Chapter - 1A
DRAWING INSTRUMENTS
AND THEIR USES
Drawing Instruments are used to prepare neat and accurate Drawings. To a greater extent, the accuracy of the Drawings depend on the quality of instruments used to prepare them. The following is the list
of Drawing Instruments and other materials required.
a)
Drawing Board
b)
T-square or Drafter (Drafting machine)
c)
Set Squares
d)
Protractor
e)
Drawing Instrument Box
f)
Drawing Sheet
g)
Drawing Pencils
h)
Drawing Pins/Clips
a) Drawing Board :
Drawing board is made from strips of well seasoned soft wood generally 25 mm thick. It is cleated
at the back by two battens to prevent warping. One of the shorter edges of the rectangular board is provided with perfectly straight ebony edge which is used as working edge on which the T-square is moved
while making Drawings.
Battens
Working edge
(ebony)
Fig. 1A.1
Note: When Minidrafter (Drafting machine) is used to prepare Drawings, the working edge is not
used.
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Computer Aided Engineering Drawing
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) is given
below
Standard size of Drawing boards
Designation
Size (mm)
Designation
Size (mm)
B0
1500 × 1000
B3
500 × 350
B1
1000 × 700
B4
250 × 350
B2
700 × 500
b) T-square :
T-squares are made from hard wood. A T-square consists of two parts namely the stock and the
blade joined together at right angles to each other by means of screws and Pins as shown in figure 1A.2. The
stock is made to slide along the working edge and the Blade moves on the Drawing board.
Working edge
Stock
Blade
Screws
Fig. 1A.2
The working edge of T-square is used to draw parallel lines, vertical lines or inclined lines at 30°, 60o
to the horizontal using set squares.
Drafting machine (or Drafter):
In a Drafting machine, the uses and advantages of T-square, set square, scales, protractors are
combined. One end of the Drafter is clamped at the left top end of the Drawing board by a screw provided
in the drafter.
An adjustable head with a Protractor is fitted at the other end of the Drafter. Two blades made of
transparent celluloid material are fitted to the adjustable head and are perfectly perpendicular to each other.
These blades are used to draw parallel, horizontal, vertical and inclined lines. The blades always move parallel
to the edges of the board. Use of Drafting machine helps in reducing the time required to prepare Drawing.
Drawing Instruments and their Uses
3
Parallel bar arm
Scale
Angle
Parallel bar arm
Scale
Potractor
Drawing sheet
Drawing board
Fig. 1A.3
c) Set Squares :
Set squares are generally made from Plastic or celluloid material. They are triangular in shape with
one corner, a right angle triangle. A pair of set squares (30°–60°) and 45° (45° set square are generally provided with Protractor) facilitate marking of angles as shown in figures 1A.4 and 1A.5.
They are used to draw lines at 30°, 60° and 45° to the vertical and horizontal.
mm
mm
11
11
10
10
9
9
8
8
7
7
6
6
5
5
4
4
3
3
2
2
3
4
5
6
7
0
8
9
Fig. 1A.4. 45 Set Square
10
11
1
2
3
4
5
6
mm
2
mm
1
1
1
Fig. 1A.5. 30 −60 Set Square
0
0
Computer Aided Engineering Drawing
3
3
2
2
1
1
4
5
4
4
5
13
0
11
10 0
170 180
20
0 15
30
180 17 0 1 6
9
0
50
0
0 16
20
12
10
60
8
10 0 11
0
80 7
0
01
40
90
100
40
30
80
15
10
10
0 110
0
13
2
01
14
40
8
9
11
0
70
60
7
6
6
7
50
m
m
m
m
Fig. 1A.6. 450 Set Square with Protractor
d) Protractor:
Protractors are used to mark or measure angles between 0 and 180°. They are semicircular in shape
(of diameter 100mm) and are made of Plastic or celluloid which has more life. Protractors with circular shape
capable of marking and measuring 0 to 360° are also available in the market.
70
60
12 0
50
10 0
90
100
80
0
14
0
30
20
20
1 60
16 0
15
30
10
1 70
0
180
10
0
0
17 0
13
50
0
15
0
12 0
60
40
180
11 0
70
0
40
80
14
13
11 0
Fig. 1A.7
e) Drawing Instrument Box :
It consists of the following
a) Large size compasses,
b) Large size divider,
c) Small size bow pen, bow divider, and
d) Lengthening bar
f) Drawing sheet:
They are available in many varieties and good quality paper with smooth surface should be selected
for Drawings which are to be preserved for longer time. Sizes of Drawing Sheets recommended by Bureau
of Indian Standards (B.I.S) is given below,
Drawing Instruments and their Uses
5
Standard size of Drawing sheet
Designation
A0
Size (mm)
1189 × 841
Designation
A3
Size (mm)
420 × 297
A1
841 × 594
A4
297 × 210
A2
594 × 420
A5
210 × 148
g) Drawing Pencils:
The accuracy and appearance of a Drawing depends on the quality of Pencil used to make Drawing. The grade of a Pencil lead is marked on the Pencil. HB denotes medium grade. Increase in hardness
is shown by value put in front of H such as 2H, 3H etc., Softer pencils are marked as 2B, 3B, 4B etc. A
Pencil marked 3B is softer than 2B and Pencil marked 4B is softer than 3B and so on. Beginning of a Drawing may be made with H or 2H. For lettering and dimensioning, H and HB Pencils are used.
h) Drawing Pins and clips:
These are used to fix the Drawing sheet on the Drawing board.
Compass:
Compass is used for drawing circles and arcs of circles. The compass has two legs hinged at one
end. One of the legs has a pointed needle fitted at the lower end where as the other end has provision for
inserting pencil lead. Circles upto 120mm diameters are drawn by keeping the legs of compass straight. For
drawing circles more than 150 mm radius, a lengthening bar is used. It is advisable to keep the needle end
about 1mm long compared to that of pencil end so that while drawing circles, when the needle end is pressed
it goes inside the drawing sheet by a small distance (approximately 1mm). Refer fig. 1A.8.
Fig. 1A.8 Compass
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Computer Aided Engineering Drawing
Divider:
A Divider has two legs of equal length with two steel needles. The legs are hinged at a common
point. A Divider is used to divide curve or straight lines into required number of equal parts. It is used to
transfer dimensions from one part of drawing to the other. Dividers are also used to set off given distances
from the scale to the drawing. Refer fig. 1A.9.
Fig. 1A.9 Divider
Chapter - 1B
LINE TYPES, DIMENSIONS
AND LETTERING
Lines:
Figure 1B.1 shows various lines used in Engineering Drawing.
(i) A
Thick line
(ii) B
A
Thick line
Medium thickness
B
Dotted line or Dashed line
(iii) C
Thin long chains
C
Centre lines
(iv) D
Thin line
D
Dimension line / Extension lines
(v) E
E
Section Plane lines
(vi) F
F
Long Break lines
(vii) G
G
Continuous thin wavy lines
Fig. 1B.1
Visible edges and surface boundaries (outlines) of objects are shown using continuous thick lines shown
by line A-A.
Line B-B shows dotted lines or dashed lines used to show hidden edges of objects. These lines are
of medium thickness.
Centre lines shown by C-C are thin and long chains used to indicate the axes of solids such as Prisms,
cones and Pyramids, lines of symmetry, pitch circles etc. These lines extend for a short distance beyond the
outlines.
Line D-D shows thin and continuous lines called Dimension lines used to show the Dimension. These
lines are terminated at the outer ends by pointed arrow heads touching the out lines. These lines are also used
to show projection lines, leader lines, hatching lines etc.
Section plane lines or cutting plane lines are thin and long chain lines. At the ends, the lines are marked
thick. Refer line E-E.
Long break lines are thin lines with short zig-zags. As the name indicates, they are used to show long
breaks.
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Computer Aided Engineering Drawing
Continuous thin wavy lines are used to show, short break lines, irregular boundary lines etc.
Dimensions of arrow head :
Arrow heads are used at the ends of the Dimension lines as shown below. The arrow heads are pointed
at the ends. The length L of arrow head is usually three times the width b. For clarity, arrow heads are filled
freehand.
b
L
100
L = 3b
Fig. 1B.2
Dimensioning:
Dimensioning is done to show the size of objects. There are two systems of Dimensioning. They are,
i) Aligned system and
ii) Unidirectional system.
i) Aligned system of dimensioning
40
20
20
40
φ10
15
20Sq
40
50
60
120Sq
Aligned system of Dimensioning
Fig. 1B.3
Linetypes, Dimensions and Lettering
9
In this system of Dimensioning, the dimensions are placed above the Dimension lines so that they may
be read either from the bottom or from the right hand side of Drawing as shown in figure 1B.3.
ii) Unidirectional system of dimensioning
As the name indicates, in this system of Dimensioning, all the Dimensions are placed in one direction
only so that they may be read from the bottom of the Drawing only. This is shown in figure 1B.4.
It is very important to note that the non-horizontal Dimension lines are interrupted in the middle to
facilitate insertion of Dimensions.
φ10
40
20
20
40
20Sq
15
40
50
60
120Sq
Unidirectional system of dimensioning
Fig. 1B.4
Principles of Dimensioning:
a)
As far as possible, the Dimensions should be placed outside the Drawing.
b)
They should be placed at least 8 mm from the outlines and also in between dimension lines.
c)
Dimensions should not be shown between dotted lines.
d)
Dimension indicated in one view usually not repeated in other view/views excepting where it is a must
for the purpose of identification, clarity or both.
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Computer Aided Engineering Drawing
e)
Dimensions should be shown in one unit only such as mm. The unit need not be shown for every Dimension. It can be shown in a prominent place in the Title block of Drawing sheet as “Dimensions in mm”
f)
While Dimensioning, it is necessary to show the minimum necessary dimensions to describe the object.
Conventions used to represent materials:
While making Drawings involving materials such as steel, concrete, glass, etc., the Engineer or
Draughtsman uses conventions as shown in figure below.
Steel, Cast Iron, Copper and its alloys,
Aluminium and its alloys, etc.
Metals
Tin, Lead, White-metal, Zinc, etc.
Marble, Porcelain, Stoneware, Slate, etc.
Glass
Glass
Packing and
Insulating material
Asbestos, Felt, Fibre, Synthetic resin products,
Paper, Rubber, Cork, Linoleum, Leather,
Wax, Insulating and Filling materials, etc.
Wood
Wood, Plywood, etc.
Liquids
Oil, Water, Petrol, Kerosene, etc.
Concrete
A mixture of Cement, Sand and Gravel