Subject: Understanding geometric tolerances



Each feature control frame consists of at least two compartments. The first compart­ment contains a geometric tolerance symbol that indicates the geometric characteris­tic to which the tolerance is applied, such as location, orientation, or form. For example, a form tolerance may indicate the flatness or roundness of a surface. The geometric tolerance symbols and their characteristics are shown in the following table.                                                               

Geometric tolerance symbols

Symbol

Characteristic

Type

 

Position

Location

Concentricity or coaxiality

Location

Symmetry

Location

Parallelism

Orientation

Perpendicularity

Orientation

Angularity

Orientation

Cylindricity

Form

Flatness

Form

Circularity or roundness

Form

Straightness

Form

Profile of a surface

Profile

Profile of a line

Profile

Circular runout

Runout

Total runout

Runout

   

The second compartment contains the tolerance value. When appropriate, the toler­ance value is preceded by a diameter symbol and followed by a material condition symbol. The material conditions apply to features that can vary in size. The material condition symbols and their meanings are shown in the following table.      

Material conditions

Symbol

Definition

 

At maximum material condition (MMC), a feature contains the maximum amount of material stated in the limits.

 

At least material condition (LMC), a feature contains the minimum amount of material stated in the limits.

 

Regardless of feature size (RFS) indicates that the feature can be any size within the stated limits.

The tolerance value can then be followed by primary, secondary, and tertiary datum reference letters, along with the material conditions of each datum. Datum reference letters are generally used as reference tolerances to one of up to three perpendicular planes from which a measurement is made, although datum reference letters can also indicate an exact point or axis.     

 

A   Geometric characteristic symbol.

B   Diameter symbol.

C   Tolerance value.

D   Material condition symbol.

E   Datum reference.

 

When two tolerances apply to the same geometry, you can also add a composite toler­ance consisting of a primary tolerance value followed by a secondary tolerance value. To make a tolerance even more specific, it can also contain a projected tolerance con­sisting of a height value followed by a projected tolerance symbol. For example, you can use a projected tolerance to indicate the perpendicularity of an embedded part.       

Understanding_geometric_tolerances
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