Circularity
Circularity
Circularity
Circularity
by GD&T Basics on December 21, 2014.
GD&T Symbol:
Relative to Datum: No
Description:
The circularity symbol is used to describe how close an object should be to a true circle.
Sometimes called roundness, circularity is a 2-Dimensional tolerance that controls the
overall form of a circle ensuring it is not too oblong, square, or out of round. Roundness is
independent of any datum feature and only is always less than the diameter dimensional
tolerance of the part. Circularity essentially makes a cross-section of a cylindrical or round
feature and determines if the circle formed in that cross-section is round.
Gauging / Measurement:
Circularity is measured by constraining a part, rotating it around the central axis while a
height gauge records the variation of the surface. The height gauge must have total variation
less than the tolerance amount.
When Used:
Circularity is a very common measurement and is uses in all forms of manufacturing. Any
time a part needs to be perfectly round such as a rotating shaft, or a bearing, circularity is
usually called out. You will see this GD&T symbol very often on mechanical engineering
drawings.
Example:
If you had a hole that was around a rotating shaft, Both pieces should be circular and have a
tight tolerance. Without circularity, the diameter of the hole and shaft would have to be very
tight and more expensive to make.
Here is a diagram showing where the surface is allowed to lie without any circularity added
for a size tolerance of 20±0.5. As you can see the max size can cause the shape of the part
to go to 20.5 – just like you would assume. However due to the rule in the GD&T standard –
the LMC size – in this case, the smallest size tolerance, only needs to be inspected with a
two-point measurement. For an odd-number lobed part – geometrically this means that the
circularity is limited by the TOTAL size tolerance. So for a size tolerance of 1.0 (±0.5), your
equivalent circularity control would be 1.0.
We go into depth on this in our GD&T Fundamentals Course when we talk about Rule #1 –
the Envelope Principle and how it needs to be inspected.
To Recap – you need to be within a perfect boundary at MMC (largest pin, smallest hole) but
for the LMC (smallest pin, largest hole size) you only need to take a 2-point measurement.
Final Notes:
Roundness:
Circularity in Geometric Dimensioning and Tolerancing is sometimes also referred to as
Roundness. Since it is a 2-Dimensional tolerance sometimes multiple sections of the same
feature must be measured to ensure that the entire length of a feature is within roundness.
Usually, two or three measurements are taken to ensure the part meets roundness for each
segment of the part.