Isometric view and orthographic projection engineering drawings

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Isometric view and orthographic projection engineering drawings

ISOMETRIC VIEWS & oRTHOGRAPHIC PROJECTION

Introduction:
An object has three dimensions like length, width and thickness. The shapes and sizes of these three dimensional objects has to be represented on a sheet of drawing paper which has only two dimensional plane. Projection is the image of an object formed on a plane. The word projection is of Latin origin and means to throw forward. For obtaining the image of an object, various points on the contour of an object, are thrown forward on to a plane by means of straight lines or visual rays. The figure formed by joining various points thus obtained on the plane, is the image of the object and is called projection.
The following are the main elements of projection.

Object to be projected
Observers’ eye or station point
The plane of projection or picture plane
Rays or lines of sight or projectors

Nomenclature

Projection is the image of an object thrown upon a plane by drawing straight lines called visual rays from the eye of an observer.
Observer’s eye or station point is a point where the eye of the observer is assumed to be located while sighting the object. Station point is called center of projection or point of sight.
Plane of projection is a plane on which the image is formed. In general, plane of projection is called picture plane.
Projector is a straight line drawn from the point on the contour of an object to the plane on which the image of the object is obtained.

Planes of projections
The plane surfaces which are used for projecting the views of an object in an orthographic projection are called planes of projection. Usually two views are required to describe an object completely. Hence, two planes which are mutually perpendicular are sufficient for projecting the views of an object.

Principle planes are the plane surfaces which are mutually Perpendicular and used for projecting the views of an object. These planes are called reference planes, co-ordinate planes or dihedral planes.
Horizontal plane is one of the principle planes which is horizontal. This plane is denoted by H.P. The view obtained on the horizontal plane is called top view or plan. The direction of viewing for getting the top view is marketed by T.
Vertical plane is one of the principle planes which is vertical. This plane is denoted by V.P. The view obtained on the vertical plane is called front view or elevation. This plane is also called frontal plane. The direction of viewing for getting the front view is marked by F.
Reference line is the line of intersection of the horizontal and vertical planes. This is also called reference line or ground line or simply line of intersection and is denoted by xy-line.
Quadrants The reference planes divide the space into four quadrants or angles of 900 or dihedral angles and can be marked as I,II,III,IV representing the first, second, third and fourth quadrants respectively, if these planes are sighted in the direction marked by A. The arrangement of the four quadrants is in the anti-clockwise order and hence this arrangement represents the anti-clockwise system.

Systems of Projections
Based upon the types of projectors or visual rays, projections may be broadly classified into two:

Parallel projections
Convergent (perspective) projections

Parallel Projection
Parallel projection is a geometric method of projection obtained on a plane of projection when the observer’s eye is imagined to be located at infinity so that the projectors are considered to be parallel to each other. The further subdivisions of parallel projections are:

Orthographic projections
Oblique projections

Orthographic Projections

The word ortho means right-angle and orthographic means right angled drawing. Orthographic projection is a geometric method of projection obtained on the plane of projection when the projectors are parallel to each other and perpendicular to the plane of projection. Here, the number of planes of projections may be one or more. The further subdivisions of orthographic projections are:

Multiview projections
Axonometric projections

Multiview Projection

Multiview projection is an orthographic projection in which the exact shape of an object is represented by two or more separate views obtained on different planes of projection which are usually at right angles to each other. Even though multiview projection is only one of the orthographic projection methods, because of its wide popularity the term orthographic projection is very commonly used to represent a multiview projection. For the clear understanding of different types of multiview projections, one should have the clear concept of the various planes of projections.
Multiview projections are classified into two:

First angle projections
Third angle projections

First Angle Projection

In the first angle projection the object is assumed to be situated in the first quadrant. Its top view is obtained on H.P. and the front view is obtained on V.P. Now, the H.P. is rotated through 900 in the clockwise direction and it becomes vertical. Now the front view is above the xy-line and the top view is below the xy-line.

Third Angle Projection

In the third angle projection, the object is assumed to be situated in the third quadrant. The top view is obtained on H.P. and the front view is obtained on V.P. The H.P. is rotated through 900 in the clockwise direction and it becomes vertical. Now, the top view is above xy-line and the front view is below xy-line

Arrangement of views
The arrangement of six views of object looked in six directions are made, based on clockwise or right hand system.

Theory of projections

For getting the front view of an object placed in any of the four quadrants, it should be viewed only from the right hand side in the anti-clockwise or left hand system.
Front view of an object is always projected on the vertical plane whatever may be the quadrant in which the object is situated irrespective of left hand or right hand system.
For getting the top view of an object placed in any of the four quadrants, it should be viewed only from the top side whatever be the system followed.
Top view of an object is always projected on the horizontal plane whatever may be the quadrant in which the object is situated irrespective of left hand or right hand system.
For getting the front view of an object placed in any of the quadrants it should be viewed only from the left hand side in a clockwise or right hand system.

The standard views used in a three-view drawing are the

Top (Plan)
Front (Elevation)
Side views

Arranged as shown in the figure

Difference between 1st angle and 3rd angle projection

The following are the important differences between the first angle and third angle projections:

In the first angle projection, the object is assumed to be situated in the first quadrant while in the third angle projection; it is to be in the third quadrant.
In first angle projection the object lies in between the observer and the plane of projection while in the third angle, the plane of projection is in between the observer and the object.
In first angle projection, the front view will be above the xy-line while in the third angle projection the front view will be below the xy-line.
In the first angle projection, the top view will be below the xy-line while in third angle projection, the top view will be above the xy-line.
In first angle projection, the side view is projected on the other side of the object. i.e. the view from left side is projected on the plane on right side while in third angle projection, the side view of the object is projected on the same side of the object. i.e. the view from the left side is projected on the plane on the left side.

Isometric Projection

Representing 3 dimensions on a flat piece of paper is a very important skill for designers enabling them to communicate their ideas to other people. This is especially useful when showing your design to non designers such as managers and marketing personnel.
There are several tried and tested 3 Dimensional drawing systems used to produce a realistic representation of an object. Some techniques such as isometric are based on mathematical systems, others a try to convey a larger degree of realism by applying perspective to the drawing. Amongst the methods covered in this tutorial are oblique, isometric, axonometric, and perspective drawing techniques.
Isometric projection is a method for visually representing three-dimensional objects in two dimensions in technical and engineering drawings.

It is an axonometric projection in which the three coordinate axes appear equally foreshortened and the angles between any two of them are 120 degrees. The term "isometric" comes from the Greek for "equal measure", reflecting that the scale along each axis of the projection is the same (unlike some other forms of graphical projection).
An isometric view of an object can be obtained by choosing the viewing direction in a way that the angles between the projection of the x, y, and z axes are all the same, or 120°. One of the things that make isometric drawings so attractive is the ease with which 60 degree angles can be constructed with only a compass and straightedge.

Isometric scale
A constructed by stepping off true measurements along line 'AB1' which is a true length line. The measurements are then transferred back to line 'AB' to get a smaller scale, in this case an isometric scale
Lines drawn using the isometric scale are approximately 80% of true size. This scale is usually marked off on a piece of paper and used to step off the foreshortened measurements along the projection of axes lines and lines parallel to them.
Lines parallel to the projection of axes are known as isometric lines.
Lines which are not parallel to theses axes are known as non-isometric lines.
It is important to note that you can only use the scales on isometric lines.
Drawing a box in isometric

Draw the front vertical edge of the cube.
The sides of the box are drawn at 30 degrees to the horizontal to the required length.

Note: All lengths are drawn as actual lengths in standard isometric.

Draw in the back verticals
Drawn in top view with all lines drawn 30 degrees to the horizontal

Drawing more complicated shapes

Initially when you first start using isometric it can be useful to use a simple box as a basic building block a guide to help you draw more complicated shapes.
	This simple example shows you how you can use a box to help you accurately draw a more complicated shape. The object we are going to draw is L-shaped as illustrated by the engineering drawing (left).

Outline of 'guide box'	The first step is to draw our guide box. This box is the size of the maximum dimensions. In this case, 50 mm long, 25 mm wide, and 50 mm high. Draw the box in very lightly. When we have the final shape we can darken the lines.

	To get the L-shape we need remove an area from this box. Draw a box 40 x 10 x 50 mm, the shape that needs to be removed from the box to create the shape we require.

The finished shape	Draw in the outline of the object using a heavier line. Using this technique you can draw complex shapes accurately because you can use the guide box as a means to measure your engineering drawing views.

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Author : NTTF Mr. Praveenkumar M Checked by Mr. S.Rao

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Isometric view and orthographic projection engineering drawings

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