03.RulesDatums 71

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1. Understand Rules of GD &T

Rule #1 and Rule #2.

2. Understand the concepts of basic dimensions, virtual condition, inner and outer boundary,worst-case boundary and bonus tolerance.

RulesThere are four rules that apply to drawings in general, and to GD&T in particular. They

specify some relationships that occur on drawing Symbols, Terms and rules are the basics of

GD&T.They are the alphabet, the definitions and the syntax of this language

When no geometric tolerance is specified, the dimensional tolerance controls thegeometric form as

well as the size. No element of the feature shall extend beyond the MMC boundary of perfect form.

The form tolerance increases as the actual size of the feature departs from MMC towards LMC

RULE # 1

Rule #1

Rule #1 is referred to as the "Individual Feature of Size Rule."

In industry the Rule #1 is paraphrased as “ perfect form at MMC” or the “envelope rule”.

“Where only a tolerance of size is specified, the limits of size of an individual feature prescribe the

extent to which variations in its form as well as in its size are allowed”.



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Rule#1 applied to an External feature of size

Each two-point measurement must be within the specified

tolerance

Size and form must allow the part to pass through the boundary



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Rule#1 applied to an Internal feature of size

Each two-point measurement must

be within the specified tolerance



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An example of effects of Rule #1 on a planar FOS.

In Rule #1, the words perfect form mean perfect flatness, straightness, circularity and

cylindricity. In other words if the feature of size is produced at MMC, it is required to

have perfect form

TECHNOTE – For features of size, where only a tolerance of size is

specified, the surfaces shall not extend beyond a boundary (envelope)

of perfect form at MMC.



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RULE # 2 (1994 standard)

RFS automatically appli es to individual tolerances and to datum feature of size.

MMC & LMC must be specif ied where Requir ed.

Rule #2a is an alternative practice of Rule #2 according to which RFS may be specified as asymbol in feature control frames if desired and applicable.



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RULE # 3(1982 Standard)

For al l other geometr ic controls, RFS automaticall y applies

Al l geometr ic tolerances specif ied for screw threads apply to the axis of the thread der ived

from the pitch diameter. Exceptions must be specif ied by a note (such as Major Di a or

Minor Dia). All geometr ic tolerances specif ied for gears and splines must designate the

specif ic f eature (such as Major Dia or M inor D ia) at whi ch each applies.

RULE # 5Where a datum feature of size is control led by a geometr ic tolerance and is specif ied as a

secondary or terti ary datum, the datum appli es at vir tual conditi on wi th respect to

orientation .

INTRODUCTION TO VIRTUAL CONDITIONAND BOUNDARY CONDITIONS

RULE # 4

Definition

Virtual Condition (VC) is a worst-case boundary generated by the collective effects of a

feature of size at MMC or at LMC and the geometric tolerance for that material condition.



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The VC of a FOS includes effects of the size, orientation, and locationfor the FOS.

Inner Boundary (IB) is a worst-case boundary generated by the smallest feature of size

minus the stated geometric tolerance (and any additional tolerance, if applicable).



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Outer Boundary (OB) is a worst-case boundary generated by the largest

feature of size plus the stated geometric tolerance (and any additional

tolerance, if applicable).

Worst-Case Boundary (WCB) is a general term to refer to the extreme boundary of a

FOS that is the worst-case for assembly. Depending upon the part dimensioning, a

worst-case boundary can be a virtual condition, inner boundary, or outer boundary.

Worst-Case Boundary when no Geometric Tolerances are specified.



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MMC Virtual Condition

The virtual condition (or WCB) is the extreme boundary that represents the worst-case for functional

requirements, such as clearance or assembly with a mating part.

In the case of an external FOS, such as a pin or a shaft, the VC (or WCB) is determined by

formula:

VC = MMC + Geometric Tol.



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VC = MMC – Geometric Tol.

In the case of an internal FOS, such as a hole, the VC (or WCB) isdetermined by formula:



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RFS inner and outer boundary

When a geometric tolerance that contains no modifiers (RFS default per Rule #2) in the

tolerance portion of the feature control frame is applied to a FOS, the inner or outer

boundary (or worst-case boundary) of the FOS is affected.In the case of an external FOS, such as a pin or a shaft, the OB (or WCB) is

determined by the formula:

OB = MMC + Geometric Tol.



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In case of an internal FOS, such as a hole, the IB (or WCB) isdetermined by the formula:

IB = MMC – Geometric Tol.



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INTRODUCTION TO BONUS TOLERANCE

The bonus tolerance concept applies to any geometric control that uses the MMC

(or LMC) modifiers in the tolerance portion of the feature control frame.

When the actual mating size of the FOS departs from MMC (towards LMC)

an increase in the stated tolerance- equal to the amount of the departure is

permitted. This increase or extra tolerance is called the bonus tolerance.



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DATUM REFERENCE FRAMES AND DATUM SYSTEMS(PLANAR DATUM)

Set of symbols and rules that communicates to the drawing user how dimensional

measurements are to be made.



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WHY DATUM SYSTEM?

• First, it allows the designer to specify which part surfaces are to contact the inspection

equipment for the measurement of a dimension.

• Second, the datum system allows the designer to specify, in which sequence the part is to

contact the inspection equipment for the measurement of a dimension.

BENEFITS OF DATUM SYSTEM

• It aids in making

repeatable dimensional

measurements.

• It aids in communicating

part functional

relationships.

• It aids in making the

dimensional measurement

as intended by the

designer.

CONSEQUENCES

• Good parts are rejected

• Bad parts are accepted



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DATUMS (PLANAR)

• DATUM

• DATUM FEATURE

• DATUM FEATURE SIMULATOR

• SIMULATED DATUM

• DATUM FEATURE SYMBOL

• DATUM SELECTION



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DATUM• A datum is a theoretically exact plane, point or axis from which a dimensional measurement is

made.

• A Datum is the true geometric counter part of a datum feature

• A true geometric counter part is the theoretical perfect boundary or best fit tangent plane of adatum feature.

DATUM FEATURE

• A datum feature is a part feature that exists on the part and contacts a datum.

SIMULATED DATUM

• A simulated datum is the plane established by the inspection equipment.

DATUM FEATURE SIMULATOR

• A datum feature simulator is the inspection equipment that includes the gage elements used to

establish the simulated datum.



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DATUM FEATURE SYMBOL

• The symbol used to specify a datum feature on a drawing is called the datum

feature symbol.



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DATUM REFERENCE IN FEATURECONTROL FRAME



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• Datum features are selected on the basis of part function and assembly

requirements. Datum features often orient (stabilize) and locate the part in it’s

assembly.



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DATUM REFERENCE FRAME• A datum reference frame is a set of three mutually perpendicular datum planes.

• The datum reference frame provides direction as well as an origin of dimensionalmeasurements.

DATUM REFERENCE FRAME



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DATUM REFERENCE FRAME(contd…)

• The planes of a datum reference frame have zero perpendicularity tolerance to each

other by definition.• The 90˚angle between datum planes are basic.



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DATUM REFERENCE FRAME(contd…)

• When making a location measurement on a part feature, the six degrees of freedom are

restricted by using a datum reference frame.

• The method of bringing a part into contact with the planes of the datum referenceframe has a significant impact on the measurement of the part dimensions.

• Primary datum: This establishes the orientation of the part(stablise the part )to the

datum reference frame.

• The part contacts the datum plane with at least three points of contact.

• The primary datum restricts three degree of freedom

• Secondary datum: This locates the part (restricts part movement) within the datum

reference frame.

• Requires a minimum of two points of contact with the secondary datum.

• The Secondary datum restricts two additional degree of freedom

• Tertiary datum: This locates the part(restricts part movement) within the datum

reference frame.

• Requires a minimum of one points of contact with the secondary datum.

• The tertiary datum restricts the last remaining degree of freedom



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Primary, Secondary and Tertiary Datums



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THE 3-2-1 RULE

• The 3-2-1 rule defines the minimum number of points of contact

required.

• The 3-2-1 rule only applies on a part with all planar datums.

• Only dimensions that are related to a datum reference frame through

geometric tolerances should measure in a datum reference frame.

• If a dimension is not associated to a datum reference frame with a

geometric tolerance, then there is no specification on how to locate the

part in the datum frame.

Datum-related versus FOS dimensions



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DATUM REFERENCE FRAME

Datum related versus FOS



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Datum-related versus FOSdimensions(contd…)



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INTRODUCTION DATUM AXIS AND CENTERPLANE

• Here Feature of Size is used as a datum features• When a diameter is used as a datum feature, It results in a datum axis • When a planar is used as a datum feature, it results in a datum center

plane

Describe the datum that results from a FOS datum feature



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3 Ways for representing an axis as datum

• Datum identification symbol can be touching the surface of a diameter to specify

axis as the datum

Describe the ways to specify an axis as a datum.



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3 Ways for representing an axis as datum(Contd….)

• Datum identification symbol can be touching the beginning

of a leader line of FOS to specify an datum axis



2 W f ti t l



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• Datum identification symbol can be inline with

dimension line to specify on axis or centre plane asdatum

2 Ways for representing a centre plane asdatum

Describe the ways to specify an centre plane as a datum.



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• Datum identification symbol can replace one side of the

dimension line and arrow head

2 Ways for representing a centre plane as

datum (Contd….)



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Datum Terminology

• Datum feature A

• Datum featuresimulator /

Gauge element

• Simulated datum

axis A• Simulated datum

Feature A



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FOS datum feature referenced at MMC



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Datum centre plane MMC primary

Datum axis MMC secondary



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Datum axis MMC secondary

Draw the datum feature simulator for an FOS datum

feature (MMC secondary with virtual condition)



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DATUM TARGETS SYMBOLS(contd….)

• A datum target point is specified by an X shaped symbol, consisting of a pair of lines intersecting at 90°.

• Basic dimensions should used be used to locate datum target points relative

each other and the other datums on the part.

A DATUM TARGET IDENTIFICATION SYMBOL B DATUM TARGET SYMBOL



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• Datum target point



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• Datum target areas



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03.RulesDatums 71