Why Roundness Tolerance Is Critical for Small Metal Parts — A Precision Manufacturing Guide
Specifying tolerances for critical features of a part is a key factor in optimizing manufacturability. When customers come to Metal Cutting Corporation and request a certain roundness tolerance for the small metal parts we manufacture, it raises some interesting questions.
What is Roundness? And How Do You Know if a Part is Truly “Round”?
Multiple Ways to Define Roundness
In the simplest terms, roundness is a measure of how closely an object resembles a true circle. In our world, roundness is a two-dimensional measurement of circularity at a specific point on a cylinder. Roundness tolerances are specified to control the shape of circular features, ensuring they aren’t too long, too square, or otherwise misshapen.
Roundness tolerance refers only to a single point on the part and not in relation to anything else or to the entire length of the cylinder. This is different from cylindricity, which involves a 3D view comparing roundness and straightness along the part’s axis, and from concentricity, which compares the roundness of two different points.
Roundness Tolerances for ID and OD
When specifying a roundness tolerance, it’s important to clarify what you’re measuring — for instance, are you referring to the inside diameter (ID) or the outside diameter (OD)? — and where along the cylinder the measurement matters most.
For example, when rolling parts (like pins) to obtain a radius, we know the diameter at the ends may differ slightly from that at the center. The larger the radius, the more the end diameters are affected. Therefore, we typically measure roundness at the center of the part. However, when working with tubing, we can only measure the ID at the ends.
In metal cutting, we often use bars and tubes — truly round objects with a uniform diameter throughout. (If the diameter isn’t consistent, the object becomes elliptical rather than round.) Whether we’re using tube or solid stock, we focus on the OD to determine roundness tolerance. Measuring diameters at several points around the part gives a good indication of how truly round it is.
Is a Round-Shaped Figure with Flat Sides Still “Round”?
While the broad definition of roundness can apply to shapes like hexagons or other flat-sided forms, the parts we typically measure are indeed round. However, we also grind and cut polygonal parts — including squares, rectangles, and even complex shapes — by length. In those cases, different measurement techniques are needed to determine if the part falls within the customer-specified roundness tolerance.
Illustrating Roundness
In Geometric Dimensioning and Tolerancing (GD&T), the symbol for roundness on engineering drawings — as you might expect — is a circle. Typically, a callout box pointing to the relevant part surface includes a circle and the appropriate roundness tolerance.
Another way to illustrate GD&T roundness is by drawing a part’s tolerance zone — two concentric circles with diameters that vary according to the specified tolerance.
These two concentric circles define the maximum and minimum sizes all points on the specified diameter must fall within to be considered sufficiently round and within tolerance.
In other words, the part must fit within the tolerance zone between the inner and outer circles. (Surprisingly, this can also apply to hexagons, stars, and other planar shapes, making it possible to assign a roundness tolerance to them.)
Specifying Roundness Tolerance
If you search for GD&T roundness examples, you might find drawings of shapes with tolerances like ±0.050” (1.270 mm) or ±0.030” (0.762 mm). At Metal Cutting, we focus on very small precision parts, where a 0.050” or 0.030” out-of-round condition is enormous!
More typically, we deal with roundness tolerances such as ±0.001” (0.0254 mm), ±0.002” (0.0508 mm), or even as tight as ±0.0002” (0.0051 mm). In fact, some of our customers specify roundness in millionths of an inch.
Specifying the Measurement Location for Roundness
For such small parts, when customers request a roundness tolerance, they typically ask us to measure the diameter at specific points around the part — usually the OD, and most commonly at the center — to verify that the part meets their specs.
Of course, uniform diameter isn’t a perfect definition of roundness. But for most of our customers, it’s sufficient to meet their roundness control requirements and helps ensure proper fit, smooth movement, and even wear.
When requested, true roundness — rather than just diameter — can be measured using special equipment. Some probes are capable of tracking all points around a part’s circumference.
Regardless of the tolerance, inspection method, or measuring equipment involved in verifying roundness or any other critical feature, including those details in a request for quote helps ensure the part is optimized for manufacturability.