Achieving Tight Tolerances for Roundness Requirements
When customers come to us with a request for a certain circularity tolerance on the small metal parts we fabricate for them, it opens up some interesting questions. What is circularity and how does one determine whether a part meets the criteria that allow it to be described as circular?
Multiple Ways to Define Circularity
In simplest terms, circularity is a description of roundness, or how closely an object resembles a true circle. In our world, circularity is a two-dimensional measurement of how round an object is at one point along a cylinder. Unlike concentricity, which compares roundness at two different points, or cylindricity, which is a three-dimensional look at both roundness and straightness along a part’s axis, circularity tolerance is concerned with only that one point on the part, not its relationship to anything else or the entire length of the cylinder. (You can read more about circularity, concentricity, cylindricity, and other related terms in our blog Dimensional Issues in Metal Cut to Length.)
When specifying a circularity tolerance, it is important to be clear what you want to be measured — for instance, are you talking about the ID or the OD? — and which point along the cylinder is most important. For example, when tumbling parts (such as pins) for radius, we know the ends will have ever so slightly different diameters than the middle will; the larger the radius, the farther in from the ends the diameter will be effected. Therefore, we typically measure circularity at the middle of the part. However, with a tube, the ID can only be measured at the ends.
At Metal Cutting, we typically work with rods and tubes — true round objects that have the same diameter throughout the part. If the diameter were not the same throughout, the object would be oval rather than round. Whether we are working with a tube or a solid, we focus on the OD to determine the circularity tolerance. Here, measuring the diameter at several points around the part does give you a good idea of whether the part is truly circular.
Although the broad definition of circularity can also describe shapes such as hexagons and other figures with flat sides, in general the parts we measure are, indeed, round. However, we do grind and cut polygonal parts to lengths, including square or rectangular sides up through complex polygons; in these cases, different measuring techniques are used to determine whether the parts are within customer specifications for circularity tolerance.
Illustrating GD&T Circularity
Under the system for Geometric Dimensioning and Tolerancing (GD&T), the symbol used to show circularity in engineering drawings is — as you would expect — a circle. Typically, the drawing will have a callout box pointing to the corresponding part surface, with the callout containing a circle and the appropriate circularity tolerance.
Another way to show GD&T circularity is draw the part’s tolerance zone — two concentric circles that differ in diameter by the part’s specified tolerance. The two concentric circles show the maximum and minimum sizes between which all points of the specified part’s diameter must fit in order for it to be deemed sufficiently round and within the required circularity tolerance. In other words, the part must fit inside the tolerance zone between the inner and outer circles. (Amazingly, this can be true even for hexagons, stars, and other flat-sided shapes, allowing them to be considered to have a circularity tolerance.)
If you search for GD&T circularity examples, you’ll likely find drawings of shapes with tolerances such as ±0.050” or ±0.030”. Here at Metal Cutting, where we specialize in very small, precision parts, 0.050” or 0.030” out of round is huge. More often, we deal with circularity tolerances such as ±0.001”, ±0.002”, or even as close as ±0.0002” — and then there are the customers for whom circularity is measured in the millionths of an inch.
Specifying Circularity Tolerance
Working with such tiny parts, when customers ask for a circularity tolerance, they typically ask us to look at the diameter around the part at a particular point — usually the OD and most often, at the middle of the part — to determine if the part meets their specifications. Although uniform diameter is not a perfect definition of what circularity is, most of our customers find it meets their requirements for controlling roundness and helping to ensure that parts fit properly, move smoothly, and wear evenly. Where customers request it, special devices (such as probes that measure all the way around a part) can be used to measure true circularity rather than diameter.