GD and T Basic tutuorial

GD&T is a means of dimensioning & tolerancing a drawing which considers the function of the part and how this part functions with related parts.
GD&T has increased in practice in last 15 years because of ISO 9000.
ISO 9000 requires not only that something be required, but how it is to be controlled. For example, how round does a round feature have to be?
GD&T is a system that 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
GD&T practices are specified in ANSI Y14.5M-1994.

1. PART PRODUCTION COMMUNICATION MODEL
MANAGEMENT
DESIGN
TOOLING
PRODUCTION
INSPECTION
ASSEMBLY
ROUTING
PLANNING
PRICING
SERVICE
PURCHASING
SALES
C
U
S
T
O
M
E
R
S
V
E
N
D
O
R
S
Geometric Dimensioning
and Tolerancing (GD&T)
Geometric Dimensioning
and Tolerancing (GD&T)

2. Dimensioning can be divided into
three categories:
•general dimensioning,
•geometric dimensioning, and
•surface texture.
The following provides
information necessary to begin to
understand geometric
dimensioning and tolerancing
(GD&T)
Three Categories of
Dimensioning
Three Categories of
Dimensioning

4. Limit Tolerancing Applied
To An Angle Block
Limit Tolerancing Applied
To An Angle Block

5. Geometric Tolerancing
Applied To An Angle Block
Geometric Tolerancing
Applied To An Angle Block

6. Geometric
Dimensioning &
Tolerancing (GD&T)
Geometric
Dimensioning &
Tolerancing (GD&T)
GD&T is a means of
dimensioning & tolerancing
a drawing which considers
the function of the part and
how this part functions
with related parts.
–This allows a drawing to
contain a more defined
feature more accurately,
without increasing tolerances.

7. GD&T cont’dGD&T cont’d
GD&T has increased in practice in
last 15 years because of ISO
9000.
–ISO 9000 requires not only that something
be required, but how it is to be controlled.
For example, how round does a round
feature have to be?
GD&T is a system that 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
GD&T practices are specified in
ANSI Y14.5M-1994.

8. For ExampleFor Example
Given Table Height
However, all surfaces have a degree of
waviness, or smoothness. For
example, the surface of a 2 x 4 is
much wavier (rough) than the surface
of a piece of glass.
–As the table height is dimensioned, the
following table would pass inspection.
If top must be flatter, you could tighten
the tolerance to ± 1/32.
–However, now the height is restricted to
26.97 to 27.03 meaning good tables would
be rejected.
Assume all 4 legs will be
cut to length at the same
time.
or

9. Example cont’d.Example cont’d.
You can have both, by using
GD&T.
–The table height may any height
between 26 and 28 inches.
–The table top must be flat within
1/16. (±1/32)
27
.06
26
.06
28
.06

10. WHY IS GD&T IMPORTANTWHY IS GD&T IMPORTANT
Saves money
–For example, if large number
of parts are being made –
GD&T can reduce or eliminate
inspection of some features.
–Provides “bonus” tolerance
Ensures design, dimension, and
tolerance requirements as they
relate to the actual function
Ensures interchangeability of
mating parts at the assembly
Provides uniformity
It is a universal understanding of
the symbols instead of words

11. WHEN TO USE GD&TWHEN TO USE GD&T
When part features are critical to
a function or interchangeability
When functional gaging is
desirable
When datum references are
desirable to ensure consistency
between design
When standard interpretation or
tolerance is not already implied
When it allows a better choice of
machining processes to be made
for production of a part

12. TERMINOLOGY REVIEW TERMINOLOGY REVIEW
Maximum Material Condition
(MMC): The condition where a size
feature contains the maximum amount
of material within the stated limits of
size. I.e., largest shaft and smallest
hole.
Least Material Condition (LMC):
The condition where a size feature
contains the least amount of material
within the stated limits of size. I.e.,
smallest shaft and largest hole.
Tolerance: Difference between MMC
and LMC limits of a single dimension.
Allowance: Difference between the
MMC of two mating parts. (Minimum
clearance and maximum interference)
Basic Dimension: Nominal
dimension from which tolerances are
derived.

13. THIS MEAN?
WHAT DOES
SIZE DIMENSION
2.007
2.003
LIMITS OF SIZE LIMITS OF SIZE

14. SIZE DIMENSION
MMC
LMC
ENVELOPE OF SIZE
(2.003)
(2.007)
ENVELOPE PRINCIPLE
LIMITS OF SIZE LIMITS OF SIZE
A variation in form is allowed
between the least material
condition (LMC) and the
maximum material condition
(MMC).
Envelop Principle defines the
size and form relationships
between mating parts.

15. ENVELOPE PRINCIPLE
LMC
CLEARANCE
MMC
ALLOWANCE
LIMITS OF SIZE LIMITS OF SIZE

16. LIMITS OF SIZE LIMITS OF SIZE
The actual size of the feature at
any cross section must be
within the size boundary.
ØMMC
ØLMC

17. No portion of the feature may
be outside a perfect form
barrier at maximum material
condition (MMC).
LIMITS OF SIZE LIMITS OF SIZE

18. PARALLEL PLANES
PARALLEL PLANES PARALLEL PLANES CYLINDER ZONE
GEOMETRIC DIMENSIONING TOLERANCE ZONES
PARALLEL LINES PARALLEL LINES PARALLEL LINES
PARALLEL PLANES PARALLEL PLANES
Other Factors
I.e., Parallel Line Tolerance Zones
Other Factors
I.e., Parallel Line Tolerance Zones

19. INDIVIDUAL
(No Datum
Reference)
INDIVIDUAL
or RELATED
FEATURES
RELATED
FEATURES
(Datum
Reference
Required)
GEOMETRIC CHARACTERISTIC CONTROLS
TYPE OF
FEATURE
TYPE OF
TOLERANCE
CHARACTERISTIC SYMBOL
SYMMETRY
FLATNESS
STRAIGHTNESS
CIRCULARITY
CYLINDRICITY
LINE PROFILE
SURFACE PROFILE
PERPENDICULARITY
ANGULARITY
PARALLELISM
CIRCULAR RUNOUT
TOTAL RUNOUT
CONCENTRICITY
POSITION
FORM
PROFILE
ORIENTATION
RUNOUT
LOCATION
14 characteristics that may be controlled

20. Characteristics & Symbols
cont’d.
Characteristics & Symbols
cont’d.
–Maximum Material Condition MMC
–Regardless of Feature Size RFS
–Least Material Condition LMC
–Projected Tolerance Zone
–Diametrical (Cylindrical) Tolerance
Zone or Feature
–Basic, or Exact, Dimension
–Datum Feature Symbol
–Feature Control Frame

21. THE
GEOMETRIC SYMBOL
TOLERANCE INFORMATION
DATUM REFERENCES
FEATURE CONTROL FRAME
COMPARTMENT VARIABLES
CONNECTING WORDS
MUST BE WITHIN
OF THE FEATURE
RELATIVE TO
Feature Control FrameFeature Control Frame

22. Feature Control FrameFeature Control Frame
Uses feature control frames to
indicate tolerance
Reads as: The position of the
feature must be within a .003
diametrical tolerance zone at
maximum material condition
relative to datums A, B, and C.

23. Feature Control
Frame
Feature Control
Frame
Uses feature control frames to indicate
tolerance
Reads as: The position of the feature
must be within a .003 diametrical
tolerance zone at maximum material
condition relative to datums A at
maximum material condition and B.

24. The of the feature must be within a tolerance
zone.
The of the feature must be within a
tolerance zone at relative
to Datum .
The of the feature must be within a
tolerance zone relative to Datum .
The of the feature must be within a
zone at
relative to Datum .
The of the feature must be within a
tolerance zone relative to datums .
Reading Feature Control Frames Reading Feature Control Frames

25. Placement of Feature
Control Frames
Placement of Feature
Control Frames
May be attached to a side, end
or corner of the symbol box to
an extension line.
Applied to surface.
Applied to axis

26. Placement of Feature
Control Frames Cont’d.
Placement of Feature
Control Frames Cont’d.
May be below or closely
adjacent to the dimension or
note pertaining to that feature.
Ø .500±.005

27. Basic DimensionBasic Dimension
A theoretically exact size, profile,
orientation, or location of a feature or
datum target, therefore, a basic
dimension is untoleranced.
Most often used with position,
angularity, and profile)
Basic dimensions have a rectangle
surrounding it.
1.000

28. Basic Dimension
cont’d.
Basic Dimension
cont’d.

29. Form FeaturesForm Features
Individual Features
No Datum Reference

Flatness Straightness
CylindricityCircularity

30. Form Features ExamplesForm Features Examples
Flatness as stated on
drawing: The flatness of the
feature must be within .06
tolerance zone.
.003
0.500 ±.005
.003
0.500 ±.005
Straightness applied to a flat surface: The
straightness of the feature must be within .003
tolerance zone.

31. Form Features ExamplesForm Features Examples
Straightness applied to the surface of a
diameter: The straightness of the feature must
be within .003 tolerance zone.
.003
0.500
0.505
Æ
Straightness of an Axis at MMC: The derived
median line straightness of the feature must be
within a diametric zone of .030 at MMC.
.030
0.500
0.505
Æ MÆ
1.010
0.990

32. BEZEL
CASE
CLAMP
PROBE
DIAL INDICATOR
6
8
10
12
10
8
6
4
22
4
Dial IndicatorDial Indicator

33. Verification of FlatnessVerification of Flatness

34. Activity 13Activity 13
Work on worksheets GD&T 1,
GD&T 2 #1 only, and GD&T 3
–(for GD&T 3 completely
dimension. ¼” grid.)

35. Features that Require
Datum Reference
Features that Require
Datum Reference
Orientation
–Perpendicularity
–Angularity
–Parallelism
Runout
–Circular Runout

–Total Runout

Location
–Position

–Concentricity

–Symmetry

36. DatumDatum
Datums are features (points, axis,
and planes) on the object that are
used as reference surfaces from
which other measurements are
made. Used in designing, tooling,
manufacturing, inspecting, and
assembling components and sub-
assemblies.
–As you know, not every GD&T
feature requires a datum, i.e., Flat
1.000

37. Datums cont’d.Datums cont’d.
Features are identified with
respect to a datum.
Always start with the letter A
Do not use letters I, O, or Q
May use double letters AA,
BB, etc.
This information is located in
the feature control frame.
Datums on a drawing of a
part are represented using
the symbol shown below.

38. Datum Reference SymbolsDatum Reference Symbols
The datum feature symbol
identifies a surface or feature
of size as a datum.
A
ISO
A
ANSI
1982
ASME
A
1994

39. Placement of DatumsPlacement of Datums
Datums are generally placed on a feature, a
centerline, or a plane depending on how
dimensions need to be referenced.
A AOR
ASME 1994
A
ANSI 1982
Line up with arrow only when
the feature is a feature of
size and is being defined as
the datum

40. Placement of DatumsPlacement of Datums
Feature sizes, such as holes
Sometimes a feature has a
GD&T and is also a datum
Ø .500±.005
A
Ø .500±.005
A Ø .500±.005

41. 6 ROTATIONAL
6 LINEAR AND
FREEDOM
DEGREES OF
UP
DOWN
RIGHT
LEFT
BACK
FRONT
UNRESTRICTED FREE
MOVEMENT IN SPACE
TWELVE DEGREES OF FREEDOMTWELVE DEGREES OF FREEDOM

42. Example Datums Example Datums
Datums must be
perpendicular to each other
–Primary
–Secondary
–Tertiary Datum

43. Primary DatumPrimary Datum
A primary datum is selected
to provide functional
relationships, accessibility,
and repeatability.
–Functional Relationships
»A standardization of size is desired in
the manufacturing of a part.
»Consideration of how parts are
orientated to each other is very
important.
–For example, legos are made in a
standard size in order to lock into
place. A primary datum is chosen
to reference the location of the
mating features.
–Accessibility
»Does anything, such as, shafts, get in
the way?

44. Primary Datum cont’d.Primary Datum cont’d.
–Repeatability
For example, castings, sheet
metal, etc.
»The primary datum chosen must
insure precise measurements.
The surface established must
produce consistent
»Measurements when producing
many identical parts to meet
requirements specified.

45. FIRST DATUM ESTABLISHED
BY THREE POINTS (MIN)
CONTACT WITH SIMULATED
DATUM A
Primary DatumPrimary Datum
 Restricts 6 degrees of freedom

46. Secondary &
Tertiary Datums
Secondary &
Tertiary Datums
All dimension may not be capable to
reference from the primary datum to
ensure functional relationships,
accessibility, and repeatability.
–Secondary Datum
»Secondary datums are produced
perpendicular to the primary datum so
measurements can be referenced from
them.
–Tertiary Datum
»This datum is always perpendicular to
both the primary and secondary datums
ensuring a fixed position from three
related parts.

47. SECOND DATUM
PLANE ESTABLISHED BY
TWO POINTS (MIN) CONTACT
WITH SIMULATED DATUM B
Secondary DatumSecondary Datum
 Restricts 10 degrees of freedom.

48. Tertiary DatumTertiary Datum
 Restricts 12 degrees of freedom.
90°
THIRD DATUM
PLANE ESTABLISHED
BY ONE POINT (MIN)
CONTACT WITH
SIMULATED DATUM C
MEASURING DIRECTIONS FOR
RELATED DIMENSIONS

49. Z
DATUM
REFERENCE
FRAME
SURFACE
PLATE
GRANITE
PROBE
COORDINATE MEASURING MACHINE
BRIDGE DESIGN
Coordinate Measuring
Machine
Coordinate Measuring
Machine

50. SIMULATED DATUM-
SMALLEST
CIRCUMSCRIBED
CYLINDER
THIS ON
THE DRAWING
MEANS THIS
PART
DATUM AXIS
A
Size Datum
(CIRCULAR)
Size Datum
(CIRCULAR)

51. Size Datum
(CIRCULAR)
Size Datum
(CIRCULAR)
SIMULATED DATUM-
LARGEST
INSCRIBED
CYLINDER
THIS ON
THE DRAWING
MEANS THIS
DATUM AXIS A
PART
A

52. Orientation TolerancesOrientation Tolerances
–Perpendicularity
–Angularity

–Parallelism
Controls the orientation of
individual features
Datums are required
Shape of tolerance zone: 2
parallel lines, 2 parallel planes, and
cylindrical

53. PERPENDICULARITY:PERPENDICULARITY:
is the condition of a surface, center plane, or
axis at a right angle (90°) to a datum plane or
axis.
Ex:
The tolerance zone is the
space between the 2
parallel lines. They are
perpendicular to the
datum plane and spaced .
005 apart.
The perpendicularity of
this surface must be
within a .005 tolerance
zone relative to datum A.

54. Practice ProblemPractice Problem
Plane 1 must be
perpendicular within .005
tolerance zone to plane 2.
BOTTOM SURFACE

55. Practice ProblemPractice Problem
Plane 1 must be
perpendicular within .005
tolerance zone to plane 2
BOTTOM PLANE

56. 2.00±.01
.02 Tolerance
Practice ProblemPractice Problem
Without GD & T this
would be acceptable
2.00±.01
.02 Tolerance
.005 Tolerance
Zone
With GD & T the overall height may end
anywhere between the two blue planes. But the
bottom plane is restricted to the red tolerance
zone.

57. PERPENDICULARITY Cont’d.PERPENDICULARITY Cont’d.
Location of hole (axis)
This means ‘the hole
(axis) must be
perpendicular within a
diametrical tolerance
zone of .010 relative to
datum A’

58. ANGULARITY:ANGULARITY:
is the condition of a surface, axis, or
median plane which is at a specific
angle (other than 90°) from a datum
plane or axis.

Can be applied to an axis at MMC.
Typically must have a basic
dimension.
The surface is at a
45º angle with a .
005 tolerance zone
relative to datum A.

59. ±0.01
PARALLELISM:PARALLELISM:
The condition of a surface or center plane
equidistant at all points from a datum plane, or
an axis.
The distance between the parallel lines, or
surfaces, is specified by the geometric
tolerance.

60. Activity 13 Cont’d.Activity 13 Cont’d.
Complete worksheets GD&T-
2, GD&T-4, and GD&T-5
–Completely dimension.
–¼” grid

61. Material ConditionsMaterial Conditions
Maximum Material Condition
(MMC)
Least Material Condition
(LMC)
Regardless of Feature
Size(RFS)

62. Maximum Material ConditionMaximum Material Condition
MMC
This is when part will weigh the
most.
–MMC for a shaft is the largest
allowable size.
»MMC of Ø0.240±.005?
–MMC for a hole is the smallest
allowable size.
»MMC of Ø0.250±.005?
Permits greater possible
tolerance as the part feature
sizes vary from their calculated
MMC
Ensures interchangeability
Used
–With interrelated features with
respect to location
–Size, such as, hole, slot, pin, etc.

63. Least Material ConditionLeast Material Condition
LMC
This is when part will weigh
the least.
–LMC for a shaft is the smallest
allowable size.
»LMC of Ø0.240±.005?
–LMC for a hole is the largest
allowable size.
»LMC of Ø0.250±.005?

64. Regardless of Feature SizeRegardless of Feature Size
RFS
Requires that the condition of
the material NOT be
considered.
This is used when the size
feature does not affect the
specified tolerance.
Valid only when applied to
features of size, such as
holes, slots, pins, etc., with
an axis or center plane.

65. Location TolerancesLocation Tolerances

–Position

–Concentricity

–Symmetry

66. Position TolerancePosition Tolerance
A position tolerance is the total
permissible variation in the location
of a feature about its exact true
position.
For cylindrical features, the
position tolerance zone is typically
a cylinder within which the axis of
the feature must lie.
For other features, the center plane
of the feature must fit in the space
between two parallel planes.
The exact position of the feature is
located with basic dimensions.
The position tolerance is typically
associated with the size tolerance
of the feature.
Datums are required.

67. Coordinate System PositionCoordinate System Position
Consider the following hole dimensioned with
coordinate dimensions:
The tolerance zone for the location of the hole
is as follows:
Several Problems:
–Two points, equidistant from true position may not
be accepted.
–Total tolerance diagonally is .014, which may be
more than was intended.
2.000
.
7
5
0

68. Coordinate System PositionCoordinate System Position
Consider the following hole dimensioned with
coordinate dimensions:
The tolerance zone for the location (axis) of the
hole is as follows:
Several Problems:
–Two points, equidistant from true position may not
be accepted.
–Total tolerance diagonally is .014, which may be
more than was intended. (1.4 Xs >, 1.4*.010=.014)
2.000
.
7
5
0
Center can be
anywhere along
the diagonal
line.

69. Position TolerancingPosition Tolerancing
Consider the same hole, but add
GD&T:
Now, overall tolerance zone is:
The actual center of the hole (axis) must lie in
the round tolerance zone. The same tolerance
is applied, regardless of the direction.
MMC =
.500 - .003 = .497

70. Bonus ToleranceBonus Tolerance
Here is the beauty of the system! The
specified tolerance was:
This means that the
tolerance is .010 if the
hole size is the MMC size,
or .497. If the hole is
bigger, we get a bonus
tolerance equal to the
difference between the
MMC size and the actual
size.

71. Bonus Tolerance ExampleBonus Tolerance Example
This system makes sense… the larger the
hole is, the more it can deviate from true
position and still fit in the mating condition!
Actual Hole Size Bonus Tol. Φ of Tol. Zone
Ø .497 (MMC) 0 .010
Ø .499 (.499 - .497 = .002).002 (.010 + .002 = .012).012
Ø .500 (.500 - .497 = .003).003 (.010 + .003 = .013).013
Ø .502 .005 .015
Ø .503 (LMC) .006 .016
Ø .504 ? ?
This means that
the tolerance is .
010 if the hole
size is the MMC
size, or .497. If the
hole is bigger, we
get a bonus
tolerance equal to
the difference
between the MMC
size and the
actual size.
.503

72. .497 = BONUS 0
TOL ZONE .010
.499 - .497 = BONUS .002
BONUS + TOL. ZONE = .012
Shaft
Hole

73. .501 - .497 = BONUS .004
BONUS + TOL. ZONE = .014
.503 - .497 = BONUS .006
BONUS + TOL. ZONE = .016

74. What if the tolerance had been specified as:
Since there is NO material modifier, the
tolerance is RFS, which stands for regardless
of feature size. This means that the position
tolerance is .010 at all times. There is no
bonus tolerance associated with this
specification.
VIRTUAL CONDITION: The worst case
boundary generated by the collective effects of
a size feature’s specified MMC or LMC
material condition and the specified geometric
tolerance.
GT = GEOMETRIC
TOLERANCE

75. PERPENDICULARITY Cont’d.PERPENDICULARITY Cont’d.

Means “the hole (AXIS) must
be perpendicular within a
diametrical tolerance zone of .
010 at MMC relative to datum
A.”
Actual Hole
Size
Bonus
Tol.
Ø of Tol.
Zone
1.997
(MMC)
1.998
1.999
2.000
2.001
2.002
2.003
Vc =

76. Activity 13 Cont’d.Activity 13 Cont’d.
Worksheet GD&T 6