Machine drawing

SYLLABUS
Section A
Introduction graphic language, classification of drawing, principle
of drawing, IS codes for machine drawing, lines, scales, section
dimensioning, standard abbreviation, – Limits , fits and Tolerance
( Dimensional and Geometrical tolerance ) , Surface finish,
Gears : Gear terminology, I.S. convention representation of
assembly of spur gears, helical gears, bevel gears , worm and
worm wheel.
Section B
Orthographic projections: principle of first and third angle
projection, orthographic views from isometric views of machine
parts / components.
Drawing of sectional views:- Coupling, Crankshaft, Pulley, Piston
and Connecting rod, Cotter and Knuckle joint. Riveted Joint and
Welded Joint.

SYLLABUS
Free hand sketching: Need for free hand sketching of standard
parts and simple machines components.
Section C
Assembly drawing with sectioning and bill of materials from
given detailed drawings of assemblies: Lathe Tail stock, Machine
vice, Pedestal bearing
Section D
Assembly drawing with sectioning and bill of materials from
given detailed drawings of assemblies Steam stop valve, Stuffing
box, Drill jigs and Milling fixture.

Introduction- Graphic Language
• Engineering drawing has its origin sometime in 500
BC in the regime of King Pharos of Egypt when
symbols were used to convey the ideas among
people.
• Irrespective of language barriers, the drawings can
be effectively used in other countries, in addition to
the country where they are prepared.
• Thus, the engineering drawing is the universal
language of all engineers.

Classification of Drawing
• Machine Drawing-
- Pertaining to machine
parts or components.
- presented through a
number of
orthographic views.
- Size & shape of
component is fully
understood.

• Production Drawing –
- Referred as working drawing.
- Should furnish all dimensions, limits & special finishing processes
such as heat treatment, honing, lapping, surface finish, etc.
- Title should also mention the material used for the product, number
of parts required.

• Part Drawing-
- Detailed drawing of a
component to facilitate its
manufacture.
- Follows principles of
orthographic projection
• Assembly Drawing-
- A drawing that shows the
various parts of a machine
in their correct working
locations.

Principle of Drawing
To provide the correct information about drawings to all
concerned people, the drawing must be prepared, following
certain standard practices, as recommended by Bureau of
Indian Standards (BIS).
• Sheet Size- For a reference size A0
having a surface area of 1
X = 841 mm and Y = 1189 mm.

• Title Block –
(i) Title of the drawing
(ii) Sheet number
(iii) Scale
(iv) Symbol, denoting the method of projection
(v) Name of the firm / institute
(vi) Initials of staff drawn, checked and approved.

• Borders & Frames –
Minimum 20 mm for size A0 & A1.
Minimum 10 mm for size A2, A3, A4.
• Centering Marks
• Metric reference
graduation
• Grid reference
system
• Trimming mark

Scales
• The various types of scales used in machine drawing are
1. Full scale 1:1
2. Reduced scale 1:X
3. Enlarged scale X:1
The standard scales are given in Table

Types of lines
& their
applications

Dimensioning
1. As far as possible, dimensions should be placed outside the
view.
2. Should be taken from visible outlines, not from hidden lines.
3. Dimensioning to a centre line should be avoided except
when the centre line passes through the centre of a hole.
4. Each feature should be dimensioned only once in a drawing.
5. Placed on the view or section that relates most clearly to the
corresponding features.
6. Each drawing should use the same unit for all dimensions,
but without showing unit symbol.
7. Minimum dimensions should be placed to define a whole
part.
8. No features of a part should be defined by more than one
dimension in any one direction.

Elements of Dimensioning
1. Dimension line — It is a thin continuous line terminated by arrowheads
touching the outlines, extension lines or centre lines.
2. Extension line (Projection line) — It is a thin line drawn outside and along the
outline. There should be a gap of about 1 mm between the extension line and
the outline.
3. Leader line —One end of the leader terminates either in an arrowhead or a dot.
The arrowhead touches the outline, while the dot is placed within the outline
of the object. The other end of the leader is terminated at a horizontal line
4. Arrowhead — An arrowhead is placed at each end of a dimension line. Its
pointed end touches an outline, an extension line or a centre line. The length
of arrowhead should be about three times its maximum width. The triangle of
the arrow should be completely filled in.

Methods of indicating Dimensions
1. Aligned system, and
2. Unidirectional system.
1. Aligned system — In the aligned system, the dimensions are
placed above the dimension lines and may be read either
from the bottom or from the right side of the drawing
2. Unidirectional system — In the unidirectional system, all
dimensions are placed with respect to the bottom of the
drawing, irrespective of the disposition of the dimension
line. In the system, the dimension lines are broken to insert
their dimensions . This system is preferred for big drawings
specially when it is not convenient to read the dimension
from the right side or any other direction.

Limits , Fits and Tolerance
• The system in which a variation in dimensions is accepted is
called the limit system.
• The allowable deviations are called tolerances.
• The relationships between the mating parts are called fits.

Limits
• The two extreme permissible sizes between which the actual
size is contained are called limits.
• The maximum size is called the upper limit and the minimum
size is called the lower limit.
• Tolerance is denoted by two symbols, a letter symbol and a
number symbol, called the grade.
• It is the difference between lower and upper deviation.
Tolerances

• Graphical
illustration of
tolerance zones

Method of placing limit dimensions
Method 1

Types of Fits
1. Clearance Fit –
It is a fit that gives a clearance between the two mating
parts.

Types of Fits
2. Transition Fit –
This fit may result in either an interference or a clearance,
depending upon the actual values of the tolerance of
individual parts.

Types of Fits
3. Interference Fit -
If the difference between the hole and shaft sizes is negative
before assembly; an interference fit is obtained.

Schematic Representation of Fits

Surface finish
The geometrical characteristics of a surface include,
1. Macro-deviations,
2. Surface waviness, and
3. Micro-irregularities.
The surface roughness is evaluated by the height, Rt and mean roughness
index Ra of the micro-irregularities.

Equivalent Surface Roughness Symbols

Indication of Surface Roughness

Gears
• Gears are machine elements, which are used for power transmission
between shafts, separated by small distance.
• While two gears are meshing, the teeth of one gear enter the spaces of
the other. Thus, the drive is positive and when one gear rotates, the other
also rotates; transmitting power from one shaft to the other.

• Pitch Circle – Outlines of imaginary smooth rollers or fiction
discs in every pair of mating gears.
• Pitch Diameter – Diameter of pitch circle (P.C.D.)
• Root Diameter – Diameter at the bottom of tooth space.
• Addendum – Height from pitch circle to the tip of the tooth.
• Dedendum – Depth of tooth space below pitch circle.
(addendum + clearance)
• Circular Pitch (C.P.) – Length of arc of pitch circle between
similar faces of successive teeth.
• Diametral Pitch (D.P.) – Number of teeth divided by pitch
diameter.
• Module (m)- Pitch diameter divided by number of teeth.
• Circular Tooth Thickness – Length of arc of the pitch circle
between opposite faces of same tooth (0.5 C.P.)

• Base Circle Diameter – Diameter of circle from which
involute is generated.
• Line of Action – Common tangent to the two base circles,
which passes through the pitch point of a pair of mating
gears.
• Pressure Angle- Acute angle formed between the line of
action and common tangent to the two pitch circles, which
passes through the pitch point.
• Tooth Fillet – Rounded corner at the bottom of tooth space.
• Centre Distance – Distance between centers of a pair of
mating gears and is equal to sum of the radii of pitch circles of
the two gears.
• Pitch Point – Point of contact between the pitch circles of
two gears in mesh & lies on the line joining their centers.

• Tooth face
• Tooth flank
• Crest of tooth
• Root of tooth
• Whole depth
• Working depth
• Addendum circle
• Dedendum / root circle
• Chordal pitch

Gear Calculation
• Circular Pitch = Circumference of P.C.D. / No. of teeth
= P.C.D. x π / No. of teeth
C.P. = P.C.D. x π / N
• Diametral Pitch = No. of teeth / Pitch Circle Diameter
= N / P.C.D.
• C.P. x D.P. = π
C.P. = π / D.P. And D.P. = π / C.P.
• Metric Module , m = Pitch Circle Diameter / No. of teeth
= P.C.D. / N
= 1 / D.P.
C.P. = π / D.P. = π x m

Outside Diameter = Pitch diameter + 2xAddendum
Root Diameter = Pitch diameter - 2xDedendum
Clearance = Dedendum – Addendum
Tooth thickness = C.P. / 2
Addendum = 1 / D.P. = C.P. / π = m
Clearance = C.P. / 20 = (π/D.P.) x (1/20) = 0.157 x m
Pressure angle, θ = 14.5 or 20 degree
Base circle diameter, B.C.D. = P.C.D. x Cos θ
Involute gears will work correctly together if they have the same
pressure angle and diametral pitch.
Gear Calculation

Machine drawing

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Machine drawing