An interchangeable part is one which can be substituted
for similar part manufactured to the same drawing.
When one component assembles properly (and which
satisfies the functionality aspect of the assembly) with any
mating component, both chosen at random, then it is
known as interchangeability.
The parts manufactured under similar conditions by any
company or industry at any corner of the world can be
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3. Before the 18th century production used to be confined to
small number of units and the same operator could adjust the
mating components to obtain desired fit.
Devices such as guns were made one at a time by gunsmith.
If single component of a firearm needed a replacement, the
entire firearm either had to be sent to an expert gunsmith for
custom repairs, or discarded and replaced by another firearm.
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4. Eli Whitney and an early attempt
Eli Whitney understood that developing "interchangeable
parts" for the firearms of the United States military is
In July 1801 he built ten guns, all containing the same
exact parts and mechanisms, then disassembled them
before the United States congress. He placed the parts in a
mixed pile and, with help, reassembled all of the firearms
right in front of Congress.
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5. Interchangeability of parts are achieved by combining a
number of innovations and improvements in machining
operations so that we will able produce components with
Modern machine tools like numerical control (NC) which
evolved into CNC. Jigs and fixtures.
Gauges to check the accuracy of the finished parts. These
helps in manufacturing the components within its specified
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6. If a plot is drawn of the actual dimensions of the
similar components produced by a well-controlled
machine, it is found to follow Normal distribution.
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σ= Standard deviation
x̄ =mean Σ X/N , f=frequency
7. Example we have 100 parts each with a hole and 100
shafts which have to fit into these holes.
If we have interchangeability then we can make any one
of the 100 shaft & fit it into any hole & be sure that the
required fit can be obtained.
Any M6 bolt will fit to any M6 nut randomly selected.
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8. The advantages of interchangeability
1. The assembly of mating parts is easier. Since any
component picked up from its lot will assemble
with any other mating part from another lot
without additional fitting and machining.
2. It enhances the production rate.
3. It brings down the assembling cost drastically.
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9. 4. Repairing of existing machines or products is
simplified because component parts can be easily
5. Replacement of worn out parts is easy.
6. Without interchangeability mass production is not
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11. Selective assembly
The discussion so far has been in connection with full
interchangeability or random assembly in which any
component assembles with any other component.
Often special cases of accuracy and uniformity arises
which might not be satisfied by certain of the fits given
under a fully interchangeable system.
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12. for example if a part at its low limit is assembled with the
mating part a high limit, the fit so obtained may not fully
satisfy the functional requirements of the assembly.
also machine capabilities are sometimes not compatible
with the requirements of interchangeable assembly.
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13. For selective assembly, components are measured and
sorted into groups by dimension, prior to the assembly
process. This is done for both mating parts.
Consider a bearing assembly
Hole with 25+0⋅02
, Shaft 25−0⋅14
Clearance should be
Randomly if we take 25−0⋅02
clearance will be
Hole and Shaft pairing respctively which gives 0.14mm
24.97 and 24.83, 25.0 and 24.86, 25.02 and 24.88
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14. If extremely tight (narrow) tolerance ranges are required,
it may not possible with machining operations. In such
case we use selective assembly
Pin and Hole with sliding fit.
Hole with 2𝑂+0⋅0
, Pin with 2𝑂−0⋅01
If pins coming with over size 20.003 need not be
scrap, they can be mated with Holes 20.013
Same for components with under sized.
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15. Process capability
The minimum toleranced components which can be
produced on a machine with more than 99% of
acceptability called as process capability
80±0.1 680/1000 accuracy.
80±0.3 991/1000 (99%)
80±0.6 1000/1000 (100%)
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16. Problem-1For Clearance Fit
Hole = 2𝑂+0⋅0
Shaft = 2𝑂−0⋅15
Tolerance for both = 0.1mm
Maximum clearance = H.L of hole - L.L of shaft = 20.1-(19.85) = 0.25 mm
Minimum clearance = L.L of hole - H.L of shaft = 20.0-(19.95) = 0.05 mm
Process capability = 0.3
Number of groups = (process capability)/Tolerance = 0.3/0.1=3
Let those groups be denoted by A, B, C
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Type Hole (mm) Shaft (mm)
17. Group C holes with, Group C shaft
Hole Tolerance =0.1mm
Shaft Tolerance =0.1mm
Type of fit required is clearance.
Maximum clearance = H.L of hole - L.L of shaft
Minimum clearance = L.L of hole - H.L of shaft
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There is a larger number of acceptable parts as original
tolerances are greater
This in turn allows the manufacture of cheaper parts as
less will be consigned to the waste bin.
Selective Assembly assures better and more accurate
assembly of parts by insuring closer tolerances between
the mating parts.
Rise the quality and lower manufacturing costs by
avoiding tight tolerances.
Reduces the rejection rate (scrap rate)
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During usage of the assembly if one component fails, first
we need manual of assembly and identify the group to
which failure component belongs to and search the
component in spare parts.
By focusing on the fit between mating parts, rather than
the absolute size of each component so there will small
deviation in size of component.
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1. A text book of production engineering By P. C.
2. Metrology & Measurement By Bewoor
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