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Department of Mechanical and Aeronautical Engineering
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MGC110
Assembly Drawings: TolerancesAfr: Toleransies
Department of Mechanical and Aeronautical Engineering
Tolerances
• Surface Tolerances
Measure of roughness or degree of finish a surface must adhere
to during the manufacturing proses
• Limits and fits
The amount of allowable variation on a dimension or a surface
of machined parts
• Geometrical tolerances
Controls the shape and positional distribution of a component
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Department of Mechanical and Aeronautical Engineering
Surface Tolerances
Surface texture is indicated by the following symbols in a detailed drawing:
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Department of Mechanical and Aeronautical Engineering
Surface Tolerances
The following detail can be indicated on a machining symbol
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Machining process specification
Sampling length (mm)
Surface
texture (µm)
Direction of
machining marks
Machining
allowance (mm)
Department of Mechanical and Aeronautical Engineering
Surface Tolerances
Machining symbols must be placed:
� Information is read from LEFT to RIGHT or BOTTOM to TOP.
� Symbols either placed on the surface of on an extension line pointing to a surface
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Department of Mechanical and Aeronautical Engineering
Surface Tolerance
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Department of Mechanical and Aeronautical Engineering
Limits and fits
• All manufactured parts are subject to manufacturing
tolerances.
• For example: a 10mm shaft may measure 9.98mm or
even 10.03 in diameter, this difference in actual size
may be described by a tolerance.
• One might be tempted to think that an outside
diameter of 10.03 could be considered equal to
10mm, which in most cases would be true, except
when this 10.03mm shaft needs to go into a
10.00mm hole, then there is a problem, that is why
tolerances are important
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Department of Mechanical and Aeronautical Engineering
Limits and fits
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Department of Mechanical and Aeronautical Engineering
Limits and fits
Categories of fits:
1. Clearance fits permit relative
freedom of motion between a
shaft and a hole (radially and
axially)
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Department of Mechanical and Aeronautical Engineering
Limits and fits
Categories of fits:
2. Transition fit
3. Interference fit
Interference and
transition fits
secure a certain
amount of tightness
between parts
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Department of Mechanical and Aeronautical Engineering
Limits and fits
ISO standards for fits between components specify a fit
as follows:
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25 (H7 – g6 )
Base or zero
line dimension
H refers to the hole and
numerical value of the
width of the tolerance
band
g refers to the shaft
and the numerical
values of the width of
the tolerance band
Department of Mechanical and Aeronautical Engineering
Limits and fits
Example:
a) Calculate the tolerances for a shaft and a hole with
ISO fit standard 25(H7-g6)
b) Show the application on a drawing
STEP 1: Use the table in your textbook to determine the
tolerances for the given fit standard.
� Diameter of 25 falls in the section 18-30 (red block)
� H7 – g6 in the green block
� Where they cross you find the answer (Remember ‘g’
for shaft and ‘H’ for hole)
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Department of Mechanical and Aeronautical Engineering
Limits and fits
Department of Mechanical and Aeronautical Engineering
Limits and fits
STEP 2: For the shaft the deviation from the zero line
for a base diameter of 25mm with g6 clearance fit is
shown as:
-7 and -20. This means:
25.000 – 0.007 = 24.993
25.000 – 0.020 = 24.980
*NOTE: Values from the table in µm
STEP 3: For the hole the deviation from the zero line for
a base diameter of 25mm with H7 clearance fit is shown
as: +0 and + 21. This means:
25.000 + 0.000 = 25.000
25.000 + 0.021 = 25.021
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Department of Mechanical and Aeronautical Engineering
Limits and fits
If the whole is manufactured to the tolerances obtained
in Step 2 we will get the correct clearance fit.
b)
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Department of Mechanical and Aeronautical Engineering
Geometrical tolerances
Geometrical tolerances are tolerances that control the
shape and positional distribution of a component.
Geometrical tolerances add to manufacturing costs and
should only be specified when:
a) the shape of the component is detrimental to the
function of a component (i.e. the shaft of a roller
press)
b) the degree of accuracy obtained by normal
manufacturing techniques is not sufficient.
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Department of Mechanical and Aeronautical Engineering
Geometrical tolerances
Single feature geometrical tolerances specify form or
shape of the component
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Department of Mechanical and Aeronautical Engineering
Geometrical tolerances
Related feature relate the attitude or position of two
features relative to each other
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Department of Mechanical and Aeronautical Engineering
Geometrical tolerances
Application of geometrical tolerances
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Department of Mechanical and Aeronautical Engineering
Geometric tolerances
Positioning of geometric tolerances
NB!! Arrows presented here are not the same as the
ones you have to draw by hand.
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Department of Mechanical and Aeronautical Engineering
Geometric tolerances
Reference feature
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Department of Mechanical and Aeronautical Engineering
Geometric tolerances
Indicating a reference plane
OR
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Department of Mechanical and Aeronautical Engineering
Single feature geometric tolerances
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Department of Mechanical and Aeronautical Engineering
Single feature geometric tolerances
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Department of Mechanical and Aeronautical Engineering
Related features geometric tolerances
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Department of Mechanical and Aeronautical Engineering
Related features geometric tolerances
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Department of Mechanical and Aeronautical Engineering
Position of tolerances
Normal dimension tolerances
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Department of Mechanical and Aeronautical Engineering
Position of tolerances
Geometric tolerances of position
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