Machine Designers Reference. J. Marrs
Figure 3-10: Parallelism Callout and Interpretation
Concentricity is a specification applied to cylindrical or spherical features. It defines a tolerance zone about the axis of the feature (or center point, in the case of a sphere) within which all center points of the feature must lie. Concentricity is a tolerance of location relative to a datum axis or center point. An illustration of this is shown in Figure 3-11.
Figure 3-11: Concentricity Callout and Interpretation
Circular runout is a specification applied to cylindrical features, or features with round cross section. It defines a circular tolerance zone on a plane perpendicular to the datum axis within which all elements of the feature surface intersecting that plane must lie. An infinite number of planes can be assumed. Circular runout, unlike total runout, does not relate measurements at each section to one another. The part will be rotated about the datum in order to measure runout. Circular runout is a functional tolerance that relates the circular form at each cross section to one or more (concentric) datum axes. Figure 3-12 illustrates circular runout and its measurement.
Figure 3-12: Circular Runout Callout and Interpretation
Total runout is a specification applied to cylindrical features. It defines a cylindrical tolerance zone parallel to the datum axis within which all elements of the feature surface must lie. The part will be rotated about the datum in order to measure runout. Total runout is a functional tolerance that relates the cylindrical form to one or more (concentric) datum axes. Figure 3-13 illustrates total runout and its measurement. This specification is particularly useful when dimensioning rotating shafts that carry components like bearings or gears that are sensitive to misalignment.
Figure 3-13: Total Runout Callout and Interpretation
Profile of a surface is a specification applied to surfaces. The surfaces can be of complex shape and must be fully defined using basic dimensions. Profile of a surface defines a tolerance zone centered on the nominal surface and following the surface shape. All points on the surface must fall within the two boundary planes. Unilateral tolerancing is possible, so consult the recommended resources for more information. Profile of a surface is a tolerance of form that may or may not reference datums. When datums are referenced, the profile form and relative position are both controlled. This specification is very powerful in that regard. Figure 3-14 illustrates this callout.
Figure 3-14: Profile of a Surface Callout and Interpretation
Position is a specification applied to center points, midplanes, or axes of features. It is most commonly applied to holes and hole patterns. Position specification defines a tolerance zone around the feature’s nominal center. This tolerance zone is cylindrical in the case of hole features, and rectangular in the case of flat features. The center, midplane, or axis of the feature must lie within the tolerance zone. Position tolerance can be applied to groups of identical features and, in such a case, the tolerance zone is set up for each individual feature in the pattern. Position is a tolerance of location relative to one or more datums. Basic dimensions must be used to provide nominal location for the features bearing position tolerances. Figure 3-15 illustrates a typical position callout for holes, and Figure 3-16 shows some possible variations allowable within position tolerance zones. Compound tolerance frames are an advanced technique that may be used to control position of features within a pattern while allowing a separate tolerance to control the location of the pattern center relative to the datums. This powerful technique is illustrated in the recommended resources as well as in Chapter 4 of this book.
Figure 3-15: Position Callout and Tolerance Zones
Figure 3-16: Position Tolerance Zones and Allowable Variation of a Hole Axis
Modifiers
Tolerance modifiers include maximum material condition, minimum material condition, and regardless of feature size. Modifers may not be added to runout, concentricity, or symmetry specifications. Maximum material condition (MMC) is the condition in which a feature is at the limit of size corresponding to the maximum material left on the part. For a hole, MMC corresponds to the smallest hole within the stated limits of size. For an external diameter, MMC corresponds to the largest diameter within the stated limits of size. Least material condition (LMC) is the condition in which a feature is at the limit of size corresponding to the least material left on the part. LMC for a hole is the largest hole within the limits of size, and for an external diameter is the smallest diameter within the limits of size.
MMC and LMC are used to modify a tolerance or datum reference based on the size of the feature as produced, rather than its theoretical size. Regardless of feature size (RFS) indicates that the tolerance or datum reference applies to a nominal feature and not to the feature as produced. RFS is assumed in all cases unless otherwise stated. MMC is commonly used, whereas LMC is seldom used. When the MMC modifier is present, the tolerance is read as “tolerance when feature is at maximum material condition.” The use of MMC permits additional tolerance when the considered feature, as produced, departs from its maximum material condition. Consult the recommended resources and Section 3.3 for guidance on the proper use of modifiers.
•All dimensions on a drawing must have tolerances specified, either implied or explicit.
•Dimensions and tolerances should convey design intent and functional relationships between features and surfaces.
•Analyze all critical tolerances to ensure proper fit and function.
•Use standard symbols when applicable rather than notes. Standard symbols have clear and universal meaning, whereas notes may be misunderstood.
•Specify the loosest tolerances possible to save cost and enable a choice of manufacturing methods. This may require explicit tolerances that are looser than the drawing’s implied tolerances in some cases.
•When checking drawings, a useful technique with paper drawings is to use a highlighting marker for checked dimensions and a red pen to make changes. The title block should also be checked, including any implied tolerance information.
•Apply position tolerance to all holes, and dimension their locations with basic dimensions.
•Threaded features and tapped holes are normally dimensioned with the modifier “Regardless of Feature Size (RFS),” and the tolerance is applied to the axis of the thread derived from the pitch cylinder. Exceptions to this practice must be noted on the drawing.
•When