Geometric Dimensioning
and Tolerancing (GD&T)
MANAGEMENT
CUSTOMERS
VENDORS
DESIGN
SALES
PRICING
TOOLING
PLANNING
PURCHASING
PRODUCTION
ROUTING
SERVICE
INSPECTION
ASSEMBLY
PART PRODUCTION COMMUNICATION MODEL
Three Categories of
Dimensioning
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)
Limit Tolerancing Applied
To An Angle Block
Geometric Tolerancing
Applied To An Angle Block
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.
GD&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.
For Example

Given Table Height
Assume all 4 legs will be
cut to length at the same
time.

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.
or

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.
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)
.06
.06
.06
26
27
28
WHY 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
WHEN 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
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.
LIMITS OF SIZE
SIZE DIMENSION
WHAT DOES
THIS MEAN?
2.007
2.003
LIMITS OF SIZE
A variation in form is allowed
between the least material
condition (LMC) and the
maximum material condition
(MMC).
SIZE DIMENSION
ENVELOPE PRINCIPLE
MMC
(2.007)
LMC
(2.003)
ENVELOPE OF SIZE
Envelop Principle defines the
size and form relationships
between mating parts.
LIMITS OF SIZE
ENVELOPE PRINCIPLE
LMC
CLEARANCE
MMC
ALLOWANCE
LIMITS OF SIZE
The actual size of the feature at
any cross section must be
within the size boundary.
ØMMC
ØLMC
LIMITS OF SIZE
No portion of the feature may
be outside a perfect form
barrier at maximum material
condition (MMC).
Other Factors
I.e., Parallel Line Tolerance Zones
GEOMETRIC DIMENSIONING TOLERANCE ZONES
PARALLEL LINES
PARALLEL LINES
PARALLEL LINES
PARALLEL PLANES
PARALLEL PLANES
PARALLEL PLANES
PARALLEL PLANES
PARALLEL PLANES
CYLINDER ZONE
GEOMETRIC CHARACTERISTIC CONTROLS
14 characteristics that may be controlled
TYPE OF
FEATURE
TYPE OF
CHARACTERISTIC SYMBOL
TOLERANCE
FLATNESS
INDIVIDUAL
(No Datum
Reference)
STRAIGHTNESS
FORM
CIRCULARITY
CYLINDRICITY
INDIVIDUAL
or RELATED
FEATURES
LINE PROFILE
PROFILE
SURFACE PROFILE
PERPENDICULARITY
ORIENTATION ANGULARITY
PARALLELISM
RELATED
FEATURES
(Datum
Reference
Required)
CIRCULAR RUNOUT
RUNOUT
TOTAL RUNOUT
CONCENTRICITY
LOCATION
POSITION
SYMMETRY
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
Feature
Frame
FEATUREControl
CONTROL FRAME
GEOMETRIC SYMBOL
TOLERANCE INFORMATION
DATUM REFERENCES
COMPARTMENT VARIABLES
THE
RELATIVE TO
OF THE FEATURE
MUST BE WITHIN
CONNECTING WORDS
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, B, and C.
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.
Reading Feature Control Frames

The
zone.
of the feature must be within a
tolerance

The
tolerance zone at
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
.
of the feature must be within a
relative
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
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
Basic 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
Basic Dimension
cont’d.
Form Features


Individual Features
No Datum Reference
Flatness
Straightness
Circularity
Cylindricity
Form Features Examples
Flatness as stated on
drawing: The flatness of the
feature must be within .06
tolerance zone.
Straightness applied to a flat surface: The
straightness of the feature must be within .003
tolerance zone.
.003
0.500 ±.005
.003
0.500 ±.005
Form 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
Straightness of an Axis at MMC: The derived
median line straightness of the feature must be
within a diametric zone of .030 at MMC.
 0.500
0.505
1.010
0.990
 .030
M
Dial Indicator
DIAL INDICATOR
BEZEL
2
CASE
2
4
4
6
6
8
8
10
12
10
CLAMP
PROBE
Verification of Flatness
Activity 13

Work on worksheets GD&T 1,
GD&T 2 #1 only, and GD&T 3
– (for GD&T 3 completely
dimension. ¼” grid.)
Features that Require
Datum Reference

Orientation
– Perpendicularity
– Angularity
– Parallelism

Runout
– Circular Runout
– Total Runout

Location
– Position
– Concentricity
– Symmetry
Datum

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 subassemblies.
– As you know, not every GD&T
feature requires a datum, i.e., Flat
1.000
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.
Datum Reference Symbols

The datum feature symbol
identifies a surface or feature
of size as a datum.
A
A
ASME
1994
ISO
A
ANSI
1982
Placement of Datums

Datums are generally placed on a feature, a
centerline, or a plane depending on how
dimensions need to be referenced.
A
OR
A
ANSI 1982
ASME 1994
Line up with arrow only when
the feature is a feature of
size and is being defined as
the datum
A
Placement of Datums

Feature sizes, such as holes
Ø .500±.005
A

Sometimes a feature has a
GD&T and is also a datum
A
Ø .500±.005
Ø .500±.005
TWELVE DEGREES OF FREEDOM
UP
BACK
LEFT
6 LINEAR AND
6 ROTATIONAL
DEGREES OF
FREEDOM
RIGHT
FRONT
DOWN
UNRESTRICTED FREE
MOVEMENT IN SPACE
Example Datums

Datums must be
perpendicular to each other
– Primary
– Secondary
– Tertiary Datum
Primary 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?
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.
Primary Datum
 Restricts 6 degrees of freedom
FIRST DATUM ESTABLISHED
BY THREE POINTS (MIN)
CONTACT WITH SIMULATED
DATUM A
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.
Secondary Datum

Restricts 10 degrees of freedom.
SECOND DATUM
PLANE ESTABLISHED BY
TWO POINTS (MIN) CONTACT
WITH SIMULATED DATUM B
Tertiary Datum

Restricts 12 degrees of freedom.
90°
MEASURING DIRECTIONS FOR
RELATED DIMENSIONS
THIRD DATUM
PLANE ESTABLISHED
BY ONE POINT (MIN)
CONTACT WITH
SIMULATED DATUM C
Coordinate Measuring
Machine
COORDINATE MEASURING MACHINE
BRIDGE DESIGN
PROBE
Z
GRANITE
SURFACE
PLATE
DATUM
REFERENCE
FRAME
Size Datum
(CIRCULAR)
THIS ON
THE DRAWING
A
MEANS THIS
PART
DATUM AXIS
SIMULATED DATUMSMALLEST
CIRCUMSCRIBED
CYLINDER
Size Datum
(CIRCULAR)
THIS ON
THE DRAWING
A
MEANS THIS
PART
DATUM AXIS A
SIMULATED DATUMLARGEST
INSCRIBED
CYLINDER
Orientation 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

PERPENDICULARITY:

is the condition of a surface, center plane, or
axis at a right angle (90°) to a datum plane or
axis.
Ex:
The perpendicularity of
this surface must be
within a .005 tolerance
zone relative to datum A.
The tolerance zone is the
space between the 2
parallel lines. They are
perpendicular to the
datum plane and spaced
.005 apart.
Practice Problem

Plane 1 must be
perpendicular within .005
tolerance zone to plane 2.
BOTTOM SURFACE
Practice Problem

Plane 1 must be
perpendicular within .005
tolerance zone to plane 2
BOTTOM PLANE
Practice Problem
2.00±.01
.02 Tolerance
Without GD & T this
would be acceptable
2.00±.01
.005 Tolerance
Zone
.02 Tolerance
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.
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’
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.
The surface is at a
45º angle with a
.005 tolerance zone
relative to datum A.


Can be applied to an axis at MMC.
Typically must have a basic
dimension.
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.
±0.01
Activity 13

Cont’d.
Complete worksheets GD&T2, GD&T-4, and GD&T-5
– Completely dimension.
– ¼” grid
Material Conditions
Maximum Material Condition
(MMC)
 Least Material Condition
(LMC)
 Regardless of Feature
Size(RFS)

Maximum 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.
Least 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?
Regardless 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.

Location Tolerances
– Position
– Concentricity
– Symmetry
Position 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.
Coordinate System Position
Consider the following hole dimensioned with
coordinate dimensions:

The tolerance zone for the location of the hole is
as follows:
2.000

.750

Several Problems:
– Two points, equidistant from true position may not
be accepted.
– Total tolerance diagonally is .014, which may be
more than was intended.
Coordinate System Position

Consider the following hole dimensioned with
coordinate dimensions:

The tolerance zone for the location (axis) of the
hole is as follows:
2.000

.750
Center can be
anywhere along
the diagonal
line.
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)
Position Tolerancing

Consider the same hole, but add
GD&T:

Now, overall tolerance zone is:
MMC =
.500 - .003 = .497

The actual center of the hole (axis) must lie in
the round tolerance zone. The same tolerance
is applied, regardless of the direction.
Bonus 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.
Bonus Tolerance Example
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

Actual Hole Size
Bonus Tol.
Φ of Tol. Zone
Ø .497 (MMC)
0
.010
Ø .499
.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
?
?
(.499 - .497 = .002)
This system makes sense… the larger the hole
is, the more it can deviate from true position
and still fit in the mating condition!
Hole
.497 = BONUS 0
TOL ZONE .010
Shaft
.499 - .497 = BONUS .002
BONUS + TOL. ZONE = .012
.501 - .497 = BONUS .004
BONUS + TOL. ZONE = .014
.503 - .497 = BONUS .006
BONUS + TOL. ZONE = .016

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
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
1.997 (MMC)
1.998
1.999
2.000
2.001
Vc =
2.002
2.003
Bonus Tol.
Ø of Tol.
Zone
Activity 13

Cont’d.
Worksheet GD&T 6