | The number written on the dimension line. |
| Units | Specifies if the value is in mm, inches, feet, etc. | Notes, title block, or next to the dimension value. |
By focusing on the written dimensions between the dimension lines, you can accurately extract the necessary size information from any technical drawing.
