Week 2 and 3 – SS2 Second Term Technical Drawing TD Notes

WEEK TWO-THREE:
Topic: Orthographic projection .
Content:
(i) Principal planes of projection.
(ii) First angle orthographic projection.
(iii) Third angle orthographic projection.

Principal planes of projection
The views of a multisided solid object are made by projecting them on to planes (flat surfaces) called planes of projection. The angle at which these views are being projected matters. For instance, if a physics student in the Laboratory decides to concentrate a beam of light rays on an object and thus producing an image (shadow) on a screen, the following can be deduced from this experiment which will further help in understanding the terms used in Orthographic projection. The screen on which the image (shadow) is formed is called the “plane of projection”, the image formed on the screen is called the “projection” or view, and the beam of light rays projected on the object is called projectors. If these projectors are parallel to each other and normal to the plane on which the views are made, such views are called orthographic (right angle) views or projections.
It should be noted that since the projectors are parallel and normal to the plane, they will show the true shape and size of the object being considered.

There are two main planes called the principal planes used in orthographic projection and these include: the horizontal plane and the vertical plane. In practice a third plane called auxiliary vertical plane is needed to adequately describe the shape and size of an object.
These two principal planes intersect to form four (4) quadrants or angles. See diagram below .The object to be drawn is assumed to be placed in one of these quadrants and the views are then projected on the planes. Note that the object may have any orientation to the planes but should be placed in such a way that its main faces are parallel to the planes in order to produce their true shapes and sizes. A view projected on the vertical plane is called the “elevation” whereas that projected on the horizontal plane is called the “plan”. The view on the auxiliary vertical plane is called the end elevation or side elevation. See diagram below

Note: The Second quadrant (angle) and Fourth quadrant (angle) are not used in practice because their views may overlap.

Evaluation Questions
1. Explain what is meant by orthographic projection.
2. Mention three types of plane of projection.

Types of orthographic projection
First angle orthographic projection:
In first angle projection, the object to be projected is assumed to be placed on the first quadrant (angle) and parallel projectors normal to the planes are used to obtain the views. The view on the vertical plane is the elevation while that on the horizontal plane is called the plan. To know how these views will appear on the drawing paper, the horizontal plane is opened or rebated about the line of intersection of the planes thus placing the “elevation” up and the “plan” down as shown in the figure below. In this situation, the line of intersection otherwise called

the ground line or XY line or folding line becomes a horizontal plane to the “elevation” and a vertical plane to the plan. The projectors of these views are parallel and normal to the XY line thus satisfying the condition for orthographic projection. See the figure below.

Symbols used in orthographic projection: To know which symbol is used for first angle or third angle orthographic projection, check out for these simple features. The symbol consists of a circle and an object that looks like a truncated triangle . If this truncated triangle comes first before the circle, the symbol is first angle. The reverse is the case for third angle. In other words, the circle represents the direction from which the object is viewed. While the truncated triangle represents the image or view obtained. Objects placed in the first quadrant can only be viewed from the right side when considering the two main principal planes of projection. But objects placed in the third quadrant can only be viewed from the left.
First angle orthographic projection symbol : Here, objects are placed in the first quadrant. Therefore, they are viewed from the right and their images placed to the left (vertical plane) as shown by these illustrations..

Evaluation Questions

Draw the figure below in first angle orthographic projection looking at the front elevation in the direction of arrow A

2. Draw full size the fulcrum support shown below in first angle orthographic projection. View the front
elevation in the direction of A. Show all hidden detail.

Third angle orthographic projection: In third angle orthographic projection, the object is placed in the third quadrant. Parallel projectors normal to the planes are used to project the elevation on the vertical plane and the plan on the horizontal plane. If the horizontal plane is rebated about the ground line, it is observed that the plan view will appear above the ground line while the elevations will appear below the ground line. See diagram below.

Third angle orthographic projection symbol : Here, objects are placed in the Third quadrant. Therefore, they are viewed from the left and their images placed to the right (vertical plane) as shown by these illustrations.

Evaluation Questions
1. Draw the figure below in third angle orthographic projection looking at the front elevation in the direction of A

2. Draw the figure below in angle orthographic projection looking at the front elevation in the direction of A

Reading assignment

Engineering drawing 1 by M.A.Parker and F.Pickup pages 43 – 64
Technical drawing by JN Green pages 160 – 171and 184 – 187

Weekend Assignment
Objective
1. Lines in orthographic drawing are projected at angle A. 30⁰ B. 90⁰ C. 120⁰ D. 360⁰
2. The two main principal planes of an orthographic projection are A. vertical and horizontal B. parallel and
horizontal C. horizontal and auxiliary D. vertical and auxiliary.
3. The orthographic drawing symbol shown below represents A. first angle B. second angle C. third angle
D. fourth angle.

4. The end view of an object is projected on which of the following planes A. Vertical B. Horizontal.
C. Auxiliary vertical. D. Auxiliary horizontal.

5 The second and fourth angles are not used in orthographic projection because of one of the following reasons.
A. their views are smaller than normal. B. their views are larger than normal. C. their views do not exist.
D. their views may overlap.

Theory
1. Draw full size the bolster block shown below in first angle orthographic projection. View the front elevation in the direction of A. Show all hidden detail.