Tolerance of Form & Profile

Tolerances of Form and Profile

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Form tolerances

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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.

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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

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Surface straightness – cylindrical object – feature control frame placement

• Rule 1 applies: Perfect form is required at MMC

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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

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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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Straightness of a surface – conical

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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.


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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


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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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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

Documents

Tolerance of Form & Profile