Tolerance

LIMITS, TOLERANCES AND FITS
• Introduction
• Need of limits and fits
• Nomenclature of limit system
• Fits
• Fundamentals of tolerances
• Considerations for tolerances
• Grades of tolerances
• Tolerance grade for different processes
• Tolerance values
• Empirical calculation of tolerance
• Linear tolerance indication
• Tolerance dimensioning
2

INTRODUCTION
• It is very difficult to manufacture machine
component to an exact size because of the
inherent limitations of men, machines and
materials.
• The workman has to be given some allowable
margin so that he can produce a part, dimension
of which will lie between two acceptable limits, a
maximum and a minimum.
• The system in which a variation is accepted is
called the limit system.
• The allowable deviations are called tolerances.
• The relationship between the mating parts are
called fits.
3

INTRODUCTION
• Two extreme permissible sizes of a part between
which the actual size is contained are called
limits
• In mass production, variations in dimensions in
the basic size is allowed depending upon the
functional requirements of the components to be
assembled, material used and the cost of
production
4

NEED OF LIMITS AND FITS
The need of limits and fits is due to;
(1)Mass production and specialization:
• Due to mass production, it is not possible for
single industry to manufacture all components
of a machinery
• To overcome this problem, the designer gives a
range of limits to the basic size within which he
expects the finished size of the job
5

NEED OF LIMITS AND FITS
(2) Standardization:
• Standardization means specifying the size of
component nationally and if possible
internationally in order to enjoy the benefit of
large scale production and specialization
• This calls for the maximum and minimum size of
the components so that it can be used in any part
of the country/ of the world
• Standardization reduces the manufacturing costs,
helps in improving the quality of products,
simplifies repair and maintenance of machines
and reduces the time and effort needed to make
the new machines
6

NEED OF LIMITS AND FITS
(3) Interchangeability:
• Interchangeability ensures the possibility of
assembling a unit or machine or replacing a worn
out component without resorting to the extra
machining or fitting operation
• Thus, if machines are provided with
interchangeable parts, they can be easily
replaced in service conditions
• This necessitates that the components must be
manufactured within the permitted variation in
size so that they can be accepted without any
change
7

NOMENCLATURE OF LIMIT SYSTEM
Following basis terms are defined as per BIS
system of limits and fits which are frequently used
in dimensioning of drawing
(1)Size of dimensions: It is a number expressed in
any particular unit which gives the numerical
value of a dimension
(2) Basic size: It is the theoretical size of a part
obtained by design. It is the size based on which
the dimensional deviations are given
(3) Actual size: It is the size of the component
obtained by actual measurement after it is
manufactured
8

Basic size
Lower Limit
Upper Limit
Lower Limit
Upper Limit
Lower Limit
Upper Limit
NOMENCLATURE OF LIMIT SYSTEM
(4) Limits of size: The two extreme permissible
sizes between which the actual size is contained
are called limits
• The maximum size is called the upper limit and
it is the greater of the two limit sizes
• The minimum size is called the lower limit and it
is the smaller of the two limit sizes
9

NOMENCLATURE OF LIMIT SYSTEM
Basic size
Lower Limit
Upper Limit
(5) Tolerance: The permissible variation of a size is
called tolerance
• It is the difference between Upper limit and
Lower limit of the size
• It is always positive and is expressed only as a
number without a sign
Tolerance
10

Basic Size
NOMENCLATURE OF LIMIT SYSTEM
• If the variation is provided on one side of the basic
size, it is termed as Unilateral tolerance
e.g. 50 +0.03 or 50 -0.03
• If the variation is provided on both sides of the
basic size, it is termed as Bilateral tolerance
e.g. 50 ±0.02
(6) Tolerance Zone: The graphical representation of
tolerance is called tolerance zone
(7) Zero line: It is a straight line to which the
deviations are referred and represents the basic
size
Zero line
Tolerance zone
11

NOMENCLATURE OF LIMIT SYSTEM
(8) Deviation: It is the algebraic difference
between a size (actual or maximum or
minimum) and the corresponding basic size
Basic size, deviations and tolerances 12

NOMENCLATURE OF LIMIT SYSTEM
Upper Deviation: It is the algebraic difference
between the upper limit and the corresponding
basic size (by symbol ES (Hole) or es (shaft)
Lower Deviation: It is the algebraic difference
between the lower limit and the corresponding
basic size (Represented by EI (Hole) or ei (shaft)
ES,es=E’ Cart Superior
EI,ei=E’ Cart Inferior
13

NOMENCLATURE OF LIMIT SYSTEM
Fundamental Deviation: It is either the upper or
the lower deviation which is nearer to the zero
line. It is chosen to locate the tolerance zone with
respect to the zero line
Actual Deviation: It is the algebraic difference
between the actual size and the corresponding
basic size
(9) Shaft: It is a term conventionally used to
designate all external features of a part including
those which are not cylindrical
(10) Hole: It is a term conventionally used to
designate all internal features of a part including
those which are not cylindrical
14

NOMENCLATURE OF LIMIT SYSTEM
(11) Basic shaft: It designates a shaft whose upper
deviation is zero
(12) Basic hole: It designates a hole whose lower
deviation is zero
(13) Allowance: The difference between the
dimensions of two mating parts is called the
allowance
(14)Maximum Metal Condition: Maximum metal
condition (MMC) is related to retention of
maximum amount of metal in any machine
part. All external features (shaft) need
maximum limit of size while all internal
features (hole) need minimum limit of size.
15

NOMENCLATURE OF LIMIT SYSTEM
(15) Least Metal Condition: Least metal condition
(LMC) is related to retention of minimum
amount of metal in any machine part. All
external features (shaft) need minimum limit
of size while all internal features (hole) need
maximum limit of size.
16

FITS
• The relationship resulting from the difference
between the actual size of the two mating parts
which are to be assembled is known as a fit.
• In assembly, the joints may be either movable or
permanent depending upon the functions
• e.g. shaft rotating in bush (movable) or flywheel
mounted on a shaft (fixed)
• Depending upon the actual limits of the hole or
shaft sizes, fits may be classified as;
(1) Clearance fit
(2) Transition fit and
(3) Interference fit
17

FITS
(1) Clearance fit: It is a fit that gives a clearance
between the two mating parts.
• Here, hole size is always greater than shaft size.
• The actual amount by which the hole exceeds
the saft is termed as the clearance.
Clearance fit
18

FITS
Minimum Clearance fit:
It is the difference
between the minimum
size of the hole and the
maximum size of the
shaft in a clearance fit.
Maximum Clearance fit:
It is the difference
between the maximum
size of the hole and the
minimum size of the shaft
in a clearance or transition
fit. 19

(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.
• It results in a clearance
fit, when shaft diameter
is smaller than hole
diameter
• It results in interference
fit, when shaft diameter
is greater than hole
diameter
FITS
Transition fit
20

(3) Interference fit:
FITS
• If the difference between the hole and shaft sizes is
negative before assembly; an interference fit is
obtained.
Interference fit
21

FITS
Minimum Interference fit:
• It is the magnitude of the
difference (negative) between
the maximum size of the hole
and the minimum size of the
shaft in an interference fit
before assembly.
Maximum Interference fit:
•It is the magnitude of the
difference between the
minimum size of the hole and
the maximum size of the shaft
in an interference or a
transition fit before assembly.
22

FITS
Schematic representation of fits
23

HOLE & SHAFT BASIS SYSTEM
• Hole basis system: In this system, the size of the shaft
is obtained by subtracting the allowance from the basic
size of the hole.
• The lower deviation of the hole is zero. The letter
symbol for this situation is ‘H’.
• This system is preferred in most cases, since standard
tools like drills, reamers, broaches, etc., are used for
making a hole.
24

HOLE & SHAFT BASIS SYSTEM
• Shaft basis system: In this system, the size of the hole
is obtained by adding the allowance to the basic size of
the shaft.
• The upper deviation of the shaft is zero. The letter
symbol for this situation is ‘h’.
• Used by industries using semi-finished shafting as raw
materials, e.g., textile industries, or when several parts
having different fits but one nominal size is required on
a single shaft
25

FUNDAMENTALS OF TOLERANCES
• The tolerance is a factor or percentage of the
nominal value, a maximum deviation from a nominal
value, an explicit range of allowed values, be
specified by a note or published standard with this
information or be implied by the numerical accuracy
of the nominal value.
• To maintain proper functionality, the largest
possible tolerance needs to be specified.
•Closer or tighter tolerances are more difficult and
costly to achieve.
•Conversely, large or looser tolerances may
significantly affect the operation of the device.
26

CONSIDERATIONS FOR SETTING TOLERANCE
Following aspects should be adhered to while
assigning the amount of tolerance;
(a)Adopting proper scientific principles, adequate
engineering knowledge, professional experience
and experimental investigation will help
determine the amount of tolerance such that it
should not affect other factors or the outcome of
the process.
(b) The choice of tolerance is also affected by the
intended statistical plan for statistical analysis.
27

CONSIDERATIONS FOR SETTING TOLERANCE
(c) The amount of engineering tolerances as provided
in any specification may not always be achieved
due to inherent variation in input and output of
operation system, system and operational errors
and statistical uncertainty in measurement.
(d) The process capabilities of systems, materials and
products must be compatible with the specified
engineering tolerances. In order to achieve these,
total quality management (TQM) should be
observed which will be reflected through process
capability index (PCI).
28

GRADES OF TOLERANCES
• Large variety of tolerance is classified by the
International Tolerance (IT) Grade.
• Lower IT number refers to low tolerance and is
used for making precision instruments.
• ISO: 286 (Part I & II) - 1988 specifies these
tolerance grades.
• For each nominal step, there are 18 grades of
tolerances, designated as IT01, IT0, IT1 to IT16,
known as fundamental tolerances.
29

TOLERANCE GRADE FOR VARIOUS PROCESSES
IT01 IT0 IT1 IT2 IT3 IT4 IT5 IT6 IT7 IT8 IT9 IT10 IT11 IT12 IT13 IT14 IT15 IT16
Most Precise tools
●
Slip Gauges ● ● ●
Lapping ● ● ● ●
Honing ● ● ●
Super finishing ● ● ●
Cylindrical Grinding ● ● ● ●
Diamond turning ● ● ● ●
grinding, broaching,
reaming
● ● ● ● ●
Boring, Turning ● ● ● ● ● ● ●
Sawing ● ● ●
Milling ● ● ● ● ●
Shaping, Planning ● ●
Extruding ● ● ● ●
Cold rolling, drawing ● ● ● ● ●
Drilling ● ● ● ●
Die Casting ● ● ● ●
Forging ● ● ● ●
Sand casting, hot rolling ● ● 30 ●

TOLERANCE VALUE FOR DIFFERENT IT GRADES
31

EMPIRICAL CALCULATION OF STD. TOLERANCE
• The fundamental tolerance is a function of the
nominal size and its unit is given by the empirical
relation,
standard tolerance unit, i 0.45
D 0.001D
Where…
i = In microns and
D = Geometrical mean of basic diameter steps in
mm
• The relation is valid for grades 5 to 16 and
nominal sizes from 3 to 500mm
• Relative magnitude is given in table below
3
32

EMPIRICAL CALCULATION OF STD. TOLERANCE
• For grades below IT5, there are other empirical
relations which are given as per IS: 1919–1963.
The expressions are;
• For IT01, i = 0.3 + 0.008D
• For IT0, i = 0.5 +0.012D and
• For IT1 to IT4, i = 0.8 + 0.02D
33

LINEAR TOLERANCE INDICATION
• Tolerance is denoted by two symbols, a letter
symbol and a number symbol, called the grade.
• The letter symbols range from A to ZC for holes
and from a to zc for shafts.
• The letters I, L, O, Q, W and i, l, o, q, w have not
been used.
• These letter symbols represent the degree of
closeness of the tolerance zone (positive or
negative) to the basic size.
34

LINEAR TOLERANCE INDICATION
(a) Basic characteristics of letter symbol for
internal (hole) features:
• The shaded length of each rectangle represents
total IT grade value
• For Positive side, the upper deviation = lower
deviation + IT grade value
• For Negative side, the lower deviation = upper
deviation + IT grade value
• For A, the magnitude of positive lower deviation
is maximum and is zero for H.
• The upper deviation becomes negative from K
onwards up to ZC
35

LINEAR TOLERANCE INDICATION
(b) Basic characteristics of letter symbol for
external (shaft) features:
• The shaded length of each rectangle represents
total IT grade value
• For Positive side, the upper deviation = lower
deviation + IT grade value
• For Negative side, the lower deviation = upper
deviation + IT grade value
• For a, the magnitude of negative upper
deviation is maximum and is zero for h.
• The upper deviation becomes positive from k
onwards up to zc
36

GRAPHICAL ILLUSTRATION OF TOLERANCE ZONES
37

TOLERANCE DIMENSIONING
There are three methods used for tolerance
individual dimensions
(1) Method 1: In this method, tolerance
dimension is given by its basic value followed
by a symbol comprising of both a letter and a
number.
e.g. Φ25H7
Φ25=Basic size is 25mm
H= Signifies the basic hole with lower deviation
zero
7 = Signifies the tolerance grade IT7
38

TOLERANCE DIMENSIONING
Method 2:
• In this method, the basic size and the tolerance
values are indicated above the dimension line;
• The tolerance values being in a size smaller than
that of the basic size and the lower deviation
value being indicated in line with the basic size.
39

TOLERANCE DIMENSIONING
Method 3:
•In this method, the maximum and minimum sizes
are directly indicated above the dimension line
•When assembled parts are dimensioned, the fit is
indicated by the basic size common to both the
components, followed by the hole tolerance symbol
first and then by the shaft tolerance symbol (e.g.,
Φ 25 H7/h6, etc…)
40

TOLERANCE DIMENSIONING
Tolerance dimensioning of assembled parts:
41

LIMITS
TOLERANCES
&
FITS
42