Simmons C.H., Dennis E.M. Manual of Engineering Drawing

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30 Manual of Engineering Drawing

Fig. 3.22

CAD organization and applications 31

Fig. 3.22 (continued)

Fig. 3.23 An assembly drawing of a fuel injector for a diesel engine. Drawn to BS and ISO standards, this is a typical professional CAD drawing which could be produced using AutoCAD LT or

software with a similar specification.

First angle projection

Assume that a small block is made 35 mm × 30 mm ×

20 mm and that two of the corners are cut away as

shown below in three stages.

Figure 4.2 illustrates a pictorial view of the block

and this has been arranged in an arbitrary way because

none of the faces are more important than the others.

In order to describe the orthographic views we need to

select a principal view and in this case we have chosen

the view in direction of arrow A to be the view from

the front.

The five arrows point to different surfaces of the

block and five views will result. The arrows themselves

are positioned square to the surfaces, that is at 90° to

the surfaces and they are also at 90°, or multiples of

90° to each other. The views are designated as follows:

View in direction A is the view from the front,

View in direction B is the view from the left,

View in direction C is the view from the right,

View in direction D is the view from above,

View in direction E is the view from below.

In first angle projection the views in the directions

of arrows B, C, D and E are arranged with reference to

the front view as follows:

The view from B is placed on the right,

The view from C is placed on the left,

The view from D is placed underneath,

The view from E is placed above.

The experienced draughtsman will commit the above

rules to memory. It is customary to state the projection

used on orthographic drawings to remove all doubt, or

use the distinguishing symbol which is shown on the

arrangement in Fig. 4.3.

Chapter 4

Principles of first and third angle

orthographic projection

Fig. 4.1

Fig. 4.2

34 Manual of Engineering Drawing

Third angle projection

The difference between first and third angle projection

is in the arrangement of views and, with reference to

the illustration in Fig. 4.4, views are now positioned

as follows:

View B from the left is placed on the left,

View C from the right is placed on the right,

View D from above is placed above,

View E from below is placed underneath.

Study the rearrangement shown below in Fig. 4.4

and remember the above rules because it is vital that

the principles of first and third angle projection are

understood. The distinguishing symbol for this method

is also shown.

If a model is made of the block in Fig. 4.1, and this

can easily be cut from polystyrene foam used in packing,

then a simple demonstration of first and third angle

projection can be arranged by placing the block on the

drawing board and moving it in the direction of the

four chain dotted lines terminating in arrows in Fig.

4.5. Figure 4.5(a) shows the positioning for first angle

and Fig. 4.5(b) for third angle projection. The view in

each case in the direction of the large arrow will give

the five views already explained.

The terms first and third angle correspond with the

notation used in mathematics for the quadrants of a

circle in Fig. 4.6 the block is shown pictorially in the

first quadrant with three of the surfaces on which views

are projected. The surfaces are known as planes and

the principal view in direction of arrow A is projected

on to the principal vertical plane. The view from D is

projected on to a horizontal plane. View B is also

projected on to a vertical plane at 90° to the principal

vertical plane and the horizontal plane and this is known

as an auxiliary vertical plane. Another horizontal plane

can be positioned above for the projection from arrow

E, also a second auxiliary vertical plane on the left for

the projection of view C. Notice that the projections to

each of the planes are all parallel, meeting the planes

at right angles and this a feature of orthographic

projection.

The intersection of the vertical and horizontal planes

give a line which is the ground line GL. This line is

often referred to as the XY line; this is useful in projection

problems since it represents the position of the horizontal

plane with reference to a front view and also the position

of the vertical plane with reference to a plan view.

Many examples follow in the text.

If the planes containing the three views are folded

back into the plane of the drawing board, then the

result is shown in Fig. 4.7 where dimensions have also

been added. The draughtsman adjusts the distances

between views to provide adequate spaces for the

dimensions and notes.

To describe a simple object, a draughtsman does

not need to draw all five views and it is customary to

draw only the minimum number which completely

illustrate the component. You will note in this particular

case that we have omitted views which contain dotted

lines in preference to those where corners and edges

face the observer. Many parts do not have a definite

‘front’, ‘top’ or ‘side’ and the orientation is decided by

the draughtsman, who selects views to give the

maximum visual information.

View E

View C View A View B

View D

Projection symbol

Fig. 4.3 First angle projection arrangement. Dotted lines indicate

hidden edges and corners

Fig. 4.4 Third angle projection arrangement

View D

View B View A View C

View E

Projection symbol

Principles of first and third angle orthographic projection 35

Traditionally, front views are also known as front

elevations, side views are often known as side or end

elevations and the views from above or beneath are

referred to as plans. All of these terms are freely used

in industrial drawing offices.

Projection symbols

First angle projection is widely used throughout all

parts of Europe and often called European projection.

Third angle in the system used in North America and

alternatively described as American projection. In the

British Isles, where industry works in co-operation

with the rest of the world, both systems of projection

are regularly in use. The current British and ISO

standards state that both systems of projection are

equally acceptable but they should never be mixed on

the same drawing. The projection symbol must be added

to the completed drawing to indicate which system

has been used.

Figure 4.8 shows the recommended proportions of

the two projection symbols.

Figure 4.9 indicates how the First angle symbol

was obtained from projections of a tapered roller. The

Third angle alternative is given in Fig. 4.10.

Please note the movement suggested by the arrow

in Fig. 4.9, Fig. 4.10 and also in Fig. 4.8, since

orientation is the main clue to understanding the

fundamental differences in projection systems.

An experienced draughtsman must be fully con-

versant with all forms of orthographic and pictorial

projection and be able to produce a drawing where no

doubt or ambiguity relating to its interpretation can

exist.

(a)

(b)

Fig. 4.5 (a) First angle arrangement (b) Third angle arrangement

AVP

Projector

Projection

Fig. 4.7

Fig. 4.6 VP is the vertical plane. HP is the horizontal plane. AVP the

auxiliary vertical plane. GL is the ground line

36 Manual of Engineering Drawing

Drawing procedure

Generally, industrial draughtsmen do not complete one

view on a drawing before starting the next, but rather

work on all views together. While projecting features

between views, a certain amount of mental checking

takes place regarding shape and form, and this assists

accuracy. The following series of drawings shows stages

in producing a typical working drawing in first angle

projection.

Stage 1 (Fig. 4.11). Estimate the space required

for each of the views from the overall dimensions in

each plane, and position the views on the available

drawing sheet so that the spaces between the three

drawings are roughly the same.

Stage 2 (Fig. 4.12). In each view, mark out the

main centre-lines. Position any complete circles, in

any view, and line them in from the start, if possible.

Here complete circles exist only in the plan view. The

heights of the cylindrical features are now measured

in the front view and are projected over to the end

view.

Stage 3 (Fig. 4.13). Complete the plan view and

project up into the front view the sides of the cylindrical

parts.

Stage 4 (Fig. 4.14). Complete the front and end

views. Add dimensions, and check that the drawing

(mental check) can be redrawn from the dimensions

given; otherwise the dimensioning is incomplete. Add

the title and any necessary notes.

It is generally advisable to mark out the same feature

in as many views as is possible at the same time. Not

only is this practice time-saving, but a continuous check

on the correct projection between each view is possible,

as the draughtsman then tends naturally to think in the

three dimensions of length, breadth and depth. It is

rarely advantageous to complete one view before starting

the others.

Reading engineering

drawings

The following notes and illustrations are intended to

assist in reading and understanding simple drawings.

In all orthographic drawings, it is necessary to project

at least two views of a three dimensional object – or

one view and an adequate description in some simple

cases, a typical example being the drawing of a ball

Fig. 4.10

Fig. 4.8

First angle projection symbol

Third angle projection symbol

1.25 D

30°

Drawing paper

Fig. 4.9

Drawing paper

Principles of first and third angle orthographic projection 37

for a bearing. A drawing of a circle on its own could

be interpreted as the end elevation of a cylinder or a

sphere. A drawing of a rectangle could be understood

as part of a bar of rectangular cross-section, or it might

be the front elevation of a cylinder. It is therefore

generally necessary to produce at least two views, and

these must be read together for a complete

understanding. Figure 4.15 shows various examples

where the plan views are identical and the elevations

are all different.

A single line may represent an edge or the change

in direction of a surface, and which it is will be

determined only by reading both views simultaneously.

Figure 4.16 shows other cases where the elevations

are similar but the plan views are considerably different.

A certain amount of imagination is therefore required

when interpreting engineering drawings. Obviously,

with an object of greater complexity, the reading of

three views, or more, may well be necessary.

Overall space for front

elevation

Overall space for

end elevation

Overall space for

plan view

Fig. 4.11 Stage 1

Fig. 4.12 Stage 2

Fig. 4.13 Stage 3

Fig. 4.14 Stage 4

Fig. 4.15

38 Manual of Engineering Drawing

Projection exercises

It is clear to us after teaching draughting and CAD for

many years that visualizing a proposed new product in

three dimensions, which is how you naturally view a

finished article, is difficult when it is necessary to read

more than one complex two dimensional drawing

simultaneously. The draughtsman also ultimately needs

to produce technically correct drawings, often from

vague initial ideas. The very action of making proposal

drawings stimulates many questions and their answers

allow development to continue. Modifications to original

ideas involve drawing amendments and changes to one

view invariably have a ‘knock on effect’. Compre-

hension, understanding and the ability to read technical

drawings fluently comes with practice.

The following simple exercises are designed to assist

in the perfection of draughting skills. They are equally

suitable for CAD and the drawing board. Produce

answers for each series and select standard sizes of

drawing sheets, taking particular care with linework

and layout.

If the CAD software program permits, move the

separate views for each exercise so that they are

positioned a similar distance from each other. Then

experiment and position the groups to give a pleasing

layout on the drawing sheet. Note how uniformity can

improve presentation and give a professional appear-

ance. Layout is a very important aspect when preparing

drawings for desk top publishing applications.

Fig. 4.16

1 Straight line examples

Figure 4.17 shows three components which have each

been machined from solid blocks. These examples have

been prepared on a grid formed by equilateral triangles.

(a)

(b)

(c)

Fig. 4.17

Principles of first and third angle orthographic projection 39

In every case, the scale is such that each side of the

triangle will be 10 mm. For each component, draw

five views in first angle projection, omitting hidden

detail, and assume that the view in the direction of the

arrow A will be the front view.

2 Examples involving radii and holes

(Fig. 4.18)

For each example, project five views in first angle

projection, taking the view in the direction of the arrow

A as the front view. Hidden detail is required in the

solutions to these problems, and note that in some

cases the position of some of the holes will be found

to coincide with centre lines. Where this occurs the

dotted line should take priority. Take each side of the

grid triangle to be 10 mm in length.

If only three views of each component were required,

which ones would you choose? The professional

draughtsman would select a front view, end view and

plan view with the least number of dotted lines. Study

your solutions carefully and where an ideal choice

exists, box this solution with a thin chain line. In some

cases more than one choice can be made and these are

indicated in the solutions.

3 Examples with missing lines (first

angle projection) (Fig. 4.19)

In the following projection examples, three views are

given. Some views are incomplete with full lines and

all dotted lines missing. Draw the given examples,

using the scale provided. Complete each view, by

inserting full lines where necessary and add all dotted

lines to represent the hidden detail.

4 Examples with missing views (first

angle projection) (Fig. 4.20)

In each of the following projection examples, two out

of three views of simple solid components are shown.

Draw the two views which are given using the scale

provided. Complete each problem by drawing the

missing view or plan in the space indicated by the

cross.

5 First angle projection examples with

plotted curves (Fig. 4.22)

In orthographic projection, all widths in the end view

are equal in size to depths in the plan view, and of

course the opposite is true that some dimensions required

to complete end views may be obtained from given

plan views. Figure 4.21 shows part of a solid circular

bar which has been cut at an angle of 30° with the

horizontal axis. Point A is at any position along the

sloping face. If a horizontal line is drawn through A

across to the end view then the width of the chord is

dimension X. This dimension is the distance across

(a)

(b)

(c)

Fig. 4.18