Short Answer:
Sectional lines in engineering graphics are used to show the internal features of a part that cannot be seen from outside. When a component is cut by an imaginary plane, the cut surface is represented using thin, evenly spaced section lines (also called hatching lines). These lines indicate where the material is solid and help in understanding the inside shape.
Sectional views with section lines make it easier to visualize holes, cavities, ribs, and wall thicknesses. They are commonly used in mechanical drawings to improve clarity and avoid confusion when parts are complex or hidden.
Detailed Explanation:
Use of sectional lines in engineering graphics
In engineering graphics, it is not always possible to show all internal features of a part using only external views like front, top, or side. Some parts have complex internal shapes—such as holes, grooves, ribs, or slots—that are hidden behind the outer surface. To clearly show these hidden features, we use a sectional view, and the most important part of that view is the sectional lines.
Sectional lines (also called hatching lines or cross-hatching) are a series of thin lines drawn at a 45° angle across the cut area of the object. These lines indicate the solid material exposed by the imaginary cutting plane. The pattern, direction, and spacing of these lines follow drawing standards like ISO or BIS, and they help in better understanding the shape and structure of the object.
Main purposes of sectional lines
- Showing internal details
- Sectional lines show what is inside the object that is not visible from outside.
- When a part is cut open in the drawing, the surface where the cut happens is filled with section lines.
- It becomes easy to see internal shapes, such as:
- Cylindrical holes
- Rectangular slots
- Complex grooves
- Indicating solid material
- The area with section lines shows that the material is solid.
- Areas with no section lines are either holes, cavities, or spaces.
- This helps in understanding what material remains and what is removed during machining or manufacturing.
- Differentiating between parts
- In assembly drawings, each part is hatched using different section line angles or spacing.
- This helps to clearly separate one part from another even if they are touching.
- For example, a nut and bolt shown in section can be easily identified if their section lines are in different directions.
- Improving clarity of drawing
- Instead of showing many hidden lines (dashed lines), a sectional view with section lines gives a clean and clear view.
- This avoids confusion and helps engineers, machinists, and fabricators read the drawing easily.
Standard rules for drawing sectional lines
- Section lines are drawn at 45° angle, thin and evenly spaced.
- Do not draw section lines across holes, ribs, shafts, or fasteners unless required.
- In castings and forgings, section lines are used to represent the shape and volume of the material.
- When the section is very small, section lines may be omitted to keep the drawing clean.
Types of sectional views using section lines
- Full Section: The part is cut completely in half.
- Half Section: Only one-half of the part is shown in section, useful for symmetrical parts.
- Offset Section: Cutting plane changes direction to pass through important features.
- Broken-out Section: Only a small area is sectioned.
In all these types, section lines are necessary to show the cut surface.
Importance in manufacturing and design
- Helps machinists understand what to cut, drill, or machine.
- Guides assembly technicians to fit parts correctly.
- Useful for inspection and quality checks.
- Reduces errors due to misinterpretation of internal details.
Conclusion:
Sectional lines are an essential part of engineering graphics that help to show the internal features of a component clearly and correctly. They represent the material exposed by a cut and provide a clear understanding of how a part looks from the inside. By using sectional views and properly drawn section lines, engineers can communicate complex shapes easily and avoid confusion in design, manufacturing, and assembly processes.