Circularity symbol svg 2The circularity symbol is used to describe how close any given cross section of a feature 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. 

Circularity Symbol and Callout 

Circularity symbol

Circularity, which is often informally called “Roundness,” is independent of any datum feature. Meaning it will never control the location of a feature with respect to any datums. Circularity can be applied to cylindrical features, spherical features, and conical features both internally as well as externally. The symbol essentially takes a cross-section of the feature and determines if the circle formed in that cross-section is round. This tolerance applies to any and all cross sections of the given feature. The feature control frame can be attached directly to the feature or also attached to the size dimension and tolerance that is associated with the feature when applicable.

Circularity Tolerance Zone 

Circularity tolerance zone

The tolerance zone is two concentric circles, one inner and one outer, in which all the cross-sectional surface elements must fall within. The tolerance zone lies on a plane that is perpendicular to the central axis of the circular feature. The value of the feature control frame for the circularity tolerance is the radial distance between the two concentric circles that establish the tolerance zone.

How to Inspect Circularity 

Circularity inspection

Circularity is measured by constraining a part, rotating it around the central axis of the feature being controlled with circularity, while the test indicator records the variation of the surface. The indicator must have total variation less than the tolerance amount. Note that the rotation must NOT be created from any datum features but rather from the specific feature where the circularity control is applied. Due to the difficult nature of using the feature to establish rotation AND inspecting the feature at the same time, it’s best to consider, for critical situations with tight tolerances, inspecting circularity with a roundness checker or runout checking machines.

Circularity Considerations 

You may not realize it, but your size tolerances already control circularity. Take a look at the two examples below. They both control the “roundness” or circularity of the cross section to a maximum of 0.030 inches. 

Circularity considerations - comparison of form control via size tolerance and via circularity control

You may be thinking, “well hang on – if it is ± Ø0.015, that’s 0.030 on the diameter, which would then mean it’s 0.015 inches on the radius. How is that equal to the 0.030 inch radial measurement that circularity controls??”  This is because of combination of WHERE the local two-point measurement of any cross section could be relative to the envelope of MMC. Rule #1 dictates we cannot be larger than the envelope of perfect form at MMC and our local sizes must also be larger than the LMC of our feature. So, if we subtract the LMC (the red value above) from the MMC (green value above) we would be left with the total size tolerance allowed as worse case form deviation (roundness or circularity). Which in this case is 0.030 inches. 

Circularity for a feature of size example

With this in mind, we see that we could have a size tolerance of 0.030 inches, for example, however we could NOT then apply a circularity of 0.030 to the same feature. It would need to be less than the size dimension for this to be a functional tolerance. 

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. We also cover the intricacies of circularity, how it’s interpreted, how it’s inspected and the miscellaneous things you should know about this symbol in our GD&T Fundamentals Course as well as our GD&T Inspection Course. These courses show you everything you need to know about circularity and then some! 

Circularity CTA image v2

 

Example and When to Use Circularity 

Circularity is a common tolerance and is used in all forms of manufacturing. It is mostly used when a part needs to be perfectly round, such as a rotating shaft or a bearing. You will see this GD&T symbol very often on mechanical engineering drawings.  

Poor circularity leads to: 

  • Uneven load distribution 
  • Excessive vibration 
  • Noise 
  • Rapid wear 
  • Premature failure 
  • Imperfect contact with shafts/housing 
  • Improper lubrication 

Additionally, you will find that a few other controls will already control the circularity of a feature. Symbols like cylindricity, circular runout and total runout will already control the circularity of a feature. However, in those instances you can still refine the circularity of the feature to a tighter tolerance if necessary. For example, in the drawing below, the size dimensions of the inner diameter are already controlling the circularity of the feature to 0.010 inches, however the circularity tolerance takes that one step further and REFINES the form to 0.003 inches.

circularity example

Circularity vs. Cylindricity 

Circularity is the 2D version of cylindricity. While cylindricity ensures all the points on a cylinder fall into a tolerance, circularity only is concerned with individual cross sections, one at a time. If you think of a stack of coins, circularity would be a measurement around one coin while cylindricity would have to measure the entire stack. (Cylindricity is actually a combination of circularity and straightness). 

The one-page GD&T reference that you will use every day

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