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6Datums and Datum Feature References
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• 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.
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• 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.
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Datum Reference Frame
• Made of three planes• Planes locate X, Y, Z axes• Perfect frame • Made from imperfect datum features
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Degrees of Freedom
• Three translational• Three rotational• Translational are along X, Y, Z• Rotational are u, v, w
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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
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Flat Surfaces (Planes)
• Commonly used as datum features• Establishes datum plane• High points establish datum• May be simulated with surface plate
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Cylindrical Features (Axes)
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Cylindrical Features
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Targets
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Target Points
• Locate and label each point• Dimensions may be basic or toleranced• 3 points primary, 2 points secondary, 1 point
tertiary
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Target Lines
• Locate and label each line• Dimensions are typically basic• Side of dowel pins are used
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Target Areas
• Phantom lines for perimeter• Continuous line for small areas• Located by dimensions or features
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Material Boundary Modifiers
• Maximum material condition (MMC)• Maximum material boundary (MMB)• No symbol indicates regardless of material
boundary (RMB)
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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
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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
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Material Boundary Modifierson Datum Features of Size
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Least Material
• Least material condition (LMC)• Least material boundary (LMB)
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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
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RMB and MMB on Surfaces
• Boundary farthest outside material is MMB• Boundary closest inside material is LMB• RMB is assumed
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RMB, MMB, and LMB Application to Datum Feature Surfaces