Fishing Craft Technology

Practical 3 - Projection

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 as “projection” of the object. The lines from the object to the planes are called “projectors.”

Orthographic Projection:

When the projectors are parallel to each other and also perpendicular to the plane of projection such type of projection is called as an “orthographic projection.”
Imagine that a person is looking at the block from an infinite distance, so that the rays of sight from his eyes are parallel to one another and perpendicular to the front surface ‘F’ as shown in the figure. If the rays of sight are extended further to meet perpendicularly a plane marked vertical plane, set up behind the block and the points at which they meet the plane are joined in proper sequence. The figure will also be exactly similar to the front surface. This figure is the projection of the block. The lines from the block to the plane are the projectors.
As the projectors are perpendicular to the plane on which the projection is obtained is the orthographic projection. It shows only two dimensions of the block, the height ‘H’ and the width ‘B’. It does not show the thickness. Thus it is felt that only one projection is insufficient for complete description of the block.
Let us assume that another plane marked as horizontal plane (H.P), the projection on the H.P. shows the top surface of the block. If a person looks at the block from above, he will obtain the same view as shown in the figure. It shows the width ‘B’ and thickness ‘t’ of the block and however, does not show the height of the block.

Planes of Projection:
Three planes which are employed for the purpose of orthographic projections are called “Reference planes” or “principal planes of projection”. They intersect each other at right angle. The vertical plane of projection is denoted by the letter V.P. The horizontal plane of projection is known as H.P. The profile plane of projection is known as P.P. Among these the most commonly used plane of projections are H.P. and V.P.

The line in which H.P. and V.P. intersect is termed as the “Ground line” and is denoted by the letter XY. The projection on the V.P. is the front elevation of the object and is commonly known as “Elevation” while that on H.P. is called as “plan”. The elevation and the plan are also known as front view and top view respectively.
Now if one of the planes is rotated or turned around on the hinges so that it lies in extension of the other plane. This can be done in two ways.
By turning V.P. in direction of the arrow ‘A’ or
By turning the H.P. in direction of arrow ‘B’.
The H.P. then turned and brought in line with V.P. is as shown by dotted lines.

Quadrants:

When the planes of projection are extended beyond the line of intersection they form four Quadrants. The object may be situated in any one of the quadrants, the position relative to the planes being described as “above or below” the H.P. and “ In front or behind the V.P.”
The projections are obtained by drawing perpendicular from the object to the planes i.e. by looking from the front and the above.

Projection of Points:
A point may be situated in space in any one of the four quadrants formed by the two principal planes of projection or may lie in any one or both of them. Their projections are obtained by extending projectors perpendicular to the planes. One of the planes is then rotated so that the first quadrant is opened out. The projections are shown on a flat surface in their respective positions either above or below or in XY.

Projection of Point in I^st Quadrant:

A point A is situated above the H.P. and in front of V.P. i.e. in the first quadrant. Now project the point A on V.P. and H.P. The front view on V.P. is its elevation (a1). Top view on H.P. is its plan (a). Rotate the H.P. with respect to V.P.
After rotation of the plane, the elevation a1 is above XY and the plan ‘a’ is below XY. The line joining ‘a’ and a1 intersects XY at right angle at the point ‘O’.
It is quite evident from the figure that a1o=Aa i.e. the distance of the elevation from XY=the distance of A from H.P. i.e. ‘h’. Similarly ao=Aa1 i.e. the distance of the plan from XY= the distance of A from the V.P.=‘d’.

Projection of point in II^nd Quadrant:

Assume a point B is above the H.P. and behind the V.P. b1 is the elevation and b is the plan. When the plane is rotated, both the views are seen above XY.

Projection of point in III^rd Quadrant:

Assume a point ‘C’ is below the H.P. and behind V.P. Its elevation c1 is below XY and plan ‘c’ is above XY.

Projection of point in IV^th Quadrant:

Assume ‘a’ point ‘D’ is below H.P. and front of the V.P. Both of its projections are below XY.