Machinery's Handbook, 31st Edition
630 STANDARD DRAFTING DEFINITIONS Maximum Material Boundary (MMB): The limit defined by a tolerance or combination of tolerances that exist on or outside the material of a feature or features. Maximum Material Condition (MMC): The condition in which a feature of size contains the maximum amount of material within the stated limits of size. For example, the lower limit of a hole is the minimum hole diameter. The upper limit of a shaft is the maximum shaft diameter. Position: Formerly called true position, position is the theoretically exact location of a feature established by basic dimensions. In ISO practice a basic dimension is called a the oretically exact dimension (TED). A positional tolerance is indicated by the position sym bol, a tolerance value, applicable material condition modifiers, and appropriate datum references placed in a feature control frame. Regardless of Feature Size (RFS): The term used to indicate that a geometric tolerance or datum reference applies at any increment of size of the feature within its tolerance limits. RFS is the default condition unless MMC or LMC is specified. The concept is now the default in ASME Y14.5-2018, unless specifically stated otherwise. Thus the symbol for RFS is no longer supported in ASME Y14.5-2018. Regardless of Material Boundary (RMB): indicates that a datum feature simulator pro gresses from MMB toward LMB until it makes maximum contact with the extremities of a feature(s). See Datum Simulator on page 627. Size, Actual: The term indicating the size of a feature as produced. Tolerance Zone Symmetry: In geometric tolerancing, the tolerance value stated in the feature control frame is always a single value. Unless otherwise specified, it is assumed that boundaries created by the stated tolerance are bilateral and equidistant about the per fect form control specified. See Fig. 10a for default zone. If desired, the tolerance may be specified as unilateral or unequally bilateral. See Fig. 10b and Fig. 10c for external and internal unilateral zones, and Fig. 10d for an example of a bilateral asymmetrical zone. Tolerance Zone Symmetry Examples
Perfect form*
Perfect form*
0.1 A
U 0.1
0.1
A
0.1 *
0.1 A Alternate practice
0.05 *
0.1 *
5
7.7°
7.7°
5
20
20
A
A
Fig. 10b. External Unilateral Tolerance Zone About Perfect Form
Fig. 10a. Default Symmetrical Tolerance Zone About Perfect Form
Perfect form*
0.1
A 0.03 U
Perfect form*
0.1 *
0.1 A U 0
0.03
0.07 *
0.1 A Alternate practice
0.1 A
0.1 *
Alternate practice
7.7°
7.7°
5
5
20
20
A
A
Fig. 10d. Bilateral Asymmetrical Tolerance Zone About Perfect Form
Fig. 10c. Internal Unilateral Tolerance Zone About Perfect Form
* Added for clarification and is not part of the specification. Tolerance, Bilateral: A tolerance where variation is permitted in both directions from the specified dimension. Bilateral tolerances may be equal or unequal.
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