Class: Senior Secondary School 1 (SS1, SS 1, SSS1, SSS 1)
Term: 3rd Term
Week: 5
Age: 15 years
Duration: 45 minutes
Subject: Technical Drawing
Curriculum Theme: Technical Drawing
Previous Lesson: Producing Models of Geometrical Solids.
Topic: METHODS OF DIMENSIONING
Subject Matter: methods of placing dimensions on drawings, dimensioning circles, dimensioning arcs, dimensioning chamfers, vertical dimensioning, horizontal dimensioning, angular dimensioning, datum dimensioning, chain dimensioning

Specific Objectives

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

Cognitive Domain:

Define dimensioning in technical drawing.
State the general rules for placing dimensions on a drawing.
Explain different methods of dimensioning.

Affective Domain:

Appreciate the importance of correct dimensioning in technical drawing.
Show interest in drawing with accurate dimensions.

Psychomotor Domain:

Apply vertical and horizontal dimensioning to given shapes.
Demonstrate the correct way to dimension circles, arcs, and chamfers.
Draw examples of chain and datum dimensioning.

Collaborate with peers to identify correctly dimensioned drawings.

Reference Materials

The following resources were used in planning this lesson:

9 Years Basic Education Curriculum for Technical Drawing.
State Unified Scheme of Work for Technical Drawing SSS 1.
New Technical Drawing for Senior Secondary Schools by A.B. Adeyemi.

Instructional Materials

The teacher will teach this lesson with the aid of:

Drawing instruments (pencils, rules, compasses, set squares).
Sample dimensioned drawings (posters or handouts).
Whiteboard and markers.

Rationale for the Lesson

This lesson helps pupils understand how to add measurements to technical drawings, which is important for communicating exact sizes and shapes. Correct dimensioning ensures that anyone can accurately manufacture or construct an object based on the drawing, avoiding errors and waste.

Prerequisite/Previous Knowledge

Pupils should have prior knowledge of basic drawing instruments, types of lines, and drawing simple geometric shapes.

Lesson Content/Board Summary

METHODS OF DIMENSIONING

Definition of Dimensioning

Dimensioning is the process of adding measurements to a technical drawing to indicate the size, location, and geometric characteristics of features on an object. It ensures that the drawing can be used for manufacturing or construction.

General Rules for Placing Dimensions

The following are general rules for placing dimensions on a drawing:

Dimensions should be placed clearly and legibly.
Avoid crossing dimension lines or extension lines.
Place dimensions outside the object outline where possible.
Avoid duplicating dimensions.
Dimensions should be placed on the view that shows the feature most clearly.
Use appropriate symbols for diameter (ø), radius (R), and square (□).

Vertical Dimensioning

This method involves placing dimensions vertically, reading from the right side of the drawing. It is used for indicating the height of features.

Horizontal Dimensioning

This method involves placing dimensions horizontally, reading from the bottom of the drawing. It is used for indicating the length or width of features.

Angular Dimensioning

Angular dimensioning is used to indicate the size of an angle. The dimension line is an arc drawn with its center at the vertex of the angle, and the dimension text is placed above the arc.

Dimensioning Circles

Circles are dimensioned by indicating their diameter. The diameter symbol (ø) precedes the numerical value. The dimension line passes through the center of the circle.

Dimensioning Arcs

Arcs are dimensioned by indicating their radius. The radius symbol (R) precedes the numerical value. The dimension line extends from the center of the arc to the arc, with an arrowhead touching the arc.

Dimensioning Chamfers

A chamfer is an angled edge or corner. Chamfers are dimensioned by specifying the chamfer angle and the setback distance from the corner, e.g., 2 x 45°. Alternatively, the two perpendicular distances can be given.

Chain Dimensioning

Chain dimensioning, also known as continuous dimensioning, involves placing dimensions in a straight line from one feature to the next. The overall length is the sum of the individual dimensions.

Datum Dimensioning (Parallel Dimensioning)

Datum dimensioning, also known as parallel dimensioning, involves all dimensions originating from a common reference point or line (datum). Each dimension is independent of the others, reducing error accumulation.

Teaching Methods/Instructional Techniques

Discussion, Lecture, Demonstration, Question and Answer, Visual Aids

Instructional Procedures

Step 1: Introduction

Time: 5 minutes
Teaching Skill: Set Induction
Teacher’s Activity: The teacher greets the pupils and asks them what they understand by “measurement” in everyday life. The teacher then introduces the topic, “Methods of Dimensioning,” explaining that in technical drawing, measurements are added in a specific way.
Pupils’ Activity: Pupils respond to the teacher’s questions and listen attentively to the introduction.
Learning Point: Pupils are prepared for the lesson and understand the context of dimensioning.

Step 2: Explanation of Dimensioning and General Rules

Time: 10 minutes
Teaching Skill: Explanation/Demonstration
Teacher’s Activity: The teacher defines dimensioning and explains its purpose. The teacher then discusses and illustrates the general rules for placing dimensions on the whiteboard, using simple shapes as examples.
Pupils’ Activity: Pupils listen, take notes, and ask questions for clarification. They observe the teacher’s illustrations.
Learning Point: Pupils understand what dimensioning is and the fundamental rules for applying it.

Step 3: Linear Dimensioning (Vertical and Horizontal)

Time: 8 minutes
Teaching Skill: Demonstration/Illustration
Teacher’s Activity: The teacher demonstrates vertical and horizontal dimensioning using simple rectangular shapes on the whiteboard or by showing sample drawings. The teacher emphasizes clarity and correct placement.
Pupils’ Activity: Pupils observe the demonstration and sketch simple examples in their notebooks.
Learning Point: Pupils learn how to apply vertical and horizontal dimensions to objects.

Step 4: Dimensioning Special Features (Angular, Circles, Arcs, Chamfers)

Time: 10 minutes
Teaching Skill: Demonstration/Explanation
Teacher’s Activity: The teacher explains and demonstrates how to dimension angles, circles (using diameter symbol), arcs (using radius symbol), and chamfers. The teacher uses appropriate symbols and illustrates each method clearly.
Pupils’ Activity: Pupils pay attention, ask questions, and practice drawing these dimensioning methods in their notebooks.
Learning Point: Pupils understand and can apply dimensioning techniques for specific geometric features.

Step 5: Advanced Dimensioning Techniques (Chain and Datum)

Time: 7 minutes
Teaching Skill: Explanation/Illustration
Teacher’s Activity: The teacher explains and illustrates chain dimensioning and datum (parallel) dimensioning, highlighting their differences and applications. The teacher shows how to avoid error accumulation with datum dimensioning.
Pupils’ Activity: Pupils listen, take notes, and observe the illustrated examples.
Learning Point: Pupils learn about different dimensioning systems and their advantages.

Step 6: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

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

What is dimensioning in technical drawing?
Mention three general rules for placing dimensions on a drawing.
How do you dimension a circle and an arc?
Explain the difference between chain dimensioning and datum dimensioning.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Pupils demonstrate understanding of the lesson.

Step 7: Conclusion

Time: 5 minutes
Teaching Skill: Summarization/Consolidation
Teacher’s Activity: The teacher summarizes the key methods of dimensioning discussed in the lesson. The teacher assigns homework: pupils are to draw a simple object and apply at least three different dimensioning methods learned in class.
Pupils’ Activity: Pupils listen to the summary and copy down the homework.
Learning Point: The lesson is reinforced, and pupils are given an opportunity to practice their new skills.

Lesson Keywords

Dimensioning – The process of adding measurements to a technical drawing.
Vertical Dimensioning – Placing dimensions vertically, reading from the right.
Horizontal Dimensioning – Placing dimensions horizontally, reading from the bottom.
Angular Dimensioning – Indicating the size of an angle using an arc.
Diameter (ø) – Symbol used for dimensioning circles.
Radius (R) – Symbol used for dimensioning arcs.
Chamfer – An angled edge or corner.
Chain Dimensioning – Dimensions placed in a continuous line from feature to feature.
Datum Dimensioning – Dimensions originating from a common reference point or line.

Differentiation

For pupils who grasp concepts quickly, the teacher can provide more complex drawings to dimension or ask them to identify errors in poorly dimensioned examples. For pupils needing extra support, the teacher can provide simpler shapes with pre-drawn extension lines to guide their dimension placement, offering more one-on-one assistance during practical exercises.

Note for teachers using this lesson plan

Ensure that pupils have a clear understanding of basic drawing principles before this lesson. Hands-on practice is essential, so provide ample time for pupils to sketch and apply different dimensioning methods. Use real-life examples of dimensioned objects or components to make the lesson more relatable.