Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Draw sectional views using correct conventions.
• Identify different types of sectional views.
• Interpret internal features from sectional views.
• Draw simple assemblies showing how parts fit together.

Lesson summary

This week, we delve into sectional views and simple assemblies. Sectional views are crucial in engineering drawings because they reveal the internal details of an object that would otherwise be hidden. Think about looking at a map of Johannesburg; you see the roads and buildings, but you don't see the underground tunnels for Gautrain. A sectional view is like cutting the object open so you can see those tunnels. Simple assemblies, on the other hand, involve combining two or more components to create a functional unit. Understanding how parts fit together is vital in manufacturing and construction, crucial sectors in South Africa.

Lesson notes

2.1 Sectional Views: Exposing the Hidden A sectional view is a drawing that shows the inside of an object as if it had been cut apart. It is used to clarify the interior construction of a part when it is too complex to be described clearly by hidden lines on an exterior view.

Cutting Plane Line: This is a line that indicates where the imaginary cut has been made. It is a thick, dashed line with arrows at the ends indicating the direction of sight. The arrows are labeled with letters (e.g., A-A, B-B) to identify the section.

Hatching (Section Lining): The cut surfaces are represented by hatching, which are thin, evenly spaced lines drawn at an angle (typically 45 degrees) to the horizontal. Different materials are represented by different hatching patterns.

However, for general purposes, a single hatching pattern is often used.

Types of Sectional Views: Full Section: The cutting plane passes entirely through the object.

Half Section: The cutting plane passes halfway through the object. This is typically used for symmetrical objects. One half of the view shows the exterior, and 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. This allows you to show more detail in a single section.

Removed Section: A section view that is drawn adjacent to the view where the cut is made but not directly in line with it.

Rules for Hatching: Hatching lines should be thin and evenly spaced. The hatching angle should typically be 45 degrees to the horizontal. Adjacent parts should be hatched at different angles (usually 30 or 60 degrees if the first part is at 45 degrees) or with different spacing to distinguish them. Large areas may be hatched around the edges to save time.

Example 1: Full Section of a Bolt Imagine a bolt. If we cut it in half lengthwise, we can see the threads inside. The cutting plane line would run down the center of the bolt. The sectional view would show the cut surface of the bolt, hatched at 45 degrees. The threads would be visible in section. Note that bolts are never sectioned through the axis. A half section or full section is used.

Example 2: Half Section of a Symmetrical Shaft with a Keyway Consider a cylindrical shaft with a keyway cut into it. A half section is ideal here. One half of the view shows the outside of the shaft, while the other half reveals the keyway inside. This method is efficient as it depicts both external and internal features without creating unnecessary views.

Example 3: Offset Section of a Bracket with Multiple Holes Suppose we have a bracket with several holes drilled at different depths. A single straight cut wouldn't reveal all the holes. An offset section can be used, where the cutting plane bends to pass through each hole. The resulting sectional view will show all the holes in a single view, improving clarity. 2.2 Simple Assemblies: Putting it Together An assembly drawing shows how multiple parts fit together to form a complete unit. It's like the instructions for building a toy. Assembly drawings are essential for manufacturing, construction, and maintenance.

Exploded Views: These show the parts of an assembly separated but aligned to show their relationship. Exploded views are very helpful for understanding how the parts fit together.

Bill of Materials (BOM): A list of all the parts required for the assembly, including their part numbers, descriptions, quantities, and materials. The BOM is usually included on the assembly drawing.

Assembly Methods: Different methods such as bolting, welding, riveting, gluing, and interference fit are used to join parts together.

Example 4: Simple Assembly of a Nut and Bolt Imagine a nut and bolt assembly. The assembly drawing would show the bolt passing through a hole, with the nut tightened on the other side. The BOM would list the bolt, nut, and any washers required. The drawing may include leader lines pointing to each part, with corresponding numbers that link them to the BO

M. Example 5: Assembly of a Pipe Flange Consider two pipes connected with a flange. The assembly drawing will show the two pipe sections, the flanges, and the bolts that hold them together. The drawing will indicate the size and type of bolts required. The sectional view would show the internal sealing mechanism. Guided Practice (With Solutions)

Question 1: Draw a full sectional view of a rectangular block with a cylindrical hole drilled through the center. The cutting plane line passes through the center of the hole. The block dimensions are 80mm x 50mm x 30mm. The hole diameter is 20mm.

Solution: Draw the rectangular block in its normal view (e.g., front view). Draw the cutting plane line down the center of the block, with arrows indicating the direction of sight. Label the cutting plane A-A. Draw the sectional view to the side (or below) the original view. Show the cut surface of the rectangular block hatched at 45 degrees.