Circularity (Roundness) vs Cylindricity: Complete GD&T Guide

The Fundamentals of Rotational Form Controls

Under ASME Y14.5 and ISO 1101, both Circularity (Roundness) and Cylindricity belong to the Form tolerance family. Crucially, neither symbol ever references a datum feature — form tolerances control the shape of a feature independently without reference to external coordinate axes.

While both controls regulate circular features, the distinction between 2D cross-sectional roundness and 3D full-length cylindrical form is one of the most widely tested concepts in quality inspection and mechanical design.

1. Circularity (Symbol: ○)

Circularity is a two-dimensional form control that evaluates individual cross-sections of a cylinder, cone, or sphere. It requires that all points on the circumference in any plane perpendicular to the feature's axis lie between two concentric coplanar circles.

• Tolerance Zone: The radial distance between two concentric circles in the same cross-sectional plane.
• Dimensioning Scope: 2D only. It checks for out-of-round conditions (lobing, ovality, egg-shape).
• What Circularity DOES NOT Control: It does not control taper, straightness along the length of the cylinder, or barrel/hourglass shape. A cylinder can have perfect circularity in every individual cross-section while being bent like a banana or tapered like an ice cream cone!

2. Cylindricity (Symbol: ⌭)

Cylindricity is a three-dimensional composite form control that simultaneously evaluates the entire surface of a cylinder. It requires that all surface points lie between two coaxial cylinders.

• Tolerance Zone: The radial distance between two coaxial cylinders extending across the entire specified length of the feature.
• What Cylindricity Controls: Cylindricity simultaneously controls three distinct geometric characteristics:
  1. Circularity (Roundness): No cross-section may be out of round by more than the tolerance value.
  2. Straightness of the Surface: No longitudinal element may bend beyond the coaxial boundary.
  3. Taper (Parallelism of Opposite Elements): The cylinder cannot taper from one end to the other.

Summary Comparison: Circularity vs Cylindricity

Top Mistakes in Blueprint Interpretation

1. Adding Datum References: Adding a datum to Circularity (e.g., [○ | 0.05 | A]) is a major drafting error under ASME Y14.5. If datum reference is needed, Runout or Total Runout must be used instead.
2. Over-specifying Cylindricity: Because inspecting cylindricity on a CMM requires thousands of surface points or helical spiral scanning, specifying tight cylindricity on non-critical parts drives up manufacturing and inspection costs unnecessarily.
3. Confusing Diametral vs Radial Tolerances: Form tolerances (both circularity and cylindricity) are radial differences ($R_{outer} - R_{inner}$), not diametrical zones!