principle of projection in engineering drawing

Engineering drawing is language of engineering .Designers record and convey their ideas by means of engineering drawing . Engineers describe the structure or machine parts with the help of drawing .Engineering drawing is graphic language
Engineering drawing another method of describing a machine .As compared to verbal or written description this method is brief and more clear .Ordinarily the idea conveyed by the word drawing is a pictorial view in which a machine is represented as the eyes see it .A pictorial view shows only the outside appearance of machine.

Principle of projection

If straight lines are drawn from various points on the contour of an object to meet a plane the object is said to be projected on that plane. The figure formed by joining in correct sequence the points at which these lines meet the plane is called a projection of that object. The lines from the object to the plane are called projector and the plane on which the projectors meet is known as the plane of projection.

Methods of projection:

In engineering drawing following four methods of projection are commonly used, they are:

(1) Orthographic projection

(2) Isometric projection

(31 Oblique projection

(41 Perspective projection.

In the above methods [2). [3] and (4) represent the object by a pictorial view '3 8Yes see it. In these mgthods of projectiqn a three dimensional object is represented on a projection plane by one view only. While in the orthographic projection an object is represented by two or three views on the mutual perpendicular proiection planes. Each projection view represents two dimensions of an object. For the complete description of the three dimensional object at least two or three views are required.

When the projectors are parallel to each other and also perpendicular to the proiection plane. the projection is called an orthographic projection.

Step 1: Imagine that a person looks at an object (fig. 1-1) from a theoretically infinite distance. so that the rays of sight from his eyes are parallel to one another and perpendicular to the front surface (shown shaded).

Step 2: If these rays are extended further to meet. perpendicularly. a vertical plane [marked VP.) set up behind the object.

Step 3: The points at which they meet the plane are ioined in proper sequence. the resulting figure (marked E ) will be the projection of the object. It will be exactly similar to the view obtained by the person and will show the front surface of the obieÃ©t in its true shape and size. The lines from the object to the plane are the projectors. As the projectors are perpendicular to the plane on which the projection is obtained. this projection is known as an orthographic projection. It is shown separately in fig. 1-2. It can be seen that parallel edges of the object are shown by parallel lines, right angles remain as such and the semi-circle also remains a semi-circle. But the figure shows only two dimensions of the object. namely. thr width W and the height H. It does not show the depth the distance from the front to the back. Thus. we find that the projection from the front only is not sufficient for wmplete description of the object.

Let us further assume that a horizontal plane (marked HP.) is hinged to the vertical plane so that. the object is in front of the VP. and above the HP. (fig. 1-3)The projection on this plane. obtained in a similar manner. is shown by the figure marked P. If a person looks at the object from above. the view of the top surface (shown shaded) of the object will be exactly the same as the figure P. This figure shows the width W and the depth 0 ol the object. It does not show the height H. This projection on the horizontal plane is known as a top view or plan.

One of the planes may now be rotated or turned around so that it lies in exteneiorI with the other plane. This can be done in two ways: (ii by turning the VP. in the direction of the arrows A or (ii) by turning the HP. in the direction of the arrow B. The HP. when turned and brought in line with the V.P. is shown by dashed linet’w Generally second way of rotation is preferable. The two projections can now be show"

From the above discussion of the orthographic projection following important conclusions can be drawn:

(i) There are three important elements of this projection system, namely (a) an object (b) plane of projection (c) an observer. ‘

(ii) The obiect is assumed to be observed from the infinite distance so that. the rays of sight are parallel to each other. The lines representing these rays of sight are known as projectors.

(iii) Observe that the planes of the object which are parallel to the plane of projection are seen as true shape and true size.

(iv) Observe that the cylindrical portion whose axis is perpendicular to the projection plane (V.P.] and parallel to the H.P.. the projection on the VP. is the semi-circle and true-shape. while on the HP. it is observed as rectangle.

(v) Observe that the plane of object which is inclined to the VP. and perpendicular to the H.P.. its projection on the VP. will be apparent shape.

DIFFERENCE BETWEEN
FIRST-ANGLE PROJECTION METHOD AND THIHD-ANGLE PROJECTION METHOD

,                 First-angle projection                                            Third angle projection

.1 The object Is kept' In the first               .             1 The object is assumed to be kept in the I

2. The object lies between the observer and ‘        2 The plane of projection lies between the                    the plane of projection.                                          observer and the object

3. The plane of proiection is assumed to be            3 The plane of projection is assumed to be                   non-transparent. ’                                                               transparent.

4. In this method. when the views are drawn          4  In this method. when the views are                          in their relative positions. the plan comes               drawn in their relative positions.the                          below the elevation. the view of the object             plan . comes above the elevation.                              as observed from the left-side is drawn ‘                 left hand side view is drawn to the                                to the right of elevation.                                     left hand side of the  elevation.
.
5. This method of projection is now                         5  This method of projection is used in USA.              recommended by "Bureau of Indian                         and other countries.
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