1. Datums and Datum Feature References6
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.
© Goodheart-Willcox Co., Inc.

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.
© Goodheart-Willcox Co., Inc.

4. Datum Reference Frame Made of three planes Planes locate X, Y, Z axes
Perfect frame
Made from imperfect datum features
© Goodheart-Willcox Co., Inc.

5. Degrees of Freedom Three translational Three rotational
Translational are along X, Y, Z
Rotational are u, v, w
© Goodheart-Willcox Co., Inc.

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
© Goodheart-Willcox Co., Inc.

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

8. Cylindrical Features (Axes)
© Goodheart-Willcox Co., Inc.

9. Cylindrical Features
© Goodheart-Willcox Co., Inc.

10. Targets
© Goodheart-Willcox Co., Inc.

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

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

13. Target Areas Phantom lines for perimeter
Continuous line for small areas
Located by dimensions or features
© Goodheart-Willcox Co., Inc.

14. Material Boundary Modifiers
Maximum material condition (MMC)
Maximum material boundary (MMB)
No symbol indicates regardless of material boundary (RMB)
© Goodheart-Willcox Co., Inc.

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
© Goodheart-Willcox Co., Inc.

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
© Goodheart-Willcox Co., Inc.

17. Material Boundary Modifiers on Datum Features of Size
© Goodheart-Willcox Co., Inc.

18. Least Material Least material condition (LMC)
Least material boundary (LMB)
© Goodheart-Willcox Co., Inc.

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
© Goodheart-Willcox Co., Inc.

20. RMB and MMB on Surfaces Boundary farthest outside material is MMB
Boundary closest inside material is LMB
RMB is assumed
© Goodheart-Willcox Co., Inc.

21. RMB, MMB, and LMB Application to Datum Feature Surfaces
© Goodheart-Willcox Co., Inc.

22. Compound datum features
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.
© Goodheart-Willcox Co., Inc.

23. 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.
© Goodheart-Willcox Co., Inc.

24. Datum feature reference
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.
© Goodheart-Willcox Co., Inc.

25. Datum reference frame (DRF)
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.
© Goodheart-Willcox Co., Inc.

26. Datum simulation Datum simulator
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.
© Goodheart-Willcox Co., Inc.

27. Datum target symbol Envelope
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.
© Goodheart-Willcox Co., Inc.

28. 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.
© Goodheart-Willcox Co., Inc.

29. 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.
© Goodheart-Willcox Co., Inc.

30. 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.
© Goodheart-Willcox Co., Inc.

31. Target area Target line
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.
© Goodheart-Willcox Co., Inc.

32. True geometric counterpart (TGC)
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.
© Goodheart-Willcox Co., Inc.

33. 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.
© Goodheart-Willcox Co., Inc.