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However, as long as they all fall within the tolerance zone the part is in spec.

Note that the derived median points do not need to form a straight line and may be scattered due to imperfections in the surface. In this example, the measured derived median points (green) all fall within the cylindrical tolerance zone surrounding datum axis A, ensuring a smooth, near-perfect rotational system.

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This would all be done with a CMM and measurement software and required special measurement programs to compare the axes. All central points would all need to fall into the cylindrical tolerance zone to be in tolerance. The tolerance zone would be established by the datum axis derived from datum feature A. This scan must then be analyzed to determine the derived median points at each location along the cylinder fall within the tolerance zone.

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Two gears with the concentricity callout.Ĭoncentricity would require side B to be measured in all dimensions several times to obtain a full dimensional scan of the surface of the reference feature. Datum A (right) is the drive side and relatively fixed with bearings to the housing, The referenced surface B is desired to be concentric with Datum A to avoid oscillations at high speed. Example:Īn intermediate shaft in a transmission is composed of two different cylindrical sections which are coaxial. In fact, in most cases, the use of runout or position should replace the need for concentricity and be much easier to measure. Equal mass or inertial concerns are one of the leading causes for the concentricity callout, however are often better designed with runout. Transmission gears, which need to always be coaxial to avoid oscillations and wear, may require concentricity to ensure all the axes line up correctly. When Used:ĭue to its complex nature, Concentricity is usually reserved for parts that require a high degree of precision to function properly. While Concentricity controls the derived median points (imperfect and scattered) of surface elements, position will control the axis (perfectly straight) of the feature. If a part is perfectly round (perfect circularity), the runout measurement will equal the concentricity, if the part is perfectly centered (perfect concentricity) the runout will equal the circularity.Ĭoncentricity is similar to position, however is not the same. Runout is a combination control that can indirectly control concentricity and circularity simultaneously.Į.g. Both tolerances measure derived median points compared to a datum plane/axis and are notoriously difficult to measure. This can only be done on a CMM or other computer measurement device and is quite time-consuming.Ĭoncentricity is considered the “circular” form of GD&T symmetry. Finally, these points are compared to the tolerance zone established by the datum axis. The median points of these diametrically opposed surface points must then be mapped out for the entire feature.

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First, you must establish a datum axis which to measure, Once the datum axis is established you must now take measure many a series of cross-sections (however many is realistic) to establish “diametrically opposed” (surface points directly opposite from each other across the diameter) surface points. All median points along the entire feature must be in this tolerance zone.Ĭoncentricity is considered one of the most difficult GD&T symbols to measure, due to its difficulty in establishing the midpoints of the feature.

Concentricity is a very complex feature because it relies on measurements from derived median points as opposed to a surface or feature’s axis.Ĭoncentricity is a 3-Dimensional cylindrical tolerance zone that is defined by a datum axis where all the derived median points of a referenced cylindrical feature must fall within.

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* In the ISO Standards, MMC is allowed with concentricity and symmetry Drawing Callout:Ĭoncentricity, (called coaxiality in the ISO Standard), is a tolerance that controls the central derived median points of the referenced feature, to a datum axis. Relative to Datum: Yes MMC or LMC applicable: No* Learn more about changes to the 2018 ASME Y14.5 standard. It is still commonly in use for those on previous versions of the standard. Note: Concentricity was removed from the 2018 ASME Y14.5 standard.

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