Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Apply cutting-plane lines correctly for various sections.
• Draw full and half sectional views to SANS 10111 standards.
• Interpret sectional views to understand internal components.
• Use conventional breaks and removed sections for clarity.

Lesson summary

This week, we delve deeper into the crucial skill of creating and interpreting sectional views of mechanical assemblies. Sectional views are essential in Engineering Graphics and Design (EGD) as they reveal the internal details of complex objects and assemblies that would otherwise be hidden. Imagine trying to understand the workings of a car engine without being able to "see" inside – sectional views provide that vital insight. This knowledge is not just academic; it's a cornerstone for careers in engineering, design, manufacturing, and even maintenance within South Africa's industrial sector.

Lesson notes

2.1 Introduction to Sectional Views Sectional views are used to expose the internal features of an object or assembly by imagining it cut by a cutting plane. They provide a clearer understanding of the internal structure than can be obtained from exterior views alone.

Cutting-Plane Line: A cutting-plane line indicates the location of the imaginary cut. It is a thick dashed line with arrows at each end, pointing in the direction of sight. The arrows are labelled with letters (e.g., A-A, B-B) to identify the corresponding sectional view.

Section Lines (Hatching): Section lines, or hatching, are used to indicate the areas that have been "cut" by the cutting plane. These lines are thin, equally spaced, and drawn at a 45-degree angle to the main outlines of the sectioned area. The same hatching convention should be used for the same material throughout the drawing. SANS 10111 provides specific hatching conventions for different materials (e.g., steel, cast iron, aluminum).

Hidden Detail: In a sectional view, hidden detail behind the cutting plane is generally omitted, focusing the viewer’s attention on the features that are actually cut.

However, features beyond the cutting plane can be shown as hidden lines if it is crucial for understanding the assembly's geometry.

Types of Sectional Views: Full Section: The cutting plane passes entirely through the object, effectively cutting it in half.

Half Section: The cutting plane extends halfway through the object. This is often used for symmetrical objects to show both the exterior and interior details in a single view. Only one quarter of the object is removed.

Offset Section: The cutting plane is bent or offset to pass through features that would not be visible in a straight cut. This allows for more information to be conveyed in a single section.

Removed Section: A section taken from a particular part of the object and placed elsewhere on the drawing for clarity.

Broken-Out Section: A small portion of the object is removed to reveal internal details. 2.2 Important Conventions and Rules (SANS 10111)

Hatching Direction: Hatching lines are typically drawn at 45 degrees. Adjacent parts should have hatching lines running in opposite directions or at different angles (but still at 45° to a major axis).

Spacing of Hatching: The spacing between hatching lines should be uniform and appropriate for the size of the drawing. Smaller details require closer spacing.

Ribs and Webs: Ribs, webs, and similar features are generally not sectioned if the cutting plane passes parallel to their length. This prevents a misleading impression of solidity. They are shown with outlines only.

However, if the cutting plane cuts across the rib or web, then it is sectioned.

Aligned Sections: In some cases, features are revolved (or "aligned") into the cutting plane to be shown true length in the sectional view. This is often done with features like spokes or bolt holes on a flange. 2.3 Worked Examples Example 1: Full Section of a Simple Valve Housing Imagine a simple globe valve used in a rural water system in South Africa. The valve body is cylindrical and has internal passages for water flow.

Problem: Draw a full sectional view of the valve housing, cutting plane A-A passing through the center of the valve.

Solution: Draw the Front View: First, sketch the front view of the valve housing, including external features.

Locate the Cutting Plane: Draw the cutting-plane line A-A through the center of the front view. Ensure the arrows indicate the direction of sight.

Imagine the Cut: Visualize the valve housing being cut along the cutting-plane line.

Draw the Sectional View: Draw the sectional view, showing the cut surfaces. Hatch the areas that have been cut. Do not hatch any ribs or webs that are cut parallel to their length. Show any features beyond the cutting plane as hidden detail only if necessary.

Label the Section: Label the sectional view "Section A-A." (Diagrams would be here showing each step. Since I can't create diagrams, visualize the process and ensure your actual drawing adheres to the descriptions.)

Example 2: Half Section of a Bushing Consider a bushing, a cylindrical component used in many machines to reduce friction, perhaps in agricultural machinery.

Problem: Draw a half-sectional view of the bushing, assuming it's symmetrical.

Solution: Draw the Front View: Draw the full front view of the bushing.

Locate the Cutting Plane: Draw a cutting-plane line that extends halfway across the bushing's front view. This should be a symmetrical cut along the center line.

Imagine the Cut: Visualize removing one-quarter of the bushing.

Draw the Sectional View: Draw the half-sectional view, showing the external features on one half and the sectioned internal features on the other half. Hatch the cut areas.

Label the Section: Since it's a half-section, no specific labeling beyond the drawing title is required. (Diagrams would be here showing each step.