System of Limits, Fits, Tolerances, and Gauging
UNIT II
SYSTEM OF LIMITS,
FITS, TOLERANCES AND
GAUGING
TOLERANCES
Tolerance
is the total amount that a specific dimension is permitted to vary in positive
or negative side
It is the difference between the maximum and the minimum limits for the dimension.
Due to the inevitable inaccuracy of manufacturing methods, a part cannot be made
precisely to a given dimension. So tolerance is a mandatory thing in designing
process of a component.
TYPES OF TOLERANCES
Unilateral tolerances
Bilateral tolerances
UNILATERAL TOLERANCES
BILATERAL TOLERANCES
Having variations in only one directions either
upper or lower
Limits are for only the larger or smaller sizes
Having variations in both the upper and lower
directions
The plus and minus limitations combine to form a
single value.
ACCUMALATION TOLERANCES
Tolerances are getting accumulated in each step
of the components
So, the total tolerance is the sum of the
individual tolerance of each component.
COMPOUND TOLERANCES
Tolerance of one dimension of the component is
dependent on the tolerance of the other dimension.
That, if tolerance of one component increases the
tolerance of the other increases or decreases.
TERMS RELATED TO
METRIC LIMITS & FITS
Basic Size
is the size from which limits or deviations are assigned. Basic sizes, usually diameters,
should be selected from a table of preferred sizes.
Nominal size
is a dimension used to describe the general size
Actual size
is the measured size of the finished part after machining
Limits
is the max and min sizes shown by the toleranced dimension
Allowance
is the min clearance or maximum interference between parts, or the tightest fit between two
mating parts
Deviation:
is the difference between the basic size and the hole or shaft size.
Upper Deviation:
is the difference between the basic size and the permitted maximum size of the part.
Lower Deviation
:
is the difference between the basic size and the minimum permitted size of the part.
Fundamental Deviation
:
is the deviation closest to the basic size.
Tolerance
:
is the difference between the permitted minimum and maximum sizes of a part.
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.
E.S. – upper deviation
E.I. – lower deviation
SCHEMATIC REPRESENTATION
OF THE PLACEMENT OF THE
TOLERANCE INTERVAL
H : lower deviation of hole is zero
h : upper deviation of shaft is zero
Formula for Fundamental deviation will be
given separately
DEFINITION OF 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.
TYPES OF FIT
CLEARANCE FIT
:
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:
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”.
SYSTEM OF FITS
BASIC SHAFT SYSTEM
the largest diameter of the shaft is assigned the basic
diameter from which all tolerances are applied.
The fundamental deviation is given by the lowercase letter
h
.
BASIC HOLE SYSTEM
the smallest hole is assigned the basic diameter from which
the tolerance and allowance are applied. (more popular
than the basic shaft system because of ease to control the
hole size)
For the generally preferred hole-basis system, the
fundamental deviation is specified by the upper-case letter
H
.
PLAIN GAUGES
PLAIN PLUG GAUGES
Used for checking inside diameter of straight hole (generally "go"
and "no-go" variety)
Consists of handle and plug on each end ground and/or lapped to
specific size
Small-diameter plug ("go" gage) checks lower limit of hole
Larger-diameter plug (“no-go” gage) checks upper limit
Used to check outside diameter of pieces
Ground and lapped internally to desired size
Size stamped on side of gage
Outside diameter knurled and "no-go" end identified by
annular groove on knurled surface
Precautions and procedures similar to those outlined for a plug
gage
PLAIN RING GAGES
SNAP GAGES
One of most common types of comparative measuring
instruments
Faster to use than micrometers
Limited in their application
Used to check diameters within certain limits by
comparing part size to preset dimension of snap gage
Have C-shaped frame with adjustable gaging anvils or
rolls set to "go" and "no-go" limits of the part
THANK YOU
Tolerances play a crucial role in design processes by allowing for variations in dimensions during manufacturing. Types of tolerances include unilateral and bilateral, with accumulation and compound tolerances affecting overall variations. Terms related to metric limits and fits help define dimensions accurately. International Tolerance Grade (IT) ensures uniform accuracy across different sizes.
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Presentation Transcript
UNIT II SYSTEM OF LIMITS, FITS, TOLERANCES AND GAUGING
TOLERANCES Tolerance is the total amount that a specific dimension is permitted to vary in positive or negative side It is the difference between the maximum and the minimum limits for the dimension. Due to the inevitable inaccuracy of manufacturing methods, a part cannot be made precisely to a given dimension. So tolerance is a mandatory thing in designing process of a component. TYPES OF TOLERANCES Unilateral tolerances Bilateral tolerances
UNILATERAL TOLERANCES Having variations in only one directions either upper or lower Limits are for only the larger or smaller sizes BILATERAL TOLERANCES Having variations in both the upper and lower directions The plus and minus limitations combine to form a single value.
ACCUMALATION TOLERANCES Tolerances are getting accumulated in each step of the components So, the total tolerance is the sum of the individual tolerance of each component. COMPOUND TOLERANCES Tolerance of one dimension of the component is dependent on the tolerance of the other dimension. That, if tolerance of one component increases the tolerance of the other increases or decreases.
TERMS RELATED TO METRIC LIMITS & FITS
Basic Size is the size from which limits or deviations are assigned. Basic sizes, usually diameters, should be selected from a table of preferred sizes. Nominal size is a dimension used to describe the general size Actual size is the measured size of the finished part after machining Limits is the max and min sizes shown by the toleranced dimension Allowance is the min clearance or maximum interference between parts, or the tightest fit between two mating parts Deviation: is the difference between the basic size and the hole or shaft size. Upper Deviation: is the difference between the basic size and the permitted maximum size of the part. Lower Deviation: is the difference between the basic size and the minimum permitted size of the part. Fundamental Deviation: is the deviation closest to the basic size. Tolerance: is the difference between the permitted minimum and maximum sizes of a part.
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.
SCHEMATIC REPRESENTATION OF THE PLACEMENT OF THE TOLERANCE INTERVAL E.S. upper deviation E.I. lower deviation H : lower deviation of hole is zero h : upper deviation of shaft is zero Formula for Fundamental deviation will be given separately
DEFINITION OF 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. TYPES OF FIT CLEARANCE FIT: 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: 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 .
SYSTEM OF FITS BASIC SHAFT SYSTEM the largest diameter of the shaft is assigned the basic diameter from which all tolerances are applied. The fundamental deviation is given by the lowercase letter h. BASIC HOLE SYSTEM the smallest hole is assigned the basic diameter from which the tolerance and allowance are applied. (more popular than the basic shaft system because of ease to control the hole size) For the generally preferred hole-basis system, the fundamental deviation is specified by the upper-case letter H.
PLAIN GAUGES PLAIN PLUG GAUGES Used for checking inside diameter of straight hole (generally "go" and "no-go" variety) Consists of handle and plug on each end ground and/or lapped to specific size Small-diameter plug ("go" gage) checks lower limit of hole Larger-diameter plug ( no-go gage) checks upper limit PLAIN RING GAGES Used to check outside diameter of pieces Ground and lapped internally to desired size Size stamped on side of gage Outside diameter knurled and "no-go" end identified by annular groove on knurled surface Precautions and procedures similar to those outlined for a plug gage
SNAP GAGES One of most common types of comparative measuring instruments Faster to use than micrometers Limited in their application Used to check diameters within certain limits by comparing part size to preset dimension of snap gage Have C-shaped frame with adjustable gaging anvils or rolls set to "go" and "no-go" limits of the part
