GD&T Symbol:
Relative to Datum: Yes
MMC or LMC applicable: No
Drawing Callout:
Description:
Concentricity, sometimes called coaxially, is a tolerance that controls the central axis of the referenced feature, to a datum
axis. The axes for the datum and referenced feature are derived from the median points of the part or feature.
Concentricity is a very complex feature because it relies on measurements from a derived axis as opposed tangible surface
or feature.
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 circular feature must fall into. the median points of the reference surface cross sections form the
theoretical axis that must be in this tolerance zone.
Gauging / Measurement:
Concentricity is considered one of the most difficult GD&T symbols to measure for due to its difficulty in establishing the
mid points 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). Once the cross sections are taken and
the exact plot of the surface is obtained, the median points of these cross sections must be determined. Then these series of
points must be plotted to see if they fall within the cylindrical tolerance zone. 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. While symmetry measured the true midpoint plane of a
feature to a datum plane or axis, concentricity measures the derived midpoint axis to a datum axis. Both are notoriously
difficult to measure.
Runout is a combination of concentricity and circularity.
Runout = Circularity + Concentricity
If a part is perfectly round, the runout will equal the concentricity.
Concentricity is also a 3D form of 2-Dimensional True Position when applied to a circular feature. While true position is
usually controlled to a fixed point in space that forms from coordinate measurements from a datum, concentricity is
controlled to the axis derived from a all the median points of a datum surface or 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. Any application where the median points of a feature need to be controlled relative to a datum would
require cylindricity. However in many cases, the use of runout or true position can replace the need for concentricity and
be much easier to measure for.
Example:
An intermediate shaft in a transmission is composed of two different diameter sections which are coaxial. Datum A 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 central axis points at each location
along the cylinder, forming the true part axis. The tolerance zone would then need to be established by measuring Datum
A to determine its axis. Both the datum tolerance zone and the measured central points from the reference surface would
be compared. The measured central axis points would all need to fall into the cylindrical tolerance zone surrounding
datum A. 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 axis falls within the cylindrical tolerance zone surrounding datum axis A, ensuring a
smooth, near-perfect rotational system.
