Lesson Notes By Weeks and Term v5 - Grade 11
Mechanical drawing: fasteners and machine components – Week 5 focus
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Mechanical drawing: fasteners and machine components – Week 5 focus
Download the Lessonotes Mobile South Africa app for faster lesson access on Android and iPhone.
Subject: Engineering Graphics and Design
Class: Grade 11
Term: 2nd Term
Week: 5
Theme: General lesson support
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This week, we delve into the critical world of fasteners and machine components in mechanical drawing. Understanding how to accurately represent and interpret these elements is fundamental to successful engineering design. From the bolts holding together a bakkie's chassis to the rivets securing sheets of corrugated iron, fasteners are all around us, ensuring stability and functionality. Similarly, comprehending how to draw and interpret machine components allows for the creation and maintenance of essential machinery. This knowledge is incredibly relevant in South Africa, where infrastructure development, manufacturing, and maintenance are crucial for economic growth and job creation.
2.1 Fasteners: The Glue Holding it All Together Fasteners are mechanical devices used to join two or more parts together. They come in various shapes, sizes, and materials, each designed for specific applications. The most common fasteners are bolts, nuts, screws, and rivets. South African National Standards (SANS) dictate how these fasteners should be represented in engineering drawings. We’ll focus on simplified representations, as they are the most efficient way to convey the necessary information.
Bolts: A bolt is a threaded fastener used with a nut to clamp parts together. They typically have a hexagonal head, though other head shapes are possible.
Nuts: A nut is a fastener with a threaded hole, designed to be used with a bolt to clamp parts together. Nuts typically have a hexagonal shape.
Screws: A screw is a threaded fastener that can be inserted directly into a material. They often have a slot or recessed head that allows them to be tightened with a screwdriver or similar tool.
Rivets: A rivet is a permanent fastener consisting of a smooth cylindrical shaft with a head on one end. It's used to join materials by passing the shaft through holes in the materials and then deforming the tail to form a second head. 2.2 Thread Representation Threads are a crucial aspect of bolts, nuts, and screws. They allow the fastener to grip the material and provide a strong clamping force. There are two main types of thread representation in engineering drawings: External Thread: This is the thread on the outside of a bolt or screw. In a simplified representation, it is shown as two parallel lines running along the length of the fastener. One line represents the major diameter (outer edge) of the thread, and the other line represents the minor diameter (inner edge).
Internal Thread: This is the thread inside a nut or tapped hole. In a simplified representation, it is shown as two parallel lines within the hole. Again, one line represents the major diameter and the other the minor diameter. When drawing a nut in section, show the internal thread using hidden detail lines where the thread is obscured. 2.3 Machine Components: The Building Blocks of Machines Machine components are the individual parts that make up a machine.
Some common machine components include: Shafts: A shaft is a rotating element used to transmit power. They are typically cylindrical in shape. In drawings, shafts are represented as circles (in end view) or rectangles (in side view). Centre lines are essential to indicate the axis of rotation.
Bearings: A bearing is a component that supports a rotating shaft and reduces friction. There are various types of bearings, such as ball bearings and roller bearings. Simplified representations are generally used, focusing on the overall dimensions and shape.
Gears: Gears are toothed wheels used to transmit rotational motion and torque. They come in various types, such as spur gears, bevel gears, and helical gears. Drawing gears accurately can be complex, so simplified representations are often used to show the basic form and dimensions. 2.4 Drawing Conventions: Ensuring Clarity and Accuracy Several drawing conventions are essential for creating clear and accurate mechanical drawings: Hidden Detail: Hidden detail lines (dashed lines) are used to show features that are not visible from the current view. This is particularly important for showing internal threads or features within machine components.
Centre Lines: Centre lines (long dash, short dash, long dash) are used to indicate the centre of circles, arcs, and symmetrical features. They are crucial for dimensioning and ensuring the correct alignment of parts.
Dimensioning: Dimensioning involves adding measurements to a drawing to specify the size and location of features. Dimensions should be clear, concise, and follow SANS standards. Always include the unit of measurement (e.g., mm).
Example 1: Drawing a simplified representation of a bolt (M12 x 50)
M12: Indicates the metric thread diameter is 12mm.
50: Indicates the length of the bolt is 50mm.
Draw the Bolt Shank: Draw a rectangle 50mm long and 12mm wide. This represents the body of the bolt.
Draw the Thread: Within the rectangle, draw two parallel lines representing the thread. The outer line should coincide with the edge of the rectangle (12mm), and the inner line should be approximately 10mm (minor diameter - this value can be assumed unless specifically given). The thread should extend along the bolt's length, excluding the head.
Draw the Hexagonal Head: At one end of the rectangle, draw a hexagon. The distance across the flats of the hexagon is typically 1.5 times the bolt diameter (1.5 x 12mm = 18mm). You can approximate the hexagon using a circle with a diameter of 18mm and then inscribing a hexagon inside it. Ensure you are representing the hexagon accurately, the flats should be parallel to the drawing board.
Add Centre Line: Add a centre line along the length of the bolt.
Example 2: Drawing a simplified representation of a nut (M12)
M12: Indicates the metric thread diameter is 12mm.
Draw the Hexagon: Draw a hexagon. The distance across the flats of the hexagon is typically 1.5 times the nut diameter (1.5 x 12mm = 18mm). As before, approximate with a circle then inscribe a hexagon.
Draw the Internal Thread (in section): If you are drawing a section view of the nut, show the internal thread. Draw two parallel lines inside the hexagon. The outer line should be 12mm apart, and the inner line should be about 10mm apart. Use hidden detail lines to indicate that the thread is inside the nut.
Example 3: Drawing a shaft supported by a bearing
Draw the Shaft: Draw a rectangle representing the shaft. Add a centre line along the length of the shaft.
Draw the Bearing: Draw a simplified representation of the bearing around the shaft. This might be a rectangle or a more stylized representation, depending on the specific type of bearing.
Dimensioning: Add dimensions to indicate the shaft diameter and length, and the bearing dimensions.
Guided Practice (With Solutions)
Question 1: Draw a simplified representation of an M10 x 40 bolt. Show all necessary dimensions.