forces are those that would be exerted in the assembly or functioning of the part. However, if the dimensions and tolerances are met in the free state, it is usually not necessary to restrain the part unless the effect of subsequent restraining forces on the concerned features could cause other features of the part to exceed specified limits. Free state variation of nonrigid parts may be controlled as described in the following paragraphs.
6.8.1 Specifying Geometric Tolerances on Features Subject to Free State Variation.
Where an individual form or location tolerance is applied to a feature in the free state, specify the maximum allowable free state variation with an appropriate feature control frame. See Fig. 6-53. The free state symbol may be placed within the feature control frame, following the tolerance and any modifiers, to clarify a free state requirement on a drawing containing restrained feature notes, or to separate a free state requirement from associated features having restrained requirements. See Figs. 3-18 and 6-54.
6.8.2 Specifying Geometric Tolerances on Features to Be Restrained. Where orientation, runout, or location tolerances are to be verified with the part in a restrained condition, select and identify the features (pilot diameter, bosses, flanges, etc.) to be used as datum surfaces. Since these surfaces may be subject to free state variation, it is necessary to specify the maximum force necessary to restrain each of them. Determine the amount of the restraining or holding forces and other requirements necessary to simulate expected assembly conditions. Specify on the drawing that if restrained to this condition, the remainder of the part or certain features thereof shall be within stated tolerances. See Fig. 6-54.
6.8.3 Average Diameter. Where form control, such as circularity, is specified in a free state for a circular or cylindrical feature, the pertinent diameter is qualified with the abbreviation AVG. See Fig. 6-53. Specifying circularity on the basis of an average diameter on a nonrigid part is necessary to ensure that the actual diameter of the feature can be restrained to the desired shape at assembly. An average diameter is the average of several diametral measurements across a circular or cylindrical feature. Normally. enough (at least four) measurements are taken to assure the establishment of an average diameter. If practicable, an average diameter may be determined by a peripheral tape measurement. Note that the free state circularity tolerance is greater than the size tolerance on the diameter. Figures 6-53(a) and (b), simplified by showing only two measurements, give the permissible diameters in the free state for two extreme conditions of maximum average diameter and minimum average diameter, respectively The same method applies when the average diameter is anywhere between maximum and minimum limits.
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