Geometric Dimensioning & Tolerancing Engineering 22 Geometric Dimensioning & Tolerancing Bruce Mayer, PE Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege.edu

Skill-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? 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 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 - 1 Feature Feature of Size (FOS) Location Dimension 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 - 2 Tolerance Zones Actual Local Size 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 - 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 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 Size A variation in form is allowed between the least material condition (LMC) and the maximum material condition (MMC). Envelope Principle defines the size and form relationships between mating parts.

Limits of Size LMC & MMC Clearance & Allowance ENVELOPE PRINCIPLE LMC

Limits of Size @ X-Section The ACTUAL size of the feature at ANY CROSS SECTION must be within the size BOUNDARY. ØMMC CROSS Sections are what we measure with Calipers or Micrometers ØLMC

Limits of Size - Boundary No portion of the feature may be outside a PERFECT FORM BARRIER at maximum material condition (MMC). The Surface can also be ROUGH Most Common

GD&T Feature Control Frame Basic Dimension - A numerical value used to describe the theoretically exact size, true profile, orientation, or location of a feature or datum target. From ASME Y14.5M-1994 Some ACAD Feature-Frames from Y14.5-1982

ANSI/ASME Y14.5 Rev.s Responsibility for Maintenance of the Standard Shifted ANSI → ASME after the 1994 Version

GD&T – Form & Profile GEOMETRIC CHARACTERISTIC CONTROLS TYPE OF 14 characteristics that may be controlled TYPE OF FEATURE TYPE OF TOLERANCE CHARACTERISTIC SYMBOL FLATNESS INDIVIDUAL (No Datum Reference) STRAIGHTNESS FORM CIRCULARITY CYLINDRICITY INDIVIDUAL or RELATED FEATURES LINE PROFILE PROFILE SURFACE PROFILE

GD&T – Orient, RunOut, Loctn GEOMETRIC CHARACTERISTIC CONTROLS 14 characteristics that may be controlled TYPE OF TYPE OF CHARACTERISTIC SYMBOL FEATURE TOLERANCE PERPENDICULARITY ORIENTATION ANGULARITY PARALLELISM RELATED FEATURES (Datum Reference Required) CIRCULAR RUNOUT RUNOUT TOTAL RUNOUT CONCENTRICITY LOCATION POSITION SYMMETRY

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

Not Well Known: Actual hole-ctr distances, angle of hole-ctrs Std-Tol vs GD&T - 1 Standard Tolerance Not Well Known: Actual hole-ctr distances, angle of hole-ctrs

Specs for Hole Centers and Angularity Std-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 Tolerance .500 Acutually Need TWO Measurement Fixtures A Go-Gage with Ø0.496 Pins A NoGo-Gage with Ø0.504 Pins

Material Conditions Maximum Material Condition (MMC) M 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 M L S Default when no “Circle” Modifier Applied

Maximum Material Condition Holes at MMC Holes at LMC Given If Gage pins are Ø0.493, 2.000 O.C., then at MMC tolerance is +/- 0.007. At LMC however, the holes are larger and the location tolerance expands to +/-0.012 Smallest Holes at narrow Position accept 0.493” Gage Pins Same Gage Pins for LMC Holes w/ Wide-Spacing allow Larger Pos Tol.

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. A ANSI 1982 ASME 1994 A A ISO Datum Reference Symbols →

ANSI Datum Frame Still Widely Used By ACAD for Example...

Datums Illustrated C C A A B B

Flatness

Straightness

Circularity (Roundness)

Cylindricity

Perpendicularity

Parallelism

Angular Tolerances Traditional methods for tolerancing angles require that angled surfaces be very accurate 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)

Profile

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

RunOut Note that the CAUSE of the RunOut is NOT Known Circular Total In CIRCULAR Case Could be some Combo of Circularity & Concentricity In TOTAL Case add Straightness to the list Circular Total

Industrial Example

GD&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…

If you Do NOT Absolutely GD&T Bottom Line If you Do NOT Absolutely NEED GD&T Then Do NOT, Repeat NOT, Use it

GD&T is Not for Everyone All Done for Today GD&T is Not for Everyone Don't Complicate Control of Your Datum Features. Datum features are just that-features. Since they are features, they need to be controlled. It seems that some people do not apply controls to datum features because they think datums are perfect, which is true. However, the features that establish the theoretical datums must be controlled. Other folks tend to over control the datum features. Start with the primary datum feature. Since everything is located and oriented from the datum established by this datum feature, in this case A, the only control required is form. Flatness is the appropriate control here. The secondary datum feature B is nominally at right angles to the first datum plane so it should be controlled with perpendicularity. Datum feature C, in this example, must be located relative to the origin of the datum reference frame established by A and B. The only geometric tolerance that locates surfaces is profile of a surface, which is the required control on datum feature C. Once these datum features are controlled, other features must be located relative to the datum reference framework. Use profile for the surfaces and position for the features of size. http://www.tec-ease.com/tips/june-05.htm

Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege.edu Engr/Math/Physics 25 Appendix  Time For Live Demo Bruce Mayer, PE Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege.edu

GD&T – Datum Surfaces and Features