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Tolerances of Form and Profile
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Form tolerances
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You are here
Form characteristics• “These characteristics are used to describe form
tolerances of single surface, element, or size features and relate to a perfect geometric counterpart of itself.” (Lowell Foster, Geo-Metrics II, 1986, p 33)
• They are never related to a datum.
3
Straightness tolerance• Perfect Straightness: A condition where an
element of a surface or an axis is in a straight line
• Straightness tolerance specifies a zone within which the required surface element or axis must lie
• A form tolerance – never related to a datum
4
Surface straightness – cylindrical object – feature control frame placement
• Rule 1 applies: Perfect form is required at MMC
5
Connection with a leader is
preferred
Connection to an extension line is also
allowed
Example 5-2
Surface straightness – shape of the tolerance zone
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Applies to each element of the surface
(360° around for a cylinder)
Example 5-2
Illustration of surface straightness
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Figure from Interpreting Engineering Drawings by Cecil Howard Jensen
Effect of surface straightness
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Example 5-2
Axis straightness• This is the exception to Rule 1 – perfect form
is not required at MMC
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Diameter symbol indicates cylindrical tolerance zone
Feature control frame is placed below diameter dimension
Example 5-3
Axis straightness – shape of tolerance zone
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Axis of feature
Example 5-3
Effect of axis straightness - RFS
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Effect of axis straightness - MMC• Again, this is the exception to Rule 1 – perfect
form is not required at MMC
12
Bonus tolerance is
added as size departs from
MMC
Unit straightness• Controls straightness of specified unit portions of the
feature• Normally used in conjunction with a straightness
tolerance for the entire feature length– Prevents extreme variations in straightness of the entire
feature
• Prevents abrupt surface or axis variations within relatively short lengths
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Unit straightness
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Straightness of non-cylindrical symmetrical features
• Center plane must lie between two parallel planes separated by distance equal to the straightness tolerance.
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No Ø symbol, so tolerance zone is two
parallel planes
Center plane of feature
Straightness of a flat surface
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Each parallel element
Each parallel element
Direction of tolerance zone determined by placement of
feature control frame
Straightness of a surface – curved in one direction
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Figure from Interpreting Engineering Drawings by Cecil Howard Jensen
Straightness of a surface – conical
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Figure from Interpreting Engineering Drawings by Cecil Howard Jensen
Application - straightness
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Application - straightness
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Application - straightness
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Diameter symbol indicates cylindrical tolerance zone (axis
control)
Flatness tolerance• Perfect Flatness: A condition of a surface
where all the elements are in one plane• Flatness tolerance establishes the distance
between two parallel planes within which all elements of the surface must lie
• A form tolerance – never related to a datum• Always regardless of feature size (RFS)
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Flatness – placement of feature control frame
• Connected by a leader or an extension line• Placed in the view where the surface appears as a
line
23Example 5-8
Flatness – the tolerance zone
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Two parallel planes
Specific area flatness• Controls flatness for only part of a surface• Can save money since only part of the surface
may require finishing
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Area is outlined by a phantom line
Specific area flatness – specifying on drawing
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Can also use toleranced dimensions
Phantom lines
Section lines
Unit flatness• Controls the flatness of specified unit portions of a
surface• Normally used in conjunction with a straightness
tolerance for the entire feature length• Prevents abrupt surface or axis variations within
relatively short lengths
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Application – flatness
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Application – flatness with unit flatness
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Circularity tolerance• Perfect Circularity: Any cross section taken
perpendicular to the axis of a cylinder, cone, or sphere is a perfect circle.
• Circularity tolerance establishes a radius zone, creating two concentric circles within which the actual cross section must lie.
• A form tolerance – never related to a datum• Always regardless of feature size (RFS)
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Circularity – the tolerance zone
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Two concentric circles, separated radically by the given circularity tolerance
Applications of circularity tolerance
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Can place in longitudinal view or view where surface appears as a
circle
Circularity and free state variation• Free state variation: The distortion of a part after removal of
forces applied during manufacture• Circularity tolerance must be less than the size tolerance,
except for parts subject to free state variation.• This can happen with thin-wall parts that may be affected by
stresses induced during manufacturing that are thus considered nonrigid.
• A nonrigid part may have to meet tolerances in its free state, in a constrained condition (e.g. connected to a simulated mating part), or in both conditions.
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Free state variation for circularity
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Free state symbol
Since part is nonrigid, average diameter is
specified
Application - circularity
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Application - circularity
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Application – circularity with free state variation
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Cylindricity tolerance• Cylindricity tolerance establishes a radius zone,
creating two concentric cylinders within which all elements of the actual surface must lie
• A form tolerance – never related to a datum• Always regardless of feature size (RFS)
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Cylindricity – the tolerance zone
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Two concentric cylinders
Application - cylindricity
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Application - cylindricity
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Could we use cylindricity?
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Could we use cylindricity?
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Hope these quotes don’t apply to your class experience (see if you can guess who said them)
• "If I seem unduly clear to you, you must have misunderstood what I said."
• "I know you believe you understand what you think I said, but I am not sure you realize that what you heard is not what I meant.“
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Answer – Alan Greenspan
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Hope this quote does apply to this class
• "The human brain is unique in that it is the only container of which it can be said that the more you put into it, the more it will hold."
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Answer – Glen Doman
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Profile tolerances
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You are here
Profile tolerance• Profile tolerances control the form and/or
orientation of straight lines or surfaces, arcs, and irregular curves.
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Profile tolerance (cont.)• Profile: The outline of an object• True profile: The actual desired shape of the
object and the basis for the profile tolerance. Normally defined by basic dimensions or electronic data (e.g. CAD model)
• Profile tolerance: Specifies a uniform boundary along the true profile within which the elements of the surface must lie
• Profile tolerances are bilateral (distributed equally about both sides of the true profile) unless otherwise noted.
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Profile tolerance (cont.)• Most versatile geometric tolerance
– May be used to control form or combinations of size, form, and orientation
– As a result, is very useful as the default tolerance for 3D electronic definition (definition with no 2D drawing)
• Two profile geometric tolerance characteristics– Profile of a line– Profile of a surface
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Profile of a line Profile of a surface
Example 5-19Example 5-19
Profile of a line
• Profile of a line: A two-dimensional or cross-sectional geometric tolerance that extends along the length of a feature.
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Profile of a line – the tolerance zone
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Example 5-21
Bilateral unless otherwise noted
Profile of a line - between two points
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Profile tolerance only applies to this surface
Example 5-20
Locating the profile tolerance zone – basic dimension vs. +/- dimension
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Locating the profile tolerance zone – basic dimension vs. +/- dimension (cont.)
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Tolerance zone can ‘float’ within limits of size…
…but tolerance zone must remain oriented to datum A
Example 5-20
Profile of a line - all around• Tolerance zone goes around entire object
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Unilateral profile of a line
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Phantom line parallel to true profile, on side
where tolerance zone is
intended
Profile of a surface• Profile of a surface: Controls an entire surface
as a single entity.
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Profile of a surface – the tolerance zone
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Profile of a surface - between two points
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Profile tolerance only applies to this surface
Profile of a surface - all around
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All around symbol
Unilateral profile of a surface• The entire profile tolerance zone is on one
side of the true profile.
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Phantom line parallel to true profile, on side
where tolerance zone is
intended
Profile as a coplanar control – two surfaces• Coplanar surfaces: Two or more surfaces on the
same plane
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Profile as a coplanar control – more than two surfaces
• May be desirable to establish a datum plane with two surfaces & reference the datum for the other surfaces
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Controlling angular orientation with surface profile
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Controlling conical features with surface profile
• May be used independently or as a refinement to size
• Used without a datum reference, controls form
• Used with a datum reference, controls orientation
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Composite profile tolerance
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Upper part locates the features from
the datums
Lower part controls the feature itself
Example 5-31
Composite profile tolerance – the tolerance zones
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0.2 tolerance zone may ‘float’ within the 0.6
zone, but must remain oriented
to A & B
Specifying basic dimensions in a note
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Example 5-25
Application –profile of a surface tolerance
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Application – profile of a surface tolerance
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Application –profile of a line tolerance
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A B