Circularity: GD&T Symbol: Relative To Datum MMC or LMC Applicable: Drawing Callout
Circularity: GD&T Symbol: Relative To Datum MMC or LMC Applicable: Drawing Callout
Circularity: GD&T Symbol: Relative To Datum MMC or LMC Applicable: Drawing Callout
GD&T Symbol:
Relative to Datum: No
MMC or LMC applicable: No
Drawing Callout:
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.
You may be thinking, “well hang on – if it is ± 0.08 and circularity is the radial
distance between the two circles, wouldn’t that mean the circularity should be only
0.08 since it would be on both sides? No – and this is because of how the two-point
measurement of any feature would work when compared to the smallest size vs the
biggest size it could be. In GD&T there is a rule that states you need perfect form at
the MMC size – meaning at the largest size for a pin (smallest for a hole), your
shape of this round feature cannot let it outside of a size of 10.08 for the first
example.
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:
Because circularity specifies the form of the surface in a specific area it needs to be
considered when calculating a statistical tolerance stack. For example, if you have a
part with a specified diameter and circularity callout, you must use both in your
statistical stack since the geometric tolerance can contribute to a large part envelope
than just the diameter tolerance alone. This will skew the statistical tolerance slightly
higher and should be considered since parts are rarely perfectly circular.