datums and datum feature references

6Datums and Datum Feature References

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

• Define the difference between a theoretically perfect datum and a datum feature.

• Explain how to create a datum reference frame through references made on a drawing.

• Utilize all methods for identifying datum features, including the use of target points, lines, and areas.


• Make datum feature references in a feature control frame using the correct order of precedence.

• Explain how a datum reference frame is established from three referenced datum surfaces.

• Use material boundary modifiers on datum feature references and be able to explain the significance of the modifiers.


Datum Reference Frame

• Made of three planes• Planes locate X, Y, Z axes• Perfect frame • Made from imperfect datum features


Degrees of Freedom

• Three translational• Three rotational• Translational are along X, Y, Z• Rotational are u, v, w


Datum Feature Symbol

• Identifies surfaces and features of size as datum features

• Typically one letter in a square• Connected by short leader line to triangle• Only applied to physical features• Datum target symbol identifies datum targets


Flat Surfaces (Planes)

• Commonly used as datum features• Establishes datum plane• High points establish datum• May be simulated with surface plate


Cylindrical Features (Axes)


Cylindrical Features


Targets


Target Points

• Locate and label each point• Dimensions may be basic or toleranced• 3 points primary, 2 points secondary, 1 point

tertiary


Target Lines

• Locate and label each line• Dimensions are typically basic• Side of dowel pins are used


Target Areas

• Phantom lines for perimeter• Continuous line for small areas• Located by dimensions or features


Material Boundary Modifiers

• Maximum material condition (MMC)• Maximum material boundary (MMB)• No symbol indicates regardless of material

boundary (RMB)


Order of Precedence

• Established from order in feature control frame• Feature control frame referencing A, B, C is

different from feature control frame referencing D, E, F

• Feature control frame referencing A, B, C is different from feature control frame referencing B, A, C


Material Boundary Modifiers on Flat Datum Features

• May have RMB, LMB, or MMB• Generally, avoid LMB and MMB on flat surfaces• No LMB or MMB on primary datum feature

reference when feature is flat• LMB or MMB on references to flat datum

features allowed only when profile creates an applicable boundary


Material Boundary Modifierson Datum Features of Size


Least Material

• Least material condition (LMC)• Least material boundary (LMB)


Compound Datum Features

• Rotating shaft is typical application• Reflects functional need• Compound datum feature reference shows two

datum letters separated by a dash• May also be used to establish a plane


RMB and MMB on Surfaces

• Boundary farthest outside material is MMB• Boundary closest inside material is LMB• RMB is assumed


RMB, MMB, and LMB Application to Datum Feature Surfaces

datums and datum feature references

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