Transcript Tolerance

Mechanical Drawing
(MDP 115)
FirstYear,
Mechanical Engineering Dept.,
Faculty of Engineering,
Fayoum University
Dr. Ahmed Salah Abou Taleb
FITS and TOLERANCES
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Control Measurements of
Finished Parts
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Indicating the Desired Dimension
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Indication of the Real Dimension of Parts
Due to the
inevitable
inaccuracy of
manufacturing
methods, a part
cannot be made
precisely to a
given dimension.
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Indication of the Real Dimension of Parts
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Indication of the Real Dimension of Parts
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Selection of Waste Parts
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Selection of Waste Parts
Waste results when the manufacturing process cannot
maintain size within prescribed limits.
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Selection of Acceptable Parts
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Selection of Acceptable Parts
There is no such thing as an "exact size".
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Important Terms – Single Part
• Nominal Size – general size, usually expressed in
mms
• Actual Size – measured size of the finished part
• Limits – maximum and minimum sizes shown by
tolerances (larger value is the upper limit and the
smaller value is the lower limit)
• Tolerance – total allowable variance in dimensions
(upper limit – lower limit) – object dimension could
be as big as the upper limit or as small as the lower
limit or anywhere in between.
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Tolerance
• 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
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Characteristics of Tolerances
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Characteristics of Tolerances
Connection btw the characteristics:
US = N + UD
LS = N – LD
T = US – LS =
+
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The Possible Positions of the Tolerance
Zone
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The ISO System of Limits and Fits (referred to as the
ISO system) is covered in national standards
throughout the world, as shown by the following list:
• Global ISO 286
• USA ANSI B4.2
• Japan JIS B0401
• Germany DIN 7160//61
• France NF E 02-100-122
• UK BSI 4500
• Italy UNI 6388
• Australia AS 1654
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• The tolerance zone is above the zero (base)
line
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• The tolerance zone is on the zero (base)
line
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• The tolerance zone is under the zero (base)
line
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Possible positions of the tolerance zone in case of
shafts
ISO standard uses tolerance position letters with lowercase letters for the
shafts.
Fundamental Deviation: is the deviation closest to the basic size.
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Possible positions of the tolerance zone in the case
of holes
ISO standard uses tolerance position letters with capital letters for the holes.
Fundamental Deviation: is the deviation closest to the basic size.
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The selection of letter freezes
one limit of hole / shaft
(how much away from Basic
size)
Representation of
Tolerance
1) Letter Symbol
Basic Size
45 E8/e7
One can have different
possible combinations; eg.
45H6g7, 45H8r6, 45E5p7
E.S. – upper deviation
E.I. – lower deviation
H : lower deviation of
hole is zero
h : upper deviation of
shaft is zero
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RANGE IN A GIVEN TOLERANCE GRADE
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Representation of Tolerance
2) Number or Grade
IT01, IT0, IT1,….IT16
Tolerance Grade defines range of
dimensions (dimensional variation)
There are manufacturing
constraints on tolerance grade
chosen
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IT: International Tolerance
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• Example: A shaft of nominal diameter 25 mm
is going to be manufactured. IT grade is
required to be IT7.
• Determine the tolerance on the shaft.
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