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 ofFinished 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 partcannot 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.9

Selection of Acceptable Parts

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Selection of Acceptable Parts

There is no such thing as an "exact size".11

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 = + 15

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

Fundamental Deviation: is the deviation closest to the basic size.

ISO standard uses tolerance position letters with lowercase letters for the

shafts.

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Possible positions of the tolerance zone in the case of holes

Fundamental Deviation: is the deviation closest to the basic size.

ISO standard uses tolerance position letters with capital letters for the holes.

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H : lower deviation of

hole is zero

h : upper deviation of

shaft is zero

E.S. – upper deviation

E.I. – lower deviation

Representation of

Tolerance

1) Letter Symbol

The selection of letter freezes

one limit of hole / shaft

(how much away from Basic

size)

45 E8/e7Basic Size

One can have different

possible combinations; eg.

45H6g7, 45H8r6, 45E5p7

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RANGE IN A GIVEN TOLERANCE GRADE

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Representation of Tolerance

Tolerance Grade defines range of

dimensions (dimensional variation)

There are manufacturing

constraints on tolerance grade

chosen

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2) Number or Grade IT01, IT0, IT1,….IT16

IT: International Tolerance30

• 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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