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CTECHJHS 1 • Term 3Topic 12Free Trial Lesson

Technical Drawing: Pictorial and Orthographic Projections

Pictorial drawings (Isometric at 30° and Oblique at 45°) and introductory Orthographic Projection (front elevation, end elevation, and plan).

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Curated Video Lesson

Visual explanation and practical step-by-step walk-through

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Comprehensive Study Notes

Aligned with Ghana NaCCA & WAEC BECE syllabus standards

Topic Introduction & Real-World Context:

Engineering communication requires both three-dimensional pictorial drawings for conceptual visualization and two-dimensional orthographic projections for precise workshop manufacturing. Mastering isometric drawings, oblique views, and first-angle orthographic projections allows students to represent any solid artifact accurately.

What You Will Master in This Lesson (NaCCA Objectives):

Distinguish between pictorial (3D) and orthographic (2D) drawings.
Construct isometric drawings on three axes (one vertical, two at 30°).
Construct oblique drawings (Cavalier and Cabinet at 45°).
Draw first-angle orthographic projections showing front elevation, end elevation, and plan.

1. Pictorial Drawings: Isometric vs Oblique

Pictorial drawings display length, width, and height simultaneously:
Isometric Projection:
- Drawn on three isometric axes meeting at 120°: one vertical axis and two receding axes inclined at 30° to the horizontal baseline.
- Lines parallel to these axes are called isometric lines and are measured at true full scale.
- Non-isometric lines cannot be measured directly; their end coordinates must be plotted from isometric reference points.
Oblique Projection:
- Front face is placed parallel to the viewer, drawn true to shape and size using horizontal and vertical axes.
- Receding depth lines are drawn at 45° to the horizontal.
- Cavalier Oblique: Full scale (100%) on the 45° depth axis.
- Cabinet Oblique: Half scale (50%) on the 45° depth axis, creating a more realistic, undistorted optical perspective.
Key Takeaway: Isometric uses two 30° receding axes; oblique uses one front face at 90° and receding depth lines at 45°.

2. Orthographic Projection Principles

Orthographic projection projects parallel rays perpendicular (at 90°) from the object onto reference planes:
Three Principal Views:
- Front Elevation: View seen when looking directly at the front of the object.
- End Elevation: View seen when looking from the side (left or right).
- Plan: View seen when looking directly down from above.
First-Angle Projection (Standard in Ghana):
- Object is imagined in the first quadrant between the observer and the projection planes.
- The Plan is projected directly BELOW the Front Elevation.
- The Left-side Elevation is drawn on the RIGHT of the Front Elevation.
- Symbol: A truncated cone with the smaller circle projected on the far side.
Key Takeaway: In First-Angle projection, what you see from the top is drawn at the bottom; what you see from the left is drawn on the right.

3. Projecting Views and Transferring Dimensions

Ensuring exact geometric alignment between orthographic views:
Use light continuous thin projection lines to transfer heights horizontally between Front and End Elevations.
Use a 45° mitre projection line drawn from the intersection of reference axes to transfer widths between the Plan and End Elevation.
Ensure hidden features in any view are represented using dashed thin lines.
Key Takeaway: The width of the plan view must always match the width of the end elevation via a 45° mitre line.
Common Mistakes Students Make in BECE Examinations:
⚠️Drawing isometric axes at 45° instead of 30°.
⚠️Placing the Plan view above the Front Elevation in First-Angle projection (that is Third-Angle projection).
⚠️Measuring dimensions directly along non-isometric lines.
Teacher's BECE Exam Pro-Tips:
⭐BECE questions frequently ask students to identify the correct Plan view corresponding to a given 3D block.
⭐Always sketch the First-Angle projection symbol (truncated cone with circle on the right).
Quick Revision Summary Checklist:
I can set up isometric axes using a 30° set square.
I know the difference between Cavalier (full scale) and Cabinet (half scale) oblique drawings.
I can arrange Front Elevation, End Elevation, and Plan correctly in First-Angle projection.

Step-by-Step Worked Examples (2)

Real BECE exam-standard problems with complete solution steps

Example 1: Setting up Isometric Axes
Problem StatementDescribe how to begin an isometric drawing of a rectangular solid measuring 60 mm long, 40 mm wide, and 30 mm high.
Step-by-Step Solution:

Step 1: Draw a light horizontal reference baseline using the T-square.

Step 2: Pick a starting vertex origin point O on this line.

Step 3: Draw a vertical line upwards from O (height axis = 30 mm).

Step 4: Using the 30° set square, draw two receding lines from O at 30° to the right (length = 60 mm) and 30° to the left (width = 40 mm).

Step 5: Complete the isometric box by drawing parallel lines along these 30° and vertical axes.

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Key Takeaway / Exam Rule: Isometric drawings are constructed on one vertical axis and two 30° receding axes.
Example 2: Positioning Views in First-Angle Orthographic Projection
Problem StatementIn First-Angle orthographic projection, where is the Plan (top view) positioned relative to the Front Elevation?
Step-by-Step Solution:

Step 1: Recall the optical metaphor of First-Angle projection: the object is between the viewer and the projection plane.

Step 2: Looking down from the top casts the shadow of the plan onto the bottom horizontal plane.

Step 3: Therefore, in First-Angle projection, the Plan is always projected directly BELOW the Front Elevation.

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Key Takeaway / Exam Rule: In First Angle projection: Plan is below Front Elevation; in Third Angle, Plan is above Front Elevation.
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