Cylindricity is a geometric tolerance that measures how closely a part’s surface matches a mathematically perfect cylinder. It captures three types of error at once: whether the surface is round at every cross-section, whether it stays straight along its length, and whether it tapers. In engineering drawings, cylindricity is defined by a 3D tolerance zone made up of two concentric cylinders, one slightly larger than the other, and every point on the part’s surface must fall between them.
How the Tolerance Zone Works
Picture two invisible cylinders sharing the same axis, one nested inside the other. The gap between the inner cylinder wall and the outer cylinder wall is the tolerance zone. If you specify a cylindricity tolerance of 0.05 mm, then the radial distance between those two concentric cylinders is 0.05 mm, and the entire surface of the part must fit within that narrow shell. The smaller the number, the closer the part must be to a perfect cylinder.
Cylindricity is a “surface control,” meaning it governs the shape of the actual surface rather than the position of the part relative to other features. Because of this, it doesn’t reference any datum. The part is evaluated purely on its own geometry. Under the ASME Y14.5 standard, the tolerance applies regardless of the part’s measured size, a principle called “regardless of feature size.” The cylindricity tolerance value also cannot exceed the dimensional tolerance already assigned to the feature’s diameter.
Cylindricity vs. Circularity
Circularity (also called roundness) is a 2D check. It looks at a single cross-sectional slice of a cylinder and asks how close that slice is to a perfect circle. You could pass a circularity check at every slice and still have a part that bows, tapers, or wobbles along its length.
Cylindricity is the 3D extension of circularity. It evaluates the entire surface at once, combining roundness at every cross-section with straightness along the axis. A part that passes cylindricity automatically satisfies circularity, but the reverse isn’t true. This makes cylindricity a stricter and more comprehensive tolerance. It’s intended for features that need to maintain the same diameter along their full length, not just at individual slices.
What Cylindricity Controls
A single cylindricity callout simultaneously constrains three types of form error:
- Circularity errors: ovality or lobing at any cross-section
- Straightness errors: bowing, bending, or waviness along the axis
- Taper: gradual change in diameter from one end to the other
This makes it distinct from tolerances like straightness or parallelism, which each address only one dimension of form error. In mathematical terms, the shape of a cylindrical surface can be broken down into variations around the circumference (described by Fourier coefficients) and variations along the axis (described by polynomial terms capturing the mean deviation, tilt, and curvature). Cylindricity error is the combined result of all these deviations.
Where It Matters in Manufacturing
Cylindricity is a common callout in mechanical and automotive applications. It shows up on shafts, pins, hydraulic and pneumatic cylinders, bearings, and bushings. Any part that rotates inside another part, or that must seal against fluid pressure, typically needs tight cylindricity control. A hydraulic piston that isn’t truly cylindrical will leak. A bearing journal that tapers will wear unevenly and fail early.
For these components, controlling roundness alone isn’t enough. A piston could be perfectly round at every cross-section but slightly barrel-shaped, and it would still lose pressure. Cylindricity catches that error because it evaluates the full surface as a unit.
How Cylindricity Is Measured
The most common approach uses radial methods, sometimes called radius-change methods. In a typical setup, the part sits on a precision turntable and a probe traces the surface while the part rotates. By collecting data at multiple heights along the part and at many points around each rotation, the measurement system builds a complete 3D picture of the surface.
Coordinate measuring machines (CMMs) are widely used for this. A CMM touches or scans the surface at hundreds or thousands of points, then software fits reference cylinders to the data and calculates the deviation. For situations where parts can’t be moved to a measurement lab, V-block methods allow in-process checks right on the shop floor. Traditional V-block measurements were considered rough estimates only, but computer-aided versions have improved their accuracy significantly, making them viable for production-line monitoring.
Calculating Cylindricity Error
Once surface data is collected, software must determine how far the part deviates from a perfect cylinder. There are four standard approaches to this calculation, and each defines “perfect” slightly differently:
- Least squares cylinder (LSC): fits a reference cylinder that minimizes the overall squared distance from all measured points. It’s the simplest and most widely used method because it always produces a single, repeatable answer.
- Minimum zone cylinder (MZC): finds the two coaxial cylinders with the smallest possible radial gap that still contain every measured point. This method aligns most closely with the ISO definition of cylindricity error.
- Minimum circumscribed cylinder (MCC): finds the smallest cylinder that fits around all the measured points, useful for evaluating external surfaces like shafts.
- Maximum inscribed cylinder (MIC): finds the largest cylinder that fits inside all the measured points, useful for evaluating internal surfaces like bores.
The MZC method typically gives the tightest (smallest) error value because it’s specifically designed to minimize the zone width. The LSC method can overestimate the error slightly, but its computational simplicity makes it the default in most measurement software. More advanced algorithms using genetic algorithms and geometric optimization techniques have been developed to improve accuracy, particularly for the minimum zone approach where finding the true optimal solution is mathematically complex.
Reading Cylindricity on an Engineering Drawing
On a drawing, cylindricity appears as a feature control frame containing a circle with two parallel tangent lines on either side. The number next to the symbol is the tolerance zone width in the drawing’s unit of measurement. For example, a callout of 0.02 mm means the entire part surface must lie between two concentric cylinders separated by 0.02 mm.
No datum reference appears in the frame because cylindricity is self-referencing. The measurement system determines the best-fit axis from the part itself. You also won’t see material condition modifiers (like maximum material condition) applied to cylindricity, since it always applies regardless of the part’s actual produced size.