Chapter 6 Datums and Datum Feature References Objectives

Chapter 6 Datums and Datum Feature References

Objectives • Define the difference between a theoretically perfect datum and a datum feature. • Explain how to create a datum reference frame through references made in a feature control frame. • Utilize all methods for identifying datum features, including the use of target points, lines, and areas. Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Objectives • Make datum feature references in a feature control frame using the correct order of precedence. • Explain how a datum reference frame may be simulated when three mutually perpendicular surfaces are referenced as datum features. • Use material boundary modifiers on datum feature references and explain the significance of the modifiers. • Identify the degrees of freedom constrained by each referenced datum feature in a datum reference frame. Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Datum Nomenclature • • • Datum feature references Datum features True geometric counterpart (TGC) Datum simulation Datum simulators Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Datum Reference Frame • • Made of three planes Planes locate X, Y, Z axes Perfect frame Made from imperfect datum features Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Degrees of Freedom • • Three translational Three rotational Translational are along X, Y, Z Rotational are u, v, w Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Datum Identification • Datum features must be identified • Pre-1982 standards allowed implied datums Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

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 Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Flat Surfaces (Planes) • • Commonly used as datum features Establishes datum plane High points establish datum May be simulated with surface plate Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Cylindrical Features (Axes) Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Targets Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Target Points • • Locate and label each point Dimensions may be basic or toleranced Spherical ended posts are used 3 points primary, 2 points secondary, 1 point tertiary Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Target Lines • Locate and label each line • Dimensions are typically basic • Side of dowel pins are used Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Target Areas • • Phantom lines for perimeter Continuous line for small areas Located by dimensions or features Consider standard tooling components Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Datum Simulation • • Tool quality hardware CMM CNC Establish theoretical locations of datums from imperfect datum features • Methods should be precise Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Material Boundary Modifiers • Maximum material condition (MMC) • Maximum material boundary (MMB) • No symbol indicates regardless of material boundary (RMB) Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Surfaces as Datum Features • • • Flat surfaces Interrupted surface Note indicating number of surfaces Surface identified as datum feature Datum feature must be referenced in a feature control frame or note Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

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 Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Primary Datum (Flat Datum Feature) • • • Primary is the first datum feature reference Located by three highest points Simulator must be as large as datum feature Workpiece must be stabilized on primary plane 3 of the 6 degrees of freedom constrained Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Secondary Datum (Flat Datum Feature) • • • Second datum feature reference Perpendicular to primary datum Located by two highest points Simulator must be as large as datum feature Workpiece must be stabilized on secondary plane 2 of the 6 degrees of freedom constrained Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Tertiary Datum (Flat Datum Feature) • • • Third datum feature reference Perpendicular to primary and secondary datums Located by highest point Simulator must be as large as datum feature Workpiece must be stabilized on tertiary plane 1 of the 6 degrees of freedom constrained Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

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 Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Cylindrical Features Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Features of Size from Parallel Surfaces • Slots, rails, tabs • Datum is at the center • Datum feature symbol in line with dimension or applied to an applicable feature control frame Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Datum Simulation • Method depends on datum feature reference • Lathe chuck or collet usually sufficient for cylindrical features Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Datum Feature References in a Feature Control Frame • Datum feature has no effect without reference to it • Number based on degrees of freedom that need to be constrained Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Order of Precedence Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Material Boundary Modifiers on Datum Features of Size Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Least Material • Least material condition (LMC) • Least material size (LMS) • Least material boundary (LMB) Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Multiple Feature Control Frames Related to the Same Datum Reference Frame • Clocking is a general term that means to constrain a rotational degree of freedom Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Datum Targets Based on Order of Precedence Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

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 Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

A Surface and Two Holes • No datum feature symbols on centerlines • No datum feature symbols on two perpendicular centerlines • Only a surface and two holes needed to establish coordinate system Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Stepped Datum Surfaces • Non-coplanar surfaces • Step distance is basic Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Equalizing Datum Targets • Cause workpiece to be centered • Movable datum target symbol used Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Targets on Diameters • Shown on multiple views • Movable datum target symbol may be used, but not required • May use points, lines, areas, or a combination • May be used internally or externally Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Dimensions Related to a Datum Reference Frame • Control feature size and orientation relative to a datum reference frame • Done by including a note • Envelope size based on MMC for dimensioned feature • Envelope has perfect orientation with respect to the noted datum reference frame Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Datum Feature Translation • Datum simulator is allowed to move Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Customized Datum Feature References • Exception to the default constraints must be noted • Notation follows datum feature reference letter • Notations made with lowercase letters within brackets • Coordinate axes are identified Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Noting Translational Degrees of Freedom • Axes X, Y, and Z labeled • Translational degrees of freedom typically not labeled on the axes • When two or more coordinate axes systems, datum reference frame noted • Applicable datum reference frame in brackets Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Specifying Applicable Degrees of Freedom in Datum References • Degrees of freedom may need to be limited • Constrained degrees of freedom are noted in brackets • Degrees of freedom noted in lowercase letters Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

RMB and MMB on Surfaces • Boundary farthest outside material is MMB • Boundary closest inside material is LMB • RMB is assumed Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Actual Mating Envelope • Simulator known as an unrelated actual mating envelope not constrained to higher precedence datums • Simulator known as a related actual mating envelope constrained to higher precedence datums Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

RMB, MMB, and LMB Application to Datum Feature Surfaces Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Material Boundary Modifiers on Offset Datum Features • RMB, MMB, or LMB may be applicable • [BASIC] or [BSC] may be noted Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.

Restrained Datum Features • Allowable restraining force may be indicated • Examples include auto body panels and aircraft structural parts • Excessive force – Preloading parts – Reduced structural integrity – Reduced fatigue life • Note may define or reference document Copyright Goodheart-Willcox Co. , Inc. May not be posted to a publicly accessible website.