Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Interpret complex assembly drawings.
• Create detailed working drawings of individual components.
• Apply appropriate sectioning techniques.
• Apply geometric tolerances and surface finish symbols.
• Produce a bill of materials (BOM).

Lesson summary

This week, we delve into the crucial skill of interpreting and creating detailed working drawings from complex assemblies. This is a cornerstone of engineering communication, allowing designers and manufacturers to collaborate effectively in bringing ideas to life. Imagine designing a bridge or a new machine – accurate working drawings are the language that ensures everyone understands the design intent, minimizing errors and saving resources. In the South African context, the ability to produce and interpret these drawings is vital for contributing to infrastructure development, manufacturing, and various engineering projects that drive our economy and improve the lives of our communities.

Lesson notes

Assembly Drawings: An assembly drawing shows how multiple parts fit together to form a complete product or system. It provides a visual representation of the final assembled state and often includes a parts list or bill of materials (BOM). Assembly drawings generally don’t contain all the dimensions and details necessary to manufacture the individual parts. They are designed for understanding how things fit together.

Detailed Working Drawings: A detailed working drawing provides all the information needed to manufacture a single component of an assembly.

It includes: Orthographic Projections: Typically, front, top, and side views (or more if necessary) are provided to fully describe the shape of the part. Remember to apply first-angle projection as per SANS standards.

Dimensions: Accurate dimensions are crucial. Use chain dimensioning, parallel dimensioning, or combined dimensioning strategically to clearly communicate the part's size and shape. Consider the manufacturing processes when deciding which dimensions are critical. For example, the diameter of a hole that will be drilled should be clearly dimensioned.

Tolerances: Tolerances specify the allowable variation in dimensions. These are essential for ensuring that parts will fit together correctly and function as intended. We use general tolerances unless otherwise specified on the drawing. Refer to SANS 22768-1 (General tolerances)

Sectioning: Sectioning is used to reveal internal features that would otherwise be hidden. Choose the appropriate sectioning technique (full, half, offset, etc.) to best illustrate the internal geometry. Remember to hatch the cut surfaces.

Surface Finish: Surface finish symbols indicate the required surface roughness of a part. This affects the part's function, appearance, and cost.

Material Specification: The material from which the part is to be made must be clearly specified.

Annotations: Notes and symbols providing additional information, such as heat treatment requirements or special instructions.

Bill of Materials (BOM): The BOM is a comprehensive list of all the components required to build an assembly.

It typically includes: Item Number: A unique identifier for each component.

Quantity: The number of each component required.

Part Number: The manufacturer's part number (if applicable).

Description: A brief description of the component.

Material: The material from which the component is made.

Revision Level: Indicates the version of the component.

Sectioning Techniques: Full Section: The cutting plane passes entirely through the object, removing one half to reveal the internal features.

Half Section: The cutting plane passes halfway through the object, removing one quarter to reveal both internal and external features. Often used for symmetrical objects.

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

Revolved Section: A section is revolved 90 degrees and superimposed on the view to show the shape of a feature.

Removed Section: A section is removed from the view and placed elsewhere on the drawing, often at a larger scale for clarity.

Geometric Tolerancing (GD&T): While not a primary focus this week, understanding the basics is important. GD&T uses symbols to specify tolerances for form, orientation, location, and runout.