Dimensional Engineering PDF
Dimensional Engineering PDF
Dimensional Engineering PDF
Truck Group
Dimensional
Engineering
Seminar
Geometric Dimensioning and Tolerancing
Variation Simulation Modeling
Date of Publication: January 12, 1998
Latest Revision Date: May 1, 1999
GM General Motors
Truck Group
DIMENSIONAL
ENGINEERING
Based on the ASME Y14.5M-
1994 Dimensioning and
Tolerancing Standard
as amended by the GM Global
Addendum-1997
GM General Motors
Truck Group
Straightness
For Flatness
Individual Form
Features Circularity (roundness)
Cylindricity
For Profile of a Line
Individual Profile
or Related Profile of a Surface
Features
Angularity
Orientation Perpendicularity
Parallelism
For Position
Related
Features Location Concentricity
Symmetry
Circular Runout *
Runout
Total Runout *
* Runout symbols may be filled or not filled
The Language of Geometric
Dimensioning & Tolerancing
Additional Symbols and Modifiers
TERM SYMBOL
Maximum Material Condition M
Diameter Symbol
1 A BC
Tolerance
Value
Tertiary
Primary
Geometric Datum
Datum
Characteristic
Symbol Secondary
Datum
Tolerance Material
Condition Symbol
Diameter Datum Material
Symbol Condition Symbol
1M A BM C
As required, additional symbols are used along with the basic feature control
frame to identify specific geometric or dimensional requirements. The above
example shows a diameter symbol and two maximum material condition (MMC)
symbols that have been added to precisely describe the feature requirements.
The diameter symbol describes the cylindrical shape of the feature tolerance
zone while the maximum material condition symbols indicate both the feature
and secondary datum material condition in which the stated tolerance applies.
Why Use GD&T ?
Manufacturing tolerances acknowledge the fact that dimensional
perfection is impossible to achieve. More importantly, from an
economic perspective, perfection may be an expensive and
inappropriate goal. Unnecessarily small tolerances do not improve
quality or performance, they do increase costs. As manufacturing
tolerances shrink, production and inspection costs increase
rapidly. Properly specified tolerances minimize manufacturing and
assembly costs, ensure product performance, and provide a means
of assessing and maintaining process controls.
To Maximize Producibility
Parts designed using GD&T methods have maximized producibility because all
available manufacturing tolerance has been included.
To Improve Productivity
Using functional tolerancing techniques improves productivity by reducing the
potential for the rejection of functional parts.
Functional Performance
Properly applied GD&T assures assembly, interchangeability, and functional
performance of all mating details.
Clear Communication
Effective GD&T identifies important dimensional relationships and offers clear
communication of functional design requirements.
Uniform Interpretation
Uniform, consistent interpretation of design requirements saves time and money by
avoiding errors and controversies resulting from misconceptions and
misunderstandings.
Rule #1
Where only a tolerance of size is specified, the limits of
size of an individual feature prescribe the extent to which
variations in its geometric form as well as size are allowed.
(ASME Y14.5M-1994, 2.7.1)
All Applicable
Geometric Tolerances
Rule #2
Regardless of Feature Size (RFS) applies, with respect to
the individual tolerance, datum reference or both, where
no modifying symbol is specified. Maximum Material
Condition (MMC) or Least Material Condition (LMC) must
be specified on the drawing where it is required
(ASME Y14.5-1994, 2.8a)
Notes:
The default condition described by Rule #2 applies only to drawings using the ASME
Y14.5M-1994 standard. Any drawing using an earlier standard will have a different default
condition.
Circular runout, total runout, concentricity, and symmetry can only be applied on an RFS
basis and cannot be modified to MMC or LMC.
Definition
Maximum Material Condition
The condition in which a feature of size contains
the maximum amount of material within the
stated limits of size -- for example, minimum
hole diameter or maximum shaft diameter
(ASME Y14.5M-1994, 1.3.20)
M
WHEN THE PART WEIGHS THE MOST!
The Maximum Material Condition symbol can be used as a tolerance modifier and/or a datum
modifier for internal or external features of size. When the MMC symbol is applied as a tolerance
modifier, the specified tolerance value applies when the feature is at its extreme limit of size (min
hole, max shaft). When the MMC symbol is applied as a datum modifier, the datum is the axis or
center plane of the datum feature at its virtual size.
Maximum Material Condition
MMC Size = 12
14.95
14.90
MMC Size = 15
Definition
Least Material Condition
The condition in which a feature of size contains
the least amount of material within the stated
limits of size -- for example, maximum hole
diameter or minimum shaft diameter
(ASME Y14.5M-1994, 1.3.19)
L
WHEN THE PART WEIGHS THE LEAST!
The Least Material Condition symbol can also be used as a tolerance modifier and/or a datum
modifier for internal or external features of size. When the LMC symbol is applied as a tolerance
modifier, the specified tolerance value applies when the feature is at its extreme limit of size (max
hole, min shaft). When the LMC symbol is applied as a datum modifier, the datum is the axis or
center plane of the datum feature at its LMC size.
Least Material Condition
LMC Size = 12
15 +0.1
0
S *
* The RFS symbol is no longer required to indicate regardless of
feature size conditions for features subject to variations in size
(See rule #2 ASME Y14.5M-1994). It is applicable only on drawings
using earlier standards.
Definition
Free State Condition
F
When applied to geometric tolerances, the free state symbol
indicates that individual or related feature tolerance(s) must
be verified with the part in an unrestrained or unclamped
condition.
When used as a datum modifier, only those datum feature(s)
specifically identified as free state (including rests and
assists) shall be unrestrained or unclamped when verifying
individual or related feature tolerance(s).
Definition
Dimensions, Features
and Tolerances
Dimension
A numeric value expressed in appropriate units of measure
and used to define the size, location, geometric characteristic,
or surface texture of a part or part feature. (ASME Y14.5-1994, 1.3.8)
Feature
The general term applied to a physical portion of a part, such
as a surface, pin, tab, hole or slot. (ASME Y14.5M-1994, 1.3.12)
Feature of size
One cylindrical or spherical surface, or set of two opposed
elements or opposed parallel surfaces associated with a size
dimension. (ASME Y14.5M-1994, 1.3.17)
Tolerance
The total amount a specific dimension is permitted to vary. The
tolerance is the difference between the maximum and minimum
limits. (ASME Y14.5M-1994, 1.3.31)
Tolerance-Bilateral
A tolerance in which variation is permitted in both directions from
the specified dimension. (ASME Y14.5M-1994, 1.3.32)
Tolerance-Unilateral
A tolerance in which variation is permitted in one direction from
the specified dimension. (ASME Y14.5M-1994, 1.3.34)
Definition
Basic Dimension
30 Basic Angle
24 Basic Diameter
Definition
Datums, Datum Targets,
Datum Features and Simulators
Datum
A theoretically exact point, axis, or plane derived from the
true geometric counterpart of a specified datum feature. A
datum is the origin from which the location or geometric
characteristics of features of a part are established.
(ASME Y14.5M-1994, 1.3.3)
Datum Feature
An actual feature of a part that is used to establish a
datum. (ASME Y14.5M-1994, 1.3.4)
Datum Target
A specified point, line, or area on a part used to establish a
datum. (ASME Y14.5M-1994, 1.3.7)
15 +/- 0.5
1M X Y Z
X
Z
XX
XX
Y As Shown on Drawing
Virtual Condition
1 Positional
Inner Boundary
Tolerance Zone at
( Maximum Inscribed
Diameter ) MMC
True (Basic)
Position of Hole
Other Possible
Extreme Locations
Boundary of MMC Hole
Shown at Extreme Limit True (Basic)
Position of Hole Axis Location of
MMC Hole Shown
Calculating Virtual Condition at Extreme Limit
12 +/- 0.5
1M L M N
L
N
XX
XX
M As Shown on Drawing
1 Positional
Virtual Condition
Tolerance Zone at
Outer Boundary
MMC
( Minimum Circumscribed
Diameter )
True (Basic)
Position of Feature
Other Possible
Extreme Locations
Boundary of MMC Feature
Shown at Extreme Limit True (Basic)
Position of Feature Axis Location of
MMC Feature Shown
at Extreme Limit
Calculating Virtual Condition
12.5 MMC Size of Feature (Maximum Size)
1 Applicable Geometric Tolerance
13.5 Virtual Condition Boundary
10.75 +0.25/- 0
.895
.890
.890
.885
Functional
Datums Should Be Consistent with Part Assembly Interfaces
Datum Features Should Minimize Assembly Variation
Datums Should Represent Actual Part Feature Relationships
Repeatable
Datum Features Must Be Dimensionally Stable
Datum Features Must Provide Secure, Repeatable Orientation
and Immobilization of a Part or Assembly as Required
Datums Planes Should Be Independent to Avoid Sensitivity
Coordinated
Datum Reference Frame Establishes a Common Basis for
Control and Measurement During All Process Phases of:
Manufacture
Inspection
Assembly
Datum Features Must Be Common and Coordinated With:
Stamping
Detail Gages
Assembly Tooling
Assembly Gages
Datum Feature
Selection and Coordination
Too many datum target areas can over constrain and distort
a panel. This could mask actual error and compromise the
integrity of inspection data. Too few and the part may not be
supported adequately, which can lead to poor or marginal
gage repeatability.
Six Degrees of Freedom
Y Axis
Z Axis Linear
Rotational
X Axis
Linear
X Axis
Rotational
Y Axis Z Axis
Rotational Linear
Datum Reference Frame
TERTIARY SECONDARY
DATUM PLANE DATUM PLANE
90 o
90 o
90 o
PART
PART
Fixed
PRIMARY DATUM
PART
PART PART
PART
Fixed Fixed
Fixed
PART
PART
PART
Free Free
Fixed
Fixed
FIRST
Free DATUM
Fixed PLANE
PRIMARY DATUM
Datum Reference Frame
The second, or secondary datum plane provides additional part
constraint and prevents free movement in one (1) additional linear
and one (1) rotational degree of freedom. Secondary planar datums
require a minimum of two points of contact on a feature surface to
constrain part movement. However, the part surface may actually
contact the datum plane or the simulated datum surface in an infinite
number of places.
Fixed Fixed
SECOND
Fixed
DATUM
PLANE
PART
PART
PART
Fixed
Free Free
Fixed
Fixed
Fixed
Fixed
SECONDARY DATUM
Datum Reference Frame
The third, or tertiary datum plane provides full part constraint and
prevents free movement along the one (1) remaining linear degree
of freedom. Tertiary planar datums require a minimum of one point
of contact on a feature surface to restrict the last degree of freedom.
However, the part surface may actually contact the datum plane or
the simulated datum surface in an infinite number of places.
Fixed Fixed
THIRD DATUM
Fixed
PLANE
PART
PART Fixed
Fixed PART
Fixed
Fixed
Fixed
Fixed
Fixed
TERTIARY DATUM
Datum Planes, Features,
and Simulated Datums
Datum Feature
(Actual Surface on Part)
Datum Plane
Part (True Geometric Counterpart of
Datum Feature)
(Workpiece)
Simulated Datum
(Surface on Gage or Fixture Locator)
Datum Feature Symbols
A AB
Datum Feature Symbol -- Former Practice
(ANSI Y14.5M-1982 and earlier standards)
A AB
Base (triangle) may
be filled or not filled
Datum Target
Datum Target
Label
Number
General
Datum Target
A1 Symbol
25 Circular
25 Datum Target
A1 A1 Area Symbol
May be filled
or not filled
10 X 20 Rectangular
Datum Target
A1 Area Symbol
Datum Targets
PARTIAL
12 SURFACE
CONTACT
A1
15 PART
15 DATUM BLOCK
PARTIAL
12 SURFACE
CONTACT
A1
15 PART
15 DATUM BLOCK
A1
120
A1
As Shown on Drawing
Means This:
PART
LINE
CONTACT
LOCATING PIN
Datum Target
Line
A1
120
A1
25
As Shown on Drawing
POINT
CONTACT
A
50
As Shown on Drawing
TRUE GEOMETRIC
COUNTERPART OF
PARTIAL SURFACE
A B
C 2.5 A-B C M D M
2.5 A-B C M D M
Multiple Datum
Features (Primary)
0.5 M A B M
B
C
C Datum Feature
Symbol
0.5 M A B M
5X M14X1-6H A
0.5 M P 20 A B M * Projected tolerance zones lie entirely
outside the boundary of the part feature
0.5 M P 20 A B M
Projected Tolerance
Zone Symbol
A
B
1 ABC
1 ABC
All Around Symbol
1 A BC
X Y
X Y
B C A
1ABC
X Y
Between Symbol
14.95 AVG
14.80
1 F
1 F
A1 A2
0.5 M A-D F B M CM
C
B
A4 D1 A3
0.5 M A-D F B M C M
2.5 M ST
ABM CM
A1 A2
C
B
A4 A3
2.5 M ST
A B M CM
Statistical
Tolerance Symbol
2.5 A B C
0.5 A B
Two Single Segment Profile Control Frames
2.5 A B C
0.5 A B
One Composite Profile Control Frame
Feature Control Frame Elements
1.5 M A B C
0.2 M A B
Two Single Segment True Position Control Frames
1.5 M A B C
0.2 M A B
One Composite True Position Control Frame
Feature Control Frame Elements
All feature elements must lie within both
specified tolerance zones simultaneously
Feature Locating
Reference
Material
Diameter
Modifier
Symbol
(Tolerance)
Datum
Reference
Frame
1M A B C
Geometric Secondary
Characteristic Tolerance
Datum
Symbol
Projected Minimum
Tolerance Projected
Symbol Zone Height
0.5 M P 20 A B M
Datum
Feature
Symbol C Material
Modifier
(Datum)
Feature Control Frame Review
Composite
Pattern Locating
True Position
Tolerance Zone
Symbol
Pattern Locating
Tolerance Zone
0.5 M A B C Framework (PLTZF)
Feature Relating
Tolerance Zone
Feature Profile
Locating Datum
2.5 A B C Reference
Feature Profile
Form/Orientation
Tolerance
Notes
E
N
D
Seminar Agenda
Objectives
Dimensional Engineering Concept
ASME Y14.5M-1994 and GM Global Addendum
Video - Introduction to GD&T
The Language of GD&T
Why Use GD&T ?
Engineering Drawings - General Review
Basic Rules and Definitions
Datum Function & Datum Reference Frames
Datum Planes, Features and Simulators
Datum Target Areas, Lines, Points
and Partial Datum Surfaces
Feature Control Frame Elements
Tolerances of Form
Tolerances of Orientation
Tolerances of Runout
Tolerances of Profile
Tolerances of Location
Rules and Definitions Quiz
Questions #1-12 True or False
11. A free state datum modifier applies to assists & rests. TRUE
4. The primary and secondary datum planes together will restrain five degrees
of freedom.