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.1
Indicator code: 3.2.1.LI.2
Theme: Design and Prototyping
Subtheme: Design and Drawing for Manufacture
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In modern manufacturing, we cannot make parts to an *exact* size every single time. There will always be small, unavoidable variations. Imagine trying to fix a bicycle with a new bolt. If the bolt and the nut are not made with these variations in mind, they might not fit together! This lesson introduces the system of Limits and Fits, which is the engineering language used on drawings to control these variations. This system ensures that parts made in different factories, even in different countries, can be assembled together correctly. This principle, called interchangeability, is the foundation of mass production, from the phone in your pocket to the tro-tro you ride to school.
This section breaks down the essential vocabulary and principles of limits and fits. 2.1 Fundamental Concepts Nominal Size (or Basic Size): The target size that a dimension is supposed to be. It's the "perfect" size written on the drawing. For example, a `Ø20 mm` shaft has a nominal size of 20 mm. Actual Size: The size of the part as measured after it has been manufactured. It will rarely be exactly the nominal size. Limits of Size: The maximum and minimum permissible sizes for a feature. Upper Limit of Size: The largest acceptable size. Lower Limit of Size: The smallest acceptable size. Tolerance: The total amount of variation allowed for a single dimension. It is the difference between the upper and lower limits. `Tolerance = Upper Limit - Lower Limit` A smaller tolerance means higher precision, which is usually more difficult and expensive to produce.
Example: A shaft is dimensioned as `20 ± 0.1 mm`. Nominal Size = 20 mm Upper Limit = 20 + 0.1 = 20.1 mm Lower Limit = 20 - 0.1 = 19.9 mm Tolerance = 20.1 - 19.9 = 0.2 mm Deviation: The difference between a size (actual, upper, or lower limit) and the basic size. Upper Deviation (ES for holes, es for shafts): `Upper Limit - Basic Size` Lower Deviation (EI for holes, ei for shafts): `Lower Limit - Basic Size` _Note the convention: Uppercase letters (ES, EI) for holes, lowercase letters (es, ei) for shafts._ Allowance: The intentional difference between the dimensions of two mating parts (e.g., a shaft and a hole). It determines the type of fit. `Allowance = Minimum Clearance` or `Maximum Interference`. 2.2 Types of Fits
A "fit" describes the relationship between a shaft and the hole it is assembled into. There are three main categories: Clearance Fit: The shaft is always smaller than the hole, creating a gap or "clearance". This allows for relative motion, like a rotating shaft in a bearing. The largest possible shaft is smaller than the smallest possible hole. *Example Application:* A car axle rotating freely in its housing. Interference Fit (or Press Fit): The shaft is always larger than the hole before assembly. The parts must be forced together (e.g., with a hydraulic press or by heating the hole/cooling the shaft). This creates a very strong, fixed joint. The smallest possible shaft is larger than the largest possible hole. *Example Application:* A gear pressed onto the shaft of an electric motor. Transition Fit: The tolerance zones of the shaft and hole overlap. Depending on the actual sizes of the parts, the fit could result in either a small clearance or a small interference. This is used when accurate location is needed, but the parts might need to be taken apart later. *Example Application:* A pulley on a shaft, located with a key. 2.3 The ISO System of Limits and Fits (The Standard)
To avoid confusion, engineers worldwide use the ISO 286 standard. A fit is specified using a combination of the basic size, a letter, and a number. For example: `Ø50 H7/g6`