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6 Datums and Datum Feature References. GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Define the difference between a theoretically.

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Presentation on theme: "6 Datums and Datum Feature References. GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Define the difference between a theoretically."— Presentation transcript:

1 6 Datums and Datum Feature References

2 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.

3 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. 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 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Datum Reference Frame Made of three planes Planes locate X, Y, Z axes Perfect frame Made from imperfect datum features

5 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Degrees of Freedom Three translational Three rotational Translational are along X, Y, Z Rotational are u, v, w

6 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. 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 targetsDatum target symbol

7 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Flat Surfaces (Planes) Commonly used as datum features Establishes datum plane High points establish datum May be simulated with surface plate

8 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Cylindrical Features (Axes)

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

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

11 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Target Points Locate and label each point Dimensions may be basic or toleranced 3 points primary, 2 points secondary, 1 point tertiary

12 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Target Lines Locate and label each line Dimensions are typically basic Side of dowel pins are used

13 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Target Areas Phantom lines for perimeter Continuous line for small areas Located by dimensions or features

14 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Material Boundary Modifiers Maximum material condition (MMC) Maximum material boundary (MMB) No symbol indicates regardless of material boundary (RMB)

15 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. 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 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. 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 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Material Boundary Modifiers on Datum Features of Size

18 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Least Material Least material condition (LMC) Least material boundary (LMB)

19 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Compound Datum Features Rotating shaft is typical application Reflects functional need Compound datum feature reference shows two datum letters separated by a dashCompound datum feature reference May also be used to establish a plane

20 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. RMB and MMB on Surfaces Boundary farthest outside material is MMB Boundary closest inside material is LMB RMB is assumed

21 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. RMB, MMB, and LMB Application to Datum Feature Surfaces

22 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Clocking –Control of the rotational position of one or more features relative to another feature on a part or in an assembly. Compound datum features –A term sometimes applied to multiple datum features. Two or more datum features that act together to establish a single datum plane or axis.

23 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Compound datum feature reference –A term sometimes applied to a multiple datum feature reference. Two or more datum letters separated by a dash and placed in one cell of a feature control frame or shown in a note. Datum feature reference B-C indicates that features B and C are to be used to establish one datum.

24 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Datum –Theoretical point, line, or plane established from datum features on a part. Datum feature –Physical feature on a part that is used to establish the location of a theoretically perfect datum. Datum feature reference –Placement of a datum letter in a feature control frame or in a note that specifies requirements relative to datums.

25 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Datum feature symbol –A square or rectangle drawn around a letter with a leader extending to a triangle. This symbol is used to identify datum features on a drawing. Datum reference frame (DRF) –Three planes that establish a coordinate system. A datum reference frame is created by datum feature references in a feature control frame or note. One part may have multiple datum reference frames.

26 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Datum simulation –The use of a tool to contact a datum feature and establish the location of the part relative to the datum. Datum simulator –A tool used to contact a datum feature. The tool probably contains some error and therefore does not establish a perfect datum, but adequately simulates the datum for practical purposes.

27 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Datum target symbol –A circle containing a letter and a number to identify datum targets and associate them with a specific datum. Envelope –A geometric boundary that contains an external feature or is contained by an internal feature. An envelope typically establishes one boundary that may not be violated.

28 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Equalizing datum targets –Targets located such that when they are contacted the forces on the part result in a centering effect. Least Material Size (LMS) –The size limit of the feature at which the least material is allowed. The least material size is the size of the least material condition.

29 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Maximum Material Boundary (MMB) –The limit outside the material that a feature may extend to as a result of size and geometric tolerances that are applicable to the feature. Surfaces and features of size have material boundaries. Movable datum target symbol –A means of indicating that the datum target simulator is movable.

30 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Related actual mating envelope –An actual mating envelope that is related to one or more datums as required for the geometric tolerances that are applied to the feature. Stepped datum surfaces –The condition of two or more non-coplanar surfaces used to establish a single datum.

31 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Target area –A portion of a surface that is identified as a target. The target area is to be contacted with a tool that is sized to the dimensions of the target area. Target line –A line on a surface that is to act as a target. The side of a dowel pin can be positioned to contact the line and establish the datum.

32 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Target point –A single point on a surface that is to act as a target. A spherical tool located to contact the point can be used to establish the datum. True geometric counterpart (TGC) –A perfect geometric shape corresponding to the shape of a feature on a part.

33 GD&T: Application and Interpretation© Goodheart-Willcox Co., Inc. Unrelated actual mating envelope –An actual mating envelope that is not related to any datums and that encompasses all size and form variations on the feature.


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