Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Extract detailed part drawings from an assembly drawing.
• Apply dimensional and geometrical tolerances.
• Specify appropriate surface finish requirements.
• Create section and auxiliary views for complex geometry.
• Annotate drawings according to SANS standards.

Lesson summary

This week, we delve deeper into the crucial skill of creating detailed working drawings for complex assemblies. In South Africa, industries ranging from manufacturing to construction rely heavily on precise engineering drawings for communication and execution. Understanding how to produce these drawings is not just about passing exams; it's about preparing you for careers that directly contribute to building and maintaining South Africa's infrastructure, manufacturing sector, and overall economy.

Lesson notes

2.1 Assembly Drawings vs.

Detailed Part Drawings: An assembly drawing shows how different parts fit together to form a complete product or machine. It's like a map showing the overall structure. A detailed part drawing, on the other hand, focuses on a single component. It provides all the information needed to manufacture that specific part: dimensions, tolerances, material specifications, surface finish, and any special instructions. Think of it as the blueprint for one specific brick in a building. The assembly drawing shows where the brick goes, but the part drawing tells the builder exactly how to make it. 2.2 Extracting Part Drawings: The first step is to carefully examine the assembly drawing. Identify the part you want to detail. Determine the best viewing direction to show its key features. Consider which views (front, top, side) are necessary to fully define its shape. Sometimes, section views or auxiliary views are needed to reveal internal details or true shapes of inclined surfaces.

Example: Imagine an assembly drawing of a simple bicycle pedal. To create a detailed drawing of the pedal body, you'd likely need a front view showing its overall shape, a side view showing its thickness and bearing bore, and possibly a section view to show the internal threads for the spindle. 2.3 Dimensioning and Tolerancing: Dimensioning is the process of adding size information to the part drawing. Use the assembly drawing to understand the part's function and how it interacts with other components. Dimensions must be complete, clear, and avoid redundancy. Apply baseline, chain, or coordinate dimensioning depending on the design intent. Always dimension to visible lines, not hidden lines unless absolutely necessary. Tolerancing is specifying the acceptable variation in dimensions. No manufacturing process is perfect, so parts will always have slight deviations from the nominal size. Tolerances ensure that parts will still function correctly when assembled.

Common types of tolerances include: General tolerances: Applied to dimensions that don't have specific tolerances assigned. Usually found in the title block.

Limit dimensions: Specify the maximum and minimum acceptable sizes (e.g., 25.00 +0.05/-0.02).

Plus/minus tolerances: Specify the allowable variation above and below the nominal size (e.g., 25.00 ±0.03). Geometric Dimensioning and Tolerancing (GD&T): Uses symbols to control the form, orientation, location, and runout of features. GD&T ensures better fit and function of assemblies.

Example: A shaft designed to fit into a bearing will need a tight tolerance on its diameter to ensure a proper fit. A general tolerance might be sufficient for the overall length of the shaft if it's not critical to the assembly. 2.4 Surface Finish: Surface finish refers to the texture of a part's surface. It is measured in micrometers (µm) or microinches. The required surface finish depends on the part's function, material, and manufacturing process. Rough surfaces can increase friction and wear, while smooth surfaces are needed for sealing and precision applications. Surface finish is indicated on the drawing using symbols and Ra values.

Example: The surface of a piston cylinder in an engine needs to be very smooth (low Ra value) to minimize friction and ensure proper sealing of the piston rings. The outside of a cast iron engine block might have a relatively rougher surface finish (higher Ra value). 2.5 Section and Auxiliary Views: Section Views: Used to reveal internal features of a part. The cutting plane line indicates where the part is "cut," and the section view shows the exposed surfaces with hatching. Common types include full sections, half sections, and offset sections.

Auxiliary Views: Used to show the true shape of an inclined surface. These views are projected from the inclined surface onto a plane that is parallel to it. This allows for accurate dimensioning and tolerancing of the inclined feature. 2.6 Drawing Standards and Conventions (SANS): South African National Standards (SANS) define the conventions for creating engineering drawings. This includes line types, lettering styles, dimensioning rules, and title block formats. Following these standards ensures that drawings are clear, consistent, and easily understood by anyone familiar with engineering drawing practices in South Africa. Familiarise yourself with SANS 10111. 2.7 Worked

Example: Detailing a Simple Bracket Let's say we have a simple L-shaped bracket in an assembly drawing. The bracket is made of mild steel and is bolted to a wall.

Step 1: Identify the views. A front view and a side view are sufficient to describe the bracket's shape.

Step 2: Dimensioning. Dimension the overall height, width, and thickness of the bracket. Also, dimension the location of the bolt holes from the edges. Use baseline dimensioning from one corner of the bracket for hole locations to avoid tolerance stack-up.

Step 3: Tolerancing.