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Datums and Datum Feature References

6. Datums and Datum Feature References. 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.

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Datums and Datum Feature References

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  1. 6 Datums and Datum Feature References

  2. 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.

  3. 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.

  4. Datum Reference Frame • Made of three planes • Planes locate X, Y, Z axes • Perfect frame • Made from imperfect datum features

  5. Degrees of Freedom • Three translational • Three rotational • Translational are along X, Y, Z • Rotational are u, v, w

  6. 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

  7. Flat Surfaces (Planes) • Commonly used as datum features • Establishes datum plane • High points establish datum • May be simulated with surface plate

  8. Cylindrical Features (Axes)

  9. Cylindrical Features

  10. Targets

  11. Target Points • Locate and label each point • Dimensions may be basic or toleranced • 3 points primary, 2 points secondary, 1 point tertiary

  12. Target Lines • Locate and label each line • Dimensions are typically basic • Side of dowel pins are used

  13. Target Areas • Phantom lines for perimeter • Continuous line for small areas • Located by dimensions or features

  14. Material Boundary Modifiers • Maximum material condition (MMC) • Maximum material boundary (MMB) • No symbol indicates regardless of material boundary (RMB)

  15. 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

  16. 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

  17. Material Boundary Modifierson Datum Features of Size

  18. Least Material • Least material condition (LMC) • Least material boundary (LMB)

  19. 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

  20. RMB and MMB on Surfaces • Boundary farthest outside material is MMB • Boundary closest inside material is LMB • RMB is assumed

  21. RMB, MMB, and LMB Application to Datum Feature Surfaces

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