Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Define the purpose and types of sectional views.
• Draw full sectional views of simple objects.
• Identify and use cutting plane lines.
• Correctly apply hatching conventions.
• Draw sectional views of simple assemblies.

Lesson summary

Sectional views are a crucial tool in Engineering Graphics and Design that allow us to see the internal details of an object. Imagine trying to understand the engine of a taxi without being able to see inside – that's the challenge sectional views overcome! This knowledge is vital in various industries across South Africa, from manufacturing and construction to automotive engineering and even architecture. Understanding how parts fit together (assemblies) and then depicting them clearly is essential for accurate communication and successful project completion. This week, we focus on understanding the principles of sectional views and applying them to simple assemblies.

Lesson notes

2.1 What are Sectional Views? A sectional view is a drawing technique used to reveal the interior features of an object. It's like slicing the object with an imaginary cutting plane and then removing the portion in front of the plane to expose the internal details. The exposed surfaces are then indicated by hatching (also known as section lines). Sectional views are extremely useful when hidden details are complex or critical to understanding the design. In South Africa, this is vital for accurately manufacturing parts for machinery used in mining, agriculture, and other key industries. 2.2 Types of Sectional Views: Full Section: The cutting plane passes completely through the object. The entire object behind the cutting plane is removed. Imagine cutting an apple directly in half.

Half Section: The cutting plane extends halfway through the object. This is usually used for symmetrical objects where the exterior and interior can be shown on one view. Imagine cutting an apple from the edge to the center.

Offset Section: The cutting plane is bent or offset to pass through features that are not in a straight line. This allows us to show multiple important internal details in a single view.

Removed Section: A thin slice of the object is removed and drawn to one side, often at a larger scale, to show a particular feature in detail. 2.3 Cutting Plane Line: The cutting plane is indicated by a thick chain line (long dash, short dash, long dash...) with arrowheads at both ends. The arrowheads indicate the direction of sight. Capital letters are used at the ends of the cutting plane line (e.g., A-A) to identify the corresponding sectional view. The sectional view is then labeled accordingly (e.g., Section A-A). 2.4 Hatching (Section Lines): Hatching indicates the surfaces that have been cut by the cutting plane.

The following conventions apply: Hatching lines are thin, continuous lines, usually drawn at 45 degrees to the horizontal. The spacing between hatching lines should be uniform. Different parts of an assembly that are in contact should be hatched with different angles (usually 30 or 60 degrees from the horizontal) or with different hatching patterns. This helps to visually distinguish between adjacent parts. Adjacent areas of the same part should be hatched in the same direction and spacing. 2.5 Simple Assemblies: An assembly drawing shows how different parts fit together to form a complete product. Before you can create a sectional view of an assembly, you must understand how the parts interact. This involves analyzing the orthographic views (front, top, and side) and any other available information, such as part lists or exploded views. 2.6 Sectioning an Assembly: When sectioning an assembly, standard practices apply for hatching to clearly show the different parts that make up the assembly. Adjacent parts should have different hatching angles or patterns.

Example 1: Full Section of a Simple Block Let's say we have a rectangular block with a cylindrical hole through it. The hole is drilled through the center of the block.

Orthographic Views: Draw the front, top, and side views of the block. The hidden detail lines in the front view will show the cylindrical hole.

Cutting Plane Line: Draw a cutting plane line (A-A) vertically through the center of the front view, passing through the cylindrical hole. The arrows should point towards the left.

Sectional View: Draw the sectional view (Section A-A). This view replaces the front view. Draw the outline of the block as if you were looking at it from the direction of the arrows on the cutting plane. The cylindrical hole is now visible, so draw it with solid lines. Hatch the areas of the block that were cut by the cutting plane. Since the hole is empty space, it is not hatched.

Example 2: Half Section of a Symmetrical Object Consider a cylindrical object with an internal cavity on one side. Because it's symmetrical, we can use a half section.

Orthographic Views: Draw the front, top, and side views.

Cutting Plane Line: Draw a cutting plane line (B-B) horizontally through the center of the front view, extending halfway across the object.

Sectional View: Draw the sectional view (Section B-B). One half of the view will be the external view, showing the outside shape of the cylinder. The other half will be the sectional view, showing the internal cavity and hatching. The central axis line should extend through the whole view.

Example 3: Sectioning a Simple Assembly - Bolt and Nut Imagine a simple assembly consisting of a bolt and a nut.

Orthographic Views: Draw the front, top, and side views of the assembled bolt and nut.

Cutting Plane Line: Draw a cutting plane line (C-C) vertically through the center of the front view.

Sectional View: Draw the sectional view (Section C-C). Draw the outline of both the bolt and the nut. Hatch the body of the bolt. Hatch the nut with a different angle than the bolt. This shows that they are two separate components.