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Concentricity
by GD&T Basics on December 15, 2014.
GD&T Symbol:
Note: Concentricity was removed from the 2018 ASME Y14.5 standard. It is still commonly
in use for those on previous versions of the standard.
Learn more about changes to the 2018 ASME Y14.5 standard.
Relative to Datum: Yes
MMC or LMC applicable: No*
* In the ISO Standards, MMC is allowed with concentricity and symmetry
Drawing Callout:
Description:
Concentricity, (called coaxiality in the ISO Standard), is a tolerance that controls the central
derived median points of the referenced feature, to a datum axis. Concentricity is a very
complex feature because it relies on measurements from derived median points as opposed
to a surface or feature’s axis.
GD&T Tolerance Zone:
Concentricity 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. All
median points along the entire feature must be in this tolerance zone.
Gauging / Measurement:
Concentricity is considered one of the most difficult Geometric Dimensioning and
Tolerancing symbols to measure, due to its difficulty in establishing the midpoints of the
feature. 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. The median points of these diametrically opposed
surface points must then be mapped out for the entire feature. Finally, these points are
compared to the tolerance zone established by the datum axis. This can only be done on a
CMM or other computer measurement device and is quite time-consuming.
Relation to Other GD&T Symbols:
Concentricity is considered the “circular” form of GD&T symmetry. Both tolerances measure
derived median points compared to a datum plane/axis and are notoriously difficult to
measure.
Runout is a combination control that can indirectly control concentricity and circularity
simultaneously.
e.g. 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.
Concentricity is similar to position, however is not the same. While Concentricity controls
the derived median points (imperfect and scattered) of surface elements, position will control
the axis (perfectly straight) of the feature.
When Used:
Due to its complex nature, Concentricity is usually reserved for parts that require a high
degree of precision to function properly. Transmission gears, which need to always be
coaxial to avoid oscillations and wear, may require concentricity to ensure all the axes line
up correctly. Equal mass or inertial concerns are one of the leading causes for the
concentricity callout, however are often better designed with runout. In fact, in most cases,
the use of runout or position should replace the need for concentricity and be much easier to
measure.
Example:
An intermediate shaft in a transmission is composed of two different cylindrical sections
which are coaxial. 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.
Two gears with the concentricity callout.
Concentricity 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. This scan must then be
analyzed to determine the derived median points at each location along the cylinder fall
within the tolerance zone. The tolerance zone would be established by the datum axis
derived from datum feature A. All central points would all need to fall into the cylindrical
tolerance zone to be in tolerance. This would all be done with a CMM and measurement
software and required special measurement programs to compare the axes.
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. Note that the derived median points do not need to form a straight line and may be
scattered due to imperfections in the surface. However, as long as they all fall within the
tolerance zone the part is in spec.
Final Notes to Remember:
Avoid Concentricity!
You will always hear from most machinists, measurement techs, and designers to avoid
concentricity like the plague. Unless it is absolutely necessary to control the distribution of
mass around a part’s median points you should look to other more applicable GD&T
symbols. A good replacement for concentricity is Circular Runout or Total Runout since they
compare the actual surface of a feature to a tolerance zone, while concentricity relates the
derived median points of the surface to a tolerance zone. You can physically touch and
measure the surface of the part to obtain a runout measurement. Controlling runout will also
control the concentricity to the same extent. (Runout = Concentricity + Circularity)
Use in Ammunition Measurement
Often you will see “concentricity gauges” that are applied to homemade bullet casings.
These gauges, however, do not measure concentricity but instead are measuring runout.
However, since runout is just a combination of circularity and concentricity, you can
technically say that you are measuring the concentricity as well indirectly.
