Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Draw a full sectional view of a simple object.
• Draw a half sectional view of a symmetrical object.
• Correctly identify and draw cutting plane lines.
• Assemble simple machine parts on paper.
• Explain the purpose of sectional views.

Lesson summary

Sectional views are a crucial tool in Engineering Graphics and Design because they allow us to see the internal details of an object that would otherwise be hidden. This is particularly important in the South African context where we are constantly involved in the design, maintenance, and repair of various structures and machines. Consider the water pumps used in rural communities, the electrical distribution boxes in our townships, or even the intricate mechanisms within a minibus taxi – sectional views help engineers and technicians understand how these components function and how to fix them when things go wrong.

Lesson notes

2. 1.

Sectional Views: Exposing Internal Details A sectional view is a drawing technique used to reveal the internal features of an object. Imagine slicing through an object with a plane – that's essentially what a sectional view represents. The area that has been "cut" is indicated with hatching (section lines).

Cutting Plane Line: This is an imaginary line that represents the plane used to cut through the object. It is a thick chain line with thin ends that are perpendicular to the view. It is labeled with letters (A-A, B-B, etc.) to indicate the location of the cut and is shown on the view before the section. Arrows at the ends indicate the direction of sight when viewing the section.

Hatching (Section Lines): These are thin, uniformly spaced lines that indicate the surface has been cut. Hatching should be at 45 degrees and consistent in direction and spacing throughout the section. Different materials can be represented by different hatching patterns (though this is less common in Grade 10). If you have multiple adjacent parts being cut, you MUST change the direction of the hatching lines to avoid confusion.

Hidden Detail: In a sectional view, hidden detail behind the cutting plane line is generally omitted (unless necessary for clarity). The cut surfaces are shown clearly.

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

Half Section: The cutting plane passes halfway through the object, removing one quarter of the object. This is 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 is drawn as a separate view, often adjacent to the view it's taken from. Useful if space is limited.

Revolved Section: The cutting plane is perpendicular to the axis of the object. The section is revolved 90 degrees in place, on the view.

Broken-Out Section (Local Section): Only a small portion of the object is sectioned, revealing a specific internal feature. 2.

2. Assemblies: Putting the Pieces Together An assembly drawing shows how multiple individual parts fit together to form a complete product or machine. It's like a "zoom out" view, showing the relationships between components.

Understanding Assembly Drawings: You need to be able to read orthographic projections of individual parts and visualize how they interact in three dimensions.

Bill of Materials (BOM): A list of all the parts needed for the assembly, usually including the part number, description, quantity, and material. (Typically introduced later but good to be aware of).

Assembly Techniques: (Not drawing these in detail at this level, but understanding the concept)

Fasteners: Bolts, nuts, screws, rivets, etc., used to join parts together.

Welding: Fusing parts together using heat.

Adhesives: Glues and other bonding agents.

Interference Fits: Parts are designed to fit tightly together, requiring force to assemble. 2.

3. Worked Examples Example 1: Full Section of a Simple Block Imagine a rectangular block with a cylindrical hole drilled through the center.

Orthographic Views: You'll have a front view, top view, and side view. The hidden detail lines in the front view show the hole.

Cutting Plane Line: Draw a cutting plane line A-A vertically through the center of the front view, showing the direction of sight.

Sectioned View: Replace the front view with a sectional view. Erase the hidden detail lines representing the hole. Draw hatching at 45 degrees within the rectangular area that has been "cut through." The cylindrical hole will now be clearly visible.

Example 2: Half Section of a Symmetrical Shaft Consider a cylindrical shaft with a keyway cut into one side.

Orthographic Views: You'll have a front view (a circle) and a side view (a rectangle with hidden detail for the keyway).

Cutting Plane Line: Draw a cutting plane line B-B horizontally, halfway across the side view (the rectangle).

Sectioned View: Replace half of the side view with a sectional view. Hatch the cut portion of the cylinder. The keyway will now be fully visible in section in the lower half of the rectangle. The upper half remains as an external view.

Example 3: Simple Assembly – Bolt and Nut You have two parts: a bolt (orthographic projections) and a nut (orthographic projections).

Understanding the Parts: Study the orthographic projections to understand the shape and dimensions of each part.

Assembly: Draw the bolt in a front view. Then, draw the nut engaged with the bolt, showing how the threads align. Guided Practice (With Solutions)

Question 1: Draw a full sectional view of the following object. The cutting plane passes vertically through the center of the object as viewed from the front. The front view is a square with a circular hole in the centre. The side view is a rectangle.

Solution: Draw the Front View with Cutting Plane: Draw the square front view.