ENGINEERING

DRAWING

LECTURE 4

Conventions

• Convention or Code:

• The representation of any matter by some sign or mark on the drawing is known

as convention or code.

• The convention make the drawing simple and easy to draw.

Convention for Lines

• Since, engineering drawing is the systematic combination of different types

of lines, it is therefore, essential for the students to have clearly in mind the

difference between the various types of lines.

Detailed Description and Uses of Various

Lines

1. Visible Outline or Object Line: The outline or object line is represented

by thick line and used to show the outer visible feature of the object in the

drawing. Every edge or surface that is visible is represented by these lines.

2. Section Line or Hatching Line: It is a thin continuous line and is used

for the purpose of sectioning an object.

(Note: The section lines are drawn at angle of 45° to the horizontal line and are

spaced uniformly from 2 to 4 mm apart depending upon the size of the object)

3. Centre Line, Locus Line, Pitch Circles, Extreme Position of

Moveable Parts and Parts Situated Infront of Cutting Plane: These

lines are represented by long and short dashes in proportion ranging from

6:1 to 4:1, closely and evenly spaced in any drawing. The Proportional once

selected should be maintained through the drawing these are used to show

the centre and location of cylindrical, conical and spherical object.

The following rules should be kept in mind while drawing centre lines of various

objects:-

i. The centre lines should not end at out line representing surfaces but should extend

approximately from 2 to 5 mm beyond the out lines of the object

ii. Where centre lines cross, the short dashes should ntersect at symmetrical. In case of very

small circles, the short dashes should be neglected while drawing centre lines.

4. Hidden Line: The hidden line is represented by short dashes, closely and evenly spaced. It is used to show the invisible for hidden parts on the drawing.

5. Construction Line: It is a thin continuous line and is used for constructing an object. These Lines do not appear in finished drawing except in geometrical drawing.

6. Dimension Line: It is thin continuous line, for giving dimension. This line terminates arrow heads where the dimension lines meet the extension lines.

7. Extension Line: It is a thin continuous line used for dimensioning an object.

8. Projectors Line: It is a thin continuation of outlines and is used for drawing projector.

9. Cutting Plane Line: The cutting plane line is represented by thick long

line at the ends with thin long and short lines at the centre. It is used to

show the edge of the cutting plane.

10. Short Break Line: The short break line is represented by thin free-hand

and is used to show the break of an object for a short length. It results in a

saving in space and time used for drawing without loss of any details.

11. Long Break Line: The long line is represented by thin ruled line provided

with free-hand zig-zags at suitable intervals and is used to show the break

for a considerable length of the object.

Conventions For Various Materials

• In engineering practice, there are different types of materials used for

manufacturing the various parts of a machine. It is therefore, desirable that

different conventions should be adopted to differentiate various materials for

convenience on the drawing. The conventions materials thus save time and

labour of the drawing work.

Dimensioning

• The art of writing the various sizes or measurements on the finished drawing

of an object is known as dimensioning.

Notation of Dimensioning

• The notation of dimensioning consists of dimension lines, extension lines, arrow heads, dimension figures, notes, symbols, etc. These notations are explained below:-

1. Dimension Line: Dimension line is a thin continuous line used to indicate the measurement which is shown by figure in a space above the dimension line or space left in the dimension line.

2. Extension Line: Extension line is also a thin continuous line extending beyond the outline of the object. It should extend about 3 mm beyond the dimension line.There should be a visible gap of 1.5 mm between the feature’s corners and the end of the extension line.

3. Arrow Heads: Arrow heads are used to terminate dimension lines. These touch

the extension lines and indicates the extent of a dimension. The length of the

arrow-head is about three times its width, the space in the arrow-heads should be

filled in. The size of the arrow-heads should be proportionate to the thickness of

the lines of the drawing.

4. Dimension Figure: A numerical that indicates the size of a particular feature of

an object is called dimension figure.

5. Leader (Pointer Line): A leader is a thin continuous line drawn from note of the

figure to show where it applies. It is terminated by an arrow-head or a dot. The

arrow-head touches the outline, whereas the dot is placed within the outline of the

object. The leader is generally drawn at any convenient angle, usually 30°, 45° and

60° but of not less than 30°.

(Note: The use of long leaders should be avoided even if it is involves repetition dimensions or notes)

6. Notes: A note on drawing gives complete information regarding specific

operation relating to a feature. It is generally placed outside a view and read

in such a way that the drawing is viewed from the bottom edge.

7. Symbol: A symbol is the representation of any object by some mark on the

drawing. It is used to save time and labour of drawing work.

Spacing of Dimensions

Theory of Dimensioning

• An object may be considered to be made up of a number of geometrical

shapes such as prism, cylinder, pyramid, cone, sphere, etc. It then becomes

very simple to dimension these geometric forms in a manner that will show

their individual sizes and location to each other. Thus, the following two

types of dimensions are commonly used in engineering drawing:

1. Size Dimensions.

2. Location Dimensions.

Types of Dimensions

• Size Dimensions:

The dimensions which indicate the various sizes of the object such as length, breadth,

diameter, etc. are known as size dimensions.

• Location Dimensions:

The dimensions which locate the position of one feature with respect to the other feature

are known as location dimensions. Distances between the centre lines of the holes from

the edges of features are given by location dimensions.

System of Placing Dimensions

• Aligned System: In this system, all dimensions are so placed that they may

be read from the bottom or the right hand edges of the drawing sheet. Here

all the dimensions are placed normal and above the dimensions lines (Latest

Method).

• Unidirectional System: In this system, all dimensions are so placed that

they may be read from bottom edge of the drawing sheet. Note, there is no

restriction on the controlling the direction of the dimension lines.

Unit of Dimensioning

• As far s possible, all dimensions should be given preferably in one unit only,

i.e in millimeters. The symbol of unit “mm” can therefore be omitted while

writing each dimensions but a note is added in a prominent place near the

tittle block that “All dimensions are in millimeters”.

Exercise