Cylindricity in GD&T: The Strictest Form Control for Cylindrical Features
Two coaxial cylinders, no datum, and the whole surface at once — what cylindricity actually controls, how it is measured, and when it earns its place on a drawing.
IsoCNC Engineering Team
CNC Machining & DFM Engineers · Updated · 14 min read
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Quick Answer
Cylindricity is a GD&T form control (ASME Y14.5) that requires every point of a cylindrical surface to lie between two coaxial cylinders separated by the tolerance value. It needs no datum and allows no MMC or LMC modifiers. Because the zone covers the whole feature at once, cylindricity simultaneously controls roundness at every cross-section and straightness of the axis — making it the strictest form control for cylinders, stricter than circularity or straightness alone. Specify it on bearing seats, hydraulic pistons, and press-fit surfaces where form over the full length is functional; measure it with a dedicated roundness/cylindricity tester or a scanning CMM. Remember that Rule #1 already bounds form at MMC, so a tight size tolerance often makes the callout unnecessary.
What Cylindricity Actually Controls
A machined cylinder is never a perfect cylinder. It may be out of round at some heights, tapered from one end to the other, barrel-shaped in the middle, or gently bowed along its axis — usually several of these at once. Cylindricity bundles all of those errors into a single number: the radial separation of the two smallest coaxial cylinders that can contain the entire measured surface. If that separation fits inside the tolerance value in the feature control frame, the part passes. The symbol is two concentric circles, and the frame carries only the symbol and the value — no datum letters, because form is about the feature’s own shape, not its relationship to anything else.
Two practical consequences follow from the definition. First, cylindricity is a surface control: it constrains the skin of the part, and it accepts any combination of roundness, taper, and bow errors as long as their total stays within the zone. Second, no material condition modifiers are allowed — MMC and LMC apply only to features of size controlled for location or orientation, and a form zone does not grow with bonus tolerance. If you are new to the feature control frame itself, our GD&T basics for CNC machining guide covers how to read one, and the GD&T symbols explained page maps all fourteen symbols to their control groups.
One subtlety worth knowing before you add the callout: under Rule #1 of ASME Y14.5 — the envelope principle — a size tolerance already limits form. A shaft held to ±0.01 mm on diameter cannot deviate from perfect form by more than that envelope allows at maximum material condition, without any cylindricity symbol at all. The explicit callout is for cases where form must be controlled tighter than the size tolerance guarantees, which is a narrower set of parts than many drawings suggest.
Cylindricity vs Circularity, Straightness, and Concentricity
The cylindrical controls form a rough hierarchy, and most drawing mistakes come from mixing the levels. Straightness works on line elements — it can keep a surface generator straight in one direction, or keep an axis from bending, but it says nothing about roundness. Circularity works on cross-sections — it keeps every slice round, but each slice is judged alone, so taper and barrel shape slip through. Cylindricity sits above both: its three-dimensional zone catches roundness, taper, barrel, and bow errors in a single check, which is why it is the most demanding — and most expensive to inspect — of the form controls.
| Control | What It Controls | Datum Required? | Typical Zone |
|---|---|---|---|
| Straightness | A line element of a surface, or the axis of a feature, staying within a straight path | No | Two parallel lines (surface) or a cylinder (axis) |
| Circularity (roundness) | Each cross-section of a cylinder or cone being round, slice by slice | No | Two concentric circles in each measured plane |
| Cylindricity | The whole cylindrical surface — roundness of every section plus straightness and taper of the axis at once | No | Two coaxial cylinders separated by the tolerance value |
| Concentricity | The median points of a feature staying centered on a datum axis (location, not form) | Yes | A cylinder around the datum axis |
Concentricity belongs to a different family entirely. It is a location control that compares the median points of one feature against a datum axis — it answers “is this cylinder centered on that one,” not “is this cylinder round.” A part can be perfectly cylindrical and badly eccentric, or concentric on average and out of round. On modern drawings, runout usually replaces concentricity because it is far easier to inspect, while cylindricity handles the pure form question on its own. Choosing the lowest control that protects the function — straightness or circularity instead of cylindricity where sections are what matter — is one of the easiest ways to cut inspection cost without giving up anything real.
How Cylindricity Is Measured
Cylindricity cannot be measured with calipers, micrometers, or a dial indicator on a V-block — those tools sample too little of the surface and cannot build the three-dimensional zone. The reference instrument is a dedicated roundness or cylindricity tester: the part is mounted on a precision rotary table, typically an air-bearing spindle, while a contact stylus traces cross-sections as the table rotates and steps along the axis. The software assembles the traces, removes tilt and eccentricity of the setup, and computes the minimum radial separation of two coaxial cylinders containing all the data points.
A coordinate measuring machine is the common alternative. A scanning CMM samples hundreds or thousands of points over the surface and fits the cylindricity zone mathematically — perfectly valid for tolerances above roughly 10 micrometres. Below that, physics pushes back: volumetric CMM accuracy is typically 1.5–5 micrometres at working volume, which is the same size as the form error being measured, while dedicated air-bearing roundness testers are 5–10 times more accurate. This has a direct drawing implication: a cylindricity callout tighter than a few micrometres commits your supplier to specialized metrology, climate-controlled inspection, and real money. Our tolerance capabilities page shows the inspection equipment behind the numbers we certify, and every lot ships with documented measurement records.
When Cylindricity Is Worth Specifying
The pattern is always the same: a cylindrical surface whose function depends on its shape over the full contact length, at a level tighter than the size tolerance alone guarantees. Bearing seats and journals are the textbook case — a seat that is round at both ends but barrel-shaped in between distorts the press-fitted race, and the bearing pays for it in vibration, noise, and early failure. Hydraulic and pneumatic pistons and bores are the second classic case: the clearance between piston and bore only seals if both parts stay straight and round along the whole stroke, which is why production engine bores are routinely specified around 0.01 mm of cylindricity with roundness tighter still.
Press-fit and shrink-fit surfaces are the third. Interference fits spread contact stress across the mating area, and form error concentrates that stress at high spots while leaving gaps elsewhere — the joint holds less torque than the calculation promised. Our press fit guide covers fit selection and interference amounts; cylindricity is the callout that makes the fit behave like the table says it should. Valve spools, precision spindles, and gauge masters round out the list.
| Application | Why Form Matters | Order of Magnitude |
|---|---|---|
| Bearing journals and seats | Out-of-round or tapered seats distort the bearing race, causing vibration, noise, and early spalling | A few micrometres on precision spindles |
| Hydraulic pistons and bores | Barrel or taper error breaks the seal film and lets pressure bypass; engine bores are commonly specified near 0.01 mm | ~0.005–0.01 mm on production engine bores |
| Press-fit and shrink-fit surfaces | Form error concentrates contact stress locally instead of spreading it across the full mating surface | See our press fit guide for fit selection |
| Valve spools and sleeves | Micron-level clearance only seals if both parts are straight and round along the whole stroke | Single-digit micrometres |
| Gauge and metrology masters | The reference artifact must be rounder than anything it verifies | Sub-micrometre on masters |
Note what these applications share: rotation or sliding contact, small controlled clearances, and load or sealing distributed along a surface. If your feature does none of those things, the next section applies to you.
When Cylindricity Is Over-Tolerancing
Cylindricity is one of the most over-used callouts on production drawings, and the cost is real: the inspection requires specialized equipment and skilled time, and suppliers either charge for it or quietly quote around it. Three situations should make you reconsider. First, when the size tolerance already does the job. Rule #1 means a shaft at Ø25 ±0.01 mm cannot be more than 0.01 mm out of form at MMC — adding “cylindricity 0.01” to that feature buys exactly nothing. Second, when only sections matter. If a seal rides at one axial position, circularity at that section is the right control, at a fraction of the inspection cost. Third, when the relationship to another feature is what matters — that is runout or position territory, not form.
The discipline that avoids all three mistakes is the same one that governs good tolerancing generally: start from function, pick the loosest control that protects it, and put tight numbers only where they earn their inspection cost. Our guide to CNC machining tolerances walks through that framework in detail, including what standard machine capability looks like before you reach for geometric controls at all.
Putting Cylindricity on Your Drawing: Practical Notes
If you have decided the callout is justified, a few habits make it work better in practice. Keep the value honest. Values between roughly 0.005 and 0.05 mm are achievable with good turning or grinding and verifiable on a scanning CMM; anything in single-digit micrometres implies ground or honed surfaces and a roundness tester in a temperature-controlled room. Pair it with a surface finish callout. Form and texture interact — a rough turned surface can pass the zone while still failing the function, and the stylus on a roundness tester filters texture differently than a CMM ball. Say what the feature does. A note like “bearing seat” or “sealing surface” tells the machinist and the inspector which errors matter, and often changes how the part is fixtured for its final pass.
Finally, send the drawing to a shop that will actually push back. A good manufacturing partner flags cylindricity callouts that duplicate Rule #1, suggests circularity where sections are sufficient, and confirms the metrology plan before cutting metal — that conversation is cheaper than a rejected lot. Upload your print through our instant quote page and an engineer reviews every geometric tolerance on it, typically within 1–2 business days.
FAQ: Cylindricity
What is cylindricity in GD&T?
Cylindricity is a form tolerance in ASME Y14.5 that controls how closely a real cylindrical surface matches a perfect geometric cylinder. Its tolerance zone is the radial space between two coaxial cylinders separated by the tolerance value, and every point of the measured surface must lie inside that zone. Because the zone wraps the entire feature, cylindricity simultaneously controls the roundness of every cross-section and the straightness and taper of the axis — which is why it is considered the strictest of the cylindrical form controls. It never references a datum, and it cannot take MMC or LMC modifiers.
What is the difference between cylindricity and circularity?
Circularity — also called roundness — is checked one cross-section at a time: each slice of the cylinder must fall between two concentric circles, but the slices are evaluated independently, so a part can pass circularity at every height and still be tapered, barrel-shaped, or bent overall. Cylindricity evaluates the whole surface at once against two coaxial cylinders, so it catches those full-length form errors that circularity misses. Think of circularity as a stack of independent two-dimensional checks and cylindricity as a single three-dimensional check. A cylindricity callout is stricter and subsumes circularity on the same feature.
Does cylindricity need a datum?
No. Cylindricity is a form control, and form controls never reference datums — the tolerance zone is defined by the shape of the feature itself, not by any external reference. The feature control frame contains only the cylindricity symbol and the tolerance value, with no datum letters after it. This is one of the clearest ways to tell form controls apart from orientation, location, and runout controls, all of which require at least one datum. It also means cylindricity says nothing about where the cylinder sits or how it is aligned with other features; it only says how perfect the cylinder’s own shape must be.
How is cylindricity measured?
The reference method is a dedicated roundness or cylindricity tester: the part sits on a precision rotary table — usually an air-bearing spindle — while a stylus traces multiple cross-sections as the table rotates and the probe steps along the axis. Software then fits the data and reports the smallest radial separation of two coaxial cylinders containing all points. A scanning CMM can approximate this by sampling points over the surface and fitting the zone mathematically, which is adequate for tolerances above roughly 10 micrometres, but volumetric CMM accuracy of 1.5–5 micrometres makes dedicated testers 5–10 times more accurate for tight callouts. A dial indicator on a V-block cannot measure cylindricity properly.
When should I specify cylindricity on a drawing?
Specify it when the function of a cylindrical surface depends on its form along the full length, not just at individual sections: bearing journals and seats, hydraulic pistons and bores, valve spools, press-fit and shrink-fit surfaces, and precision spindles are the classic cases. Before adding the callout, check whether your size tolerance already does the job — under ASME Y14.5 Rule #1, the envelope principle already limits form error at maximum material condition, so a tight size tolerance bounds cylindricity for free. Add the explicit callout only when the form must be controlled tighter than the size tolerance allows, or when the size tolerance had to be loosened and form still matters.
What is the difference between cylindricity and concentricity?
They answer different questions. Cylindricity is a form control: it asks whether one cylindrical surface is shaped like a perfect cylinder, with no reference to anything else. Concentricity is a location control: it asks whether the derived median points of a feature stay centered on a datum axis, so it requires a datum and compares two features. A part can be perfectly cylindrical and badly off-center from its datum, or perfectly concentric on average while being out of round. In practice, runout is usually a better and more inspectable choice than concentricity, while cylindricity handles the form side on its own.
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