3 Datums
3 Datums
• Explain the purpose of datums • Define common terms related to datum features • Interpret information provided by datum feature symbols • Describe methods for representing datum surfaces on drawings • Explain how to establish datum reference frames
• • • •
Identify the degrees of freedom of a part Explain how to establish datum targets Identify datum target points, areas, and lines Describe methods for representing datum axes on drawings • Describe methods for representing datum center planes on drawings
Datums • Represent true geometric counterpart of a datum feature • Reference features of an obect • Establish location and si!e dimensions
Datum Feature Simulator • "pposite shape of datum feature • #wo types$ % #heoretical datum feature simulator % Physical datum feature simulator
• &anufacturing examples$ % &achine tables % 'urface plates % (auge surfaces % 'urface tables % 'pecially designed rotation devices
Datum Feature Symbol • Do not apply to centerlines, center planes, or axes • Draw using thin lines • )se any letter to identify a datum feature except I, ", or * • )se double letters when number of datums exceeds number of letters in alphabet
Datum Feature Symbol
Datum Feature Datum plane
Datum feature
Simulated datum plane
Part
Physical datum feature simulator
Surface of manufacturing or verification equipment
Datum Feature Terms • +ctual mating envelope •'imulated datum •#angent plane
Datum Feature Symbol Placement for Surfaces
Datum Surface • an be controlled by geometric tolerance • Datum plane measurements do not ta-e variations in datum surface from datum plane into account
Geometric Control of Datum Surface
Datum Reference Frame (DRF) Concept • 'elect three perpendicular datum features based on their importance to part design • +ssign precedence and datum reference order % Primary datum % 'econdary datum % #ertiary datum
Datum Reference Frame Concept
Datum Reference Frame Concept: Inspecting te Part • +ll parts have six degrees of freedom •#hree degrees of translation •#hree degrees of rotation
•&ovement is either translational or rotational
Degrees of Free!om
"stablising #ultiple Datum Reference Frames • Datums , /, and 0 establish one datum reference frame • Datums 1 and & establish second datum reference frame
Datum Features Specifie! In!i$i!ually • Place a note next to datum feature symbols indicating how many datum features to consider separately • Place the note 23 I4DI5ID)+11/6 next to datum feature symbols of two separate datum features identified by same letter
Datum Target Symbols • Identify datum targets • )seful on parts with surface or contour irregularities • onnect to datum target point, line, or area with a leader
Datum Target Symbols
Datum Target Points • Establish primary datum plane by locating at least three points on primary datum surface • Establish secondary datum plane by locating at least two points on related secondary datum surface • Establish tertiary datum plane by locating at least one point on related tertiary datum surface • Dimension using baseline or chain dimensioning • 1ocation dimensions originate from datums
Datum Target Points
Datum Target Points • Datum plane established by locating pins at datum target points
Datum Target %reas • )sed to establish datums using areas of contact • 1ocating pins are flat end tooling pins with pin diameter e7ual to specified si!e of target area • #op half of the datum target symbol identifies si!e of target area • )se datum target point at center location when area is too small to accurately or clearly display on drawing
Datum Target %reas
Datum Target &ines • Indicated by a target point on edge view of surface and a phantom line on surface view • 1ocating pins are cylindrical or -nife8edged with datum target line along tangency where pins meet the part • 'urface is often placed 9: to the pin to create datum reference frame °
Datum Target &ines
Partial Datum Surface
Coplanar Surface Datums • )sed to establish coplanar surfaces as one datum plane • 'pecify number of surfaces with a note such as$ ;3 ')R<+E'; below related feature control frame
Coplanar Surface Datums
Datum %'is • Represents two theoretical planes intersecting at 9: • Represented in drawings with centerlines °
• Pitch cylinder for screw threads establishes datum axis unless otherwise specified % =hen not using pitch cylinder for screw threads, place note such as ;&+>"R DI+; or ;&I4"R DI+; next to datum feature symbol
• 'imulated datum axis established by inspection e7uipment
Datum %'is
Datum Feature Symbol Placement for a Datum %'is
Coa'ial Datum Features • 'eparate datum reference letters by a dash in one compartment of the feature control frame
Datum %'is "stablise! it Datum Target Symbols • Primary datum axis established by two sets of three e7ually spaced targets • Identify datum target points in correlation to adacent cylindrical datum feature when two cylindrical features of different diameters establish a datum axis
Datum %'is "stablise! it Datum Target Symbols • ylindrical datum target areas and circular datum target lines can be used to establish datum axis % #arget area represented by two phantom lines with section lines between % Datum target line represented by phantom line all around part
• Establish secondary datum axis by placing three e7ually spaced targets on cylindrical surface
#o$able Datum Target Symbols an! Datum Target Points • =hen datum targets establish a center point, axis, or center plane on a R&? basis, datum feature simulator movement is normal to true profile
#o$able Datum Target Symbols an! Datum Target Speres • ircular phantom line represents spherical datum feature simulator to establish datum axis
Datum Center Plane • +xis and center plane datum feature symbols align with@replace dimension line arrowhead or appear on feature, leader shoulder, dimension line, or feature control frame
Datum Center Plane
Translation #o!ifier • +dded to feature control frame following datum feature reference and any other applicable modifiers • 'pecifies that basic location of datum feature simulator is unloc-ed and free to translate within specified geometric tolerance
Translation #o!ifier
sing Contoure! Surface as Datum Feature • AD mathematical coordinate system defines compound surface feature on a part • Datum simulator results from mathematical data • ommon when design copies contour of an actual obect that has few or no flat surfaces to use as datums
Identify the datum feature, part, simulated datum plane, physical datum feature simulator, and datum plane.
+B Datum feature ?B Datum plane B Part
DB 'imulated datum plane EB Physical datum feature simulator
Part II$ +B Datum plane + ?B C3.3 minimum B C3. minimum DB :.C geometric tolerance
Identify items +%<.
+B ?asic dimension ?B Datum feature symbol B #arget point DB Datum target symbol EB Datum reference
Identify items +%.
+B Datum target area ?B Datum target symbol B #arget area si!e
Identify whether the datum reference symbol represents a datum surface, datum axis, or datum center plane in items +%.
+B Datum surface ?B Datum center plane
• +ctual mating envelope % #he smallest si!e that can be contracted about an external feature or the largest si!e that can be expanded within an internal feature.
• oaxial datum features % Exist when a single datum axis is established by two datum features that are coaxial.
• oplanar surfaces % #wo or more surfaces that are on the same plane.
Permission granted to reproduce for educational use only.
• Datum axis % #he center axis established when a cylindrical datum feature is used.
• Datum center plane % #he plane that splits a symmetrical feature, such as a slot or tab.
• Datum feature % +n actual feature on a part that is used to establish the datum.
Permission granted to reproduce for educational use only.
• Datum feature simulator % + perfect theoretical boundary or a physical boundary used to establish a datum or simulated datum from a specified datum feature.
• Datum feature symbol % + symbol used to identify an obect feature specified as a datum and referred to as a datum feature.
• Datum plane % #he theoretically exact plane established by the simulated datum of the datum feature. Permission granted to reproduce for educational use only.
• Datum reference frame DR
• Datum target lines % Datum targets indicated by the target point symbol ;; on the edge view of the surface and by a phantom line on the surface view.
• Datum target % + point, line, or area of contact on a part used to establish a datum when it is not possible to use a surface. Permission granted to reproduce for educational use only.
• Datums % + surface, plane, point, or axis considered theoretically perfect. + datum is assumed to be exact.
• Degrees of freedom % #he number of coordinates it ta-es to exclusively control the position of a part.
Permission granted to reproduce for educational use only.
• 4ormal % #erm used to describe a feature that is perpendicular to a plane surface and radial to a curved surface.
• "rder of precedence % #he order in which the datums appear in the last three compartments of the feature control frame. #he primary datum is given first, followed by the secondary datum and then the tertiary datum.
• Primary datum % #he primary datum reference established in the datum reference frame. #he primary datum is first in the order of precedence. Permission granted to reproduce for educational use only.
• Primary datum plane % + datum plane established by at least three points on the primary datum surface. #hese points are used to provide stability on the primary plane.
• Rotational % &ovement around an axis.
• 'econdary datum % #he secondary datum reference established in the datum reference frame. #he secondary datum is second in the order of precedence.
Permission granted to reproduce for educational use only.
• 'econdary datum plane % + datum plane established by at least two points on the related secondary datum surface. #wo points provide the re7uired stability for the secondary plane.
• 'imulated datum % + point, axis, line, or plane consistent with or resulting from processing or inspection e7uipment, such as a surface plate, inspection table, gage surface, or a mandrel.
Permission granted to reproduce for educational use only.
• 'imulated datum axis % #he axis of a perfectly cylindrical inspection device that contacts the datum feature surface.
• 'imulated datum center plane % #he center plane of a perfectly rectangular inspection device that contacts the datum feature surface.
• 'imulated datum plane % #he plane derived from the physical datum feature simulator. It coincides with the datum plane when the datum plane is in contact with the simulated datum plane. Permission granted to reproduce for educational use only.
• #angent plane % + plane that contacts the high points of the specified feature surface.
• #ertiary datum % #he tertiary datum reference established in the datum reference frame. #he tertiary datum is third in the order of precedence.
• #ertiary datum plane % + datum plane established by at least one point on the related tertiary datum surface. "ne point of contact at the tertiary datum plane is all that is re7uired to complete the datum reference frame and provide stability. Permission granted to reproduce for educational use only.
