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

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40 Manual of Engineering Drawing

(a)

0 20406080100

millimetres

(b)

(c)

Front view

(a)

0 204060 80100

millimetres

End view

(b)

Plan

(c)

Fig. 4.19 Fig. 4.20

Principles of first and third angle orthographic projection 41

the cut face in the plan view and this has been marked

on the vertical line from A to the plan. If this procedure

is repeated for other points along the sloping face in

the front view then the resulting ellipse in the plan

view will be obtained. All of the examples in this group

may be solved by this simple method.

A word of warning: do not draw dozens of lines

from points along the sloping face across to the end

view and also down to the plan view before marking

any dimensions on your solution. Firstly, you may be

drawing more lines than you need, and in an examination

this is a waste of time. Secondly, confusion may arise

if you accidently plot a depth on the wrong line. The

golden rule is to draw one line, plot the required depth

and then ask yourself ‘Where do I now need other

points to obtain an accurate curve?’ Obviously, one

needs to know in the plan view the position at the top

and bottom of the slope, and the width at the horizontal

centre line and at several points in between.

In the examples shown in Fig. 4.22 three views are

given but one of them is incomplete and a plotted

curve is required. Redraw each component using the

scale provided. Commence each solution by establishing

30°

Fig. 4.21

Fig. 4.22

(a)

30° 45°

0 20 40 60 80 100

millimetres

30°

20°

(b)

R60

(c)

R60

(d)

42 Manual of Engineering Drawing

projection. Make a pictorial sketch of each component

and arrange that the corner indicated by the arrow is in

the foreground. No dimensions are given but estimate

the proportions of each part assuming that the largest

dimension is every example is 100 mm.

7 Geometric solids in third angle

projection

Figure 4.24 shows three views of each of three geometric

solids. Sufficient dimensions are given to define the

shapes but in each case two of the views are incomplete.

Redraw the details provided and complete the views

in third angle projection.

8 Sectional views in third angle

projection

In Fig. 4.25 there are three components and two views

are provided for each one. Copy the views given, using

the scale provided, and project the missing view which

will be a section. Your solution should include the

section plane, cross hatching and the statement A–A.

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 4.23

the extreme limits of the curve and then add intermediate

points.

6 Pictorial sketching from

orthographic views

Figure 4.23 shows six components in first angle

Principles of first and third angle orthographic projection 43

30°

(a) Cube

Across flats

45°

(b) Frustum of hexagonal pyramid

30°

45°

Across corners

Plan

(a)

End view

(b)

Plan

(c)

0 20 40 60 80 100

millimetres

Fig. 4.24

Fig. 4.25

44 Manual of Engineering Drawing

9 Dimensioning examples

(first angle projection)

In Fig. 4.26 a scale is provided to enable each of the

components to be redrawn. Redraw each example and

add any dimensions which you consider necessary and

0 204060 80100

millimetres

(a)

(b)

Fig. 4.26

(c)

(d)

which would be required by the craftsman. Bear in

mind that if an object has sufficient dimensions to

enable it to be drawn, then it can most likely be made.

Hence, any sizes which are required to enable you to

draw the part are also required by the manufacturer.

For additional information regarding dimensioning refer

to Chapter 14.

Drawing paper sizes

The British Standard BS8888 recommends that for

normal practical purposes the area of the largest sheet

is one square metre and the sides are in the ratio of

1:√2. The dimensions of the sheet are 841 mm × 1189

mm. For smaller sheets the longest side is progressively

halved giving the designations and dimensions in Table

5.1. Since the A0 size has the area of one square metre,

paper weights are conveniently expressed in the unit

‘grams per square metre’.

Table 5.1

Designation Size (millimetres) Area

A0 841 × 1189 1 m

A1 594 × 841 5000 cm

A2 420 × 594 2500 cm

A3 297 × 420 1250 cm

A4 210 × 297 625 cm

Drawing sheets may be obtained from a standard

roll of paper or already cut to size. Cut sheets sometimes

have a border of at least 15 mm width to provide a

frame and this frame may be printed with microfilm

registration marks, which are triangular in shape and

positioned on the border at the vertical and horizontal

centre lines of the sheet.

Title blocks are also generally printed in the bottom

right-hand corner of cut sheets and contain items of

basic information required by the drawing office or

user of the drawing. Typical references are as follows:

Name of firm,

Drawing number,

Component name,

Drawing scale and units of measurement,

Projection used (first or third angle) and or symbol,

Draughtsman’s name and checker’s signature,

Date of drawing and subsequent modifications,

Cross references with associated drawings or

assemblies.

Other information will vary according to the branch

and type of industry concerned but is often standardized

by particular firms for their own specific purposes and

convenience.

Presentation

Drawing sheets and other documents should be

presented in one of the following formats:

(a) Landscape; presented to be viewed with the longest

side of the sheet horizontal.

(b) Portrait; presented to be viewed with the longest

side of the sheet vertical.

Lines and linework

Two thicknesses of line are recommended for manual

and CAD drawings. A wide line and a narrow line in

the ratio of 2:1.

Standard lead holders, inking pens for manual use,

and those for CAD plotters are all available in the

following millimetre sizes: 0.25, 0.35, 0.5, 0.7, 1.0,

1.4 and 2.0.

Line thicknesses of 0.7 and 0.35 are generally used

and will give good quality, black, dense and contrasting

lines.

Table 5.2, shows applications for different line types

which are designed to obtain a good professional finish

to a drawing.

Various combinations of line thickness and type are

shown on the mechanism in Fig. 5.2. (Circled numbers

relate to the line types in Table 5.2.)

Figure 5.3 shows part of a cone and if the complete

cone was required, for example for dimensioning

purposes, then the rest would be shown by adding

narrow continuous lines which intersect in a dot.

Chapter 5

Linework and lettering

Fig. 5.1 Standard size reductions from A0 to 35 mm microfilm

1189

841

594

420

Microfilm

297

210

841

594

420

297

46 Manual of Engineering Drawing

If it is necessary to show the initial outline of a part

before it is bent or formed, then the initial outline can

be indicated by a chain thin line which is double dashed.

Figure 5.4 shows the standard applied to a metal strip.

Figure 5.5 shows the method of detailing a long

strip of metal which has 60 holes in it at constant

pitch. There would be no need to detail all of the

component and this illustration gives one end only.

The line to show the interruption in the drawing is

narrow continuous and with the zig-zag cutting line

indicated by the letter J.

Chain lines

Particular care should be taken with chain lines to

ensure that they are neatly applied and attention is

drawn to the following points:

(a) All chain lines should start and finish with a long

dash.

(b) When centre points are defined, then the chain

lines should cross one another at solid portions

of the line.

beyond the feature unless they are required for

dimensioning or other purpose.

Table 5.2 Types of line

Example Description & Representation Application

A Continuous wide line Visible edges and outlines

B Continuous narrow line 1 Dimension, extension and projection lines

2 Hatching lines for cross sections

3 Leader and reference lines

4 Outlines of revolved sections

5 Imaginary lines of intersection

6 Short centre lines

7 Diagonals indicating flat surfaces

8 Bending lines

9 Indication of repetitive features

C Continuous narrow irregular line Limits of partial views or sections provided the line is not an axis

D Dashed narrow line Hidden outlines and edges

— — — — — — — — —

E Long dashed dotted narrow line 1 Centre lines.

2 Lines of symmetry

3 Pitch circle for gears

4 Pitch circle for holes

F Long dashed dotted wide line Surfaces which have to meet special requirements

G Long dashed dotted narrow line with wide line at Note BS EN ISO 128-24 shows a long dashed dotted wide line for this

ends and at changes to indicate cutting planes application

H Long dashed double dotted 1 Preformed outlines

narrow line 2 Adjacent parts

3 Extreme positions of moveable parts

4 Initial outlines prior to forming

5 Outline of finished parts

6 Projected tolerance zones

J Continuous straight narrow line with zig zags Limits of partial or interrupted views;

Suitable for CAD drawings provided the line is not an axis

Linework and lettering 47

Gasket part

N° 62/3

90°

A–A

(d) Centre lines should not extend through the spaces

between views and should never terminate at

another line on the drawing.

Fig. 5.2

Fig. 5.3 Example showing imaginary lines of intersection

Fig. 5.4 Initial outline applications

898

Pitch

60 × φ12

Equally spaced

as shown

Fig. 5.5 Interrupted view application

48 Manual of Engineering Drawing

(e) If an angle is formed by chain lines, then the

long dashed should intersect and define the angle.

(f) Arcs should meet straight lines at tangency points.

(g) When drawing hidden detail, a dashed line should

start and finish with dashes in contact with the

visible lines from which they originate.

(h) Dashed lines should also meet with dashes at

corners when drawing hidden detail.

Lettering

It has previously been mentioned that technical drawings

are prepared using only two line thicknesses and if

reasonable care is taken a pleasing result can easily be

obtained. Drawings invariably need dimensions and

notes and if these are added in a careless and haphazard

manner, then a very poor overall impression may be

given. Remember that technical drawings are the main

line of communication between the originator and the

user. Between a consultant and his client, the sales

manager and his customer, the designer and the

manufacturer, a neat well executed technical drawing

helps to establish confidence. The professional

draughtsman also takes considerable pride in his work

and much effort and thought is needed with respect to

lettering, and spacing, in order to produce an acceptable

drawing of high standard.

The following notes draw attention to small matters

of detail which we hope will assist the draughtsman’s

technique of lettering.

1 Lettering may be vertical or slanted, according to

the style which is customarily used by the draughts-

man. The aim is to produce clear and unambiguous

letters, numbers and symbols.

2 If slanted lettering is used, the slope should be

approximately 65°–70° from the horizontal.

Legibility is important. The characters should be

capable of being produced at reasonable speed

and in a repeatable manner. Different styles on

the same drawing spoil the overall effect.

3 Use single stroke characters devoid of serifs and

embellishments.

4 All strokes should be of consistent density.

5 The spacing round each character is important to

ensure that ‘filling in’ will not occur during

reproduction.

6 Lettering should not be underlined since this

impairs legibility.

7 On parts lists or where information is tabulated,

the letters or numerals should not be allowed to

touch the spacing lines.

8 All drawing notes and dimensions should remain

legible on reduced size copies and on the screens

of microfilm viewers.

9 Capital letters are preferred to lower case letters

since they are easier to read on reduced size copies

of drawings. Lower case letters are generally used

only where they are parts of standard symbols,

codes or abbreviations.

10 When producing a manual drawing the

draughtsman should take care to select the proper

grade of pencil for lettering. The pencil should be

sharp, but with a round point which will not injure

the surface. Mechanical pencils save time and

give consistent results since no resharpening is

necessary.

11 Typewritten, stencilled or letters using the ‘Letraset’

adhesive letter system may be used since these

provide uniformity and a high degree of legibility.

Minimum character height for capital

letters and numerals

Table 5.3 gives the minimum recommended character

heights for different sizes of drawing sheet and it is

stressed that these are minimum sizes. If lower case

letters are used then they should be proportioned so

that the body height will be approximately 0.6 times

the height of a capital letter.

The stroke thickness should be approximately 0.1

times the character height and the clear space between

characters should be about 0.7 mm for 2.5 mm capitals

and other sizes in proportion.

The spaces between lines of lettering should be

consistent and preferably not less than half the character

height. In the case of titles, this spacing may have to

be reduced.

All notes should be placed so that they may be read

from the same direction as the format of the drawing

but there are cases, for example when a long vertical

object is presented, where it may be necessary to turn

the drawing sheet through 90° in the clockwise direction,

in effect, to position the note which is then read from

the right hand side of the drawing sheet.

Linework and lettering 49

The shape and form of an acceptable range of letters

and numbers is illustrated in Fig. 5.6.

Drawings on a computer, of course, leave no trace

when parts are deleted but this is not necessarily the

case if the work is undertaken manually on tracing

film or paper. The point to remember is that on the

area covered by the erasure, part of a new drawing

will be added and the quality of this drawing must be

identical in standard with the original. Obviously, if

the surface of the drawing sheet has been damaged in

any way during erasure, then the draughtsman per-

forming the work starts with a serious disadvantage.

The following suggestions are offered to assist in

the preservation of drawings when erasures have to be

made.

1 Use soft erasers with much care. Line removal

without damaging the drawing surface is essential.

2 An erasing shield will protect areas adjacent to

modifications.

3 Thoroughly erase the lines, as a ghost effect may

be observed with incomplete erasures when prints

are made. If in any doubt, a little time spent

performing experimental trial erasures on a sample

of a similar drawing medium will pay dividends,

far better than experimenting on a valuable original.

Care and storage of

original drawings

Valuable drawings need satisfactory handling and

storage facilities in order to preserve them in first class

condition. Drawings may be used and reused many

times and minimum wear and tear is essential if good

reproductions and microfilms are to be obtained over

a long period of time. The following simple rules will

assist in keeping drawings in ‘mint’ condition.

1 Never fold drawings.

2 Apart from the period when the drawing is being

prepared or modified, it is good policy to refer to

prints at other times when the drawing is required

for information purposes.

3 The drawing board should be covered outside normal

office hours, to avoid the collection of dust and

dirt.

4 Too many drawings should not be crowded in a

filing drawer. Most drawing surfaces, paper or

plastics, are reasonably heavy and damage results

from careless manipulation in and out of drawers.

5 Do not roll drawings tightly since they may not lie

flat during microfilming.

6 Do not use staples or drawing pins. Tape and drawing

clips are freely available.

7 When using drawings, try to use a large reference

table. Lift the drawings rather than slide them, to

avoid smudging and wear.

8 Drawings should be stored under conditions of

normal heat and humidity, about 21°C and 40 to

60% relative humidity.

Table 5.3

Application Drawing sheet size Minimum character

height

Drawing numbers A0, A1, A2 and A3 7 mm

etc. A4 5 mm

Dimensions and A0 3.5 mm

notes

A1, A2, A3, and A4 2.5 mm

(a) ABCDEFGHIJKLMNOPQRSTUVWXYZ

abcdefghijklmnopqrstuvwxyz

1234567890

(b)

ABCDEFGHIJKLMNOPQRSTUVWXYZ

abcdefghijklmnopqrstuvwxyz

1234567890

(c)

ABCDEFGHIJKLMNOPQRSTUVWXYZ

aabcdefghijklmnopqrstuvwxyz

1234567890

(d)

ABCDEFGHIJKLMNOPQRSTUVWXYZ

abcdefghijklmnopqrstuvwxyz

1234567890

(e) 1234567890

(f) 12334567890

Fig. 5.6

Open styles are often used on drawings which are

to be microfilmed, as increased clarity is obtainable

on small reproductions.

Drawing modifications

After work has been undertaken on a drawing for a

reasonable amount of time, then that drawing will

possess some financial value. The draughtsman

responsible for the drawings must be concerned with

the reproducible quality of his work as prints or

photographic copies are always taken from the originals.

Revisions and modifications are regularly made to

update a product, due for example, to changes in

materials, individual components, manufacturing

techniques, operating experience and other causes

outside the draughtsman’s control.

When a drawing is modified its content changes

and it is vital that a note is given on the drawing

describing briefly the reason for change and the date

that modifications were made. Updated drawings are

then reissued to interested parties. Current users must

all read from a current copy. Near the title block, on a

drawing will be placed a box giving the date and Issue

No., i.e. XXXA, XXXB, etc. These changes would usually

be of a minimal nature.

If a component drawing is substantially altered, it

would be completely redrawn and given an entirely

new number.