Technical Drawing · Lesson 5

First- and third-angle projection, symbols, views, hidden detail and dimensioning

WAECNECO

What the syllabus asks

WAEC: WAEC/NECO Technical Drawing, Solid Geometry item 14 (orthographic projection in first and third angle; principal and auxiliary planes; projection symbols; arrangement of views; hidden detail). (Not a JAMB UTME subject: Technical Drawing is absent from JAMB’s official IBASS e-syllabus list.)

NECO: WAEC/NECO Technical Drawing, Solid Geometry item 14 (orthographic projection in first and third angle; principal and auxiliary planes; projection symbols; arrangement of views; hidden detail). (Not a JAMB UTME subject: Technical Drawing is absent from JAMB’s official IBASS e-syllabus list.)

What you will learn

Explain the principal planes, quadrants and the difference between first- and third-angle projection.
Arrange views correctly and draw the correct projection symbol.
Project front, plan and end views with correct visible, hidden and centre lines.
Read given views and find missing views.
Represent holes, cylinders, fillets and conventional features.
Dimension orthographic drawings correctly and plan the sheet layout.

Key points

Views: front (L × H), plan (L × D), end (D × H).
First angle: plan below, left view on right.
Third angle: plan above, left view on left.
Symbol: first angle circles right; third angle circles left.
Front of object: next to the front view in third angle, farthest in first angle.
Line precedence: visible over hidden over centre.
Use a 45° mitre line to transfer depth.
Plan sheet space before drawing.
Dimension once, outside, from datums.
Always add the projection symbol.

Start here: Pictorial versus orthographic drawing

A photograph or pictorial sketch shows an object as the eye sees it, but it cannot show true lengths, true angles or hidden detail clearly. A fabricator in Nnewi, a carpenter in Ibadan or a machinist at a Lagos workshop needs exact sizes. Orthographic projection solves this by drawing separate flat views of the object, each looking straight at one face, with lines of sight (projectors) parallel to each other and perpendicular to the plane of projection. Each view shows two of the three dimensions in true size.

The three principal dimensions

Length (width): left to right.
Height: top to bottom.
Depth: front to back.

The front view (front elevation) shows length and height; the plan (top view) shows length and depth; the end view (side elevation) shows depth and height. Every dimension appears in two views, which is why the views must be aligned by projection lines: length is shared by the front view and plan, height by the front and end views, and depth by the plan and end view.

Why “orthographic”

Ortho means “right” or “straight”: the projectors meet the picture plane at right angles. Contrast this with perspective (td6), where the projectors converge to the eye.

Choosing the front view

Choose the view that shows the most characteristic shape of the object.
It should contain the fewest hidden lines.
Place the object in its natural or functional position (a chair standing, a bracket bolted to a wall).
Usually the longest dimension runs horizontally, which fits the sheet better.

For a car viewed for a service manual, the front view is normally the side profile, because it shows the most characteristic shape; for a cupboard it is the face with the doors. Choosing well saves hidden lines and time.

The direction of the front view is often given in the exam by an arrow labelled with the letter of the view.

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Practice questions

1. In first-angle projection, the plan is drawn

below the front view
above the front view
to the left of the front view
to the right of the front view

Show answer

A: below the front view
First angle: plan below.

2. In third-angle projection, the view from the left is placed

on the right of the front view
below the front view
above the plan
on the left of the front view

Show answer

D: on the left of the front view
Third angle: views go on the side they are seen from.

3. In the first-angle symbol, the circles (end view) are drawn

above the trapezium
inside the trapezium
to the right of the trapezium
to the left of the trapezium

Show answer

C: to the right of the trapezium
ISO 5456-2 figure 4.

4. In the ISO projection symbol, the large end of the cone has diameter

half the small end
twice the small end
equal to the small end
three times the small end

Show answer

B: twice the small end
H = 2h (ISO 5456-2 Table A.1).

5. The front view shows the

length and height
length and depth
depth and height
true shape of all faces

Show answer

A: length and height
Plan: length × depth; end view: depth × height.

Key words

Orthographic projection: Views made with parallel projectors perpendicular to the plane.
Vertical plane: Plane receiving the front view.
Horizontal plane: Plane receiving the plan.
Profile plane: Plane receiving the end view.
XY line: Intersection of the VP and HP.
Quadrant: One of four spaces made by the VP and HP.
First-angle projection: Plan below, left view on right.
Third-angle projection: Plan above, left view on left.
Front elevation: View from the front.
Plan: View from above.
End elevation: View from the side.
Hidden line: Dashed line for unseen edges.

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