Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Interpret full and half sectional views of mechanical assemblies.
• Create full and half sectional views from assembly drawings.
• Differentiate between various types of sections.
• Annotate sectional views according to SANS standards.

Lesson summary

Mechanical assemblies are fundamental to engineering design. They represent how individual components come together to form a functional unit. Sectional views are crucial for understanding the internal workings of these assemblies, revealing hidden details that would otherwise be obscured in a conventional external view. In South Africa, this understanding is vital for various sectors, from manufacturing and mining to infrastructure development and maintenance. Imagine designing a water pump for a rural community – understanding its internal assembly via sectional views is essential for efficient design, repair, and maintenance.

Lesson notes

2. 1.

Mechanical Assemblies: A mechanical assembly is a collection of two or more parts joined together to perform a specific function. Understanding how these parts interact is crucial for design, manufacturing, and maintenance. Drawings of mechanical assemblies typically show how the different parts fit together. 2.

2. Sectional Views: Sectional views are used to reveal the internal details of an object that would otherwise be hidden by the exterior. They are created by imagining a cutting plane passing through the object and removing the front (or top, or side) portion. The cut surface is then shown with section lines (also known as hatching). 2.2.

1. Cutting Plane Line: The cutting plane is an imaginary plane that cuts through the object. It is represented by a cutting plane line on the view from which the section is taken. The cutting plane line is usually chain thin with thick ends and arrows indicating the direction of sight. 2.2.

2. Section Lines (Hatching): Section lines are used to indicate the surfaces that have been cut by the cutting plane. They are generally drawn at a 45-degree angle and should be uniformly spaced. Different materials are often represented by different hatching symbols, as defined in SANS standards (although for practical purposes in Grade 12 EGD, a single standard hatching is used). 2.2.

3. Hidden Detail: Hidden detail is generally not shown in sectional views unless it is absolutely necessary for clarity. The primary purpose of a sectional view is to show the cut surfaces. Showing too much hidden detail can clutter the drawing and make it difficult to interpret. 2.

3. Types of Sectional Views: Full Section: The cutting plane passes completely through the object. This is the most common type of section.

Half Section: The cutting plane passes halfway through the object, removing one-quarter of the object. Half sections are commonly used for symmetrical objects. One half of the view shows the exterior, while the other half shows the interior.

Offset Section: The cutting plane is bent or offset to pass through features that are not in a straight line.

Revolved Section: A cross-section of a feature is revolved 90 degrees and drawn directly on the view.

Removed Section: A cross-section is drawn separately from the view, usually to a larger scale. 2.

4. Sectioning Conventions: Adjacent Parts: When two adjacent parts are sectioned, the section lines should be drawn in opposite directions or at slightly different angles. This helps to distinguish between the parts. Ribs, Webs, and Spokes: Ribs, webs, and spokes are generally not sectioned if the cutting plane passes through them lengthwise. This is because sectioning them would give a false impression of thickness and solidity.

However, if the cutting plane cuts across a rib, web, or spoke, it should be sectioned.

Aligned Sections: In certain cases, features that are not actually in the cutting plane may be "aligned" or rotated into the plane for clarity. This is particularly common with circular features. 2.

5. Worked

Examples: Example 1: Full Section of a Simple Bracket: Imagine a simple L-shaped bracket. A vertical cutting plane cuts through the entire bracket. The resulting full section shows the thickness of the material and any internal features (e.g., holes, fillets). The section lines are drawn at 45 degrees. If there are fillets or rounds, these also need to be properly drawn.

Example 2: Half Section of a Shaft Support: Consider a symmetrical shaft support. A cutting plane cuts halfway through the support. One half of the drawing shows the external view, while the other half shows the sectioned view, revealing the inner diameter of the bearing housing. No hidden detail is added to the sectional view unless it is vital.

Example 3: Ribs in a Sectional View: Imagine a casing that has a rib running along its inner surface for structural support. The cutting plane cuts lengthwise down the rib. In this case, the rib is not sectioned.

However, if the cutting plane had cut across the rib, it would be sectioned.

Example 4: Sectioning Adjacent Parts of a Flange Coupling: Consider a flange coupling consisting of two flanges bolted together. When creating a sectional view of the assembled flanges, the section lines on the two flanges should be drawn in opposite directions (e.g., one at 45 degrees, the other at -45 degrees) to clearly distinguish them. The bolts themselves would usually not be sectioned if the cutting plane runs along their length.

Example 5: Applying SANS standards The width of the cutting plane lines should be clearly thicker than the outline of the object. Section lines must be uniformly spaced. Consider appropriate application of dimensioning for sectional views Guided Practice (With Solutions)

Question 1: A cylindrical bushing has an outer diameter of 50mm, an inner diameter of 30mm, and a length of 60mm. Draw a full sectional view of the bushing, assuming a vertical cutting plane passes through its center.