# Cylindricity

Interpreting the full cylindrical form zone and planning measurements that reveal variation around and along a surface.

Reviewed: 2026-11-01

Tags: parts-and-design, specifications, gdt, cylindricity

## Meaning and scope

Cylindricity controls the whole cylindrical surface between coaxial cylinders. Their radial separation is the tolerance. [KEYENCE form tolerances](https://www.keyence.com/ss/products/measure-sys/gd-and-t/type/form-tolerance.jsp)

The ideal barrel example has perfect circular sections but radii from 9.980 to 10.020 mm. About its construction axis, the enclosing cylinders have **0.040 mm radial separation**. The diameter range is 0.080 mm. Round sections alone do not establish cylindrical form.

Read the drawing's governing standard and edition before interpreting the complete requirement. ISO lists **ISO 1101:2017**, edition 4, February 2017, confirmed in 2022, for geometrical-specification symbols and interpretation. ASME lists **Y14.5-2018 (R2024)** for dimensioning and tolerancing. These records identify separate frameworks; they do not establish that all defaults and related size requirements are interchangeable. [ISO record](https://www.iso.org/standard/66777.html), [ASME record](https://www.asme.org/codes-standards/find-codes-standards/y14-5-dimensiones-y-tolerancias/2018)

Use cylindricity when the cylindrical form itself matters. Determine separately what size, location, orientation and relationships to other features the assembly requires. A cylinder can have excellent form while being the wrong size or positioned incorrectly.

## The full surface matters

Circularity addresses a circular section; cylindricity requires a common cylindrical envelope over the specified surface extent. Independently satisfactory circular sections do not establish a satisfactory cylinder: perfectly round sections can have changing radii or displaced centers. A single roundness trace cannot represent all possible axial variation. [KEYENCE circularity and cylindricity comparison](https://www.keyence.com/ss/products/measure-sys/gd-and-t/type/form-tolerance.jsp)

Distinguish the measured surface from an axis derived from it. A fitted axis alone cannot describe lobes, local surface damage or a barrel-shaped exterior. Preserve enough surface information to evaluate the requested form. Likewise, a rotation setup using another feature as its reference answers a relationship question that should not silently replace this datum-independent form requirement.

## Original radial-zone illustration

Consider an ideal axisymmetric barrel surface, **50 mm** long, whose radius varies with axial coordinate `z` as:

`r(z) = 10.000 − 0.020 × cos(2πz / 50) mm`, for `0 ≤ z ≤ 50 mm`.

Every normal circular section is a perfect circle, but the radius varies from **9.980 to 10.020 mm**. Coaxial cylinders about the assumed construction axis enclose the surface with a radial separation of:

`10.020 − 9.980 = 0.040 mm`.

The corresponding section diameters range from **19.960 to 20.040 mm**, a difference of **0.080 mm**. That diameter difference is twice the radial envelope width in this deliberately symmetric example; it is not a general way to calculate cylindricity from two diameter readings.

This constructs one enclosing zone about a known axis. It does not perform an arbitrary measured point cloud's minimum-zone optimization, establish measurement uncertainty or supply a drawing acceptance threshold. It demonstrates why roundness of individual sections and variation of section diameters answer different questions from the full cylindrical envelope.

## Measured result and fitting method

Hexagon's **PC-DMIS 2021.1** help distinguishes the actual enclosing zone for the entire surface from a zone calculated using sampled points. It warns that uncertainty, point count and point locations can make the measured result larger or smaller than the actual value, and illustrates underestimation from too few points. Its cylindrical feature must contain surface data. [Hexagon cylindricity, dated 22 February 2021](https://docs.hexagonmi.com/pcdmis/2021.1/en/helpcenter/mergedProjects/core/geometric_tolerances/Cylindricity.htm)

That documented release offers **DEFAULT**, a minimum-zone/min-max fit, and **LSQ**, a least-squares fit. The former minimizes the enclosing radial zone; the latter minimizes squared deviations and can produce a larger reported zone width. Keep the chosen fitting method with the result. This is a named software example, not a claim about every CMM's defaults or every standard's evaluation provisions. [Hexagon fitting options](https://docs.hexagonmi.com/pcdmis/2021.1/en/helpcenter/mergedProjects/core/geometric_tolerances/Cylindricity.htm)

Do not compare results from two programs solely by their displayed precision. Resolve whether they used equivalent point coverage, surface extent, fitting method, filtering and rejection settings before treating a numerical difference as a change in the part.

## Inspection planning

KEYENCE illustrates both a rotary roundness instrument with a contacting stylus and a CMM approach. These are possible instrument arrangements; its introductory point-count example is not a universal sampling plan for acceptance. [KEYENCE measuring cylindricity](https://www.keyence.com/ss/products/measure-sys/gd-and-t/form-tolerance/cylindricity.jsp)

Select coverage around the circumference and along the axis to reveal the expected manufacturing errors. Consider taper, barrel/hourglass form, lobing and local transitions when planning traces or scans. Check access near shoulders and ends, and document any excluded area. A measured region must correspond to the drawing's intended extent.

Keep support and clamping suitable for the part: distortion during measurement can change the surface being evaluated. Resolve contamination, burrs and unstable fixturing before repeating a discrepant measurement. Choose instrument capability and measurement uncertainty appropriate to the actual tolerance rather than assuming every cylinder fit is equally suitable.

## Proposed inspection record

| Field | Information to retain |
| --- | --- |
| Requirement | Drawing revision, governing standard, tolerance and controlled extent |
| Acquisition | Instrument, probe, fixture, angular/axial coverage and excluded regions |
| Evaluation | Surface-data feature, fitting method, filters and point rejection |
| Result | Linear radial-zone value, units and uncertainty/decision rule |
| Reproducibility | Program/settings version and retained measurement data |

These are practical handoff fields rather than a normative report template. Evaluate size and other geometric requirements separately under the complete drawing definition. Resolve disagreement about scope or evaluation before accepting a pass/fail result.

## Source scope

Standards records above identify editions and confirmation/reaffirmation, not a new technical edition in the confirmation year. The Hexagon example is explicitly PC-DMIS 2021.1. KEYENCE's live tutorials have no verified technical revision date here. The barrel calculation and proposed inspection record are original explanatory guidance.

[Dimensions, tolerances and specifications](index.md)