GD&T
Basic Geometric Tolerancing
Before Geometric Tolerancing can be fully understood,
some of the basic terms used in dimensioning and tolerancing
much be clearly defined.
Dimension
A dimensions are numerical values used to define the characteristics
of objects. Such as size, location, position etc.
Basic
Dimension
Basic dimensions are identified as a rectangular block
around the dimension. A basic dimension is theoretically exact
dimension of a feature, title block tolerances do not apply
to basic dimensions and it DO NOT accumulate tolerances.
The use of the basic dimension requires a geometric tolerance
which is specified in a feature control frame or a specific
note.
Reference
Dimension
A
reference dimension is used only for information purpose.
Usually, a dimension within a bracket or a abbreviation REF
indicated beside or under the dimension.
Nominal
Dimension
A nominal dimension is the value designated to an object.
Block tolerance do not apply to a nominal dimension.
A nominal dimension shall not be used for manufacturing
or inspection purpose.
Likewise,
a abbreviation NOM shall be indicate immediately following
or under the dimension.
Coordinate
Tolerancing
Coordinate
tolerancing Is the tolerance used to specify the allowable
variation of a dimension. It is the total dimension that may
vary.
Feature
A
feature is a specific portion of the part, such as, holes,
location, slot etc.
Maximum
Material Condition ( MMC )
A
maximum Material Condition ( MMC ) is the condition of the
feature where it contains the maximum amount of material.
MMC can be thought as the condition when the part weight the
most. e.g.. Maximum size of a shaft and minimum size of
a
hole.
Allowable
tolerance of MMC maximum material condition of the
6.0 hole is 0.2
True position tolerance = 0.2 in diameter
Allowable hole position =0.2, therefore position tolerance
will be 12.2+/ 0.1
If the hole measured at 6.4
Allowable position tolerance =( 0.4+0.2=0.6 ) therefore
position tolerance =12.2+/0.3

Least
Material Condition ( LMC )
Least Material
Condition (LMC) is the condition of a feature where it contains
least amount of material. It is direct opposite of the MMC,
and may be consider as the condition of the feature when it
weight the least.
Allowable
tolerance of LMC least material condition of the 24.1
hole is 0.5 in diameter
The hole at it maximum size, the location tolerance
allowable is 30+/ 0.25
If the hole measured at 23.9
Allowable position tolerance =( 0.5+0.2=0.7 ) therefore
position tolerance =30+/0.35

