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INTRODUCTION
TO
ENGINEERING
DRAWING
Chapter 1
Chapter Details 1.1 Drawing Instruments and their
Uses
1.2 Letters and Numbers
1.3 Convention of Lines and their Applications
1.4 Scale (Reduced, Enlarged and Full Size). Plain Scale and Diagonal Scale
1.5 Assignments on Scales
1.6 Sheet Layout
1.7 Introduction to CAD
1.8 Geometric Construction
1-2 Engineering Graphics
1.1 DRAWING INSTRUMENTS AND THEIR USES
Engineering drawing is a language used in engineering field. Drawings are
prepared as per the ideas of the engineers and the work is carried out as per
those drawings by them or other engineers. Thus communication in engineering
field is done by drawing.
To prepare engineering drawings special drawing instruments are required.
The instruments should be of good quality. Engineering personnel should know
how to use, handle and maintain the drawing instruments.
The following drawing equipments and materials are required to prepare
drawings.
i. Drawing Board
ii. Mini-Drafter/T-square
iii. Set squares 45° and 30°-60°
iv. Instrument Box
v. Scales
vi. Protractor
vii. French curves
viii. Proportional Divider
ix. Clutch pencils/Wood pencils
x. Pencil sharpener
xi. Sand paper pad
xii. Pencil eraser
xiii. Erasing shield
xiv. Drawing Pins/Clips/Adhesive Tape
xv. Dusting Cloth/Hanker chief.
xvi. Drawing Paper
xvii. Roll-N-Draw
xviii. Templates
xix. Tracing Paper
xx. Tracing Cloth
Introduction to Engineering Drawing 1-3
i. Drawing Board
Drawing board is rectangular in shape and is made of strips of well
seasoned soft word or white pine of 25 mm thick and glued edge to edge to
prevent warping. Two battens are fixed at the back by means of screws fitted in the slots to allow seasonal contraction and expansion. One of the
edges of the board is used as working edge, on which
T-square is made to slide. The left hand edge of the board is grooved
throughout its length and perfectly straight ebony edge is fitted inside the
groove. This provides a perfect guide for the movement of T-square. The
top surface of the board should be free from cracks and it should be
absolutely flat. (Refer figure 1.1).
Drawing boards are made in various sizes. Its selection depends upon the size of the drawing paper to be used. For convenience of working, drawing
board is kept tilted about 15° to 20°.
TOP
WORKING EDGE
EBONY
BATTENS
Figure 1.1
The standard sizes of drawing boards available are as follows:
Sr. No. Designation Measurement Name
1. B0 1500 × 1000 Antiquarian
2. B1 1000 × 700 Double elephant
3. B2 700 × 500 Imperial
4. B3 500 × 350 Half Imperial
5. B4 350 × 250 Quarter Imperial
Drawing boards having sizes B2 and B3 are more common for the use in
Technical Schools and Engineering Colleges. Large size boards are used
in drawing offices and engineering firms.
1-4 Engineering Graphics
ii. Mini Drafter
Mini drafter is preferred by engineering students since it is a time saving
equipment for drawing work. Drafting machines are used extensively in
commercial drafting rooms because their use leads to 25 to 50 percent saving in time. It serves the purpose of T-square, Set-square, Protractor
and Scales in a single unit. It consists of arms, scale, grooved clamp and
locking knob (G). An aluminum scale blades are accurately set at right
angles to each other. They are graduated in mm and circular portion is in
degrees from 0° to 360°.
The scales can be rotated about the vertical axis of the knob. The locking
knob clamps the scale to the required angle.
Step 11 : To grip-clamp is inserted on the top edge of the board at left hand
side and is tightened just sufficient by the grip screw (k) to prevent slipping. Hard tightening may damage the drafter alignment or
board.
Step 22 : Drawing paper is fixed to the drawing board with the help of
drawing clips or pins, keeping drawing paper in line, with the
horizontal scale.
Step 33 : During operation, two blades are always remain parallel to the set
position if they are moved anywhere on the sheet.
Step 44 : For drawing lines at an angle, set required degree with reference
mark. (Refer figure 1.2)
K
G
Figure 1.2
Introduction to Engineering Drawing 1-5
T-square
T-square is made of two parts
a. The blade and,
b. The stock or Head.
Both are fastened together rigidly at right angles to each other. T-square is
made out of hard quality wood, plastic or acralic material. The stock is
placed adjoining the working edge of the board and is made to slide on it,
as and when required. The blade lies on the surface of the board. Its
working edge is made beveled. The length of the blade is selected as per
the size of the drawing board. Refer figure 1.3.
90°WORKING EDGE
STOCK BLADE
Figure 1.3
T-square is used to draw horizontal lines
The stock of the T-square is held firmly by left hand against the working
edge of the board and line is drawn from left to right. The pencil should be
used slightly inclined in the direction of line (to the right) while the pencil point should be kept close to the working edge of the blade.
(Refer figure 1.4)
Figure 1.4
1-6 Engineering Graphics
The working edge of the T-square is also used as a base for set-squares to
draw vertical, inclined or mutually parallel lines. T-square should never be
used on edge other than working edge of the board. It should always be
kept on the board, even when not in use.
iii. Set Squares: 45°°°° and 30°°°°-60°°°°
Now-a-days set squares are made of transparent colourless plastic or
celluloid material. There are two forms of set squares. They are triangular in
shape, having one right angled corner in each. The length of the set-square
is given by the longer edge except hypotenuse. 45° set squares are available
in 150,200,250 and 300 mm. lengths while the lengths of 30°-60° set squares
are 200,250,320, and 420 mm, Thickness of set squares varies from 1.5 to 2.5
mm. They are made with straight edge for pencil work and with beveled
edge for ink work. Now a days set squares are available along with
protractor or French curve carved within it. Adjustable set squares are also
available in the market. The 30°-60° set square of 250 mm length and 45° set
squares of 200 mm length are convenient sizes for use in schools and
colleges. (Refer figure 1.5)
Set squares are used for drawing straight lines except horizontal lines,
which are generally drawn with the T-square. Set squares are used with
T-square to draw lines at various angles with the horizontal. With
combination of T-square and two set squares angles of 15°, 30°, 45°, 60°, 75°,
90°, 105°, 120°, 150°, 165° (i.e. multiples of 15°) can be drawn. The above
combination can divide circle into 8, 12, and 24 equal parts without using
protractor. Parallel lines inclined at any angle can be drawn by using two set
squares. Set-squares must be kept clean and their edges should be protected
from getting damaged. Refer 1.6 (a, b, c, d).
Introduction to Engineering Drawing 1-7
30°
90° 60° 90° 45°
45°
Figure 1.5
30 DIVISIONS°
T-SQUARE
30°
60°
(a)
45 DIVISIONS°
T-SQUARE
(b)
45° 45°
Figure 1.6 (a & b)
Figure 1.6 (c & d)
15 DIVISIONS°
(d)
75°
15°
PARALLEL LINE
GIVEN LINE
(c)
15°
1-8 Engineering Graphics
iv. Instrument Box
The instrument box generally contains the following instruments.
Refer figure 1.7.
a. Large size compass in which pencil leg, pen leg and needle attachment are interchangeable.
b. Large size divider.
c. Small bow pencil.
d. Small bow pen.
e. Small bow divider.
f. Lengthening bar.
g. Inking pen.
h. Pen leg, needle attachment, croquill and compass leads etc.
Figure 1.7
Compass
Circles and arc of circles are drawn with the help of a compass. It consists
of two legs hinged together at its upper end. The pivoted steel needle is fitted at the lower end of the leg, while a pencil lead is inserted at the end
of other leg. The lower part of the pencil leg in detachable and similar
piece containing inking pen can be fitted at that place. [Refer figure 1.8 (a)
and figure 1.8 (b)]
Introduction to Engineering Drawing 1-9
(a)
75°
(b)
Figure 1.8 (a & b)
Both the legs are provided with knee joints. Circles up to 120 mm diameter can be drawn by keeping the legs straight. Figure 1.9 for drawing larger
circles, both the legs should be bent at knee joints thus they are
perpendicular to the surface of paper. (Refer figure 1.10)
Figure 1.9 Figure 1.10
1-10 Engineering Graphics
Step 11 : To draw the circle, adjust the opening of the legs of the compass to
the required radius.
Step 22 : Hold the compass with thumb and first two fingers of right hand
and place the needle lightly on the centre with the help of left
hand.
Step 33 : Rotate compass about the needle leg in clockwise direction, until
the circle is completed.
Step 44 : The compass should be kept slightly inclined in the direction of
rotation.
Step 55 : Smallest circle should be drawn first while drawing concentric
circles.
Step 66 : One grade lighter lead is used for compass work than usual line
work to have equal darkness of drawing. (Refer figure 1.11)
Figure 1.11
Step 77 : The metal needle is to be inserted slightly inside the paper
(about 1 mm).
Step 88 : It is kept longer than the lead point.
Step 99 : The setting of lead and needle and the shape of lead is shown in
figure 1.12. The flat side of lead is kept facing outward.
Introduction to Engineering Drawing 1-11
Figure 1.12
Circles more than 150 mm radius is drawn with the aid of lengthening bar.
The length of the pencil leg is increased by fitting the lengthening bar
between the pencil leg and detachable lower part of the pencil leg. It is
necessary to guide the pencil leg with other hand while drawing large
circles. (Refer figure 1.13)
LENGTHENING
BA
R
9090 SURFACE OF PAPER
Figure 1.13
Beam compass is used to draw very large size circles (more radius than
previous case). Pencil and ink attachments can be used for pencil and ink
work. While operating large compass, both the legs should be kept
perpendicular to the surface of drawing paper.
Bow Compass
This compass is provided with side wheel or central wheel adjustment. The
spring bow compass comprises two legs, one with a pencil or ink point and
the other with a detachable needle. The bow compass can be adjusted for
drawing circles and their arcs of less than 25 mm radius, both in ink and
1-12 Engineering Graphics
pencil. Refer figure 1.14 (a) and figure 1.14 (b). This instrument with its
even tension and positive adjustment mechanism is very useful in getting
accurate results. Bow compass is used for drawing large number of circles
of same diameter. (Refer figure 1.15)
(a) (b)
Figure 1.14 (a & b)
Figure 1.15