298 GD&T: Application and Interpretation Copyright Goodheart-Willcox Co., Inc. Position tolerances are only applied to features of size and bounded features. The feature control frame may be connected to the appropriate feature with a leader, it may be placed adjacent to the feature dimension, or it may be placed on the dimension line for the size dimension. ASME standards do not support using posi- tion tolerances on surfaces. ISO standards do permit position tolerance application to surfaces, but the ISO practice can result in confusion and should not be used on product defi nition that is based on ASME standards. ASME establishes clarity of location requirements for surfaces using profi le tolerances as explained in a later chapter. Many position tolerancing applications may be met using the single-segment (single-line) fea- ture control frame already described. More complex applications sometimes require the use of multiple single-segment feature control frames or a compos- ite feature control frame. The format of these feature control frames is shown in Figure 8-4. These are explained in the next chapter. Tolerance Value The tolerance value is preceded by a diame- ter symbol (∅) if the zone is circular or cylindrical. The tolerance value is preceded by the spherical diameter symbol if the zone is spherical. No modifi er is shown on the tolerance value if RFS is applicable. The tolerance value is followed by a material condi- tion modifi er if MMC or LMC is applicable. If tolerances are determined using statistical cal- culations, the statistical symbol may be inserted in the feature control frame. It is placed after any shown material condition modifi er. If the statistical symbol is shown, notations may be added outside the feature control frame to defi ne any required statistical pro- cess control requirements that are applicable to the tolerance. Should there be a need to control the maximum allowable tolerance when the MMC or LMC modifi er is applied, a maximum tolerance value may follow the material condition modifi er (∅.010 at MMC ∅.014 MAX). If the tolerances are applied for both a restrained and free state, the tolerance that is appli- cable in the free state includes the free state symbol following the material condition modifi er. Datum Feature References As with all other feature control frames, the datum feature references in a position tolerance feature control frame are read from left to right. The primary datum feature is shown fi rst, followed by the second- ary and tertiary datum features. All datum feature references are applicable regardless of boundary con- dition when no modifi er is shown. Where it is desired to indicate a datum feature reference is applicable at the maximum material boundary or the least material boundary, the MMB or LMB modifi er must be shown following the datum feature reference. Explicit datum feature references are required. Implied datums are not permitted. Beginning with the 2009 standard, an exception was identifi ed where it is permissible to apply a position tolerance without datum feature references. This is not an application in which datum features are implied. Position tolerances without datum feature references are only permitted where a pattern of features has a position tolerance and the pattern of features becomes the primary datum feature to which other features are toleranced. This is explained later in this chapter. The applica- tions where datum references are not required are rare exceptions to the general requirement for datum feature references in position tolerances. Use of implied datum feature references is not permitted. Implied datum feature references cause confusion. If implied datum features were used, a datum reference frame would need to be assumed. Being required to assume a datum reference frame introduces risk of inconsistency because two peo- ple might assume different datum reference frames. Two different datum reference frames will not create the same results when fabricating or measuring parts. PAST PRACTICE The requirements regarding datum feature refer-- ences in positional tolerances have matured the development of several revisions of standards. If working with old drawings created prior to the 1982 standard, datum references may be omitted, and the intended interpretation of requirements may be diffi cult to determine. T l tol h d dduring I 1 references a Composite (One feature control frame) (Two feature control frames) Two single segments Goodheart-Willcox Publisher Figure 8-4. Single-segment, multiple single-segment, or composite position tolerance feature control frames may be used to achieve the required level of control.