Orthographic and Isometric Projections

Orthographic and Isometric Projections –

A) Basic Terminology –

                                                            Fig. (1) Isometric view 

1) Plane formed by X & Y coordinates called as Horizontal Reference Plane H.R.P (or Horizontal Plane).

2) Plane formed by X & Z coordinates called as Frontal Reference Plane F.R.P (or Vertical Plane).

3) Plane formed by Y & Z coordinates called as Profile Reference Plane P.R.P (or Auxiliary Vertical Plane A.V.P).

Such system forms four quadrants. The space clearly seen called 1’st quadrant. The space behind F.R.P. and above H.R.P. is called 2’nd quadrant. The space below H.R.P. and behind F.R.P. is called 3’rd quadrant. The space below H.R.P. and front of F.R.P. is called 4’rth quadrant.  

Que. What is the Orthographic and Isometric Projection ?

Ans. Firstly understand what is projection ?

Creating views of an object on a Plane. There are various projection principle in engineering Drawing.

1. Perspective

2. Orthographic/Parallel

3. Oblique

4. Axonometric

                                                                         Fig. (2)

The Orthographic projection enables to create single or multiple views of the object. So , sometimes it called multi-view drawing. This gives true shape & dimensions of the object.

To understand this, see fig.3,

1. An Observer

2. An Object

3. Plane  

Fig. (3)

An Isometric drawing is meant to depict a 3D image of an object in what appears to be a perspective view. However, similar to an orthographic perspective, all of the lines in an isometric drawing can be measured to their true length.

                                                                              Fig. (4)

What it makes difference from an orthographic perspective is that its angled lines are drawn at 30° or 60° or divisions of them. Drawing this by hand you would use a 30°/60°/90°.

B)  1’st Angle Projection System –

                                                                               Fig. (5)

Here, the LHSV is seen to be on right of the F.V and the T.V. is seen to be on the lower side of the F.V. Similarly, any RHSV, if drawn, shall be seen to be on the left side of the F.V.; any bottom side view; if drawn; shall be on the upper side of the F.V.

                                                                             Fig. (6)

C) 3’rd Angle Projection System –

                                                                                Fig. (7)

As per similar imagining the object in 3’rd quadrant results into the LHSV to be seen on the left on the F.V. The T.V. to be seen on the upper side of the F.V. The RHSV to be seen on the right of the F.V. and bottom view to be seen on the lower side of F.V.

                                                                         Fig. (8)

 D) 2’nd or 4’th Angle Projection System –

They are not exist, even they are not allowed to exist. If any orthographic drawing is to be prepared assuming in 2’nd or 4’th quadrant, it can create two types of confusion as its T.V. formation.

                                                                        Fig. (9)

1) The F.V. and T.V. are available at H.P. and F.R.P. The problem start while converting 3D view into a 2D. The H.R.P. is required to be revolved around the X-axis. If axis revolved in clockwise, then the T.V. may have an overlapping or fouling tendency with the F.V. already available. Such overlapping shall create confusion.

                                                                   Fig. (10)

2) If the H.P. revolved in anti-clockwise then the F.V. and T.V. are tried to be made co-planar, then the T.V. gives wrong sense of orientation. The features on the frontal side of the object appear to be as on the rear side as the T.V. brought below the F.V.

(E) Symbol of the 1’st and 3’rd angle Projection –

In the 1’st angle projection, left hand side view shall be to the right of the Front View. Accordingly, symbol gets developed as shown below,

                                                                        Fig. (11)

In the 3’rd angle projection, left hand side view shall be to the left of the Front View. Accordingly, symbol gets developed as shown below,

                                                                   Fig. (12)