BMayer@ChabotCollege.edu ENGR-22_Lec-19_GDT-1.ppt 1 Bruce Mayer, PE Engineering 22 – Engineering...

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BMayer@ChabotCollege.edu • ENGR-22_Lec-19_GDT-1.ppt1

Bruce Mayer, PE Engineering 22 – Engineering Design Graphics

Bruce Mayer, PELicensed Electrical & Mechanical Engineer

BMayer@ChabotCollege.edu

Engineering 22

GeometricGeometricDimensioninDimensionin

TolerancingTolerancing

Skill-Development GoalSkill-Development Goal

To learn how to effectively tolerance parts such that • The Parts Function Correctly

• Fabrication Cost Is Kept To A Minimum

Apply Geometric Dimensioning & Tolerancing (GD&T); in particuluar• Position, Size

• Flatness, Circularity

• Perpendicularity, Parallelism

Geometric Dims & Tolerancing

Uses Standard Symbols To Indicate Tolerances That Are Based On The Feature’s Geometry.

Sometimes Called Feature-Based Dimensioning & Tolerancing, Or True Position Dimensioning & Tolerancing

Uses “Feature Control Frames” To Indicate Tolerance(s)

“State of the Art” for Tolerances

Geometric?Geometric?

The “G” in GD&T refers to Geometric Forms • e.g., plane, circle, cylinder, sq, or hexagon

Theoretically these forms are Perfect – but any REAL Form will be Imperfect

In GD&T The Limits of Real Variation (tolerance) are Specified by the Diameter/Width of a Planer, Cylindrical, Annular, or Spherical Zone

History of TolerancingHistory of Tolerancing

In the 1800’s, manufacturing used the “cut & try, file & fit” approach.

The plus-minus (or coordinate) system of tolerancing was next developed.

In the 1900’s, the first GD&T standards came out to improve the quality & utility of engineering drawings.

In 1966, the united GD&T standard was published → ANSI - Y14.5M

GD&T Definitions - 1GD&T Definitions - 1 Feature

• General term applied to a physical portion of a part, such as a surface, hole, or slot.

Feature of Size (FOS)• One cylindrical or spherical surface, or a set

of parallel surfaces, associated with a size dimension. (Can be external or internal)

Location Dimension• Locates the centerline or centerplane of a

part feature relative to: another part feature, centerline, or datum.

GD&T Definitions - 2GD&T Definitions - 2

Tolerance Zones• all geometric tolerances have imaginary

tolerance zones that are the basis for acceptance or rejection of the product

• have specific shapes depending on the geometric tolerance and feature being controlled

Actual Local Size• the value of any individual distance at any

cross section of a FOS

GD&T Definitions - 3GD&T Definitions - 3

Actual Mating Envelope (AME)• a similar perfect feature counterpart that

can be circumscribed/inscribed about/within the feature so it just contacts the surfaces at the highest & lowest points

• It is derived from an actual part

• Used When Calculating a “Bonus Tolerance”– More on this Next Time

Envelope PrincipleEnvelope Principle

Proper Tolerancing establishes the ENVELOPE of the “perfect” part

Any deviation in FORM is acceptable, as long as it remains within the limits of size

Limits of SizeLimits of Size

A variation in form is allowed between the least material condition (LMC) and the maximum material condition (MMC).

SIZE DIMENSION

ENVELOPE OF SIZE

(2.003)

(2.007)

ENVELOPE PRINCIPLE

Envelope Principle defines the size and form relationships between mating parts.

Limits of Size LMC & MMCLimits of Size LMC & MMC

Clearance & AllowanceENVELOPE PRINCIPLE

LMCCLEARANCE

MMCALLOWANCE

Limits of Size @ X-SectionLimits of Size @ X-Section

The ACTUAL size of the feature at ANY CROSS SECTION must be within the size BOUNDARY.

CROSS Sections are what we measure with Calipers or Micrometers

Limits of Size - BoundaryLimits of Size - Boundary

No portion of the feature may be outside a PERFECT FORM BARRIER at maximum material condition (MMC).

Most CommonThe Surface can also be ROUGH

GD&T Feature Control FrameGD&T Feature Control Frame

From ASME Y14.5M-1994• Some ACAD Feature-Frames from Y14.5-1982

ANSI/ASME Y14.5 Rev.sANSI/ASME Y14.5 Rev.s

Responsibility for Maintenance of the Standard Shifted ANSI → ASME after the 1994 Version

GD&T – Form & ProfileGD&T – Form & Profile

INDIVIDUAL (No Datum Reference)

INDIVIDUAL or RELATED FEATURES

GEOMETRIC CHARACTERISTIC CONTROLS

TYPE OFFEATURE

TYPE OFTOLERANCE CHARACTERISTIC SYMBOL

FLATNESS

STRAIGHTNESS

CIRCULARITY

CYLINDRICITY

LINE PROFILE

SURFACE PROFILE

PROFILE

14 characteristics that may be controlled

GD&T – Orient, RunOut, LoctnGD&T – Orient, RunOut, Loctn

RELATED FEATURES (Datum Reference Required)

GEOMETRIC CHARACTERISTIC CONTROLS

TYPE OFFEATURE

TYPE OFTOLERANCE CHARACTERISTIC SYMBOL

SYMMETRY

PERPENDICULARITY

ANGULARITY

PARALLELISM

CIRCULAR RUNOUT

TOTAL RUNOUT

CONCENTRICITY

POSITION

ORIENTATION

RUNOUT

LOCATION

14 characteristics that may be controlled

Understanding Tolerance Zones Traditional ± type of tolerancing describes a

SQUARE zone for acceptable locations. GD&T describes a CIRCULAR zone around

the theoretically exact location for the feature.

Basic Dimension A theoretically exact

dimension used to locate features in GD&T• The Dimension From

Which the Limits of Variation are Derived

Basic dimensions are UNtoleranced• These NOMINAL Dims

are THEORETICALLY Exact

Basic Dims Identified by Enclosure in a FRAME

Std-Tol vs GD&T - 1Std-Tol vs GD&T - 1 Standard Tolerance

Not Well Known: Actual hole-ctr distances, angle of hole-ctrs

Std-Tol vs GD&T - 2Std-Tol vs GD&T - 2 GD&T

Specs for Hole Centers and Angularity

Cylindrical Tolerance Zone

Line Connecting the Centers of the Circles at the Top & Bottom Surfaces Must Fall Completely Within The Tolerance Cylinder

Measure Position ToleranceMeasure Position Tolerance

Acutually Need TWO Measurement Fixtures• A Go-Gage with Ø0.496 Pins

• A NoGo-Gage with Ø0.504 Pins

Material ConditionsMaterial Conditions

Maximum Material Condition (MMC)• largest acceptable size for external feature

• smallest acceptable size for internal feature

• object weighs the most

Least Material Condition (LMC) Regardless of Feature Size (RFS)

• No “Bonus Tolerance” Applied

• Default when no “Circle” Modifier Applied

Maximum Material ConditionMaximum Material Condition

Same Gage Pins for LMC Holes w/ Wide-Spacing allow Larger Pos Tol.

Holes at MMC Holes at LMC

Smallest Holes at narrow Position accept 0.493” Gage Pins

Datums Datums are features

on the object that are used as reference surfaces from which other measurements are made.

Not every GD&T feature requires a datum.

ANSI1982

ASME1994

Datum Reference Symbols →

ANSI Datum FrameANSI Datum Frame

Still Widely Used• By ACAD for Example...

Datums IllustratedDatums Illustrated

FlatnessFlatness

StraightnessStraightness

Circularity (Roundness)Circularity (Roundness)

CylindricityCylindricity

PerpendicularityPerpendicularity

ParallelismParallelism

Angular Tolerances Traditional

methods for tolerancing angles require that angled surfaces be veryaccurate near the vertex of the angle, but can vary more along the length of the angled feature.• That is, the allowable DISPLACEMENT in inches or mm

INCREASES with DISTANCE from the VERTEX

GD&T Angular Tolerance Zone

In (b) Notice How the Width of the Tolerance Zone Expands with Distance From the Vertex

GD&T Eliminates The Zone Expansion• Angles Typically Given as “Basic” or Theoretical (c)

• Tolerance Zones are Then CONSTANT Width (d)

ProfileProfile

Concentricity

Similar to Cylindrical Tolerance, but related to a DATUM Cylinder • The ENTIRE Axis of the Concentric Feature

Must Lie within the Tolerance Zone Relative to the Datum centerline

RunOutRunOut Note that the

CAUSE of the RunOut is NOT Known• In CIRCULAR

Case Could be some Combo of Circularity & Concentricity

• In TOTAL Case add Straightness to the list

Circular

Industrial ExampleIndustrial Example

GD&T Caveat → Use with CareGD&T Caveat → Use with Care

GDT is VERY Powerful, BUT…It it can be Quite CONFUSING and ESOTERIC

Many Degreed Engineers, as well as Most Drafters/Designers, and Some Machinists have only a Vague Notion About Meaning of GDT Symbols• MisApplication and Confusion-Induced

Delays are COMMON– e.g. Try asking what MMC or RFS means…

GD&T Bottom LineGD&T Bottom Line

All Done for TodayAll Done for Today

GD&Tis Not forEveryone

Bruce Mayer, PELicensed Electrical & Mechanical Engineer

BMayer@ChabotCollege.edu

Engr/Math/Physics 25

AppendiAppendixx

6972 23 xxxxf

GD&T – Datum Surfaces and FeaturesGD&T – Datum Surfaces and Features

BMayer@ChabotCollege.edu ENGR-22_Lec-19_GDT-1.ppt 1 Bruce Mayer, PE Engineering 22 – Engineering...

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