datums

3Datums

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

Permission granted to reproduce for educational use only.

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 object Establish location and size dimensions


Datum Feature Simulator Opposite shape of datum feature Two types: Theoretical datum feature simulator Physical datum feature simulator

Manufacturing examples: Machine tables Surface plates Gauge surfaces Surface tables Specially designed rotation devicesPermission granted to reproduce for educational use only.

Datum Feature Symbol Do not apply to centerlines, center planes, or axes Draw using thin lines Use any letter to identify a datum feature except I, O, or Q Use double letters when number of datums exceeds number of letters in alphabet


Datum Feature Symbol


Datum FeatureDatum plane Part

Datum feature

Simulated datum plane

Surface of manufacturing or verification equipment

Physical datum feature simulator


Datum Feature TermsActual mating envelope Simulated datum Tangent plane


Datum Feature Symbol Placement for Surfaces


Datum Surface Can be controlled by geometric tolerance Datum plane measurements do not take variations in datum surface from datum plane into account


Geometric Control of Datum Surface


Datum Reference Frame (DRF) Concept Select three perpendicular datum features based on their importance to part design Assign precedence and datum reference order Primary datum Secondary datum Tertiary datum


Datum Reference Frame Concept


Datum Reference Frame Concept: Inspecting the PartAll parts have six degrees of freedomThree degrees of translation Three degrees of rotation

Movement is either translational or rotational


Degrees of Freedom


Establishing Multiple Datum Reference Frames Datums X, Y, and Z establish one datum reference frame Datums L and M establish second datum reference frame


Datum Features Specified Individually Place a note next to datum feature symbols indicating how many datum features to consider separately Place the note 2X INDIVIDUALLY next to datum feature symbols of two separate datum features identified by same letter


Datum Target Symbols Identify datum targets Useful on parts with surface or contour irregularities Connect 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 Location dimensions originate from datums


Datum Target Points


Datum Target Points Datum plane established by locating pins at datum target points


Datum Target Areas Used to establish datums using areas of contact Locating pins are flat end tooling pins with pin diameter equal to specified size of target area Top half of the datum target symbol identifies size of target area Use datum target point at center location when area is too small to accurately or clearly display on drawingPermission granted to reproduce for educational use only.

Datum Target Areas


Datum Target Lines Indicated by a target point on edge view of surface and a phantom line on surface view Locating pins are cylindrical or knife-edged with datum target line along tangency where pins meet the part Surface is often placed 90 to the pin to create datum reference frame


Datum Target Lines


Partial Datum Surface


Coplanar Surface Datums Used to establish coplanar surfaces as one datum plane Specify number of surfaces with a note such as: "2 SURFACES" below related feature control frame


Coplanar Surface Datums


Datum Axis Represents two theoretical planes intersecting at 90 Represented in drawings with centerlines Pitch cylinder for screw threads establishes datum axis unless otherwise specified When not using pitch cylinder for screw threads, place note such as "MAJOR DIA" or "MINOR DIA" next to datum feature symbol

Simulated datum axis established by inspection equipmentPermission granted to reproduce for educational use only.

Datum Axis


Datum Feature Symbol Placement for a Datum Axis


Coaxial Datum Features Separate datum reference letters by a dash in one compartment of the feature control frame


Datum Axis Established with Datum Target Symbols Primary datum axis established by two sets of three equally spaced targets Identify datum target points in correlation to adjacent cylindrical datum feature when two cylindrical features of different diameters establish a datum axis


Datum Axis Established with Datum Target Symbols Cylindrical datum target areas and circular datum target lines can be used to establish datum axis Target 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 equally spaced targets on cylindrical surfacePermission granted to reproduce for educational use only.

Movable Datum Target Symbols and Datum Target Points When datum targets establish a center point, axis, or center plane on a RMB basis, datum feature simulator movement is normal to true profile


Movable Datum Target Symbols and Datum Target Spheres Circular phantom line represents spherical datum feature simulator to establish datum axis


Datum Center Plane Axis 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 Modifier Added to feature control frame following datum feature reference and any other applicable modifiers Specifies that basic location of datum feature simulator is unlocked and free to translate within specified geometric tolerance


Translation Modifier


Using Contoured Surface as Datum Feature 3D mathematical coordinate system defines compound surface feature on a part Datum simulator results from mathematical data Common when design copies contour of an actual object 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.

A) Datum feature B) Datum plane C) Part

D) Simulated datum plane E) Physical datum feature simulatorPermission granted to reproduce for educational use only.

Fill in blanks for Part I A and B and Part II A, B, C, and D. Provide dimensions for Part II B, C, and D. Part I: A) Datum feature symbol B) Feature control frame

Part II: A) Datum plane A B) 12.2 minimum C) 12.8 minimum D) 0.1 geometric tolerancePermission granted to reproduce for educational use only.

Identify items AF.

A) Basic dimension B) Datum feature symbol C) Target point D) Datum target symbol E) Datum reference F) Target number G) Datum target line


Identify items AC.

A) Datum target area B) Datum target symbol C) Target area size


Identify whether the datum reference symbol represents a datum surface, datum axis, or datum center plane in items AC.

A) Datum surface B) Datum center plane C) Datum axis


Actual mating envelope The smallest size that can be contracted about an external feature or the largest size that can be expanded within an internal feature.

Coaxial datum features Exist when a single datum axis is established by two datum features that are coaxial.

Coplanar surfaces Two or more surfaces that are on the same plane.


Datum axis The center axis established when a cylindrical datum feature is used.

Datum center plane The plane that splits a symmetrical feature, such as a slot or tab.

Datum feature An actual feature on a part that is used to establish the datum.


Datum feature simulator A perfect theoretical boundary or a physical boundary used to establish a datum or simulated datum from a specified datum feature.

Datum feature symbol A symbol used to identify an object feature specified as a datum and referred to as a datum feature.

Datum plane The theoretically exact plane established by the simulated datum of the datum feature.Permission granted to reproduce for educational use only.

Datum reference frame (DRF) The "frame" created by three datum features that are perpendicular to each other and used for layout purposes.

Datum target lines Datum targets indicated by the target point symbol "X" on the edge view of the surface and by a phantom line on the surface view.

Datum target A point, line, or area of contact on a part used to establish a datum when it is not possible to use a surface.


Datums A surface, plane, point, or axis considered theoretically perfect. A datum is assumed to be exact.

Degrees of freedom The number of coordinates it takes to exclusively control the position of a part.


Normal Term used to describe a feature that is perpendicular to a plane surface and radial to a curved surface.

Order of precedence The order in which the datums appear in the last three compartments of the feature control frame. The primary datum is given first, followed by the secondary datum and then the tertiary datum.

Primary datum The primary datum reference established in the datum reference frame. The primary datum is first in the order of precedence.Permission granted to reproduce for educational use only.

Primary datum plane A datum plane established by at least three points on the primary datum surface. These points are used to provide stability on the primary plane.

Rotational Movement around an axis.

Secondary datum The secondary datum reference established in the datum reference frame. The secondary datum is second in the order of precedence.Permission granted to reproduce for educational use only.

Secondary datum plane A datum plane established by at least two points on the related secondary datum surface. Two points provide the required stability for the secondary plane.

Simulated datum A point, axis, line, or plane consistent with or resulting from processing or inspection equipment, such as a surface plate, inspection table, gage surface, or a mandrel.


Simulated datum axis The axis of a perfectly cylindrical inspection device that contacts the datum feature surface.

Simulated datum center plane The center plane of a perfectly rectangular inspection device that contacts the datum feature surface.

Simulated datum plane The 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.

Tangent plane A plane that contacts the high points of the specified feature surface.

Tertiary datum The tertiary datum reference established in the datum reference frame. The tertiary datum is third in the order of precedence.

Tertiary datum plane A datum plane established by at least one point on the related tertiary datum surface. One point of contact at the tertiary datum plane is all that is required to complete the datum reference frame and provide stability.Permission granted to reproduce for educational use only.

Three degrees of rotation Occurs when a part can rotate about each axis in the datum reference frame.

Three degrees of translation Occurs when a part can move without rotation in each of the three directions from the mutually perpendicular planes.

Translational Refers to uniform movement without rotation.


datums

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