Morling K. Geometric and Engineering Drawing

Подождите немного. Документ загружается.

Geometric and Engineering Drawing284

two different drawings can ever raise exactly the same problems. Each drawing that

you do needs to be studied very carefully before you begin to dimension it.

Examination questions often ask for only five or six ‘ important ’ dimensions to

be inserted on the finished drawing. The overall dimensions – length, breadth and

width – are obviously important but the remaining two or three are not so obvious.

The component or assembled components need to be studied in order to ascertain the

function of the object. If, for instance, the drawing is of a bearing, then the size of

the bearing is vitally important because something has to fit into that bearing. If the

drawing is of a machine vice, then the size of the vice jaws should be dimensioned

so that the limitations of the vice are immediately apparent. These are the types of

dimensions that should make up the total required.

Conventional Representations

There are many common engineering details that are difficult and tedious to draw.

The screw thread is an example of this type of detail and it has been shown earlier in

this part of the book that there are conventional ways of drawing screw threads which

are very much simpler than drawing out helical screw threads in full.

Figure 18.22 shows some more engineering details and alongside the detailed

drawing is shown the conventional representation for that detail. These conventions

are designed to save time and should be used wherever and whenever possible.

These are not all the conventional representations that are available but the rest

are beyond the scope of this book. The interested student can find the rest in BS 308.

There is a way of representing compression and tension springs diagrammatically.

The coils of these springs can be represented by straight lines.

Engineering Drawing 285

Title

Break

lines

Straight

knurling

Diamond

knurling

Square

on shaft

Subject

Round (Solid)

Round (Tubular)

Rectangular

Rectangular (Wood)

Convention

Figure 18.22 (a)

Geometric and Engineering Drawing286

Title

Compression

springs

Tension

springs

Splined

shafts

Serrated

shafts

Holes

circular

pitch

Holes

linear

pitch

Subject Convention

Figure 18.22 (b)

Engineering Drawing 287

Machining Symbols

The shape of an engineering component can be determined in several ways. The

component may be forged, cast, drawn, etc. After one or more of these processes, it

is quite likely that some machining will have to be done. It is therefore important that

these machined faces be indicated on the drawing. The method recommended by BS

308 is shown in Fig. 18.23 but this is not the only method in use. Sometimes, the let-

ter ‘ f ’ is written over the face to be machined. This letter ‘ f ’ stands for ‘ finish ’ .

The small tick shows only that that particular face has to be machined. It does

not show how it is to be machined, nor does it show how smooth the finish is to be.

The method of machining – turning, milling, grinding, etc. – is not normally put on a

drawing but the standard of finish is.

Title

Treatment

symmetrical

parts

Subject

(Note the short thick double line at the end of each centre line.)

Convention

Figure 18.22 (c)

Geometric and Engineering Drawing288

Surface Roughness

This short section on roughness symbols is beyond the scope of this book, but it is

well worth looking at.

The standard of finish, or roughness of a surface, is of vital importance in engineer-

ing. The degree of roughness permitted depends on the function of the component. When

two pieces of metal slide against each other, as in the case of a bearing, the finish on both

parts must be very smooth or the bearing will overheat and ‘ seize ’ . On the other hand,

smooth finishes are expensive to produce and should be kept to a sensible minimum.

If the surface of a piece of machined metal is magnified it will look like a range

of very craggy mountains. The surface roughness is the distance from the highest

‘ peak ’ to the lowest ‘ valley ’ . This roughness is measured in micrometres and one

micrometre is one millionth part of a metre. Not only can a surface be made smooth

to one micrometre but it can also be measured to one micrometre.

The British Standard index numbers of surface roughness are 0.025, 0.05, 0.1, 0.2,

0.4, 0.8, 1.6, 3.2, 6.3, 12.5 and 25.0. A surface roughness from 0.025 to 0.2 can be

obtained by lapping or honing, 0.4 can be obtained by grinding and 0.8 by careful turn-

ing, rough grinding, etc. The surface roughness number is shown within the vee of the

machining symbol. A tolerance on surface roughness is shown as a fraction, with the

maximum roughness number above the minimum roughness number ( Figure 18.24 ).

60°

Machining

symbol

First angle projection

Figure 18.23 Application of machining symbol.

Engineering Drawing 289

3.2

0.8

0.05

3.2

0.2

Figure 18.24 Application of surface roughness symbols.

Abbreviations

A list of the standard abbreviations for some general engineering terms is shown

below. These abbreviations are used to save time, and space on drawings.

Although the abbreviations are shown in block capital letters, small letters may

be used. For unit abbreviations always use small letters. The abbreviations are the

same in the singular and the plural. Note that full stops are not used except when the

abbreviation makes another word, e.g. number becomes No. and figure becomes Fig.

These are by no means all the standard abbreviations. They should contain all that

are required at this stage but should the students require the full list of abbreviations,

they will find them in BS 1991.

Term Abbreviation

Across-flats AF

British Standard BS

Centimetre cm

Centre line CL or

Chamfered CHAM

Cheese head CH HD

Countersunk CSK

Counterbore CBORE

Degree (of angle) °

Diameter (preceding a dimension) φ

Drawing DRG

Figure FIG.

First angle projection

Hexagon HEX

Internal diameter I/D

Kilogram kg

Left hand LH

Machine MC

Geometric and Engineering Drawing290

Material MATL

Maximum MAX

Metre m

Millimetre mm

Minimum MIN

Minute (of angle) 

Number NO.

Outside diameter O/D

Per /

Pitch circle diameter PCD

Radius (preceding a dimension) R

Right hand RH

Roundhead RD HD

Second (of angle)  

Square (in a note) SQ

Square (preceding a dimension)

䊐 or



Square metre m

Standard STD

Third angle projection

Screw threads

British Standard Pipe BSP

International Organization for

Standardization

ISO

Syst è me International SI

Undercut U ’ CUT

Unified coarse UNC

Unified fine UNF

Unified selected UNS

Framing and Title Block

Most paper comes in standard sizes. The largest sheet you are likely to use is A0 and

the smallest A4. If your drawing paper has no frame then draw one. A minimum of

20 mm is used on A0 and A1 from the edge of the paper to the frame line and a mini-

mum of 10 mm on A2, A3 and A4.

In order to space out the views that you will draw on your paper use the following

formulas (A, B and C are the maximum sizes of your views) and the p and q dimen-

sions are the distances between the views.

You do not have to use exact dimensions which might complicate the sums; use

sensible approximations for A, B and C ( Fig. 18.25 ).

You may well have to add information to your finished drawing and this should be

shown in blocks. This information could include:

Title

Your name

Engineering Drawing 291

p = X – (A + B)

q = Y – (B + C + p)

Edge of paper

Frame

Figure 18.25 Positioning of views to be drawn.

Your location (college or company)

The scale of the drawing

The system of projection used

Date

You might also have many parts assembled on your drawing and you will need to

list them, show where they can be found on the drawing by using a grid reference,

describe what the parts are called and say how many there are. This information can

be done as shown in Fig. 18.26 (see pages 292 and 293).

Geometric and Engineering Drawing292

Centring mark

Edge of sheet

Projection

Border

Grid

reference

Part

info

Title

block

12 34

Descrip.

SCALE NAME

TITLE

DATE LOCATION

Grid

Ref.

No.

off

4321

(A)

Figure 18.26 Layout of title boxes. (A) Portrait format. (B) Landscape format.

Engineering Drawing 293

Edge of sheet Frame

Projection

SCALE NAME

TITLE

LOCATIONDATE

FEDCBA

ABCD

Descrip.

Grid

Ref.

No.

Off.

(B)

Figure 18.26 (Continued)