Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Identify different types of sectional views (full, half, offset).
• Create sectional views applying correct hatching (SANS 10111).
• Interpret engineering drawings with sectional views.
• Apply standard conventions for lines and dimensioning.
• Explain the purpose of using sectional views.

Lesson summary

This week, we're diving into mechanical assemblies and sectional views, crucial skills for any aspiring engineer or designer. Understanding how individual components fit together to form a functional machine or structure is fundamental. Sectional views are the tool we use to visualize the internal details of these assemblies, allowing us to clearly communicate complex designs. In South Africa, from the bustling automotive industry in Gqeberha (formerly Port Elizabeth) to the mining sector needing robust machinery, a strong grasp of mechanical assemblies and sectional views is highly valuable.

Lesson notes

2.1 Mechanical Assemblies: A mechanical assembly is a collection of two or more parts fastened or joined together to perform a specific function. Understanding how these parts interact is crucial for design, manufacturing, and maintenance. Assemblies can range from simple (e.g., a hinge) to extremely complex (e.g., an engine). We represent assemblies in drawings showing how individual components fit together, their relative positions, and often their functionality. 2.2 Sectional Views: Sectional views are used to reveal the internal features of an object or assembly that are difficult or impossible to see in a regular view. Imagine trying to understand the intricate workings of a borehole pump without being able to "see inside" – sectional views give us that crucial internal perspective. 2.2.1 Cutting Plane Line: The cutting plane line is an imaginary line that "cuts" through the object to create the sectional view. It's represented by a thick, dashed line with arrows at each end indicating the direction of sight. The arrows are labeled with letters (e.g., A-A) to identify the sectional view. 2.2.2 Section Lines (Hatching): Section lines (or hatching) are used to indicate the surfaces that have been "cut" by the cutting plane. Different materials are represented by different hatching patterns as defined by SANS

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1. General Use (Cast Iron, Steel): Equally spaced lines at 45 degrees.

Other Materials: Varying angles and spacing depending on the material. Consult SANS 10111 for specific material representations.

Adjacent Parts: When two adjacent parts are sectioned, the hatching lines should be drawn at different angles to distinguish them. 2.2.3 Types of Sectional Views: Full Section: The cutting plane passes completely through the object. This is the most common type.

Half Section: The cutting plane cuts halfway through the object. Typically used for symmetrical objects. One half shows the outside view, and the other half shows the inside.

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

Broken-Out Section: Only a small portion of the object is sectioned to reveal a specific detail. The sectioned area is "broken out" from the regular view using a jagged line.

Removed Section: The section view is drawn adjacent to the view of the object and not in direct projection. 2.3 Conventions and Rules: Hidden lines are generally omitted from sectional views, as the section reveals the internal features.

However, in some cases, where a hidden feature is crucial for understanding, it can be included. Ribs, webs, and spokes are generally not sectioned when the cutting plane passes along their length. This is to avoid giving a false impression of solidity.

Aligned Sections: In some cases, features may be "aligned" to appear in the section view even though the cutting plane does not actually pass through them in their true location. This is done to show important features more clearly.

Dimensioning: Follow standard dimensioning practices according to SANS

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1. Dimensions should be placed outside the sectioned area whenever possible. 2.4 Worked

Examples: Example 1: Full Section of a Simple Bracket Imagine a simple L-shaped bracket used to support a shelf. The bracket has two holes drilled through it for mounting. Let's create a full section view, assuming the cutting plane passes vertically through the center of one of the holes. Draw the front view of the bracket. Add the cutting plane line vertically through the center of one of the holes, with arrows pointing to the left (indicating the direction of sight for the section view). Label the cutting plane line A-A. Draw the section view to the left of the front view. The hole that was cut by the cutting plane will now appear as a circle. The material of the bracket that was "cut" by the cutting plane is hatched at 45 degrees, assuming it's made of steel. Hidden lines from the front view are generally omitted in the section view. Label the section view as "SECTION A-A".

Example 2: Half Section of a Shaft Collar Consider a cylindrical shaft collar with a set screw. Due to its symmetry, a half section is appropriate. Draw the front view of the shaft collar as a circle. Indicate the position of the set screw. Add the cutting plane line that cuts halfway through the collar. The cutting plane line is vertical and stops at the centre line of the front view. Label the cutting plane line B-B. Draw the half section view. One half of the view is the regular front view showing the outside of the collar and the set screw. The other half is the sectional view, showing the hole for the shaft and the tapped hole for the set screw. The cut surfaces are hatched at 45 degrees.

Example 3: Offset Section of a Lever Arm A lever arm has a hole at one end and a keyway along its length. The hole and keyway are not aligned. An offset section can show both. Draw the front view of the lever arm. Add the cutting plane line.