Fits and tolerances in engineering

P R E S E N T E D B Y :
A N K I T K U M A R ( 1 4 5 7 0 7 8 )
N A Y A N K U M A R ( 1 4 5 7 0 8 0 )
P R A N T I K C H O W D H U R Y ( 1 4 5 7 0 8 2 )
Fits and Tolerances in
Engineering
Heritage Institute of Technology
Department of Mechanical Engineering
SEMINAR-II MECH-3221

Contents
 History
 Introduction
 Interchangeability
 System Terminologies
 Tolerances
 Fits
 Hole and Shaft-basis System
 Limit Systems
 Designation of holes and shafts
 Conclusion

History
 Replacement of hand made tools with machine made
tools.
 This change in production, is now known as the
Industrial Revolution.
 Began in England in 18th century.
 Instead of utilizing artisans to produce hand-made
items, machines started to help and eventually take
the place of the artisans.
 Inventions aided in speeding up the production of
manufactured items.

Introduction
 In the manufacturing of a machine, quality is a
primary consideration.
 Manufacturing precision taken into the product
determines its quality, its cost and selling price.
 Precision is the degree of accuracy necessary to
ensure the functioning of a part as intended.
 There are two types of parts:
o Non-mating parts
o Mating parts

Interchangeability
 The concept of mass production originated with the
automobile industry.
 MODEL-T of Ford Motors was the first machine to
be mass-produced.
 Till the 1940’s, every component was manufactured
in-house.
 When one component assembles properly (and
which satisfies the functionality aspect of the
assembly/product) with any mating component,
both chosen at random, then it is known as
interchangeability.

Ford Motor Co./AP The 1914 Ford Model T touring car is
shown in this Ford Motor Co. handout. The 1914 model was
the first version built on Henry Ford's moving assembly line.

System Terminologies
 Shaft: The term ‘shaft’ used in this standard has a wide meaning and serves for
specification of all outer elements of the part, including those elements, which
do not have cylindrical shapes.
 Hole: The term ‘hole’ can be used for specification of all inner elements
regardless of their shape.
 Basic Size: The basic size or normal size is the standard size for the part and is
the same both for the hole and its shaft. This is the size which is obtained by
calculation of strength.

System Terminologies (Contd.)
 Actual Size: Actual size is the dimension as measured on a manufactured
part. As already mentioned, the actual size will never be equal to the basic size
and it is sufficient if it is within predetermined limits.
 Limits of Size: These are the maximum and minimum permissible sizes of
the part (extreme permissible sizes of the feature of the part).
 Maximum Limit: The maximum limit or high limit is the maximum size
permitted for the part
 Minimum Limit: The minimum limit or low limit is the minimum size
permitted for the part.
 Zero Line: In a graphical representation of limits and fits, a zero line is a
straight line to which the deviations are referred to. It is a line of zero deviation
and represents the basic size. When the zero line is drawn horizontally, positive
deviations are shown above and negative deviations are shown below this line.
 Deviation: It is the algebraic difference between a size (actual, limit of a size,
etc.) and the corresponding basic size.

 Upper Deviation: It is designated as ES (for hole) and es (for shaft). It is the
algebraic difference between the maximum limit of the size and the
corresponding basic size. When the maximum limit of size is greater than the
basic size, it is a positive quantity and when the maximum limit of size is less
than the basic size then it is a negative quantity.
 Lower Deviation: It is designated as EI (for hole) and ei (for shaft). It is the
algebraic difference between the minimum limits of size and the corresponding
basic size. When the minimum limit of size is greater than the basic size, it is a
positive quantity and when the minimum limit of size is less than the basic size
then it is a negative quantity.
 Fundamental Deviations (FD): This is the deviation, either upper or the
lower deviation, which is the nearest one to the zero line for either a hole or a
shaft. It fixes the position of the tolerance zone in relation to the zero line.
 Actual Deviation: This is the algebraic difference between an actual size and
the corresponding basic size.
 Mean Deviation: It is the arithmetical mean between the upper limit and the
lower limit.

 Tolerance: It is the difference between the upper limit and the lower limit of a
dimension. It is also the maximum permissible variation in a dimension.
 Tolerance Zone: It is a function of basic size. It is defined by its magnitude
and its position in relation to the zero line. It is the zone bounded by the two
limits of size of a part in the graphical presentation of tolerance.
 Tolerance Guide: It is the degree of manufacturing. It is designated by the
letters IT (stands for International Tolerance). Numbers, i.e., IT0, IT01, IT1,
follow these letters up to IT16; the larger the number, the larger the tolerance.
 Tolerance Class: The term is used for a combination of fundamental
deviation and tolerance grade.
 Allowance: It is an intentional difference between the maximum material
limits of mating parts. For a shaft, the maximum material limit will be its high
limit and for a hole, it will be its low limit.
 Fits: The relationship existing between two parts, shaft and hole, which are to
be assembled, with respect to the difference in their sizes is called fit.

Tolerances
 Tolerance is the total amount that a specific dimension is
permitted to vary;
 It is the difference between the maximum and the minimum
limits for the dimension.
 For Example a dimension given as 1.625 ± .002 means that the
manufactured part may be 1.627” or 1.623”, or anywhere
between these limit dimensions.
 How to decide tolerance?
 Functional requirements of mating parts
 Cost of production
 Available manufacturing process
 Choose as coarse tolerance as possible without compromising
functional requirements
 Proper balance between cost and quality of parts

Tolerances
The Tolerance is 0.001” for the Hole as well as for the Shaft

Allowances and Clearances
Interchangeable Fit

Specifications of Tolerances
 Limit Dimensioning
The high limit is placed above the
low limit.
In single-line note form, the low
limit precedes the high limit
separated by a dash

Specifications of Tolerances (Contd.)
 Plus-or-minus Dimensioning
• Unilateral Tolerance
• Bilateral Tolerance

Fits
 Fit is the general term used to signify the range of tightness or
looseness that may result from the application of a specific
combination of allowances and tolerances in mating parts.
 A fit is the relationship between two meeting parts, viz., shaft and
hole. This relationship is nothing but the algebraic differences
between their sizes
 There are four types of fits between parts:
 Clearance Fit
 Interference Fit
 Transition Fit
 Line Fit

Clearance Fit
 When an internal member fits in an external member (as a shaft
in a hole) and always leaves a space or clearance between the
parts.
Minimum air space is 0.002”. This is the allowance and is always positive
in a clearance fit

Interference Fit
 The internal member is larger than the external member such
that there is always an actual interference of material. The
smallest shaft is 1.2513” and the largest hole is 1.2506”, so that
there is an actual interference of metal amounting to at least
0.0007”. Under maximum material conditions the interference
would be 0.0019”. This interference is the allowance, and in an
interference fit it is always negative.

Transition Fit
 This type of fit may result in either a clearance or interference
condition. In the figure below, the smallest shaft 1.2503” will fit
in the largest hole 1.2506”, with 0.003” to spare. But the largest
shaft, 1.2509” will have to be forced into the smallest hole,
1.2500” with an interference of metal of 0.009”.

Line Fit
 The limits of size are so specified that a clearance or surface
contact may result when mating parts are assembled.

Hole-basis System
 Minimum hole is taken as the basic size, an allowance is
assigned, and tolerances are applied on both sides of and away
from this allowance.
1. The minimum size of the hole 0.500”
is taken as the basic size.
2. An allowance of 0.002” is decided on
and subtracted from the basic hole
size, making the maximum shaft as
0.498”.
3. Tolerances of 0.002” and 0.003”
respectively are applied to the hole
and shaft to obtain the maximum
hole of 0.502” and the minimum
shaft of 0.495”.
Minimum clearance: 0.500”-
0.498” = 0.002”
Maximum clearance: 0.502” –
0.495” = 0.007”

Shaft-basis System
 Maximum shaft is taken as the basic size, an allowance is
assigned, and tolerances are applied on both sides of and away
from this allowance.
1. The maximum size of the shaft
0.500” is taken as the basic size.
2. An allowance of 0.002” is decided on
and added to the basic shaft size,
making the minimum hole as 0.502”.
3. Tolerances of 0.003” and 0.001”
respectively are applied to the hole
and shaft to obtain the maximum
hole of 0.505” and the minimum
shaft of 0.499”.
Minimum clearance: 0.502”-
0.500” = 0.002”
Maximum clearance: 0.505” –
0.499” = 0.006”

International Tolerance Grade (IT)
They are a set of tolerances that varies according to the basic size
and provides a uniform level of accuracy within the grade.

Indian Standards Specifications and Application
 In India we have IS: 919 recommendation for limits and fits for
engineering. This standard is mostly based on British Standards
BS: 1916-1953.
 Sizes up to 500 min are covered in IS: 919 and sizes above 500
mm, up to 3150 mm, are covered in IS: 2101
 However, it is yet to adopt several recommendations of ISO: 286.

Conclusion
 In manufacturing process after manufacturing the product we
need to check the dimensions of the product.
 No manufacturing process is perfect to get perfect dimensions
for a required product.
 After manufacturing process even if you are getting the errors in
the dimensions of the product, those errors can be acceptable up
to certain limit. These limits are decided during the designing
process.

Bibliography
 Kulkarni, V.A. , Bewoor, A.K: Metrology and
Measurement
 I.S. 1871: Commentary on Indian Standard wrought
steels for general engineering purposes.
 ISO 286: ISO System of limits and fits (in two parts)

Fits and tolerances in engineering

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