STANDARD DRAFTING DEFINITIONS Machinery's Handbook, 31st Edition
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20 0.3
4.1 4.2
8 0.3
20 0.3
30 0.1˚
Dimension origin symbol Fig. 7. Dimension Origin Symbol
Primary datum reference
Tolerance
Geometric control symbol
Datum modifier
A B
C
∅ 0.25
M
M
A - B
Tertiary datum reference
Tolerance modifier
Co-datum (both primary)
Secondary datum reference
Fig. 8. Feature Control Frame and Datum Order of Precedence When an ISO tolerance frame leader line terminates on the outline of the feature, it indicates that the control is a line or the surface itself (Fig. 9a.) When an ISO tolerance frame leader line terminates on a dimension, the axis or medium plane of the dimensioned feature is being controlled. Either inside or outside dimension lines may be used (Fig. 9b.)
0,1
0,1
0,1
Fig. 9a. ISO Tolerance Frame Leader Line Terminating on the Outline of the Feature
Fig. 9b. ISO Tolerance Frame Leader Line Terminating on a Dimension
Feature of Size, Regular: One cylindrical or spherical surface, a circular element, and a set of two opposed parallel elements or opposed parallel surfaces, each of which is associ ated with a directly toleranced dimension. Feature of Size, Irregular: A directly toleranced feature or collection of features that may contain or be contained by an actual mating envelope that is a) a sphere, cylinder, or pair of parallel planes; or b) other than a sphere, cylinder, or pair of parallel planes. Least Material Boundary (LMB): The limit defined by a tolerance or combination of tolerances that exist on or inside the material of a feature or features. Least Material Condition (LMC): The condition in which a feature of size contains the least amount of material within the stated limits of size, for example, upper limit or maxi mum hole diameter and lower limit or minimum shaft diameter. Limits, Upper and Lower (UL and LL): The arithmetic values representing the maxi mum and minimum size allowable for a dimension or tolerance. The upper limit represents the maximum size allowable. The lower limit represents the minimum size allowable.
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