Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Construct full sectional views of assemblies.
• Identify and represent cutting planes correctly.
• Apply correct hatching for different materials.
• Interpret sectional views to understand internal parts.
• Explain the advantages of using sectional views.

Lesson summary

Mechanical assemblies and sectional views are fundamental concepts in Engineering Graphics and Design (EGD). They are crucial for understanding how different components fit together to form a functional machine or structure, and for communicating design intent clearly and unambiguously. This week's focus will be on mastering the techniques of creating and interpreting sectional views of mechanical assemblies. Sectional views are particularly important because they expose the internal details of complex objects, which are often hidden in standard orthographic projections.

Lesson notes

2.1 Sectional Views: The Basics A sectional view is a drawing that shows the inside of an object as if it had been cut through with a saw. It allows us to see the internal features that would otherwise be hidden by the object's external surfaces. This is especially important for complex mechanical assemblies.

Cutting Plane Line: This line indicates the location where the object is theoretically cut. It's represented by a thick chain line (long dash, short dash) with arrows at the ends pointing in the direction of sight. The arrows are usually labeled with letters (e.g., A-A).

Section Lines (Hatching): These lines are used to indicate the surfaces that have been cut by the cutting plane. They are thin, parallel lines, usually drawn at a 45-degree angle to the horizontal. The spacing between the lines should be uniform. Different hatching patterns can be used to represent different materials. Consult SANS 10111 for specific material hatching conventions. For example, cast iron typically uses standard 45-degree hatching, while steel may use a different variation.

Removed Sections: These are sections that are drawn outside of the orthographic view. They can be used to show the shape of a small feature more clearly without cluttering the main view.

Offset Sections: The cutting plane line is bent to pass through important features that are not in a straight line. 2.2 Types of Sectional Views Full Section: The cutting plane passes completely through the object, dividing it into two halves.

Half Section: The cutting plane extends halfway through the object. This is typically used for symmetrical objects to show both the internal and external features in a single view.

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

Broken-Out Section: Only a small portion of the object is sectioned to reveal a particular internal feature.

Revolved Section: A section is taken perpendicular to the longitudinal axis and then rotated 90 degrees into the plane of the drawing.

Removed Section: A section is taken and drawn at a different location on the drawing sheet, typically to a larger scale, to show the detail more clearly. 2.3 Hatching Conventions (Material Symbols) The type of hatching used indicates the material of the component. Although SANS 10111 provides a comprehensive list, some common materials and their corresponding hatching symbols include: Cast Iron: Equally spaced lines at 45 degrees.

Steel: Same as cast iron, but lines may be slightly closer together.

Brass/Bronze: Groups of lines at 45 degrees, separated by larger gaps.

Aluminum: Alternating groups of closely spaced lines at different angles.

Rubber/Plastic: Hatching with a wider spacing than metal. Important

Note: Adhere strictly to SANS 10111 for official hatching conventions. 2.4 Ribs, Webs, and Spokes: Ribs, webs, and spokes are often not sectioned when the cutting plane runs parallel to their longitudinal axis. This is because sectioning them would falsely imply a thicker, solid section. Instead, they are typically shown as if they weren't cut.

However, if the cutting plane cuts perpendicular to a rib, web, or spoke, it should be sectioned. 2.5 Aligned Sections: Aligned sections are used when features are located off the center axis of a symmetrical object. These features are "revolved" into the cutting plane before the section view is created. This is done to show the true shape of the features. 2.6

Example: Simple Flange Coupling with Full Section Imagine a flange coupling, used to connect two shafts together. It consists of two flanges bolted together. Let’s create a full section view.

Orthographic Projection: Begin with the standard orthographic views (front, top, and side). Ensure all dimensions are accurate.

Cutting Plane: Draw a cutting plane line on the front view, passing through the center of the flange coupling. Label it A-

A. Sectioned View: In the top view, which will now be the sectional view, imagine cutting the flange along the cutting plane A-

A. Hatching: Apply the appropriate hatching to the cut surfaces. Let's assume the flanges are made of cast iron. Use the standard 45-degree hatching. Remember that the bolts and nuts, if included in the section, are typically not hatched (they are considered "standard parts" and are often not sectioned to improve clarity). If the shaft is part of the assembly, apply the hatching conventions according to its material.

Hidden Detail: Remove any hidden detail lines in the sectioned portion of the view. Only show the visible edges of the cut surfaces.

Dimensions: Add necessary dimensions to the sectional view to fully define the internal features. 2.7

Example: Simple Bracket with Broken-Out Section Consider a simple L-shaped bracket. It has a hole drilled through one arm. A full section isn’t needed. A broken-out section would suffice to show the details of the hole.

Orthographic Projection: Create a front and side view of the bracket.