Straightness, roundness and cylindricity are three different form characteristics, and which one to specify depends on the feature and on what the part has to do. Straightness applies to a feature of type line, roundness to a feature of type circle, and cylindricity to a feature of type cylinder. A diameter tolerance covers none of them, so the drawing has to name the characteristic you actually need.
What is the difference between straightness, roundness and cylindricity?
They describe form on different feature types, which is why they are not interchangeable on a drawing.
| Characteristic | Applies to | What the callout covers | How it is evaluated |
|---|---|---|---|
| Straightness | Features of type line | A line element, over the length you state | Over the stated length, per meter or over the full bar |
| Roundness | Features of type circle | One circular cross-section | At the named cross-section or sections |
| Cylindricity | Features of type cylinder | The cylindrical surface as a whole | Over the whole cylindrical surface |
Each of the three characteristics is covered by its own standard series, with a part that sets out vocabulary and parameters and a part that defines the complete specification operator. Chinese standards address the same three characteristics.
These standards define terminology and the way a specification is operated. They do not give tolerance values, so the number on your drawing still has to come from the function of the part and from what your process can hold.
Which one should my drawing call out?
Name the characteristic that matches the function. If the whole cylindrical surface has to work as a surface, cylindricity is the single callout that applies to it; roundness and straightness each cover only a cross-section or a line.
Size, straightness, roundness, cylindricity and run-out are different characteristics and should be specified according to function. A shaft can sit inside its diameter tolerance and still not be straight, and a straight shaft can still be out of round at a critical section. That is why a diameter band alone is not a form specification.
Three practical situations:
- The function is concentrated at one cross-section, such as a seal lip, a bearing seat or a housing bore. Roundness stated at that cross-section is the direct requirement, and it tells the supplier where to check.
- The function depends on the bar running true along its length, for example long-stroke sliding or a shaft passing through several supports. Specify straightness and state the length it applies to, because a value per meter and a value over the full bar are different requirements.
- Both matter on the same cylindrical surface. Cylindricity describes that surface as a whole rather than at one section or along one line, and it is the appropriate single callout when the drawing cannot separate the two.
Where two characteristics are called out on the same surface, state which one governs in case of dispute, and agree the measuring method in the inquiry.
Is a size tolerance such as h6 enough to control form?
No. A size tolerance controls the diameter only. A conforming diameter does not prove that a shaft is straight, round or cylindrical.
The tolerance class itself is also incomplete on its own: h6 without a basic size, mating hole or function does not define a requirement. ISO 286-1 provides the code system for cylindrical linear sizes and explains the basic-hole and basic-shaft principles, but it does not choose the fit for the designer, and it does not set form.
For bar stock this matters at the RFQ stage. A drawing that carries only a diameter band leaves the supplier free to deliver bars that pass the diameter check but do not meet the function. Add the form characteristic you need, with its value and its evaluation length or cross-section, and keep the roughness requirement separate — Ra is a surface texture requirement, not a form requirement.
Does plating change which form characteristic I should state?
State form on the finished part, because the plated surface is the one your bearing, seal or guide actually runs on.
At Yixin Precision Machinery (Changzhou), hard chrome on the 45 Steel Chrome-Plated Soft Shaft, the 45 Steel Induction-Hardened Chrome Shaft and the 45 Steel Chrome-Plated Linear Shaft is 10–50 μm per side, chosen per order. The chrome layer is applied on top of the ground surface, so the drawing should say whether form and roughness are verified before plating, after plating, or both.
Yixin lists straightness of about 0.1 mm/m from Ø25 and about 0.2 mm/m below Ø25 as a standard capability, with final values confirmed on the drawing and order. Catalogue roughness is Ra ≤ 0.4 μm on chrome-plated bars and Ra ≤ 0.8 μm on ground bars. Straightness, cylindricity and Ra are separate requirements; meeting one does not demonstrate the others, so list each one you need.
How should the three characteristics be inspected?
Agree the method before the order: the length or the cross-sections measured, the instrument, and whether the check is made on the ground or the plated surface.
The ISO Part 2 documents define the complete specification operator, which is why the same surface can give different numbers under different agreed procedures. A drawing note that names the characteristic, the value and the evaluation length removes most of that ambiguity; a note that only says the bar must be straight does not.
| Item | What to agree before production |
|---|---|
| Diameter and tolerance | Basic size and tolerance zone, for example −0.02/−0.05 mm at Ø4–11 mm or 0/−0.02 mm from Ø12 mm on catalogue bars, plus the measuring position |
| Straightness | Value, and the length it applies to: per meter or over the full bar |
| Roundness | The named cross-section or sections, and how many are checked |
| Cylindricity | The zone value and the surface it covers |
| Surface roughness | Ra value, and whether it applies to the ground or the chrome-plated surface |
| Plating | Chrome thickness per side, and whether form is verified before or after plating |
Where a third-party inspection report is needed, it is arranged on request and covers the samples tested.
What to put in your RFQ
- Basic size and the tolerance zone required, with the measuring position.
- Which characteristic governs: straightness, roundness or cylindricity, each with its own value.
- For straightness, the length the value applies to — per meter or over the full bar.
- For roundness, the cross-sections to be checked and how many.
- For cylindricity, the surface covered and the zone value.
- Ra separately, and whether it is required on the ground or the chrome-plated surface.
- Chrome thickness per side if plated, and whether form is checked before or after plating.
- Grade and condition: Grade 45 to GB/T 699-2015, which may be induction hardened or quenched and tempered; 40Cr and 42CrMo to GB/T 3077-2015, quenched and tempered only.
Summary
Straightness applies to a line, roundness to a circle at one cross-section, and cylindricity to a cylinder as a whole. Choose the one that matches the function, state it with a value and an evaluation length or cross-section, and keep the diameter tolerance and Ra as separate requirements. On plated bars, state form and roughness on the finished surface and agree when they are verified. Standards define the terms and the operation; the numbers remain a design decision that has to be confirmed on the drawing and order.
References
- The ISO series for straightness, roundness and cylindricity — vocabulary and parameters in the first part, the complete specification operator in the second.
- The corresponding Chinese standards for the same three characteristics.
- ISO 286-1 — code system for cylindrical linear sizes; explains the basic-hole and basic-shaft principles and does not choose the fit for the designer.
- GB/T 699-2015 — Grade 45 steel.
- GB/T 3077-2015 — 40Cr and 42CrMo steel.