1. Geometric Dimensioning & Tolerancing
Engineering 22
Geometric Dimensioning & Tolerancing
Bruce Mayer, PE
Licensed Electrical & Mechanical Engineer

2. 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

3. 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

4. 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

5. 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

6. 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.

7. 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

8. 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

9. 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

10. 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.

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

12. 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

13. 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

14. 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 Y

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

16. 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

17. 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

18. 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.

19. 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

20. 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

21. Specs for Hole Centers and Angularity
Std-Tol vs GD&T - 2
GD&T
Specs for Hole Centers and Angularity

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

23. Measure Position Tolerance
.500
Acutually Need TWO Measurement Fixtures
A Go-Gage with Ø0.496 Pins
A NoGo-Gage with Ø0.504 Pins

24. 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

25. Maximum Material Condition
Holes at MMC
Holes at LMC
Given
If Gage pins are Ø0.493, O.C., then at MMC tolerance is +/ 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.

26. 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 →

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

28. Datums Illustrated C C A A B B

29. Flatness

30. Straightness

31. Circularity (Roundness)

32. Cylindricity

33. Perpendicularity

34. Parallelism

35. 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

36. 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)

37. Profile

38. 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

39. 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

40. Industrial Example

41. 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…

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

43. 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.

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

45. GD&T – Datum Surfaces and Features