Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

This page supports the lesson note with a companion video and a short classroom-ready summary.

For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.

Performance objectives

• Correctly apply cutting plane lines.
• Create full and half sectional views.
• Apply section lining (hatching) accurately.
• Identify and avoid common sectioning errors.
• Interpret sectional assembly drawings.

Lesson summary

This week, we delve into the crucial skill of creating and interpreting mechanical assembly drawings with sectional views. Understanding how parts fit together and visualizing internal features is paramount in engineering and design. Whether it's designing a new type of water pump for rural communities or improving the efficiency of mining equipment, the ability to accurately represent and understand assemblies is essential for South African engineers and designers. Sectional views are vital because they allow us to see the internal features of an object without having to draw hidden lines, which can make drawings cluttered and difficult to understand.

Lesson notes

2.1 Introduction to Mechanical Assemblies: A mechanical assembly is a collection of two or more parts that are fastened together to form a unit. Assembly drawings show how these parts fit together, providing all the information necessary for assembly. In South Africa, these assemblies could be anything from the engine of a taxi to the components of a maize thresher used in agriculture. 2.2 Sectional Views: Revealing Internal Details: Sectional views are used to reveal the internal features of an object or assembly. They are created by imagining that the object has been cut by a cutting plane. The cutting plane line indicates where the cut is made and the direction in which the view is taken.

Types of Sectional Views: Full Section: The cutting plane passes entirely through the object. The entire area behind the cutting plane is shown.

Half Section: The cutting plane passes halfway through the object. One half of the object is shown in section, and the other half is shown as an external view. Half sections are typically used for symmetrical objects.

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

Removed Section: A section that is removed from its normal projected position and placed elsewhere on the drawing. This is often done to avoid cluttering the main view.

Broken-out Section: A portion of the object is broken away to reveal an internal feature. 2.3 Cutting Plane Lines: Cutting plane lines are thick, dashed lines with arrows at the ends. The arrows indicate the direction of sight for the sectional view. Letters are placed at each end of the cutting plane line to identify the corresponding sectional view (e.g., A-A, B-B). 2.4 Section Lining (Hatching): Section lining, also known as hatching, is used to indicate the surfaces that have been cut by the cutting plane. The lines are thin, parallel, and evenly spaced. The angle of the lines is typically 45 degrees to the horizontal.

Material Representation: Different hatching patterns are used to represent different materials. Although specific hatching patterns are standardized, a legend should always be included on the drawing to identify the materials. In exams, you will typically use a generic hatching for the same material.

Adjacent Parts: If two adjacent parts are sectioned, the hatching lines should be drawn at different angles to distinguish them. Typically, they will differ by 30 degrees.

Large Areas: For large areas, the hatching may be omitted from the center to save time and reduce clutter. The hatching will be along the outer edges to show the boundaries of the section. 2.5 Important Considerations and Conventions: Ribs and Webs: Ribs and webs are often not sectioned if the cutting plane runs parallel to them along their length. This is to avoid giving the false impression that they are solid. The exception is when the rib/web adds crucial information. Shafts, Bolts, Nuts, and Washers: These components are typically not sectioned if the cutting plane runs along their axis of symmetry. This is because sectioning them would not reveal any useful information. They are drawn as if they are solid.

Alignment: Ensure that sectional views are properly aligned with the corresponding orthographic views. 2.6 Worked Example 1: Full Section of a Simple Bracket Imagine a simple L-shaped bracket used to support a water tank in a rural community. Let's say we need to create a full section view to show the thickness of the metal and the shape of a reinforcing rib.

Step 1: Draw the front view of the bracket.

Step 2: Draw the cutting plane line through the center of the bracket, indicating the desired cut. Label it A-

A. Step 3: Draw the sectional view A-A, showing the area behind the cutting plane. The bracket material is hatched at 45 degrees. The reinforcing rib, since the cutting plane passes perpendicular to it, is fully hatched.

Step 4: Add dimensions and annotations as needed. 2.7 Worked Example 2: Half Section of a Symmetrical Valve Body Consider a symmetrical valve body used in irrigation systems. A half section is ideal for showing both the internal and external features.

Step 1: Draw the front view of the valve body, showing the axis of symmetry.

Step 2: Draw the cutting plane line, extending halfway through the valve body along the axis of symmetry. Label it B-

B. Step 3: Draw the half section view B-B. One half of the view shows the internal features with hatching, while the other half shows the external features.

Step 4: Add centerlines, dimensions, and annotations. 2.8 Worked Example 3: Offset Section of a Gear Housing Imagine a gear housing containing gears that need to be displayed in section. An offset section is useful when the key features are not in the same plane.

Step 1: Draw the front view of the gear housing.

Step 2: Draw the cutting plane line, offsetting it to pass through the center of different gears. Label it C-

C. Step 3: Draw the offset section view C-C.