Engineering Drawing - Part 1

Orthographic projection

Orthographic projection is the graphical method used in modern engineering drawing. In order to interpret and communicate with engineering drawings a designer must have a sound understanding of it's use and a clear vision of how the various projections are created. 

There are two predominant orthographic projections used today. They are based on Monge's original right angle planes and are shown fully in Figure 1. They define four separate spaces, or quadrants. Each of these quadrants could contain the object to be represented. Traditionally however, only two are commonly used, the first and the third.

Projections created with the object placed in the first quadrant are said to be in First Angle projection, and likewise, projections created with the object placed in the third quadrant are said to be in Third Angle projection.

Figure 1: Quadrants and Projections

First angle projection

Consider the first quadrant in Figure 1. The resultant drawing of the cone would be obtained by flattening the two perpendicular projections planes, as shown in Figure 2.

Figure 2: First angle projection

For this example, you could say that the right hand side image is the plan or top elevation and the image to the left is the side elevation.

Whether you view the objects from the left or the right, the order in which the drawing views are arranged puts the image that you see after the object, object first then the image. This is always true for First Angle projection.

Put another way:

• Viewing from the left: The drawn image on the right is your view of the drawn object on the left.
• Viewing from the right: The drawn image on the left is your view of the drawn object on the right.

This can get confusing, particularly when also considering other drawings created using other projections. You may develop your own way of recognizing First Angle projection.

e.g.
The OBJECT is FIRST for FIRST Angle projection.
or...
EYE > OBJECT > IMAGE
or...
You look through the object and place the image

Figure 3: An example of a component represented in a multiview drawing, in First Angle projection

Third angle projection

Consider the third quadrant in Figure 1. The resultant drawing of the cone would be obtained by flattening the two perpendicular projections planes, as shown in Figure 4.

Figure 4: Third angle projection

For this example of the cone, you would say that the left hand image is the plan or top elevation and the image to the right is the side elevation.

Whether you view the objects from the left or the right, the order in which the drawing views are arranged puts the image that you see before the object, image first then the object. This is always true for Third Angle projection.

Put another way:

• Viewing from the left: The drawn image on the left is your view of the drawn object on the right.
• Viewing from the right: The drawn image on the right is your view of the drawn object on the left.

Again, you may develop your own way of recognizing Third Angle projection.

Perhaps: EYE > IMAGE> OBJECT

Figure 5: The same component shown using Third Angle projection

Orthographic projection symbols

Both systems of projection, First and Third angle, are approved internationally and have equal status. The system used must be clearly indicated on every drawing, using the appropriate symbol shown in Figure 6 below.

Figure 6: Projection system symbols and recommended proportions

Pictorial Drawing

Orthographic projection is used as an unambiguous and accurate way of providing information, primarily for manufacturing and detail design. This form of representation can however make it difficult to visualize objects. Pictorial views can be created to give a more three dimensional impression of the object. There are three types of pictorial projections commonly used, as shown below.

Figure 7: Perspective, isometric and oblique pictorial projections

Perspective: 

• Used more with freehand sketching.
• Parallel lines appear to converge and meet at what is referred to as the vanishing point.
• You can have one, two or three vanishing points (VP).

Isometric: 

Receding lines drawn at 30º and are usually kept at true measured lengths.

Oblique: 

Front face sketched as a true shape. Starts with two axes, one horizontal, one vertical. The third axis is usually drawn at 45º and lengths are reduced by 50% of true lengths. Sometimes called 'cabinet' projection.

Creating orthographic projection drawings

This is an introduction into how to create and interpret multiview orthographic projection drawings.

First angle projection

The component: Your drawing will, for this example consist of four views:

• Front F
• Left L
• Right R
• Plan (Top) P

Usual practice is to orient the component in a position that it is most likely to be found in.

Your aim is to create, from the front view, an orthographic projection drawing as shown below in Figure 8. Note how the views are constructed in line with each other, allowing the features to be 'projected' between the views.

Figure 8: A completed First angle projection drawing

So, the stages are:

1. Choose which view direction or face will be used as the front view of the component.
2. Draw the outline of the front view, leaving room for the other views.
3. Draw feint construction lines out from the front view.
4. Start to draw the outlines of the other views, using sides you know the length of.
5. Complete the details of the views by adding any required hidden detail lines, other outlines and center lines.

With first angle projection the plan view is below the front view. If you had placed the plan view above the front view it would actually have to become the bottom or underside view!

Third angle projection

The construction method used is the same. The difference between first and third angle projection when creating or reading really lies with the positions of the views. For the same component, an orthographic projection drawing with the same front, side and plan views would look like Figure 9 below.

Figure 9: Third angle projection

Observe how, in third angle, the views give the image then the object. In other words, what you see then what you are looking at.

In first angle you are given the object then the image, or what you are looking at, then what you see.

Drawing conventions

Introduction
In order for anyone to be able to understand exactly what a drawing represents, sets of precise rules and conventions have to be followed, much like a language. These rules are usually referred to as Standards.

When a designer works with an engineering drawing they must be familiar with the precise meaning of the various line styles, abbreviations, drawing simplifications and terminology as specified in the relevant standards. This section introduces you to some of the conventions defined in British Standard.

Standards are developed both privately by companies and by internationally recognized institutions.

Two such international standards are:

• British Standard Institution
• American National Standards Institute

Line styles or types

Each line on a drawing represents specific precise information regarding the components
design.

Lettering

All characters on a drawing must be legible and consistent, with consideration being given to the possibility of drawing reductions and poorer quality reproductions being made.

No particular style is required, but characters should all be consistent on the same drawing. Capital letters are preferred to lower case ones. Size of lettering is given as a minimum height, relating to drawing size, as shown below:

Application	Drawing sheet size	Min. character height (in mm)
Drawing numbers	A0, A1, A2 & A3	7
Titles, etc.	A4	5
Dimensions & Notes	A0	3.5
	A1, A2, A3 & A4	2.5