gd&t - overview

Geometric Dimensioning & Tolerancing

What is GD&T?

• Rules based method of placing dimensions and tolerances on a drawing

• Considers actual part function and interfaces

• Tolerances relative to functional features

• Not in a vacuum. Must consider role in an assembly

Why use GD&T?

• Saves money– Maximum producibility

– Maximum component tolerances - over conservative designs

– Bonus tolerances

• Focus on functional relationships

• Insures interchangeability

Why use GD&T? (cont’d)

• Drawing uniformity, no misinterpretations

• Control of complex geometry

• Becoming the standard

When to use GD&T?

• Features critical to function or interchangeability

• Functional gaging desired

• Need consistency between tooling, manufacturing and inspection operations

• Standard interpretation or tolerance not implied

Two key elements of GD&T

• Datums - The reference features that the tolerances are given with respect to.

• Geometric Tolerances - The zone in which a controlled feature must lie.

Datums

A theoretically exact point, axis, or plane derived from the true geometric counterpart of

a specified datum feature.

Datum Feature

The actual feature (hole, slot, surface, etc.) on a part that is used to establish the datum.

1982 vs. 1994 Datum Standards

-A-

-D-

ANSI Y14.5-1982

AD

ASME Y14.5-1994

2009 Datum Standard

Datum Targets

• Specify specific zone of contact as a datum as opposed to an entire surface.

• Zones are typically correlated to some process or assembly fixture.

• May be points, lines or areas

Datum Target

C1

Datum Reference Frame

• A set of three mutually orthogonal planes created from a combination of one or more datums.

• The origin from which geometric tolerances are evaluated.

• Chosen relative to part function and mating component interfaces.

Datum Reference Frames (cont’d)

• Must lock up the part in six degrees of freedom.

Yaw

Pitch

Roll

X

Y

Z

3-2-1 Principle

• 3 Points - Primary Datum

• 2 Points - Secondary Datum

• 1 Point - Tertiary Datum

Datum Order Specification

BC

A

Theoretical Part Actual Part

Datum Order Specification

Datum Reference Frame Creation3 Planar Datums

A

C

B

Datum Reference Frame CreationPlane, Hole, Slot Datums

A

B

C

Datum Reference Frame CreationPlane, 3 Slot Datums

A

C

B

Datum Reference Frame CreationAxis, Plane, Median Plane Datums

B

A

C

Implied Datums4X 1.0 0.1

1.0 0.1

5.0 0.1

6.0 0.1

2.0 0.1

6.0 0.1

10.0 0.1

Implied Datums4X 1.0 0.1

1.0

5.0

6.0 0.1

2.0

6.0

10.0 0.1

.2 M A B C

A

B

C

Feature Control Frame

• Specifies the geometric tolerance for a given feature

• Tolerance symbol

• Tolerance value

• Datum references etc.

0.2 A B CM M

Geometric Characteristic Symbol (Position)Diameter symbol

Tolerance Value

Feature Modifier

Primary Datum

Secondary DatumTertiary Datum

Datum Modifier

Feature of Size

A feature to which a size dimension can be applied such as a cylinder or a set of two

opposed elements or parallel surfaces

Maximum Material Condition

• The condition in which a feature of size contains the maximum amount of material within the stated limits of size.

• Tolerance at MMC means the tolerance is dependent on the size of the feature.

• Ensures interchangeability.

M

Regardless of Feature Size

• The geometric tolerance or datum reference applies at any increment of size of the feature within the allowable size tolerance.

• Implied if no other modifier is specified.

S

Least Material Condition

• The condition in which a feature of size contains the least amount of material within the stated limits of size.

• The opposite of MMC.

• Use is rare relative to MMC and RFS.

L

Geometric Tolerances

• Form Tolerances

• Profile Tolerances

• Orientation Tolerances

• Runout Tolerances

• Locations Tolerances

Geometric Characteristic Symbols

Line Profile

Surface Profile

Flatness

Straightness

Cylindricity

Angularity

Parallelism

Circular Runout

Total Runout

Position

Concentricity

Symmetry

Circularity

GeometricCharacteristic

SymbolFeature Type Tolerance Type

Individual Features

Individual or Related Features

Related Features

Form

Profile

Orientation

Runout

Location

Perpendicularity

Form Tolerances• Form tolerances control how far an actual

surface or feature is permitted to vary from the desired (theoretical) from implied by the drawing.

• Applied to individual features with no datum references.

• Refinements of size

Form - Straightness (2D)• 2D control when applied to line elements of a surface.

• Each line element of the surface must fall within the tolerance zone consisting of two parallel lines.

0.05 0.05 tolerance zone

Form - Straightness (3D)• 3D control when applied to a axis or median plane

feature.

• For an axis - tolerance zone is cylindrical

0.05 tolerance zone

0.05 M

12 0.02

Straightness Example

Actual Part Tolerance

Size Zone

12.02 MMC 0.05

12.01 0.06

12.00 0.07

11.99 0.08

11.98 LMC 0.09

Form - Flatness• 3D tolerance controlling the deviation of a planar surface

• Tolerance zone is two parallel planes

0.05

0.05 tolerance zone

Form - Circularity• 2D tolerance controlling the roundness of each

circular cross section through a feature.

• Tolerance zone is two concentric circles.

0.02 0.02 tolerance zone

Form - Cylindricity

• 3D tolerance controlling both circular and longitudinal elements of a cylinder.

• Tolerance zone is two concentric cylinders.

Tolerance Zone0.05

Orientation Tolerances

• Orientation tolerances allow a surface or feature to vary in orientation relative to a datum.

• Datum reference required.

Orientation - Angularity• Condition of a surface, axis or median plane that is at an

angle (other that 90º or 180º) to a datum plane or axis.

• Tolerance zone is two parallel planes for planar features or a cylinder for axis features.

.005 tolerance zone

40

.005 A

A

Angularity applied to feature of size

80

A

5.1 0.1

80

.15 M ATolerance Zone

Orientation - Perpendicularity

• A special case of angularity where the angle is 90º between the feature and the datum.

A

0.05 A0.05 tolerance zone

Simulated plane A

Orientation - Parallelism

• A special case of angularity where the angle is 180º between the feature and the datum.

A

0.05 A0.05 tolerance zone

Simulated plane A

Runout Tolerances

• A runout tolerance control how far a surface or is allowed to deviate from its theoretical form and orientation during a full 360º rotation of the part about the datum axis.

• Runout is applied at RFS.

• A datum axis is required.

Runout - Circular Runout

• 2D tolerance controlling a surface with respect to an axis.

• The tolerance zone is two concentric circles for cylindrical surface and two circles separated axially for planar surfaces perpendicular to the datum axis.

Circular Runout

A

2.0042.000

.004 A

.004 tolerance zone

Runout - Total Runout

• 3D tolerance controlling a surface with respect to an axis.

• The tolerance zone is two concentric cylinders for cylindrical surfaces and two parallel planes for planar surfaces perpendicular to the datum axis.

Total Runout

A

2.0042.000

.004 A

.004 tolerance zone

Profile Tolerances

• Controls a surface within a uniform boundary along the theoretical profile.

• Applied to individual features or related features relative to a datum.

• Feature modifiers are not applicable.

Profile - Line Profile• 2D tolerance controlling the line elements of a surface.

• The tolerance zone is two parallel profile boundaries offset from the nominal surface.

10.0 .05

10.0

BA

1.0 A B

5.0

Tolerance Zone

Profile - Surface Profile• 3D tolerance controlling the entirety of a surface.

• The tolerance zone is two parallel profile boundaries offset from the nominal surface.

0.05 tolerance zone0.05 A B C

A

B C

Location Tolerances

• Controls the allowable deviation in a specified location of a feature of size relative to some other feature or datum.

• Require datum reference in most cases.

Location - Position• 3D tolerance used to locate features of size.

• The tolerance zone is a cylinder for round features and two parallel planes for median plane features.

.010 M A B C

4X 1.25 +/- .005C

A

B

.010 tolerance zone

Actual

Hole

Tolerance

Zone

Bonus

Tolerance

1.245 MMC .010 0.000

1.246 .011 0.001

1.247 .012 0.002

1.248 .013 0.003

1.249 .014 0.004

1.25 .015 0.005

1.251 .016 0.006

1.252 .017 0.007

1.253 .018 0.008

1.254 .019 0.009

1.255 LMC .020 0.010

Position Tolerance at MMC

.010 M A B C

4X 1.25 +/- .005

Position Tolerance at RFS

.010 A B C

4X 1.25 +/- .005

Actual

Hole

Tolerance

Zone

Bonus

Tolerance

1.245 MMC .010 0.0

1.246 .010 0.0

1.247 .010 0.0

1.248 .010 0.0

1.249 .010 0.0

1.25 .010 0.0

1.251 .010 0.0

1.252 .010 0.0

1.253 .010 0.0

1.254 .010 0.0

1.255 LMC .010 0.0

Location - Concentricity• 3D tolerance controlling the location of one axis

relative to another datum axis.

• Tolerance zone is a cylinder

1.0 .03

0.1 A

A

0.10 tolerance zone

Location - Symmetry• 3D tolerance mimicking concentricity, but applied

to non-cylindrical features of size.

• Tolerance zone is two parallel planes.

0.10 tolerance zone

1.0 .010

0.10 A

A