Screw Threads, Fasteners and Their Types for SS 3

Class: SS 3
Term: First Term
Week: 3
Age: 17 years
Duration: 60 minutes
Subject: Technical Drawing
Previous Lesson: Engineering Design Process and Preparation of Working Drawings
Topic: SCREW THREADS, FASTENER AND DEVICES
Subject Matter: type of screw threads e.g. v-threads,square buttress and active etc type of fastener and their user e.g. bolt, nuts, studs screws etc

Specific Objectives

By the end of the lesson, pupils/students should be able to:

Cognitive Domain

• Define screw threads.
• Identify different types of screw threads (V-threads, square threads, buttress threads, Acme threads).
• Define fasteners.
• Identify various types of fasteners (bolts, nuts, studs, screws).
• State the uses of different fasteners.

Affective Domain

• Appreciate the importance of screw threads and fasteners in mechanical assemblies.
• Develop an interest in the accurate representation of threaded components.

Psychomotor Domain

• Sketch common forms of screw threads.
• Sketch different types of fasteners.

Social Domain

• Collaborate with peers to identify different fasteners.

Reference Materials

The following resources were used in planning this lesson:

• 2014 Senior Secondary Education Curriculum (SSEC)
• Relevant State Unified Scheme of Work
• Technical Drawing for Senior Secondary Schools by P.N. Okeke
• FCT ERC/NAPPS Scheme of work

Instructional Materials

The teacher will teach this lesson with the aid of:

• Actual examples of various screw threads and fasteners (e.g., bolts, nuts, studs, screws).
• Bench vice.
• Drawing board, T-square, set squares, pencils, eraser, drawing paper.
• Charts illustrating different screw thread profiles and fastener types.

Rationale for the Lesson

This lesson is essential for students to understand the fundamental components used in joining parts in engineering and construction. Knowledge of screw threads and fasteners is critical for accurate technical drawing, enabling students to correctly represent and interpret mechanical designs. This understanding forms a basis for further studies in engineering and related technical fields.

Prerequisite/Previous Knowledge

Students should have basic knowledge of technical drawing principles, including orthographic projection and dimensioning, and an understanding of simple mechanical components.

Lesson Content/Board Summary

SCREW THREADS, FASTENER AND DEVICES

Screw Threads

A screw thread is a continuous helical ridge of uniform section formed on the external or internal surface of a cylinder or cone. They are primarily used for fastening, adjusting, and transmitting power.

Types of Screw Threads

Screw threads are classified based on their profile or shape. Common types include:

1. V-threads: These threads have a V-shaped profile. They are widely used for fastening due to their strong gripping power and resistance to loosening. Examples include:
   1. Unified Thread Standard (UTS): Common in North America, with a 60-degree thread angle.
   2. ISO Metric Thread: The most common international standard, also with a 60-degree thread angle.
   3. Whitworth Thread: An older British standard with a 55-degree thread angle.
   4. American National Thread: Similar to Unified but with flat crests and roots.
2. Square Threads: These threads have a square profile, making them very efficient for transmitting power. They are typically used in applications requiring heavy loads, such as lead screws in lathes and presses, and jacks.
3. Buttress Threads: These threads have an asymmetrical profile, with one flank perpendicular to the axis and the other angled. They are designed to transmit heavy loads in one direction only, such as in vices, C-clamps, and screw jacks.
4. Acme Threads: These threads have a trapezoidal profile with a 29-degree included angle. They are a modification of the square thread, offering easier machining and greater strength. Acme threads are used for lead screws, valve stems, and other power transmission applications.

Fasteners

Fasteners are mechanical devices used to join two or more parts together, either permanently or temporarily. They are crucial in assembling structures and machines.

Types of Fasteners and Their Uses

Different types of fasteners serve specific purposes:

1. Bolts: A bolt is a threaded cylindrical rod with a head on one end and a thread on the other. It is designed to be inserted through holes in assembled parts and secured by a nut.
   1. Uses: Used for joining heavy components, structural connections, and where strong, removable joints are required (e.g., engine parts, machine frames).
2. Nuts: A nut is a block of metal, usually hexagonal or square, with an internal thread. It is designed to mate with a bolt to create a clamping force.
   1. Uses: Used in conjunction with bolts to secure parts, provide adjustable clamping, and prevent loosening (e.g., hexagonal nuts, cap nuts, lock nuts).
3. Studs: A stud is a cylindrical rod threaded at both ends, or threaded at one end and plain at the other. It does not have a head. One end is usually screwed permanently into one component, and the other end receives a nut to secure the second component.
   1. Uses: Used where a bolt head would be inaccessible, for permanent attachment to a casting, or to align components before assembly (e.g., cylinder head studs in engines).
4. Screws: A screw is a threaded fastener that forms its own mating thread in the material it is driven into, or it mates with a pre-tapped hole. Unlike bolts, screws are typically driven directly into the material.
   1. Uses:
      1. Machine Screws: Used for joining metal parts, often with a pre-tapped hole or a nut.
      2. Wood Screws: Designed with a tapered shank and coarse threads for fastening wood components.
      3. Self-Tapping Screws: Create their own threads as they are driven into materials like plastic, sheet metal, or wood.
      4. Set Screws: Used to prevent relative motion between two parts, often by clamping against a shaft.
5. Washers: Flat, thin discs placed under the head of a bolt or nut to distribute the load, prevent loosening, or provide spacing.
6. Rivets: Permanent fasteners used to join plates or sheets by deforming one or both ends.
7. Pins: Used for alignment, shear resistance, or to prevent relative motion between parts (e.g., dowel pins, cotter pins).

Teaching Methods/Instructional Techniques

Discussion, Demonstration, Guided Practice, Question and Answer, Explanation, Observation.

Instructional Procedures

Step 1: Introduction

Time: 5 minutes

Teaching Skill: Recalling/Motivation

Teacher’s Activity: The teacher greets the students and asks them to recall previous lessons on joining methods. The teacher then introduces the topic of screw threads and fasteners, highlighting their importance in everyday life and technical drawing.

Pupils’ Activity: Pupils respond to questions and listen attentively to the introduction.

Learning Point: Importance of fasteners

Step 2: Introduction to Screw Threads

Time: 10 minutes

Teaching Skill: Explanation/Demonstration

Teacher’s Activity: The teacher defines screw threads and displays various examples of components with screw threads (e.g., bolts, screws). The teacher explains the basic principle of a helical ridge.

Pupils’ Activity: Pupils observe the displayed examples and listen to the explanation.

Learning Point: Definition of screw threads

Step 3: Discussing V-threads and Square Threads

Time: 10 minutes

Teaching Skill: Description/Illustration

Teacher’s Activity: The teacher describes V-threads (e.g., ISO Metric, Unified) and square threads, using charts and actual examples. The teacher explains their characteristics and common applications, guiding students to identify them.

Pupils’ Activity: Pupils identify V-threads and square threads from examples and charts, noting their features.

Learning Point: V-threads and square threads

Step 4: Discussing Buttress and Acme Threads

Time: 8 minutes

Teaching Skill: Explanation/Comparison

Teacher’s Activity: The teacher describes buttress and Acme threads, highlighting their unique profiles and specific uses, especially in power transmission. The teacher compares them with previously discussed thread types.

Pupils’ Activity: Pupils observe and differentiate between buttress and Acme threads, understanding their applications.

Learning Point: Buttress and Acme threads

Step 5: Introduction to Fasteners

Time: 5 minutes

Teaching Skill: Definition/Categorisation

Teacher’s Activity: The teacher defines fasteners and explains their general purpose in joining components. The teacher introduces the main categories of fasteners.

Pupils’ Activity: Pupils listen to the definition and understand the concept of fasteners.

Learning Point: Meaning of fasteners

Step 6: Types of Fasteners and Their Uses

Time: 10 minutes

Teaching Skill: Demonstration/Discussion

Teacher’s Activity: The teacher displays various fasteners like bolts, nuts, studs, and screws. The teacher describes each type, demonstrates how they are used (e.g., using a bench vice to show a bolt and nut assembly), and explains their specific applications.

Pupils’ Activity: Pupils identify different fasteners, observe their uses, and ask questions for clarification.

Learning Point: Fastener types and uses

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

1. What is a screw thread?
2. Mention two types of V-threads.
3. Differentiate between a bolt and a stud.
4. Give two uses for square threads.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Understanding screw threads and fasteners

Step 8: Note-Taking

Time: 10 minutes

Teaching Skill: Guided Writing

Teacher’s Activity: The teacher guides pupils/students to copy the essential Board Summary notes on screw threads and fasteners into their notebooks.

Pupils’ Activity: Pupils/students copy the notes carefully into their notebooks.

Learning Point: Recording lesson information

Step 9: Conclusion

Time: 2 minutes

Teaching Skill: Summarisation

Teacher’s Activity: The teacher briefly summarises the main points of the lesson, reinforcing the definitions and types of screw threads and fasteners, and their importance in technical drawing and engineering applications.

Pupils’ Activity: Pupils listen and ask any final questions.

Learning Point: Lesson consolidation

Continuous Assessment/Further Study

Type: Homework

Instruction: Answer the following questions in your technical drawing notebook.

1. Sketch and label the profile of a V-thread and a square thread.
2. List three types of fasteners and state one specific use for each.
3. Explain why buttress threads are suitable for transmitting heavy loads in one direction.

Lesson Keywords

• Screw thread – A continuous helical ridge on a cylinder or cone.
• V-thread – A screw thread with a V-shaped profile, common for fastening.
• Square thread – A screw thread with a square profile, efficient for power transmission.
• Buttress thread – An asymmetrical thread for heavy loads in one direction.
• Acme thread – A trapezoidal thread used for lead screws and power transmission.
• Fastener – A mechanical device used to join parts.
• Bolt – A threaded rod with a head, secured by a nut.
• Nut – An internally threaded block that mates with a bolt.
• Stud – A rod threaded at both ends, often permanently fixed at one end.
• Screw – A threaded fastener that forms its own thread or mates with a tapped hole.

Differentiation

Support: Provide simplified diagrams and pre-labelled charts for struggling learners. Pair them with more advanced students for identification activities. Focus on identifying the main types of threads and fasteners rather than detailed characteristics.

Extension: Challenge advanced learners to research specific applications of each thread type and fastener in complex machinery. Encourage them to draw detailed cross-sections of different thread profiles or design a simple assembly using various fasteners.

Suggested Lesson Videos

For further understanding of screw threads and fasteners, students can search on YouTube for:

• types of screw threads technical drawing SS3
• types of fasteners technical drawing SS3