MODULE
3
GEOME TRIC TOLERANCE
After studying the material in this module, student should be
able to :
Ø
List tolerance charateristic symbols
Ø
Draw geometric tolerancing symbols
Ø
Specify position and geometric tolerances
Ø
Indicate datum targets
Ø
Apply geometric tolerance on a working drawing
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GEOMETRIC TOLERANCE
1.0
2
INTRODUCTION
Geometric tolerances are used to control more precisely the shape and form of a
component. They are use only when the shape or form has a particular function
and errors would result in poor performance. Geometrical tolerances are applied in
addition to dimensional tolerances.
Before the introduction of geometrical tolerancing most drawings which required
the control of shape and form would have simple instructions printed against the
required feature, e.g. ‘surface to flat and parallel’. It will be appreciated that
this expression leaves itself open to many interpretations.
Interpretation of the designers requirements is critical and any errors could be
expensive, but the use of symbols which have a fairly precise meaning has
illuminated many of the misunderstandings.
Definitions : Geometrical tolerancing is the system used to control deviations in
geometry. It defines the form and size of a tolerance zone within which a feature
is to be contained.
2.0
SYMBOLS
The symbols used to control the geometrical shape and form of a component are
themselves simple but their application needs to be considered very carefully. The
application of a geometrical tolerance symbol to control a feature or shape may
affect control of another; e.g. positional tolerance may also control squareness and
straightness
Fig 1. Table of geometric tolerance symbols
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GEOMETRIC TOLERANCE
2.0
3
TOLERANCE FRAME
Tolerance frame is a rectangular symbol which contain indications that define the
geometrical tolerance for a feature.
The frame should be divided into
compartments, containing form left to right :
a.
b.
c.
the symbol for the characteristic to be toleranced;
the tolerance value in the unit used for linear dimensions; this value
should be preceded by the diameter symbol if the tolerance zone is
circular or cylindrical;
the letter or letters identifying the datum feature or features,
where appropriate.
Fig 2. Use of Symbols for Tolerance
The basic dimension (size) is the value used to describe the theoretically exact
size, shape or location of a feature. It is the basis from which permissible
variations are established by tolerances on other dimensions in notes, or in feature
control frame ( Fig 2a).
3.0
TOLERANCE FEATURE
The tolerance frame should be connected to the toleranced feature by a leader
line which, in general, is aligned perpendicular to the toleranced feature. At the
toleranced feature the leader should be t erminated by an arrowhead.
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GEOMETRIC TOLERANCE
4
Fig 3. Position of leader line
4.0
TOLERANCE ZONES
A geometrical tolerance defines the form and size of a tolerance zone within which
a feature is to be contained.
Tolerance zone may be one of the following :
a.
the area within a circle
b.
c.
d.
e.
f.
5.0
the area between two concentric circles
the area between two parallel lines
the space within a cylinder
the space between two coaxial cylinders
the space between two parallel surfaces or two parallel
planes the space within a parallelepiped
DATUM FEATURE
A datum should be indicated by an equilateral triangle symbol identify by a capital
letter enclosed in a frame connected to a datum symbol. When a toleranced
feature is related to a datum, the same letter which defines the datum should be
repeated in the tolerance frame. A different letter should be used for each
datum identification.
Fig 4. Datum & datum feature
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GEOMETRIC TOLERANCE
5
When the tolerance frame can be directly connected in a clear and simple manner
with the datum symbol by a leader line, the datum letter may be omitted.
Fig 5. Direct identification of datum
Where the datum is established by a single
feature, it should be indicated by a
letter in
the third compartment of the tolerance frame.
Where a common datum is established by two
features, it should be indicated in the third
compartment of the tolerance frame by two
letters separated by a hyphen.
Datum established by two or more datum features.
datums should be placed in separate compartments.
The letters identifying the
Table shows example of a) indication of datums, b) the datum features c) how
datums are established by means of simulated datum features.
6.0
DATUM TARGETS
A datum targets may be a point, a line, or an area. A datum target should be
indicated by datum target frame and a datum target symbol.
The datum target frame should be a circle divided into two compartments by the
horizontal diameter.
Fig 6. Datum Target Frame
When the datum target is a point, it should be indicated by a cross.
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GEOMETRIC TOLERANCE
6
When it is a line it should be indicated by a line
terminated by a cross.
A leader from the datum target frame terminating a in a arrowhead should point
towards the datum symbol.
Fig 7. Location of Datum Targets
7.0
APPLICATION OF DATUM
Table shows examples of : a) the indication of datum b) the datum features and
c) how datums are established by means of simulated datum feature.
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GEOMETRIC TOLERANCE
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Fig 8. Example of Application of Datum
8.0
DEFINITION, INDICATION AND INTERPRETATION OF
TOLERANCE ZONE
STRAIGHTNESS
A straightness tolerance
specified a tolerance zone
within which an axis or
element must lie Fig 9.
Straightness
is
a
condition in which an
element of a surface or
an axis is a straight line.
Fig 9. Specifying Straightness
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FLATNESS
A
flatness
tolerance
specifies a tolerance zone
defined by two parallel planes
within which the surface
must lie (Fig 10). Flatness is
the condition of a surface
having all elements in one
plane.
Fig 10. Specifying Flatness
ROUNDNESS
A roundness (circularity) tolerance specifies a tolerance zone bounded by two
concentric circles within each circular element of the surface must lie. Roundness
is a condition of a surface of revolution in which, for a cone or cylinder, all points
of the surface intersected by any plane perpendicular to a common axis are
equidistant from that axis.
Fig 11. Specifying Roundness For a Cylinder or Cone
CYLINDRICITY
A cylindricity toerance specifies a tolerance zone bounded by two concentric
cylinders within which the surface must ile. Cylindricity is a condition of a surface
of revolution in which all points of the surface are equidistant from a common axis.
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GEOMETRIC TOLERANCE
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PROFILE
A Profile tolerance specifies a uniform boundary or zone along the true profile
within which all elements of the surface must lie. Profiles are formed by
projecting a three-dimensional figure onto a plane or by taking a cross section
through the figure.
Fig 11. Specifying Profile of a Surface All Round
Fig 12. Specifying Profile of a between Points
ANGULARITY
An
Angularity tolerance specifies a
tolerance zone defined by two parallel
plane at the specified basic angle (other
than 90O) from a datum plane or axis
within which the surface or the axis of
the feature must lie.
Fig 13. Specifying Angularity
For a Plane Surface
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GEOMETRIC TOLERANCE
10
PARALLELISM
A Parallelism tolerance specifies a tolerance zone defined by two parallel plane or
lines parallel to a datum plane or axis, respectively, within which the surface or
axis of the feature must lie.
Fig 14. Specifying Parallelism for a Plane Surface
A Parallelism tolerance specifies a tolerance zone specify a cylindrical tolerance
zone parallel to a datum axis, within which the axis of the feature must lie.
Fig 15. Specifying Parallelism for an Axis Fe ature
Fig 16. Specifying Parallelism for an Axis Feature at MMC
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GEOMETRIC TOLERANCE
11
PERPENDICULAR
A Perpendicular tolerance is a condition of a surface, median plant, or axis at 90O
to a datum plane or axis.
a)
A tolerance zone is defined by two parallel plane perpendicular to a datum
plane, datum axis, or axis is within which the surface of the feature must
lie.
Fig 17. Specifying Perpendicularity
b) A cylindrical tolerance
zone perpendicular to a
datum
plane
within
which the axis of the
feature must lie.
Fig 18. Specifying
Perpendicularity
for an Axis, Pin or Boss .
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GEOMETRIC TOLERANCE
12
CONCENTRICITY
Concentricity is a condition
in which the axes of all
cross-section elements of a
feature’s
surface
of
revolution are common to
the axis of a datum
feature.
A concentricity
tolerance
specifies
a
cylindrical tolerance zone
whose axis coincides with a
datum axis and within which
all cross-sectional axes of
the
feature
being
controlled must lie.
Fig 19. Specifying Concentricity
9.0
APPLICATION OF GEOMETRIC TOLERANCING
The use of various feature control symbols in lieu of notes for position and form
tolerance dimensions as abstracted from ANSI Y14.5-1994 is illustrated in Fig 15.
Fig 20. Application of Symbols to Position and Form Tolerance Dimension
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