Coaxiauty Controls

Coaxiality is that condition where the axes of two or more surfaces of revolution are coincident. The amount of permissible variation from coaxiality may be expressed by a positional tolerance or a runout tolerance. Selection of the proper control depends on the nature of the functional requirements of the design.

5.11.1 Positional Tolaranca Control. Where the surfaces of revolution are cylindrical and the control of the axes can be applied on a material condition basis, positional tolerancing is recommended. Coaxial Relationships. A coaxial relationship may be controlled by specifying a positional tolerance at MMC. See Fig. 5-48. A coaxial relationship may also be controlled by specifying a positional tolerance at RFS. See Fig. 5-55. The datum feature may be specified on either an MMC or an RFS basis, depending upon the design requirements. In Fig. 5-48, the datum feature is specified on an MMC basis. In such cases, any departure of the datum feature from MMC may result in an additional displacement between its axis and the axis of the considered feature. See the condition shown in Fig. 5-49(c). Where two or more features are coaxially related to such a datum — for example, a shaft having several diameters — the considered features are displaced as a group relative to the datum feature, as explained in para. for a pattern of features. Verification. The positional tolerance control shown in Fig. 5-48 usually permits, but does not dictate the use of a simple receiver gage for inspection. The application of such a gage is illustrated in Fig. 5-49. that shows:

(a) both the considered feature and the datum feature at MMC;

(b) the considered feature at LMC and the datum feature at MMC;

(c) both the considered feature and the datum feature at LMC. displaced in opposite extremes. Coaxial Features Controlled Within Limits of Size. Where it is necessary to control coaxiality of related features within their limits of size, a zero positional tolerance at MMC is specified. The datum feature is normally specified on an MMC basis. See Fig. 5-50. Boundaries of perfect form are thereby established that are truly coaxial, where both features are at MMC. Variations in coaxiality are permitted only where the features depart from their MMC size toward LMC. Alignment of Coaxial Holes. A

composite positional tolerance may be used to control the alignment of two or more coaxial holes. This method allows specific control of feature-to-feature

Composite Positional Dimension

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