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The Curious Bag
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Basic dimensions are part of a dimensioning practice that conforms to ANSE/ANSI Y14.5, geometric dimensioning and tolerancing. It is part of a complete system to locate features on mechanical parts.
Basic dimension
A basic dimension is one that is inside an enclosed rectangular box. It is not associated with any tolerance, and often referred to as a "dead nuts" number.
Tolerances
On a manufacturing drawing, a basic number is an exact number. To allow manufacturability, feature control frames are added.
Feature control frame
If a hole in a plate is given basic dimensions of 2 inches off a corner, vertical and horizontal, that is the exact location. Adding a feature control frame with a position tolerance allows the machinist flexibility to manufacture the part.
Position tolerance
In the above example, if a position tolerance of 5/1000s of an inch, for example, is added in the feature control frame, that applies to the location of the hole. This position tolerance allows the position of the hole to "float" within a circle the diameter of the tolerance.
ASME/ANSI Standards
The basic dimension is part of a plan put together and published in ANSI Y14.5. The object is to manufacture consistent and repeatable parts.
Source:
Engineers Edge: Geometric Dimensioning & Tolerancing (GD&T) Definitions
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