Lesson Notes By Weeks and Term v4 - SHS 3
Design and Drawing for Manufacture
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Design and Drawing for Manufacture
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Subject: Manufacturing Engineering
Class: SHS 3
Term: 2nd Term
Week: 13
Grade code: 3.2.1.LI.2
Strand code: 2
Sub-strand code: 1
Content standard code: 3.2.1.CS.2
Indicator code: 3.2.1.LI.2
Theme: Design and Prototyping
Subtheme: Design and Drawing for Manufacture
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This lesson introduces the creation of professional part drawings, also known as detail drawings. In Ghana, from the skilled welders in Kokompe to the mechanics in Suame Magazine, people make and fix machine parts every day. Often, they work by copying an old part. However, for modern manufacturing, we need a precise 'language' to communicate exactly how a part should be made. A part drawing is that language. It is a technical instruction manual for the machinist or fabricator, telling them everything they need to know—the shape, size, material, and required quality—to produce a part correctly the first time.
A Part Drawing (or Detail Drawing) is a technical drawing that provides all the information needed to manufacture a single component of a machine or product. It is not an artistic sketch; it is a precise set of instructions. A. Core Components of a Part Drawing Orthographic Views: These are 2D drawings showing the object from different angles, typically the Front, Top, and Right Side views. This method gives a complete and unambiguous representation of the part's geometry. Remember to align the views correctly as learned in SHS 1 & 2. Dimensions: These are numerical values that define the size and location of features on the part. Rules of Dimensioning: Place dimensions clearly outside the object outlines. Use dimension lines, extension lines, and arrowheads. Do not repeat dimensions (no over-dimensioning). Dimension features where their shape is most clearly shown. Example: A simple block might have its length, width, and height dimensioned. A hole is dimensioned by its diameter and its location from two reference edges. Tolerances: It is impossible to manufacture a part to an *exact* size (e.g., precisely 50.0000 mm). A tolerance is the acceptable range of variation for a dimension. Why are they needed? To ensure parts that are made separately can still fit and function together (interchangeability). How they are written: Bilateral Tolerance: Variation is allowed in both directions. Example: 50 ± 0.1. This means the part is acceptable if its size is between 49.9 mm (lower limit) and 50.1 mm (upper limit). Unilateral Tolerance: Variation is allowed in only one direction. Example: 50 +0.1 / -0.0. This means the part is acceptable between 50.0 mm and 50.1 mm. General Tolerances: Often, a note is placed in the title block, like "ALL DIMENSIONS ± 0.5 UNLESS OTHERWISE SPECIFIED". This saves cluttering the drawing.
Worked Example (Calculation): A shaft has a specified diameter of 25 ± 0.05 mm. Nominal Size: 25 mm Upper Limit: 25 + 0.05 = 25.05 mm Lower Limit: 25 - 0.05 = 24.95 mm Total Tolerance: Upper Limit - Lower Limit = 25.05 - 24.95 = 0.1 mm Any shaft produced with a diameter between 24.95 mm and 25.05 mm is acceptable. Surface Finish (Surface Roughness): This specifies the smoothness of a surface. Some surfaces need to be very smooth (e.g., where a bearing will sit), while others can be rough (e.g., the outside of a cast iron block). Symbol: The basic symbol looks like a tick (✓). Measurement: Roughness is often measured in micrometres (µm) and indicated by a value like Ra (Roughness Average). A smaller Ra value means a smoother surface. Example on a Drawing: A basic tick (✓) might mean a standard machined finish. A tick with a number, e.g., Ra 1.6, specifies a particular smoothness achieved by grinding. Common Applications in Ghana: Ra 12.5: Rough casting or sawn finish (very rough). Ra 3.2: General machining on a lathe or milling machine (common finish). Ra 0.8: Grinding, for surfaces where parts slide or seal (e.g., hydraulic ram). Material Specification: The drawing must state the exact material to be used. This affects the part's strength, weight, cost, and how it is manufactured. Example: Mild Steel, Aluminium Alloy 6061, Brass, Cast Iron, PVC. This is typically written in the Title Block. The Title Block: This is the information box, usually in the bottom-right corner of the drawing. It contains all the administrative information. Essential Fields: Part Name: e.g., "Mounting Bracket" Drawn By: Your Name
Material: e.g., "Mild Steel" Scale: e.g., "1:1" or "1:2" General Tolerances: e.g., "± 0.5 mm" Drawing Number:
Guided Practice (With Solutions) Question 1: The Spacer Block