Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Identify and classify at least five common types of threaded fasteners.
• Draw conventional representations of at least three different threaded fasteners.
• Explain the purpose of washers and locking devices.
• Identify basic geometric shapes in common machine components.

Lesson summary

Mechanical drawing of fasteners and machine components is a fundamental skill in Engineering Graphics and Design. Fasteners hold things together, and machine components are the things that are held together. This week, we will be focusing on understanding different types of fasteners and common machine components, and how to accurately represent them in technical drawings. This is crucial for anyone aiming to work in engineering, manufacturing, construction, or any field where designs need to be communicated precisely.

Lesson notes

2.1 Fasteners: Threaded Fasteners Threaded fasteners are used to clamp parts together by means of a screw thread.

The most common types include: Bolts: Externally threaded fasteners designed to be inserted through holes in assembled parts, and are normally intended to be tightened or released by torquing a nut.

Screws: Externally threaded fasteners capable of being inserted into holes in assembled parts, mating with a preformed internal thread or forming its own thread and are tightened or released by torquing the head. Examples are machine screws, wood screws, and self-tapping screws.

Nuts: Internally threaded fasteners designed to mate with bolts. Common types include hex nuts, square nuts, and wing nuts.

Studs: A headless, externally threaded fastener, threaded at both ends. One end is screwed into a component and the other is used with a nut to clamp another component. Conventional Representation of Threaded Fasteners (SANS Standards) Instead of drawing the threads exactly as they are, we use a simplified conventional representation. This saves time and makes drawings easier to read. Here’s how: Major Diameter: The largest diameter of the thread. In drawings, it is represented by a thick (visible) line.

Minor Diameter: The smallest diameter of the thread (at the root of the thread). In drawings, it is represented by a thin line inside the major diameter. This thin line terminates at the end of the thread.

Thread Length: The length of the threaded portion.

Example 1: Bolt and Nut Combination (Conventional Representation) Let's draw a conventional representation of a bolt and nut combination in elevation. We will assume the bolt is an M10 bolt. This means the major diameter is 10mm.

Bolt Head: Draw a hexagon for the bolt head (use a template for accuracy). The height of the bolt head is usually 0.8 times the diameter (0.8 x 10mm = 8mm). The width across flats is typically 1.5 times the diameter (1.5 x 10mm = 15mm).

Bolt Shank (Threaded Part): Draw two parallel lines representing the major diameter (10mm apart) for the desired thread length. Let’s say the thread length is 30mm. Use a thick (visible) line.

Minor Diameter: Draw a thin line parallel to the major diameter lines inside them, representing the minor diameter. A good approximation for the minor diameter is 0.85 times the major diameter (0.85 x 10mm = 8.5mm).

Therefore, the thin lines are 8.5mm apart. These thin lines terminate at the end of the threaded portion.

Nut: Draw a hexagon for the nut similar to the bolt head, but the height of the nut is often slightly smaller than the bolt head (approximately 0.7 times the diameter, so 0.7 x 10mm = 7mm).

Washer (Optional): A thin washer can be drawn between the nut and the surface of the component being fastened.

Example 2: Set Screw (Conventional Representation) A set screw is used to prevent relative movement between two parts, like a shaft and a pulley. It is often headless.

Screw Body: Draw the major diameter of the screw as a thick line.

Thread Representation: Use the same convention as the bolt - a thin line inside the major diameter to represent the minor diameter.

Tip: The tip of the set screw is often conical or cupped. Draw this tip according to the specific type of set screw. 2.2 Washers and Locking Devices Washers: Thin, flat, or sometimes conical rings used to distribute the load of a fastener, reduce friction, or prevent loosening.

Types include: plain washers, spring washers (like Belleville washers and split washers), and locking washers.

Locking Devices: Devices used to prevent nuts and bolts from loosening due to vibration or other factors.

Examples include: lock washers (split washers, star washers), nyloc nuts (nuts with a nylon insert), and cotter pins. 2.3 Basic Machine Components Many machine components are based on combinations of basic geometric shapes: Cylinder: Shafts, pins, and housings often contain cylindrical elements.

Prism: Blocks, bases, and housings can be based on prisms.

Cone: Conical sections are used in tapers, nozzles, and some types of fasteners.

Sphere: Ball bearings, spherical seats, and some housings incorporate spherical shapes.

Example 3: Representing a Simple Shaft in Orthographic Projection Let’s draw a simple cylindrical shaft with a diameter of 20mm and a length of 50mm in orthographic projection (front view and top view).

Front View: Draw a rectangle. The width of the rectangle is the length of the shaft (50mm), and the height is the diameter of the shaft (20mm). Use thick, visible lines.

Top View: Draw a circle with a diameter of 20mm. Center the circle above the front view rectangle, maintaining alignment. Use thick, visible lines.

Hidden Detail: There are no hidden details in this simple example.

However, if the shaft had a hole drilled through it, we would represent the hole in both views using dashed lines (hidden detail lines).