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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.

basic dimension

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.

referance 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.

tolerance

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.

MMC maximium material condition
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

 

 

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