Short Answer:

Section lines are thin, slanted lines used in engineering drawings to show the cut surface of an object in a sectional view. These lines are drawn inside the area where the imaginary cutting plane passes through the object, helping to reveal the internal shape or features that are otherwise hidden.

Section lines also help in identifying the type of material used in the component. Different materials like steel, wood, bronze, and concrete have standard patterns for section lines. This helps engineers, manufacturers, and workers know the material properties at a glance from the drawing.

Detailed Explanation:

Section Lines and Material Indication in Technical Drawings

In mechanical drawings, objects are often cut through by imaginary planes to show their inner details. This process is known as sectioning, and the resulting view is called a sectional view. To clearly show the surface where the object is cut, engineers use section lines.

Section lines (also called hatching or cross-hatching) are a very important part of this process. They not only indicate that the surface has been cut but also provide information about the material of the object through the style and angle of the lines used.

What Are Section Lines

Section lines are:

• Thin and light lines drawn at a regular angle of 45°, unless specified otherwise.
• Placed evenly and parallel inside the area that is cut by the section plane.
• Used only in cut surfaces—not drawn in uncut or visible surfaces.
• Never touch the boundary lines and are kept a small distance from the outline for neatness.

They are drawn using pencil or CAD tools, and the spacing between them is usually 1 to 2 mm, depending on the drawing scale.

How Section Lines Show Material Properties

One of the special uses of section lines is to represent different types of materials. This is done by using standard line patterns or symbols. These patterns are recognized worldwide and help the reader understand the material used without needing extra explanation.

Here are some common examples:

1. Steel / General Metal:
   • Uniform 45° parallel lines, closely spaced.
2. Cast Iron:
   • Similar 45° lines but spaced slightly wider.
3. Bronze / Brass / Copper:
   • Series of parallel lines mixed with broken lines.
4. Aluminum:
   • Lines with dots or dots between lines.
5. Wood:
   • Wavy or grain-like lines to represent natural texture.
6. Concrete / Masonry:
   • Cross-hatching or stone pattern to show rough texture.

These patterns are defined by standards such as BIS (Indian), ISO, ANSI, and help in global understanding of engineering drawings.

Why Material Indication Is Important

• Helps in material selection during manufacturing.
• Aids in cost estimation and procurement.
• Ensures that correct machining, welding, or casting process is used.
• Provides clear information without using long written descriptions.
• Saves time for readers like workers, supervisors, and inspectors.

In complex assemblies, section lines also help distinguish different parts by using different directions or spacing.

Correct Usage of Section Lines

• Section lines should be uniform throughout the part unless the material changes.
• In assemblies, use opposite directions or different patterns for each part to make them visually separate.
• Do not draw section lines over standard parts like nuts, bolts, shafts—these are usually left unhatched.
• Avoid using random patterns—stick to standard material symbols for clarity.

In CAD software, these patterns are built-in and can be applied easily using commands like “Hatch” or “Section View.”

Conclusion

Section lines are a key part of engineering drawings used to show cut surfaces in sectional views. Beyond just marking the section, they also indicate material properties using standard patterns. This helps everyone involved in the production or design process to identify materials, understand functions, and make the correct decisions. Proper use of section lines improves clarity, communication, and quality in technical drawings.