What are the 7 Types of Lines in Technical Drawing?
Technical drawings use different types of lines to represent visible edges, hidden features, dimensions, centerlines, and cutting planes. According to standards such as ISO 128 and ASME Y14.2, each line type has a specific meaning.
The seven most common types of lines are:
| Line Type | Appearance | Purpose |
|---|---|---|
| Visible (Object) Line | Thick continuous | Shows visible edges and outlines |
| Hidden Line | Thin dashed | Shows hidden edges and features |
| Center Line | Thin long dash–short dash | Indicates centers of circles, arcs, and symmetry |
| Dimension Line | Thin continuous with arrowheads | Shows the size of features |
| Extension Line | Thin continuous | Extends from the object to dimension lines |
| Section (Hatching) Line | Thin, evenly spaced diagonal lines | Indicates surfaces cut by a section plane |
| Cutting Plane Line | Thick chain line with arrows | Shows where an object is imagined to be cut for a sectional view |
7 Types of Lines in Technical Drawing?
1. Visible (Object) Line
Appearance
- Thick continuous line
Purpose
Represents all visible edges and boundaries of an object.
Uses
- Object outlines
- Visible corners
- Visible surfaces
Example: The outer edges of a mechanical component.
2. Hidden Line
Appearance
- Thin dashed line
Purpose
Represents edges or features that cannot be seen in the current view.
Uses
- Holes behind surfaces
- Internal slots
- Hidden grooves
Example: A hole passing through a block that is not visible from the front.
3. Center Line
Appearance
- Thin line with alternating long and short dashes
Purpose
Indicates the center of circles, arcs, cylinders, and symmetrical parts.
Uses
- Hole centers
- Shaft axes
- Symmetrical components
Example: The centerline of a drilled hole.
4. Dimension Line
Appearance
- Thin continuous line with arrowheads at both ends
Purpose
Shows the distance or size of a feature.
Uses
- Length
- Width
- Diameter
- Radius
- Angle measurements
Example: A dimension line labeled 50 mm between two arrowheads.
5. Extension Line
Appearance
- Thin continuous line
Purpose
Extends from the object to the dimension line to indicate the feature being measured.
Uses
- Dimensioning
- Measurement references
Example: Lines extending from the edges of a rectangle to a dimension line.
6. Section (Hatching) Line
Appearance
- Thin, evenly spaced diagonal lines (usually at 45°)
Purpose
Represents surfaces that have been cut in a sectional view.
Uses
- Sectional drawings
- Internal features
- Material identification (special hatch patterns may indicate different materials)
Example: A cross-section of a bearing or engine block.
7. Cutting Plane Line
Appearance
- Thick chain line with arrows at the ends
Purpose
Shows the location where an imaginary cut is made to create a sectional view.
Uses
- Section views
- Internal component visualization
Example: A cutting plane labeled A–A through a machine part.
Additional Line Types (Commonly Used)
Besides the seven basic lines, technical drawings often include:
| Line Type | Purpose |
|---|---|
| Leader Line | Connects notes or labels to features |
| Phantom Line | Indicates alternate positions or adjacent parts |
| Break Line | Shows that a portion of an object has been removed for clarity |
| Construction Line | Used as a guide during drawing; usually not part of the final drawing |
Importance of Line Types
Using the correct line type helps:
- Improve drawing clarity.
- Avoid misinterpretation.
- Communicate design intent accurately.
- Ensure compliance with engineering drawing standards.
- Facilitate manufacturing and inspection.
Summary
| Line Type | Thickness | Function |
|---|---|---|
| Visible (Object) Line | Thick | Shows visible edges and outlines |
| Hidden Line | Thin | Shows hidden features |
| Center Line | Thin | Indicates centers and axes |
| Dimension Line | Thin | Shows measurements |
| Extension Line | Thin | Extends from the object to dimensions |
| Section (Hatching) Line | Thin | Represents cut surfaces in section views |
| Cutting Plane Line | Thick | Indicates the location of an imaginary cut |
These seven line types form the foundation of technical drawing and are universally used in engineering, architecture, and manufacturing to communicate designs clearly and accurately.
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