Introduction
1.
Engineering drawing is the language of engineers and technicians. The accuracy
and neatness of engineering drawing depends on the quality of the instruments
used. With good instruments, the drawing can be prepared easily, quickly
and accurately. Therefore, the knowledge of different drawing instruments
and materials, methods of using them and the preparatory works to be completed
before commencing the drawing is very important.
Instruments
for Drawing
2.
The following drawing instruments are required for preparing a neat and correct
drawing.
(a) Basic Instruments.
(i) Drawing board
(ii) Drawing sheet
(iii) Drawing pencil
(iv) Drawing clips or pins
(v) Eraser
(vi) Eraser shield
(b) Instruments for Drawing
Straight Lines.
(i)
T- square.
(ii)
Set- squares
(c) Instruments for Drawing
Curved Lines.
(i)
Large size compass
(ii)
Small bow compass
(iii)
French curve
(d) Instruments for Measuring
Distance.
(i)
Large size divider
(ii)
Small bow divider
(iii) Scales
(e) Instruments for Measuring
Angles.
(i)
Protractors
(ii)
Set-squares
(f) Special Tool.
Mini drafter
3.
Drawing Board. Fig. 1
shows a drawing board with its working surface upward. The top surface of the
board is perfectly smooth and level. Fig. 2 shows the bottom of the drawing
board. A drawing board is rectangular in shape and is made of well seasoned
soft wood such as oak or pine. A straight ebony edge is fitted on the left side
on the board against which the head of the T- square moves.
Fig.
1 Drawing Board (Top)
Fig. 2 Drawing Board (Bottom)
4.
Drawing Sheet.
The drawing is frequently made in pencil on the drawing sheet. The best drawing
sheet has the following qualities:
(a)
Light cream buff in colour to have good appearance.
(b)
Fine grains to pick up the graphite and produce clean, dense black lines.
(c)
Strong fibers.
(d)
Superior erasing qualities.
(e)
Folding strength
(f)
Toughness
(g)
Smooth surface
(h)
Hard surface
5. Drawing Pencil. Neatness, quality and accuracy of the drawing greatly depends upon the type and conditions of the pencil used for drawing. Pencil leads are made of graphite with clay added in varying amounts to make 18 grades from 9H to 7B. These grades can be divided in three groups:
(a)
Hard :
9H to 4H
(b)
Medium : 3H to
B (3H, 2H, H, F, HB and B)
(c)
Soft :
2B to 7B
Pencil of 9H is
the hardest and that of 7B is the softest. Harder pencils have leads of small
diameters and softer pencils of larger diameters to give adequate strength. The
choice of grade of pencil depends upon the type of work, texture of paper,
atmosphere, humidity, etc. Following pencils should be used for drawing work in
class:
2H Pencil
- For drawing outlines, Centre
lines, Break lines, etc.
H Pencil - For dimensioning,
arrowheads, hatching lines, lettering, sketching, circles, arcs, etc.
Micro tip pencil - 0.5mm for drawing outlines and 0.8mm for shading and
sketching
6. Drawing Clips
or Pins.
Drawing clips or pins are used to fix the drawing sheet on the drawing
board at the required place. Frequent use of pins cause formation of
impressions of pin pricks on the board, thus spoiling the surface of the board.
The present trend is to go in for steel clips, if the size of the drawing paper
is the same as that of the drawing board. Clips are used at all the four
corners of the drawing board to clamp the paper. Adhesive tapes are also used
for fixing the drawing sheet.
7
Eraser. Eraser is used
to remove the extra lines, lines/marks drawn by mistake and to clear soiled
spots on the drawing. Only pencil eraser is used. Soft India-rubber is
the most suitable kind of eraser for pencil drawings. The eraser used should be
such that the surface of the drawing paper is not spoiled in anyway. It is
desirable to use erasing shield to protect the near by lines from being erased.
The rubber crumbs formed after erasing should be swept away with a clean duster
and should never be brushed off with hands. Use of eraser should be minimized
by proper planning. (Refer Fig No. 3).
Fig. 3 Eraser and Eraser Shield
8. Erasing Shield.
It is a thin metal or plastic plate cut with slots, circles and curves
of different dimensions. It helps to erase unwanted pencil lines without
erasing the surrounding lines. (Refer Fig No. 3)
9. T-
Square. It is composed of a long strip called blade, which is
screwed rigidly at right angle to a shorter piece called head or stock.
It is made of mahogany or pear wood, which is harder than the board wood. The
head also has an ebony edge which slides against the working edge of the board.
T- Square is used for making horizontal, vertical, inclined or parallel lines
on the drawing sheet. (Refer fig. 4).
Fig. 4 T-
Square
10.
Set-Squares.
Set-squares are made of transparent plastic and are available in the shape
of triangles, having a French curve or simply a gap cut in the body. These are
used for drawing short straight lines, measuring
and drawing certain angles. A good combination of set-squares is 30O
x 60O set square with a long edge of 250 mm and a 45O set
squares with each edge of 200 mm. (Refer Fig 5).
Fig. 5 Set- Square
11. Large Size Compass.
The compass is used for drawing circles and arcs. It consists of two legs
hinged together at its upper end. A pointed needle is fitted at the lower end
of one leg, while a pencil lead is inserted at the end of the other leg. The
lower part of the pencil leg is detachable and it can be interchanged with a
similar piece containing an inking pen. Both the legs are provided with knee
joints. Circles up to about 120 mm diameter can be drawn with the legs of the
compass kept straight. For drawing smaller circles, both the legs should be
bent at the knee joints so that these are perpendicular to the surface of the
paper. (Refer Fig. 6).
Fig. 6 Large Size Compass
12.
Small Bow Compass. Small bow compass is
conveniently used for drawing circles and arcs of small diameters. It is very
handy when a number of small circles of the same diameter are to be drawn. The
adjusting nut of the small compass may be on the side or at the centre. This
adjusting nut is provided to make fine adjustment for accurate small circles.
(Refer Fig 7).
Fig. 7 Bow Compasses
13. French
Curves. French curves are used to draw irregular curved
lines, which can not be drawn with a compass. A light pencil curve is first
drawn free hand through the known points. Neat continuous curve is finally
drawn with the longest possible curve coinciding exactly with the free hand
curve. Proper care must be taken to ensure that no corners are formed anywhere
on the curve. Proper use of French curves requires skill. French curves are
made of transparent celluloid or plastic. These are available in various
shapes. One of the french curves is shown in Figure 8.
Fig.
8 French Curves
14.
Large Size Divider. The dividers
has two legs hinged at the upper end and is provided with steel pins at both
the lower ends, but it does not have the knee joints. (Refer Fig 9).
Fig. 9 Large Size Divider
The dividers are used to
(a) Divide straight or curved
lines into desired number of equal parts.
(b) Set off distances from the
scale to the drawings.
(c) Transfer measurements from
one part of the drawing to
another.
15.
Small Bow Divider. The small bow
divider is adjusted by a nut and is very convenient for marking minute
divisions and large number of short equal distances. (Refer Fig 10)
Fig.10 Small
Bow Divider
16.
Scales.
Scales are made of wood, steel, celluloid or plastic. Stainless steel
scales are more durable. Scale may be flat or of triangular cross- section. 15
cm long and 2 cm wide or 30 cm long or 3 cm wide flat scales are commonly used.
These are usually about 1 mm thick. The longer edges of the scale are marked
with inch and its sub-divisions on one side and centimeter and its
sub-divisions on the other side. (Refer Fig. 11)
Fig. 11 Scale17. Protractors. Protractors or Pro-circles are used for drawing any desired angle. These are made of hard transparent plastic. The edges are either squared or beveled. Figure 12 shows semi-circular type protractor.
Fig. 12 Protractor
18.
Mini Drafter. A T-square, protractor and set squares
can be replaced by a drawing drafter. With this, lines can be drawn at any
desired angle. A mini drafter is made with several links. The scale is attached
at the working end of the links. The scale unit can be rotated and set at any
desired angle. The clamp end is fixed to the upper or lower edge of the drawing
board. There is no need to have a working edge on a drawing board when a mini
drafter is used. Mini drafter saves considerable time. (Refer Fig 13).
Fig. 13 Mini Drafter
19.
Guidelines for use. The mini drafter is clamped on the board
for use as follows (Refer Fig 14):
(a)
Set the protractor head such that zero on protractor coincides with the
reference mark on index plate. Lock the head by locking knob.
(b)
Insert the clamp at the left-top corner of the drawing board along horizontal
or vertical edge.
(c)
Align the bottom of horizontal scale along the bottom edge of the board. In
this position tighten the clamp screw.
(d)
Place the drawing sheet, with already drawn border lines, underneath the scales
of mini drafter and align the bottom borderline of sheet with the edge of
horizontal scale of mini drafter.
(e)
Fix the drawing sheet in the same position by drawing clips or adhesive tape.
(f)
The protractor head along with scales can be moved to any place on the drawing
sheet.
(g)
To draw horizontal and vertical line the reference mark should coincide with
the zero on protractor head. To draw inclined line the protractor can be set to
any desired angle coinciding with the reference mark on the index plate.
(h)
All the positioning is done by one hand while the other is used for drawing the
lines.
Fig.
14 Use of Mini Drafter
20.
Precautions for Neatness in Drawing Work. Cleanliness
and neatness in drawing work are very important requirements. Following
precautions are required to be taken to keep a drawing neat and clean:
(a) The hands
should be kept clean at all times during work.
(b) All the
drawing instruments should be kept clean by wiping with a cloth/towel.
(c) Special
emphasis is to be given to sliding instruments on the drawing sheet, such as T-
square and set squares. These instruments must be cleaned properly every time.
(d) Pencil should
always be kept sharp and used properly. It should be sharpened away from the
drawing sheet and other instruments.
(e) Dirt and
graphite particles from the pencil will make the drawing dirty. Hence, every
care should be taken to remove them from the drawing sheet.
(f) Direct
contact of hand with the drawing sheet should be avoided.
(g) Rubbing or
erasing should be done properly with soft eraser.
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