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First Angle Projection & Third Angle Projection Symbol (Orthographic Projection)

What is orthographic projection

Orthographic views consist of one, two, or more separate aspects of an object taken from different directions & different sizes, generally at right angles to each other and arranged in a definite manner.

These views collectively describe the object. Orthographic views of any purpose can be represented by any one of the two systems of projection

The first angle projection and the third angle projection.

It is named according to the quadrant in this which the object is imagined no place for purposes of line projection.

As far as any shape and size of the views are concerned, there is no difference between these two systems.

The only difference lies in the relative positions of the various views. Indian Standards Institution recommends• (IS: 696-1972) both these systems leaving the choice to the discretion of the organization concerned.

However, there are specific merits and demerits associated with each system. Students are advised to study both the methods to meet the varying demands of different industries in the country.

To meet this end. Some chapters in this book are exclusively dealt with in the first angle projection, and some in the third-angle projection.

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Types of orthographic projection?

Orthographic projection two different parts and also a different type of symbol as per below detail mention.

    1. First angle projectionFirst angle projection symbol 
    2. Third angle projection Third angle projection symbol 

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1. First angle projection & First angle projection symbol 

In this, the object is imagined to be in the first quadrant. Because the observer normally looks from the right side of the quadrant to obtain the front view.

The objects will come in between the observer and the plane of projection. Therefore, in this case, the object is to be transparent, and the projectors are imagined ta be extended from various points of the object to meet the projection plane. 1 these meeting points when joined in the order form an image

First plane

Principle first angle projection view 

This is the principle of the first angle projection. Thus in the first angle projection, any view is so placed that it represents the side of the object away from it.

First angle projection is mesurly used throughout all parts of Europe so that called European projection.

Most important of First angle projection symbol 

FIRST ANGLE PROJECTION SYMBOL

First angle projection symbol

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2. Third angle projection Third angle projection symbol 

In this, the object is imagined to be placed in the third quadrant.

Again, as the observer is normally supposed to look from the right side of the quadrant to obtain the front view, in this method, the projection plane comes in between the observer and the object.

Therefore, the plane of projection has to be assumed to be transparent. The intersection of this plan with the projectors from all the points of the object would form an image on the transparent plane.

This is the principle of the third angle projection

third section plan

Principle third angle projection view 
Thus it is seen that in the third angle projection any view is so placed that it represents the side from the object nearest to it
Most important of Third angle Projection symbol 
THIRD ANGLE PROJECTION SYMBOL

Third angle Projection symbol

The third angle is the system in used North America and alternatively described as American projection.

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Making Orthographic Views 

    1. Front view 
    2. Side view 
    3. Top View 
    4. Presentation of view 

1. Front View

To draw the front view of an object in the first angle projection, the object which is assumed to be transparent is imagined to be in front of the vertical plane (first quadrant) as shown in.

First plane

Front View Detail Drawing No 1

The shape of the object, when viewing from the front.

Is obtained by joining in order all the meeting points of the projectors extended from the visible edges of the object. This is called a front view.

A similar treatment is repeated for the front view in the third angle projection, but with a transparent vertical plane imagined to be held in front of the object.

Here the shape of the object when viewed from the front is obtained by joining in order all the intersection points on the transparent plane of the projectors to the visible edges of the object

third section plan

Front View Detail Drawing No 2

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2. Side View

The shape of some objects cannot be interpreted completely from the from and top views only.

In such cases, more information may be needed, and for this purpose, a third plane is imagined. This is called an auxiliary vertical plane and is placed perpendicular to the first two planes.

A view can be obtained on to this plane in a way similar to that explained above. In the first angle method, if the auxiliary vertical plane is the place to the right of the object, a view is obtained on it by looking at the object from its left side.

It is then called a left side view. On the other hand, if the auxiliary vertical plane is placed to the left of the

Side view

Side View Detail Drawing 

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3. Top View

The projection on to a plane placed horizontally below the transparent object will reveal the shape of the object as viewed from below.

Top View

Top View Drawing No 1

This view is called a top view, and the principle that is being followed is that of the first angle projection. Figure deals with a similar treatment in the third angle projection with a transparent plane placed horizontally below the object.

Top View plan

Top View Drawing No 2

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4. Presentation of view 

For presenting the views on a drawing sheet, irrespective of the method of projection, the horizontal plane, and the auxiliary vertical plane arc rotated till they come in the plane with the original vertical plane.

Now the three views of the object will be in the same plane, viz., the plane of the paper. These three views normally provide enough information to describe the shape and size of the object.

Presentation of view

Presentation of view drawing  No 1/  First angle projection symbol

above drawing shows the relative positions of the views in the first angle projection and below drawing n the third angle projection.

Presentation of view

Presentation of view drawing  No 2/  Third angle projection symbol

The method used for the projection is indicated by a distinctive symbol (presentation of view drawing No 1 .C and Presentation of view drawing No 2 C)

(ft may be observed that while rotating the horizontal plane and the side vertical plane to come in plane with the vertical plane, the first and third quadrants are always opened and consequently the second and fourth quadrants are closed. Hence, in the second or fourth angle projection, the top, front, and side views get superimposed on one another. That is why the second and the fourth angle projections are never followed to represent the orthographic views of an object).

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