datums

3Datums

Permission granted to reproduce for educational use only.© Goodheart-Willcox Co., Inc.

• 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.

L earning Objectives


• 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.

L earning Objectives


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 devices


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 Feature

Datum plane

Simulated datum plane

Datum feature

Part

Physical datum feature simulator

Surface of manufacturing or verification equipment


Datum Feature Terms

• Actual 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 Part

• All parts have six degrees of freedom– Three 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 drawing


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 equipment


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 surface


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 simulator

R eview


Identify Parts A and B.

A. Datum feature symbol

B. Feature control frame

R eview


Identify parts A and D and provide dimensions for B, C, and D.

A. Datum plane A

B. 12.2 minimum

C. 12.8 minimum

D. 0.1 geometric tolerance

R eview


Identify items A–G.

A. Basic dimension

B. Datum feature symbol

C. Target point

D. Datum target symbol

E. Datum reference

F. Target number

G. Datum target line

R eview


Identify items A–C.

A. Datum target area

B. Datum target symbol

C. Target area size

R eview


Identify whether the datum reference symbol represents a datum surface, datum axis, or datum center plane in items A–C.A. Datum surfaceB. Datum center planeC. Datum axis

R eview

datums

Documents

datum surfaces

datum surfacecan

datum targets

datum axes

datum feature symbols

datum feature symboldo

educational use

datum reference frames