tvet certificate iii in fitting and welding

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TVET CERTIFICATE III in FITTING AND WELDING

TECHNICAL DRAWING

WLDTD301 Apply technical drawing

Credits: 8 Learning hours: 80hrs

Sector: Manufacturing.

Sub-sector: Welding

Module Note Issue date: June, 2020

Purpose statement

This general module describes the performance outcomes, skills and knowledge required to

carry out basic technical drawing for an engine mechanic. In order to perform many of

particular competences successfully, a mechanic must apply principles of technical drawing.

Table of Contents

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Elements of competence and performance criteria Page No.

Learning Unit Performance Criteria

1. Introduce technical

drawing.

1.1Identification of the different types of drawing

instruments, equipment and materials.

3

1.2 Understanding of graphic language

techniques.

1.3 Classification of different types of drawing.

2.Apply principles of drawing

2.1 Identification of drawing sheets. 18

2.2 Application of drawing scales.

2.3 Use of lines.

2.4 Respect of drawing lettering standard rules.

2.5 Design of sections.

2.6 Application of drawing conventional

representations dimensioning and standard

abbreviations.

Total Number of Pages: 56

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Learning Unit 1 – Introduce technical drawing

LO 1.1 – Identify the different types of drawing instruments, equipment and

materials.

Content/Topic 1: Drawing instruments, equipment and materials.

Drawing Instruments are used to prepare neat and accurate Drawings. To a greater extent,

the accuracy of the drawings depends on the quality of instruments used to prepare them.

The following is the list of Drawing Instruments and other materials required:

− Drawing Board

− T-square

− Set Squares

− Protractor

− Drawing Instrument Box

− Drawing Sheet

− Drawing Pencils

A. Drawing Board: Is an essential piece of equipment in technical graphic. Paper will be

attached to the drawing board with the aid of a T-square so that it is kept straight

and still, so that the drawing can be completed with accuracy.

− It is a board or platform rectangular in shape.

− Top surface should be smooth.

− Drawing board is made from strips of well seasoned soft wood generally 25

mm thick.

− It is made of wood.

Size of drawing board need to be larger than that of drawing paper.

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Drawing board size: Drawing boards are made in various sizes. The selection of

Drawing board depends on the size of drawing paper used. The sizes of Drawing

board recommended by Bureau of Indian Standards (B.I.S) are given below

The standard size of drawing board according to Indian Standard Institution (I.S.I.) is

given below

The standard size of drawing board according to Indian Standard Institution

(I.S.I.) S,

NO. Designation Size in mm length x width x

thickness

To be used with sheet sizes

1

2

3

4

D0

D1

D2

D3

1500 X 1000 X 25

1000 X 700 X 25

700 X 500 X 15

500 X 350 X 15

A0

A1

A2

A3

B. T-square: Is a key piece of equipment used in Technical Graphics. Its primary

function is to draw horizontal lines on your drawing sheet but it is also used as a

guide for set squares in order to draw vertical or angled lines. It gets its name from

its resemblance of the letter 'T'.

A T-square has two components, the long shaft which is called the 'blade' and the

short shaft called the 'stock' or the 'head'. The 'stock' uses the edge of the drawing

board for support. When using the T-square, the edge must be up against the edge

of the drawing board, otherwise your lines may be inaccurate.

C. The Set Square: There are two types of set squares and they are named according to

the angles present on each.

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Set squares are useful for drawing parallel lines and perpendicular lines

i. Drawing Parallel Lines: Lines that lie in the same plane and do not meet one another

are said to be parallel lines. In the accompanying diagram, the line AB is parallel to

the line CD. This is indicated by the similar arrows.

A ruler and set square can be used to draw parallel lines as described below.

- Position an edge of the set square against a ruler and draw a line along one of

the other edges.

- Slide the set square into a new position while keeping the ruler fixed exactly at

the same position.

- Draw a line along the same edge that was used in Step 1.

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Example

Use a ruler and set square to draw a line that is parallel to a given line, AB, and passes

through a given point, P.

Solution:

- Position an edge of the set square along the given line, AB.

- Place a ruler against one of the other edges.

- Slide the set square along the ruler until the edge used in Step 1 passes through

the given point P.

- Draw the line CD through P.

The line CD passes through the given point, P, and is parallel to the given line AB.

ii. Drawing Perpendicular Lines: Lines that are at right angles to each other are said to

be perpendicular lines. Note that a vertical line is perpendicular to the horizontal,

where as perpendicular lines can be drawn in any position. Bricklayers use a plumb-

bob to set out vertical lines and a spirit level to set out horizontal lines.

A set square can be used to draw a perpendicular at a point on a given line as

described below.

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- Set an edge of the set square on the given line so that the other edge is just in

contact with the point.

- Draw a line that passes through the given point with the help of the set square.

Example: Use a set square to draw a perpendicular to a given line, AB, through a point, P,

not on the line.

Solution:

- Set an edge of the set square on the given line so that the other edge is just in

contact with the point.

- Draw a line that passes through the given point with the help of the set square.

D. Adjustable Set Squares: Are set squares that you can adjust to any angle you want to

between 0 and 90°. They are incredible useful especially when you are working with

drawings that have angles other than the standard set square angles.

E. A protractor: Is a measuring instrument, typically made of transparent plastic or

metal, for measuring angles. Protractor can be flat, circular or semi circular. There

are two types of protractors.

- 180° Protractor used to draw angles from 0 to 180°

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- 360° Protractor used to draw angles from 0 to 360°

.

F. Instrument Box: Are useful in geometry and other allied subjects’ projects.

These boxes contain protractor, divider, compass, 2 set angles, sharpener, eraser,

pencil and a 15 cm ruler. The box is designed specifically to fit in all these products

easily.

G. Eraser (Rubber): An Eraser is used to remove unwanted lines form a drawing sheet.

Similar to pencils, eraser can come in hard and soft types. It is key that you get a Soft

eraser. Not only will a hard eraser remove your unwanted line it could possibly tear

your drawing sheet.

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H. Erasing Shield: Erasing shield is used to protect the adjacent lines on the drawing

when some part of a line is being erased. It is usually made of thin metal in which

gaps of different widths, curves, small circles, arcs, etc. are cut according to the lines

to be eraser.

I. Pencil: Pencils are used to draw different lines, shapes, symbols and to write texts in

engineering drawing. Based on the hardness of lead pencils are classified in three

major grades as hard, medium and soft.

They are further subdivided and numbered as mentioned in table below:

Pencils of Different Grades

Grades Items arranged ordering harder to softer

Hard 9H> 8H> 7H>6H>5H>4H

Medium 3H>2H>H>F>HB>B

Soft 2B>3B>4B>5B>6B>7B

The following grades are used in engineering drawing:

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J. Pencil Sharpener (Topper): Pencil Sharpener is used to sharpen the point of your

pencil before use and during use if you notice the point getting blunt. A good steel

pencil sharpener would achieve a good point.

K. Compass: It is used to draw circles and arcs both in pencil and ink. It consists of two

legs pivoted at the top. One leg is Equipped with a steel needle attached with a

screw, and other shorter leg is, provided with a socket for detachable inserts.

L. Dividers: Used chiefly for transferring distances and occasionally for dividing spaces

into equal parts. I.e. for dividing Curved and straight lines into any number of equal

parts, and for transferring measurements.

M. French curve: It is used to draw irregular curves that are not circle arcs. The shape

varies according to the shape of irregular curve.

N. Drawing Paper: Drawing paper is the paper, on which drawing is to be made. All

engineering drawings are made on sheets of paper of strictly defined sizes, which are

set forth in the respective standards. The use of standard size saves paper and

ensures convenient storage of drawings.

Desirable properties a good drawing paper:

- It should be smooth and uniform in thickness.

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- It should be thick, strong and tough.

- Fibers of drawing paper should not be disintegrated when a good eraser is used

on it.

LO 1.2 –Understand the graphic language techniques

Content/Topic 1: Importance of Graphic Language.

You can easily understand that …

- Graphics are visual images or designs on some surface, such as a wall, canvas,

screen, paper, or stone to inform, illustrate, or entertain

- The word languages are inadequate for describing the size, shape and features

completely as well as concisely.

- Composition of Graphic Language: Graphic language in “engineering

application” use lines to represent the surfaces, edges and contours of objects.

-

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Graphic design is an important tool that enhances how you communicate with other

people. It serves to convey your ideas in a way that is not only effective, but also beautiful.

Here are just a few factors to consider before investing in graphic design.

The graphic language had its existence when it became necessary to build new structures

and create new machines or the like, in addition to representing the existing ones. In the

absence of graphic language, the ideas on technical matters have to be conveyed by speech

or writing, both are unreliable and difficult to understand by the shop floor people for

manufacturing.

This method involves not only lot of time and labour, but also manufacturing errors.

Without engineering drawing, it would have been impossible to produce objects such as

aircrafts, automobiles, locomotives, etc., each requiring thousands of different components.

The language is known as “drawing” or “drafting”. A drawing can be done using freehand,

instruments or computer methods.

A. Freehand drawing: The lines are sketched without using instruments other than

pencils and erasers. Example:

B. Computer drawing: The drawings are usually made by commercial software such as

AutoCAD, solid works etc.

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Content/Topic 2: Need for Correct Drawings.

The drawings prepared by any technical person must be clear, unmistakable in meaning and

there should not be any scope for more than one interpretation, or else litigation may arise.

In a number of dealings with contracts, the drawing is an official document and the success

or failure of a structure depends on the clarity of details provided on the drawing. Thus, the

drawings should not give any scope for miss interpretation even by accident.

It would not have been possible to produce the machines/automobiles on a mass scale

where a number of assemblies and sub-assemblies are involved, without clear, correct and

accurate drawings. To achieve this, the technical person must gain a thorough knowledge of

both the principles and conventional practice of drawing. If these are not achieved and or

practiced, the drawings prepared by one may convey different meaning to others, causing

unnecessary delays and expenses in production shops.

Hence, an engineer should posse’s good knowledge, not only in preparing a correct drawing

but also to read the drawing correctly. The course content of this book is expected to meet

these requirements. The study of machine drawing mainly involves learning to sketch

machine parts and to make working and assembly drawings. This involves a study of those

conventions in drawings that are widely adopted in engineering practice.

Content/Topic 3: Drawing Terminology.

A. Lines: Are straight elements that have no width, but are infinite in length

(magnitude), and they can be located by two points which are not on the same spot

but fall along the line. Lines may be straight lines or curved lines

B. Angle: An angle is formed by the intersection of two lines.

C. Technical Drawings: It is the art of representation of an object by systematic lines on

paper. The graphical representation of any object or idea can be termed as drawing.

It is a formal and precise way of communicating information about the shape, size,

features and precision of physical object.A clear, precise language used in the design

process for communicating, solving problems, quickly and accurately visualizing

objects, and conducting analyses

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D. Dimensioning: Is the process of specifying part’s information by using of figures,

symbols and notes.

E. Scale: The proportion by which we either reduce or increase the actual size of the

object on a drawing is known as Scale.

F. Projection: Is the process in which the rays of sight are taken in a particular direction

from an object to form an image on a plane.

G. View: The image formed on a picture plane by projecting rays of sight is called a

view.

H. Projector: The lines or rays drawn from the object to the plane are called projectors.

I. Sectioning: It is defined as an imaginary cut made through an object to expose the

interior or to reveal the shape of a portion.

J. Cutting Plane: Cutting plane is a plane that imaginarily cuts the object to reveal the

internal features.

K. Engineers: People who use technical means to solve problems. They design

products, systems, devices, and structures to improve our living conditions.

LO 1.3 – Classify different types of drawing.

Content/Topic 1 Machine drawing.

A. Machine drawing: It is pertaining to machine parts or components. It is presented

through a number of orthographic views, so that the size and shape of the

component is fully understood. Part drawings and assembly drawings belong to this

classification. An example of a machine drawing is given in Fig.

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Content/Topic 2: Production drawing.

A. Production drawing: A production drawing, also referred to as working drawing,

should furnish all the dimensions, limits and special finishing processes such as heat

treatment, honing, lapping, surface finish, etc., to guide the craftsman on the shop

floor in producing the component. The title should also mention the material used

for the product, number of parts required for the assembled unit, etc.

Since a craftsman will ordinarily make one component at a time, it is advisable to

prepare the production drawing of each component on a separate sheet. However,

in some cases the drawings of related components may be given on the same sheet.

Figure 1.2 represents an example of a production drawing.

Content/Topic 3: Parts drawing.

Component or part drawing is a detailed drawing of a component to facilitate its

manufacture. All the principles of orthographic projection and the technique of graphic

representation must be followed to communicate the details in a part drawing. A part

drawing with production a detail is rightly called as a production drawing or working

drawing.

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Content/Topic 4: Assembly drawing.

A drawing that shows the various parts of a machine in their correct working locations is an

assembly drawing (Fig. 1.3). There are several types of such drawings.

A. Design assembly drawing: When a machine is designed, an assembly drawing or a

design layout is first drawn to clearly visualise the performance, shape and

clearances of various parts comprising the machine.

B. Detailed assembly drawing: It is usually made for simple machines, comprising of a

relatively smaller number of simple parts. All the dimensions and information

necessary for the construction of such parts and for the assembly of the parts are

given directly on the assembly drawing. Separate views of specific parts in

enlargements, showing the fitting of parts together, may also be drawn in addition

to the regular assembly drawing.

C. Sub- assembly drawing: Many assemblies such as an automobile, lathe, etc., are

assembled with many pre-assembled components as well as individual parts. These

pre-assembled units are known as sub-assemblies. A sub-assembly drawing is an

assembly drawing of a group of related parts that form apart in a more complicated

machine. Examples of such drawings are: lathe tail-stock, diesel engine fuel pump,

carburetor, etc.

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Learning Unit 2- Apply principles of drawing.

LO 2.1 –Identify drawing sheets.

Standards are set of rules that govern how technical drawings are represented.

Drawing standards are used so that drawings convey the same meaning to everyone who

reads them.

Content/Topic 1: Lines.

Lines: Are straight elements that have no width, but are infinite in length (magnitude), and

they can be located by two points which are not on the same spot but fall along the line.

Lines may be straight lines or curved lines

Lines of different types and thicknesses are used for graphical representation of objects. The

types of lines and their applications are shown in Table 2.4. Typical applications of different

types of lines are shown in Figs. 2.5 and 2.6.

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Types of lines and their application

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A. Meaning of Lines,

a) Visible lines/outline: represent features that can be seen in the current

view(drawn dark and continuous)

b) Hidden lines: represent features that cannot be seen in the current view.

c) Center line: represents symmetry, path of motion, centers of circles, axis of

ax symmetrical parts.

d) Dimension line:Thin continuous line terminated by arrowheads touching the

outlines, extension lines or center lines (used to specify end points of a

dimension)

e) Extension lines:An extension of an outline (drawn light and continuous, used

to indicate the entity being dimensioned)

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B. Precedence of Lines

1. When a Visible Line coincides with a Hidden Line or Center Line, draw the Visible

Line. Also, extend the Center Line beyond the outlines of the view.

2. When a Hidden Line coincides with a Center Line, draw the Hidden Line.

3. When a Visible Line coincides with a Cutting Plane, draw the Visible Line.

4. When a Center line coincides with a Cutting Plane, draw the Center Line and

show the Cutting Plane line outside the outlines of the view at the ends of the

Center Line by thick dashes.

Content/Topic 2: Sheet size.

Engineering drawings are to be prepared on standard size drawing sheets. The correct shape

and size of the object can be visualized from the understanding of not only the views of it

but also from the various types of lines used, dimensions, notes, scale, etc.

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To provide the correct information about the drawings to all the people concerned, the

drawings must be prepared, following certain standard practices, as recommended by

Bureau of Indian Standards (BIS).

The basic principles involved in arriving at the sizes of drawing sheets are:

(a) X: Y = 1:2, (b) XY = 1 where X and Y are the sides of the sheet. For a reference size A0

(Table 2.1) having a surface area of 1 m2, X = 841 mm and Y = 1189 mm. The

successive format sizes are obtained either by halving along the length or doubling

along the width, the areas being in the ratio 1:2

Drawing sheet formats

Content /Topic 3: Designation of Sizes.

The original drawing should be made on the smallest sheet, permitting the necessary clarity

and resolution. The preferred sizes according to ISO-A series (First choice) of the drawing

sheets are given in Table 2.1.

When sheets of greater length are needed, special elongated sizes (Second choice) are used

(Table 2.2).

These sizes are obtained by extending the shorter sides of format of the ISO-A series to

lengths that are multiples of the shorter sides of the chosen basic format.

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Content /Topic 4: Title block.

The title block should lie within the drawing space such that, the location of it, containing

the identification of the drawing, is at the bottom right hand corner. This must be followed,

both for sheets positioned horizontally or vertically.

The direction of viewing of the title block should correspond in general with that of the

drawing. The title block can have a maximum length of 170 mm. Figure 2.3 shows a typical

title block, providing the following information:

1. Title of the drawing

2. Sheet number

3. Scale

4. Symbol, denoting the method of projection

5. Name of the firm

6. Initials of staff drawn, checked and approved.

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NOTE: According to Bureau of Indian Standards, SP-46:1998, ‘‘Engineering Drawing Practice

for Schools and Colleges’’, First angle projection is preferred.

Content /Topic 5: Borders and frames.

Borders enclosed by the edges of the trimmed sheet and the frame, limiting the drawing

space, should be provided with all sheet sizes. It is recommended that these borders have a

minimum width of 20 mm for the sizes A0 and A1 and a minimum width of 10 mm for the

sizes A2, A3 and A4 (Fig. 2.4). A filing margin for taking perforations, may be provided on the

edge, and far left of the title block.

Content/Topic 6 Centering Marks.

Four centering marks may be provided, in order to facilitate positioning of the drawing

when reproduced or microfilmed. Two orientation marks may be provided to indicate the

orientation of the drawing sheet on the drawing board (Fig. 2.4).

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Content/Topic 7: Metric Reference Graduation.

It is recommended to provide a figure-less metric reference graduation, with a minimum

length of 100 mm and divided into 10 intervals on all the drawing sheets (Fig. 2.4) which are

intended to be microfilmed. The metric reference graduation may be disposed

symmetrically about acentring mark, near the frame at the border, with a minimum width of

5 mm.

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Content/Topic 8: Grid Reference System (Zoning).

The provision of a grid reference system is recommended for all the sizes, in order to permit

easy location on the drawing of details, additions, modifications, etc. The number of

divisions should be divisible by two and be chosen in relation to the complexity of the

drawing.

It is recommended that the length of any side of the grid should not be less than 25 mm and

not more than 75 mm. The rectangles of the grid should be referenced by means of capital

letters along one edge and numerals along the other edge, as shown in Fig. 2.4. The

numbering direction may start at the sheet corner opposite to the title block and be

repeated on the opposite sides.

Content/Topic 9: Trimming Marks.

Trimming marks may be provided in the borders at the four corners of the sheet, in order to

facilitate trimming. These marks may be in the form of right-angled isosceles triangles or

twoshort strokes at each corner (Fig. 2.4).

LO 2.2 Apply drawing scales

Content/Topic 1 Designation.

The complete designation of a scale should consist of the word Scale, followed by the

indication of its ratio as:

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− SCALE 1: 1 for full size,

− SCALE ×: 1 for enlarged scales,

− SCALE 1: × for reduced scales.

The designation of the scale used on the drawing should be shown in the title block.

Content/Topic 2: Recommended Scales.

The recommended scales for use on technical drawings are given in Table 2.3. The scale and

the size of the object in turn, will decide the size of the drawing.

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Content/Topic 3: Scale Specification.

If all drawings are made to the same scale, the scale should be indicated in or near the title

block. Where it is necessary to use more than one scale on a drawing, the main scale only

should be shown in the title block and all the other scales, adjacent to the item reference

number of the part concerned or near the drawings.

LO 2.3: Use Lines.

Content/Topic1: Thickness of Lines.

Two thicknesses of lines are used in draughting practice. The ratio of the thick to thin line

should not be less than 2:1. The thickness of lines should be chosen according to the size

and type of the drawing from the following range:

0.18, 0.25, 0.35, 0.5, 0.7, 1, 1.4 and 2

It is recommended that the space between two parallel lines, including hatching, should

never be less than 0.7 mm.

Content/Topic2: Order of Priority of Coinciding Lines.

When two or more lines of different types coincide, the following order of priority should be

observed:

1. Visible outlines and edges (Continuous thick lines, type A),

2. Hidden outlines and edges (Dashed line, type E or F),

3. Cutting planes (Chain thin, thick at ends and changes of cutting planes, type H),

4. Centre lines and lines of symmetry (Chain thin line, type G),

5. Centroidal lines (Chain thin double dashed line, type K),

6. Projection lines (Continuous thin line, type B).

The invisible line technique and axis representation should be followed as per the

recommendations given in Table 2.5.

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Table 2.5 Invisible lines

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Content/Topic 3: Termination of Leader Lines.

A leader is a line referring to a feature (dimension, object, outline, etc.). Leader lines should

terminate (Fig. 2.7),

a) With a dot, if they end within the outlines of an object,

b) With an arrow head, if they end on the outline of an object,

c) Without dot or arrow head, if they end on a dimension line.

A. Leader lines and notes

− Leader (or pointer) line – Thin continuous line connecting a note or

dimension figure with the feature to which it applies. One end of the leader

terminates in an arrowhead or dot.

− The arrowhead touches the outline while the dot is placed within the object

or on the outline

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− The other end of a leader is terminated in a horizontal line underlining the

note

B. Rules for leader lines

- A leader line is never drawn horizontal, vertical or curved

- It is drawn at an angle not less than 30o to the line that it touches

- When pointing to a circle or arc, it is drawn radially

L O 2.4: Respect drawing lettering standard rules

A. LETTER NUMBERING: The art of writing the alphabets A, B, C…Z and numbers such

as 1, 2, 3…0 etc, is known as lettering. It is an important part of drawing and is used

to write letters, dimensions, notes and other necessary information required to

complete execution of machine or structure, etc

I. Feature of lettering.

1. Uniformity

2. Neatness

3. Rapidity and proper spacing

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All lettering works are done either by freehand or by using drawing instruments. Skill

and proficiency in freehand lettering can be achieved by the proper selection of

appropriate sizes and style of lettering.

II. Height of letters and numerals: The height of letters and numerals

recommended for use in engineering drawing are 2.5, 3.5, 5, 7, 10, 14 and

20mm. Height of the letters and numerals will be different for different

purposes and may be selected suitably for their purpose.

III. Style of freehand lettering

− Vertical or upright freehand lettering

a) Single stroke vertical freehand lettering

b) Lower case vertical freehand lettering

Single stroke: The lettering in which the alphabets are written with a single

stroke of pencil or pen is called a single stroke.

Double stroke: The lettering in which the alphabets are written by double

stroke of the pencil or pen with uniform spacing in between strokes is called

double – stroke.

− Inclined or Italic freehand lettering

a) Single stroke italic freehand

b) Lower case Italic freehand

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a. Spacing of letters: The spacing means the distance which is to be left between the

two adjacent letters in all types of lettering.

Note:

- The space between each word should be kept equal to height of letter

- The space between the two lines should be left equal to twice the height of

letter.

- The space between the two lines should be kept not less than half or more than

one and a half times the height of letter

Legend:

H: Lettering height (capital letters)

C: Height of lower-case letters

A: Spacing between characters

B: Minimum spacing of base line

E: Minimum spacing between words

D: Thickness of lines

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Content/Topic 1: Dimensions.

The following specifications are given for the dimensions of letters and numerals:

i. The height of capital letters is taken as the base of dimensioning (Tables 2.6 and 2.7).

ii. The two standard ratios for d/h, 1/14 and 1/10 are the most economical, as they

result in a minimum number of line thicknesses.

iii. The lettering may be inclined at 15° to the right, or may be vertical.

Lettering A (d = h/14)

Characteristic Parameter Ratio Dimensions (mm)

Lettering Height (Height of capitals)

h (14/14)h 2.50 3.50 5.00 7.0 10.0 14 20.0

Height of lower case letters (without stem or tail)

c (10/14)h — 2.50 3.50 5.0 7.0 10 14.0

Spacing between characters a (2/14)h 0.35 0.50 0.70 1.0 1.4 2 2.8

Minimum spacing of base characters

b (20/14)h 3.50 5.00 7.00 10.0 14.0 20 28.0

Minimum spacing between words

e (6/14)h 1.05 1.50 2.10 3.0 4.2 6 8.4

Thickness of lines d (1/14)h 0.18 0.25 0.35 0.5 0.7 1 1.4

https://en.wikipedia.org/wiki/Cap_height

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Lettering B (d = h/10)

Characteristic Parameter Ratio Dimensions (mm)

Lettering Height

(Height of capitals) h (10/10)h 2.50 3.50 5.0 7.0 10 14.0 20

Height of lower case letters

(without stem or tail) c (7/10)h — 2.50 3.5 5.0 7 10.0 14

Spacing between characters a (2/10)h 0.50 0.70 1.0 1.4 2 2.8 4

Minimum spacing of base

characters b (14/10)h 3.50 5.00 7.0 10.0 14 20.0 28

Minimum spacing between

words e (6/10)h 1.50 2.10 3.0 4.2 6 8.4 12

Thickness of lines d (1/10)h 0.25 0.35 0.5 0.7 1 1.4 2

LO 2.5 Apply dimension rules

Content/Topic 1: General principles.

A Dimension is a numerical value expressed in appropriate units of measurement and used

to define the size, location, orientation, form or other geometric characteristics of a part.

In other words, indicating on a drawing, the sizes of the object and the other details

essential for its construction and function using lines, numerals, symbols, notes, etc., is

called dimensioning, Therefore, an engineering drawing, illustrating the shape, size and

relevant details most essential for the construction of the object.

The dimensions are given to indicate the sizes of the various features of the object and their

location. It is mainly used to know the object size, the name of parts and diameter of the

hole etc.Dimensions also communicate the tolerance (or accuracy) required

https://en.wikipedia.org/wiki/Cap_height

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- As far as possible, dimensions should be placed outside the view.

- Dimensions should be taken from visible outlines rather than from hidden lines.

- Dimensioning to a centre line should be avoided except when the centre line

passes through the centre of a hole.

- Each feature should be dimensioned once only on a drawing.

- Dimensions should be placed on the view or section that relates most clearly to

the corresponding features.

- Each drawing should use the same unit for all dimensions, but without showing

the unit symbol.

- No more dimensions than are necessary to define a part should be shown on a

drawing.

- No features of a part should be defined by more than one dimension in any one

direction.

Content/Topic 2: Method of Execution.

The elements of dimensioning include the projection line, dimension line, leader line,

dimension line termination, the origin indication and the dimension itself.

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1. Projection and dimension lines should be drawn as thin continuous lines.

2. Projection lines should extend slightly beyond the respective dimension lines.

3. Projection lines should be drawn perpendicular to the feature being dimensioned.

Where necessary, they may be drawn obliquely, but parallel to each other (Fig. 2.30).

However, they must be in contact with the feature.

4. Projection lines and dimension lines should not cross each other, unless it is

unavoidable (Fig. 2.31).

5. A dimension line should be shown unbroken, even where the feature to which it

refers, is shown broken (Fig. 2.32).

6. A center line or the outline of a part should not be used as a dimension line, but

maybe used in place of projection line (Fig. 2.31).

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Content /Topic 3: Termination and Origin Indication.

Dimension lines should show distinct termination, in the form of arrow heads or oblique

strokes or where applicable, an origin indication.

Two-dimension line terminations and an origin indication are shown in Fig. 2.33. In this,

1. The arrow head is drawn as short lines, having an included angle of 15°, which is

closed and filled-in.

2. The oblique stroke is drawn as a short line, inclined at 45°.

3. The origin indication is drawn as a small open circle of approximately 3 mm in

diameter.

The size of the terminations should be proportionate to the size of the drawing on which

they are used. Where space is limited, arrow head termination may be shown outside the

intended limits of the dimension line that is extended for that purpose. In certain other

cases, an oblique stroke or a dot may be substituted (Fig. 2.34).

Where a radius is dimensioned, only one arrow head termination, with its point on the arc

end of the dimension line, should be used (Fig. 2.35). However, the arrow head termination

may be either on the inside or outside of the feature outline, depending upon the size of

feature.

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Content/Topic 4: Methods of Indicating Dimensions.

Dimensions should be shown on drawings in characters of sufficient size, to ensure

complete legibility. They should be placed in such a way that they are not crossed or

separated by another line on the drawing. Dimensions should be indicated on a drawing,

according to one of the following two methods. However, only one method should be used

on any one drawing.

METHODS:

1. (Aligned System): Dimensions should be placed parallel to their dimension lines and

preferably near the middle, above and clear-off the dimension line (Fig. 2.36). An

exception may be made where superimposed running dimensions are used (Fig. b)

Dimensions may be written so that they can be read from the bottom or from the right side

of the drawing. Dimensions on oblique dimension lines should be oriented as shown in next

figure

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Fig. 2.37. Angular dimensions may be oriented as shown in Fig. 2.38.

2. (Uni-directional System): Dimensions’ should be indicated so that they can be read

from the bottom of the drawing only. On-horizontal dimension lines are interrupted,

preferably near the middle, for insertion of the dimension (Fig. 2.39).Angular

dimensions may be oriented as in Fig. 2.40.

Dimensions can be,

i. Above the extension of the dimension line, beyond one of the terminations, where

space is limited (Fig. 2.34) or

ii. At the end of a leader line, which terminates on a dimension line, that is too short to

permit normal dimension placement (Fig. 2.34) or

iii. Above a horizontal extension of a dimension line, where space does not allow

placement at the interruption of anon-horizontal dimension line (Fig. 2.41).Values of

dimensions, out of scale (except where break lines are used) should be underlined as

shown in Fig. 2.41.

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A. Placing of dimension

The following indications (symbols) are used with dimensions to reveal the shape

identification and to improve drawing interpretation. The symbol should precede the

dimensions (Fig. 2.42).

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B. Symbols and abbreviations used in Engineering Drawing

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Content/Topic 5: Arrangement of Dimensions.

The arrangement of dimensions (Dimensioning techniques) on a drawing must indicate

clearly the design purpose.

- Keep dimensions off the part to be dimensioned where possible.

- Arrange extension lines so the larger dimensions are outside of the smaller

dimensions.

- Stagger the dimension value labels to ensure they are clearly defined.

The following are the ways of arranging the dimensions.

A. Chain dimensions: Chains of single dimensions should be used only where the

possible accumulation of tolerances does not endanger the functional requirement

of the part (Fig. 2.43).

B.

C. Parallel dimensions: In parallel dimensioning, a number of dimension lines, parallel

to one another and spaced-out are used. This method is used where a number of

dimensions have a common datum feature (Fig. a). (Parallel dimensioning consists of

several dimensions originating from one projection line.)

i. Super –imposed running dimensions: These are simplified parallel dimensions

and may be used where there are space limitations (Fig. b).

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D. Combined dimensions: These are the result of simultaneous use of chain and

parallel dimensions (Fig. 2.45).

E. Co-ordinate dimensions: The sizes of the holes and their co-ordinates may be

indicated directly on the drawing; or they may be conveniently presented in a

tabular form, as shown in Fig. 2.46.

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F. Dimensioning Small Features: When dimensioning small features, placing the

dimension arrow between projection lines may create a drawing which is difficult to

read. In order to clarify dimensions on small features any of the above methods can

be used.

Content /Topic 6 Special Indications

A. Diameters: Diameters should be dimensioned on the most appropriate view to

ensure clarity. All dimensions of circles are preceded by the symbol. There are

several conventions used for dimensioning circles:

i. Shows two common methods of dimensioning a circle. One method dimensions

the circle between two lines projected from two diametrically opposite points.

The second method dimensions the circle internally.

ii. Is used when the circle is too small for the dimension to be easily read if it was

placed inside the circle. A leader line is used to display the dimension.

iii. The final method is to dimension the circle from outside the circle using an arrow

which points directly towards the center of the circle. The dimension value should

be the next figure shows the method of dimensioning diameters.

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iv. Chords, Arcs, Angles and Radius: The dimensioning of chords, arcs and

angles should be as shown in Fig. 2.48. Where the centre of an arc falls

outside the limits of the space available, the dimension line of the radius

should be broken or interrupted according to whether or not it is necessary

to locate the centre (Fig.2.35).

Where the size of the radius can be derived from other dimensions, it may be

indicated by a radius arrow and the symbol R, without an indication of the

value (Fig. 2.49).

v. Equidistant features: Linear spacings with equi-distant features may be

dimensioned as shown in Fig.2.50.

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vi. Chamfers and countersunk: Chamfers may be dimensioned as shown in Fig.

2.51 and countersunks, as shown in Fig. 2.52.

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LO 2.6: Design sections.

The technique of representing an object in a drawn form is referred to as projection.

Projection can be divided into pictorial (3- dimensional) projection and orthographic (2-

dimensional) projection.

- Pictorial projection is further divided into isometric, oblique and perspective

projections; while the

- Orthographic projection is divided into 1st angle and 3rd angle projection.

A. Orthographic projection: Is a means of representing a three-dimensional object in

two dimensions (2D). It uses multiple views of the object, from points of view

rotated about the object's center through increments of 90°.

The projection or view obtained on a plane of projection when the projectors are

parallel to each other, but perpendicular to the plane of projection, is known as

orthographic projection. While drawing the orthographic projections, the following

items should invariably exist:

1) The object to be projected

2) The projector

3) The plane of projections

4) The observer’s eye or station point.

The orthographic projection system is used to represent a 3D object in a 2D plane.

The orthographic projection system utilizes parallel lines, to project 3D object views

onto a 2D plane. According to the rule of orthographic projection. To draw a

projection view of a 3D object on a 2D Plane. The horizontal plane is rotated in the

clockwise direction.

i. Types of Orthographic projection systems. Types of Orthographic projection

systems are first angle and third angle projection.

− First Angle Projection: In the first angle projection, the object is

placed in the 1st quadrant. The object is positioned at the front of a

vertical plane and top of the horizontal plane. First angle projection is

widely used in India and European countries. The object is placed

between the observer and projection planes. The plane of projection

is taken solid in 1st angle projection.

Symbol –

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− Third Angle Projection: In the third angle projection, the object is

placed in the third quadrant. The object is placed behind the vertical

planes and bottom of the horizontal plane. Third angle projection is

widely used in the United States. The projection planes come

between the object and observer. The plane of projection is taken as

transparent in 3rd angle projection.

Symbol –

Difference between First Angle Projection and Third Angle Projection:

B. PLANE OF PROJECTION: The plane which is used for the purpose of projection is

called plane of projection, types of projection plane are:

1. REFERENCE PLANE: In general, two planes are employed for projection and are

known as reference planes or principal planes of projection. These planes

intersect at right angle to each other.

2. VERTICAL PLANE (V.P): The plane which is vertical is called vertical plane and is

denoted by V.P. Vertical plane is also known as frontal plane since front view is

projected on this plane

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3. HORIZONTAL PLANE (H.P): The plane which is horizontal but at right angle to the

V.P

4. AUXILIARY PLANE (A.P): Any other plane, placed at any angles to the principle

planes

5. PROFILE PLANE (P.P): The plane which is at right angles to the two-principle

plane is called auxiliary vertical plane (A.V.P) PP

C. Principle Views: In orthographic projection there are 6 principle views of an object,

front, top, L side, R side, rear, and back views. The three most commonly views

drawn on a technical drawing are the front, back, and side views most other views

are not needed. Other views may be needed in order for the per-son who is creating

the Part, to better visualize it in order to properly manufacture it. A well viewed

Type of Views.

− Front view or elevation

− Rear view

− Top view or plan

− Bottom view

− Right view

− Left view or Side view or Side elevation or profile view

− Auxiliary view: The object is projected on an auxiliary plane

The views are named as follows:

− F − Front view or main view/elevation

− Sl − Side view from the left or left side view

− T − Top view

− R − Rear view

− Sr − Side view from the right- or right-side view

− B − Bottom view

D. Principle of the first Angle Projection: In first angle projection, the object is

imagined to be positioned in the first quadrant. The view from the front of the object

is obtained by looking at the object from the right side of the quadrant and tracing in

correct sequence, the points of intersection between the projection plane and the

rays of sight extended. The object is between the observer and the plane of

projection (vertical plane). Here, the object is imagined to be transparent and the

projection lines are extended from various points of the object to intersect the

projection plane. Hence, in first angle projection, any view is so placed that it

represents the side of the object away from it.

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E. Methods of Obtained Orthographic Views

a) View from the front: The view from the front of an object is defined as the

view that is obtained as projection on the vertical plane by looking at the

object normal to its front surface. It is the usual practice to position the

object such that its view from the front reveals most of the important

features. Figure 3.1 shows the method of obtaining the view from the front

of an object.

b) View from Above: The view from above of an object is defined as the view

that is obtained as projection on the horizontal plane, by looking the object

normal to its top surface. Figure 3.2 shows the method of obtaining the view

from above of an object.

c) View from the side: The view from the side of an object is defined as the

view that is obtained as projection on the profile plane by looking the object,

normal to its side surface. As there are two sides for an object, viz., left side

and right side, two possible views from the side, viz., view from the left and

view from the right may be obtained for any object. Figure 3.3 shows the

method of obtaining the view from the left of an object.

F. Presentation of view: The different views of an object are placed on a drawing sheet

which is a two dimensional one, to reveal all the three dimensions of the object.

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For this, the horizontal and profile planes are rotated till they coincide with the vertical

plane. Figure 3.4 shows the relative positions of the views, viz., the view from the front,

above and the left of an object.

G. Designation and relative position of view: An object positioned in space may be

imagined as surrounded by six mutually perpendicular planes. So, for any object, six

different views may be obtained by viewing at it along the six directions, normal to

these planes. Figure 3.5 shows an object with six possible directions to obtain the

different views which are designated as follows:

1) View in the direction a = view from the front

2) View in the direction b = view from above

3) View in the direction c = view from the left

4) View in the direction d = view from the right

5) View in the direction e = view from below

6) View in the direction f = view from the rear

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Figure 3.6a shows the relative positions of the above six views in the first angle projection

and Fig.3.6b, the distinguishing symbol of this method of projection. Figure 3.7 a show the

relative position of the views in the third angle projection and Fig. 3.7b, the distinguishing

symbol of this method of projection.

NOTE A comparison of Figs. 3.6 and 3.7 reveals that in both the methods of projection, the

views are identical in shape and detail. Only their location with respect to the view from the

front is different

It is important to understand the significance of the position of the object relative to the

planes of projection.

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To get useful information about the object in the orthographic projections, the object may

be imagined to be positioned properly because of the following facts:

1. Any line on an object will show its true length, only when it is parallel to the plane of

projection.

2. Any surface of an object will appear in its true shape, only when it is parallel to the

plane of projection.

In the light of the above, it is necessary that the object is imagined to be positioned such

that its principal surfaces are parallel to the planes of projection.

H. Selection of views: For describing any object completely through its orthographic

projections, it is important to select a number of views. The number of views

required to describe any object will depend upon the extent of complexity involved

in it. The higher the symmetry, the lesser the number of views required.

One view drawing: Some objects with cylindrical, square or hexagonal features or, plates of

any size with any number of features in it may be represented by a single view. In such

cases, the diameter of the cylinder, the side of the square, the side of the hexagon or the

thickness of the plate may be expressed by a note or abbreviation. Square sections are

indicated by light crossed diagonal lines. Figure 3.12 shows some objects which may be

described by one-view drawings.

I. Two -view drawing: Some objects which are symmetrical about two axes may be

represented completely by two views normally; the largest face showing most of the

details of the object is selected for drawing the view from the front. The shape of the

object then determines whether the second view can be a view from above or a side

view. Figure 3.13 shows the example of two-view drawings.

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J. Three-view drawing: In general, most of the objects consisting of either a single

component or an assembly of a number of components are described with the help

of three views. In such cases, the views normally selected are the views from the

front, above and left or right side. Figure 3.14 shows an object and its three

necessary views.

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K. Development of missing view: When two views of an object are given, the third

view may be developed by the use of a miter line. To construct the view from the

left, from the 2 given views-Construction (Fig. 3.15)

1. Draw the views from the front and above.

2. Draw the projection lines to the right of the view from above.

3. Decide the distance, D from the view from the front at which, the side view is

to be drawn.

4. Construct a miter line at 45°.

5. From the points of intersection between the miter line and the projection

lines, draw vertical projection lines.

6. Draw the horizontal projection lines from the view from the front to intersect

the above lines. The figure obtained by joining the points of intersection in

the order is the required view. Figure 3.16 shows the steps to be followed in

constructing the view from above of an object, from the given views from the

front and left.

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NOTE These exercises are aimed at improving the practice in reading and

developing the imagination of the student.

Spacing the views

The views of a given object must be positioned on the drawing sheet so as to give a good

and balanced appearance. Keeping in view, (i) number of views, (ii) scale and (iii) space

between the views, the draughtsman should decide about the placement of views on the

drawing sheet. Sufficient space between the views must be provided to facilitate placement

of dimensions, notes, etc., on the drawing without overcrowding.

Content/Topic 1: Hatching of Sections.

Hatching is generally used to show areas of sections. The simplest form of hatching is

generally adequate for the purpose, and may be continuous thin lines (type B) at a

convenient angle, preferably 45°, to the principal outlines or lines of symmetry of the

sections (Fig. 2.11).

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Precautions in hatching: Hatching lines are spaced at 1mm to 3mm and inclined at 30, 45,

or 60 depending on the needs, but 45 degrees is the most preferred.

i. Hatching a single object: When you are hatching an object, but the objects have

areas that are separated. All areas of the object should be hatched in the same

direction and with the same spacing.

ii. Hatching Adjacent objects: When hatching assembled parts, the direction of the

hatching should ideally be reversed on adjacent parts. If more than two parts

are adjacent, then the hatching should be staggered to emphasise the fact that

these parts are separate.

A. Section Lines:

- Section lines can be used to show different types of materials or different parts of

the same material.

- Refer to Technical Graphics text for a complete list

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Content/Topic 2: Cutting Plane.

A long chain thick line is used to indicate the cutting plane as well as the direction of view.

On the two ends of this line are indicated a letter which gives the name of the section. This

letter (L-L) in our example below is very important and helps to differentiate among

sections, especially when we have many sections represented on a drawing. The cutting

plane should be identified by capital letters and the direction of viewing should be indicated

by arrows.

For example, on the following drawing they are two sections H-H and G-G. So, each will be

represented apart.

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Content /Topic 3: Revolved or Removed Sections Cross sections may be revolved in the relevant view or removed. When revolved in the

relevant view, the outline of the section should be shown with continuous thin lines. When

removed, the outline of the section should be drawn with continuous thick lines. The

removed section may be placed near to and connected with the view by a chain thin line or

in a different position and identified in the conventional manner.

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Content/Topic 4: Half section.

A half sectional view is preferred for symmetrical objects. For a half section, the cutting

plane removes only one quarter of an object. For a symmetrical object, a half sectional view

is used to indicate both interior and exterior details in the same view. Even in half sectional

views, it is a good practice to omit the hidden lines. Figure 4.4a shows an object with the

cutting plane in position for obtaining a half sectional view from the front, the top half being

in section. Figure 4.4b shows two parts drawn apart, exposing the inner details in the

sectioned portion. Figure 4.4c shows the half sectional view from the front. It may be noted

that a centre line is used to separate the halves of the half section. Students are also advised

to note the representation of the cutting plane in the view from above, for obtaining the

half sectional view from the front.

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Content/Topic 5 local section.

A local section may be drawn if half or full section is not convenient. The local break may be

shown by a continuous thin free hand line (Fig. 2.24).

Content /Topic 6Arrangement of Successive Sections.

LO 2.7: Apply drawing conventional representations dimensioning and

standard abbreviations

Content/Topic 1: Material

Certain draughting conventions are used to represent materials in section and machine

elements in engineering drawings.

A. Material: As a variety of materials are used for machine components in engineering

applications, it is preferable to have different conventions of section lining to

differentiate between variousmaterials. The recommended conventions in use are

shown in next table.

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Content/Topic 2 Machine Components.

When the drawing of a component in its true projection involves a lot of time, its

convention may be used to represent the actual component. Figure 2.27 shows typical

examples of conventional representation of various machine components used in

engineering drawing.

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Reference(s):

1. DR.K.L. Narayana, (2006). Machine Drawing. INDIAN INSTITUTE OF

TECHNOLOGYCHENNAI-600 036, INDIA: New Age International (P) Ltd

2. Ulrich F, (2008). Mechanical and Metal Trades Handbook. Germany, Media Print

International technologies.

tvet certificate iii in fitting and welding

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