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LESSON 2

ENGINEERING DRAWING AND ITS INSTRUMENTS

I n t r o d u c t i o nNow after a small introduction to engineering graphics let us

proceed with t he basics that are engineering drawing instru-

ments an d their uses. But first of all we must know what isengineering drawing and what is its significance.

An engineering drawing is a type of drawing that is techn ical

in nature, used to fully and clearly define requirement s for

engineered items, and is usually created in accordance with

standardized conventions for layout, nomenclature, interpreta-

tion, app earance (such as t ypefaces and line styles), size, etc.

Engineering drawings are often referred to as blueprints.

Ho wever, the term is an anachronism, and is due to the fact that

most copies of engineering drawings were formerly made usinga chemical printing process that yielded graphics on blue-colored

paper. The process of producing engineering drawings, and the

skill of producing them, is often referred to as technical

drawing , although tech nical drawings are also required for

disciplines that would not ordinarily be thought o f as parts of

engineering

Engineering drawing is the language of engineers. It is meant

for comm unicating your ideas, thoughts and designs to others.

A drawing drawn b y engineer, having engineering knowledge

for th e engineering purposes is an engineering drawing.

Engineering drawing is the starting point of all branches-

mechanical, product ion, civil, electrical, and metallurgical,

instrumentation, agriculture etc.

It is a un iversal graphic language of en gineers, spoken read and

written in its own way. It is used by them to develop and record

their ideas and transmit them to oth ers for execution. Like

every language has its own rules of grammar also Engineering

drawing also has its theory of projections, its idioms in

convent ional practices, its pun ctuation in t he types of lines, its

abbreviations, symbols and its descriptions in the construc-

tions.

Now w hy it is required i.e. wh at is its Significance?

The en gineer like you (would be) to whatever branch m ay

belong to must have perfect knowledge and skill in drawing. As

bad the language is unpleasant to read and communicate, soalso a wrong d rawing will worry a trained eye. Even a sm all error

comm itted by an engineer in his / her drawing may misguide

the workman/ operator and the ultimate product / construction

will be a flop.

As we all know engineering drawing has big classification which

we have discussed in introduction also there is a found ation of

all drawings I think you all know foundation yes its the base

called geometrical drawing which can also be further catego-

rized. So let us first pr oceed with geometrical drawing

What is Geometrical drawing?

Geometrical drawing is the foundation of all engineering

drawings. It is a subject, difficult to learn or to teach without theaid of a good textboo k. A good speed in drawing work should

be achieved, because time is money in the ind ustry. Accuracy,

neatness and legibility are of great import ance in engineering

drawing.

G eometrical drawing consists of:

1. Plain geometrical drawing

2. Solid geometrical drawing

Geometrical drawing is the art of representation of geometrical

objects such as square, rectangle, cylinder, cone, sphere etc on a

drawing sheet. as I h ave already told you so its further are

defined as

Plain geometrical drawing is the art of representation of

objects having two dimensions i.e., length and breadth such as

square, rectangle, and quadrilateral etc. on a drawing sheet.

Solid geometrical drawing is the art of representation of

objects of three dimensions, i.e., length, breadth and height,

such as cube, cylinder, and sphere etc. on a drawing sheet.

Engineering drawing, particularly, geometrical drawing is the

basis for various sp ecialized types of d rawings applied to

various fields: machine drawing in the machine industries;

structural drawing in the structural constructions such as

buildings, bridges and dam s; electrical drawing in the electrical

industries; aeronautical drawing in the aircraft manufacturing;

marine drawing in ship construction and so on.

En g i n e e r i n g D ra w in g In s t r u m e n t s a n dUsesD rawing equipment and instruments are needed t o record

information on drawing paper or any other suitable surface.

D rawing composed of straight lines, curves, circles and arcs is

prepared with certain instruments. The quality of a drawing

depends to a large extent on the quality, adjustment and care of

the instruments used. Reliable and accurate equipment is

essential as good results increase and enthusiasm.


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ENGINEE

RING

GRAPHICS

List o f Draw ing Instru me nt s

Before comm encing the course o f engineering drawing, it is

essential that a studen t shou ld obtain the fo llowing drawing

instruments:

1. Drawing board

2. Drawing sheets

3. Mini-drafter/ drafting machine

4. Instrument bo x

5. Set squares (45, 30-60 triangle)6. Protractor

7. Drawing pencils (HB, H and 2H grades)

8. Pencil eraser

9. Clips or ad hesive tape

10. Pencil sharpener

11. Emery paper and tracing paper

12. Dusting cloth.

13. French curves

Comm on Featu res of Eng inee ring Draw ings

A variety of line styles are used to graphically represent physical

objects. Types of lines include the fo llowing:

Visible - are continuo us lines used to d epict edges directly

visible from a particular angle.

Hidden - are short-dashed lines that m ay be used to

represent edges that are not directly visible.

Center - are alternately long- and short -dashed lines that may

be used to represent the axes of circular features.

Cutting plane - are thick, medium-dashed lines, or thick

alternately long- and double short-dashed that may be used

to define sections for section views.

Section - are thin lines in a p arallel pattern u sed to indicate

surfaces in section views resulting from cutting. Sectionlines are com mon ly referred to as cross-hatching.

Mul t ip le View s an d Pro ject ion sIn m ost cases, a single view is not sufficient to show all

necessary features, and several views are used. Types of views

include the following:

Orthograph ic Projection - show the object as it looks

from th e front, right, left, top, botto m, or back, and are

typically positioned relative to each oth er according to the

rules of either first angle or third angle. The form er is

primarily used in E urop e and Asia; the latter is primarily

used in the United States and Canada. N ot all views are

necessarily used, and det ermination of what surfaceconstitutes the front, etc., varies from object to object.

Orthographic comes from the Greek for straight writing

(or drawing).

Section - depict what t he ob ject would look like if it were

cut perfectly along cutting plane lines defined in a particular

view, and rotated 90 degrees to directly view the resulting

surface(s), which are indicated with section lines. They are

used to show features n ot extern ally visible, or not clearly

visible.

Detail - show po rtions of o ther views, magnified for

clarity.

Auxiliary p rojection - similar to orthographic projections,

however the directions of viewing are other than those for

orthographic projections.

Isometric- shows th e object from angles in which the scales

along each axis of the object are equal. It correspond s to

rotation of the object by + / - 45 degrees about the vertical

axis, followed by rotation of appro ximately + / - 35.264

degrees [= arcsin (tan (30 degrees))] about the h orizont al axisstarting from an ort hographic projection view. Isom etric

comes from th e G reek for same measure.

Sh o w in g D im e n s i o n sThe required sizes of features are conveyed through use of

dimensions. Distances may be indicated with either of two

standardized form s of dimension: linear and ordinate.

With linear dimension s, two p arallel lines, called extension

lines, spaced at th e distance between two features, are

shown at each of the features. A line perpendicular to the

extension lines, called a dimension line, with arro ws at its

endpo ints, is shown between, and term inating at, the

extension lines. The distance is indicated num erically at themidpoint o f the d imension line, either adjacent to it, or in a

gap provided for it.

With ordinate dimensions, one ho rizontal and o ne vertical

extension line establish an origin for the ent ire view. The

origin is identified with zeroes placed at the ends of these

extension lines. Distances along th e x- and y-axes to oth er

features are specified using other extension lines, with the

distances indicated num erically at th eir ends.

Sizes of circular features are indicated using either diametral or

radial dimensions. Radial dimensions use an R followed by

the value for th e radius; Diametral dimensions u se a circle with

forward-leaning diagonal line thro ugh it, called th e diametersymbol, followed by the value for t he diameter. A radially

aligned line with arrowhead p ointing to t he circular feature,

called a leader, is used in con junction with b oth diametral and

radial dimensions. All types of dimensions are typically

composed of two parts: the no minal value, which is the ideal

size of the feature, and th e tolerance, which specifies the

amount that th e value may vary above and b elow the no minal.

Notes

Notes-textual inform ation-are also typically included in

drawings, specifying details not otherwise conveyed. N otes are

almost always in co mpletely uppercase characters, for un iformity

and m aximal legibility after du plication of t he d rawing, which

may involve substantial reduction in size. Leaders may be used

in conjunction with notes in order to point to a particular

feature or object that the note concerns.

Sizes of Drawings

Sizes of drawings typically comply with either of t wo d ifferent

standards, metric or U.S. customary, according to the fo llowing

tables:


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METRIC Drawing Sizes (mm)

A4 210 X 297

A3 297 X 420

A2 420 X 594

A1 594 X 841

A0 841 X 1189

U.S. Customary Drawing Sizes

A 8.5" X 11"

B 11" X 17"

C 17" X 22"

D 22" X 34"

E 34" X 44"

The metric drawing sizes correspond to international paper

sizes. The U .S customary A-size correspo nds to letter size,

and B-size corresponds to ledger size. ANSI Y14.2, Y14.3,

and Y14.5 are standards that are commonly used in the U.S.Type s o f Lines

Engineering drawing uses various types of lines to describe

different objects and different purposes. Each type of line in the

drawing has different meaning

IS: 10714-1983 adop ted fro m I SO: 128-1982 specifies the types

of lines and their applications are as:

Chap te r Learn ing Outcome s

After reading th is chapter, you will be able to the fo llowing

questions.

D iscuss the use of engineering graphics in d etail.

Explain types of engineering drawing and its uses.

Discuss common features of engineering drawing.

Discuss principles of lettering and dimensioning.

Explain geometrical construction of some pentagon,

hexagon etc.

Notes

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