W A T K I N S - J O H N S O N C O M P A N Y Semiconductor Equipment Group

1
Engineering 22
GeometricDimensioning Tolerancing
Bruce Mayer, PE Licensed Electrical Mechanical
EngineerBMayer_at_ChabotCollege.edu
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
  (GDT) in particuluar
• Position, Size
• Flatness, Circularity
• Perpendicularity, Parallelism

3
Geometric Dims Tolerancing

• Uses Standard Symbols To Indicate Tolerances That
  Are Based On The Features 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 GDT 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 GDT 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 1800s, manufacturing used the cut try,
  file fit approach.
• The plus-minus (or coordinate) system of
  tolerancing was next developed.
• In the 1900s, the first GDT standards came out
  to improve the quality utility of engineering
  drawings.
• In 1966, the united GDT standard was published ?
  ANSI - Y14.5M

6
GDT 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.

7
GDT 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

8
GDT 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

12
Limits of Size _at_ 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
GDT Feature Control Frame

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

15
ANSI/ASME Y14.5 Rev.s

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

16
GDT Form Profile
GEOMETRIC CHARACTERISTIC CONTROLS
14 characteristics that may be controlled
CHARACTERISTIC
SYMBOL
FLATNESS
INDIVIDUAL (No Datum Reference)
STRAIGHTNESS
FORM
CIRCULARITY
CYLINDRICITY
INDIVIDUAL or RELATED FEATURES
LINE PROFILE
PROFILE
SURFACE PROFILE
17
GDT Orient, RunOut, Loctn
GEOMETRIC CHARACTERISTIC CONTROLS
14 characteristics that may be controlled
TYPE OF
TYPE OF
CHARACTERISTIC
SYMBOL
FEATURE
TOLERANCE
PERPENDICULARITY
ANGULARITY
ORIENTATION
PARALLELISM
RELATED FEATURES (Datum Reference Required)
CIRCULAR RUNOUT
RUNOUT
TOTAL RUNOUT
CONCENTRICITY
POSITION
LOCATION
SYMMETRY
18
Understanding Tolerance Zones

• Traditional type of tolerancing describes a
  SQUARE zone for acceptable locations.
• GDT describes a CIRCULAR zone around the
  theoretically exact location for the feature.

19
Basic Dimension

• A theoretically exact dimension used to locate
  features in GDT
• 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
Std-Tol vs GDT - 1

• Standard Tolerance

Not Well Known Actual hole-ctr distances, angle
of hole-ctrs
21
Std-Tol vs GDT - 2

• GDT

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

• 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)
• 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
Given

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

• Smallest Holes at narrow Position accept 0.493
  Gage Pins

26
Datums

• Datums are features on the object that are used
  as reference surfaces from which other
  measurements are made.
• Not every GDT feature requires a datum.

• 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 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

36
GDT Angular Tolerance Zone

• In (b) Notice How the Width of the Tolerance Zone
  Expands with Distance From the Vertex
• GDT 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
• In CIRCULAR Case Could be some Combo of
  Circularity Concentricity
• In TOTAL Case add Straightness to the list

Circular
Total
40
Industrial Example
41
GDT Caveat ? Use with Care

• GDT is VERY Powerful, BUTIt 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
GDT Bottom Line
If you Do NOT Absolutely NEED GDT Then Do
NOT, Repeat NOT, Use it
43
All Done for Today
GDTis Not forEveryone
44
Engr/Math/Physics 25
Appendix
?
Time For Live Demo
Bruce Mayer, PE Licensed Electrical Mechanical
EngineerBMayer_at_ChabotCollege.edu
45
GDT Datum Surfaces and Features