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Geometric Dimensioning and Tolerancing (GD&T)
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
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
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
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
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 insure 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
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.
ENVELOPE PRINCIPLE
LMC CLEARANCE
MMC ALLOWANCE
LIMITS OF SIZE
LIMITS OF SIZE
The actual size of the feature at any cross section must be within the size boundary.
ØMMC
ØLMC
No portion of the feature may be outside a perfect form barrier at maximum material condition (MMC).
LIMITS OF SIZE
Other Factors I.e., Parallel Line Tolerance Zones
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
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
THE
GEOMETRIC SYMBOL TOLERANCE INFORMATION
DATUM REFERENCES
FEATURE CONTROL FRAME
COMPARTMENT VARIABLES
CONNECTING WORDS MUST BE WITHIN OF THE FEATURE
RELATIVE TO
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, 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.
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
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
Cylindricity Circularity
Form 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.
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.
.030 0.500 0.505 ∅ M ∅
1.010 0.990
Dial Indicator
Verification of Flatness
Features that Require Datum Reference
Orientation – Perpendicularity – Angularity – Parallelism
Runout – Circular Runout
– Total Runout
Location – Position – Concentricity
– Symmetry