The question constantly comes up in our exchanges with design offices: *"Can I expect ± 0.1 mm on this dimension out of plasma cutting?"*, *"Should rework machining be planned after bending?"*, *"Which ISO 2768 tolerance class should I specify?"*. This article provides quantified, industrially grounded answers, process by process, based on the standards in force (ISO 2768, ISO 13920, EN 10025, EN 22768) and on forty years of industrial experience at Baguet Métal Parachèvement.
In any technical drawing, two levels of tolerance coexist:
A well-chosen general tolerance allows cleaning up the drawing, reducing manufacturing costs and avoiding interpretation ambiguity. A general tolerance that is too strict forces systematic rework; too wide, it weakens critical functions.
This standard defines four general tolerance classes for linear and angular dimensions (part 1) and four classes for geometric tolerances (part 2):
For linear dimensions from 30 to 120 mm, typical deviations are:
For welded structures (steel frames, chassis, mechanical assemblies), ISO 13920 is more appropriate. It distinguishes classes A (fine), B (medium), C (coarse) and D (very coarse) for linear and angular dimensions, and specifies tolerances for flatness, straightness and squareness after welding.
Fiber laser cutting is today the most precise cutting process for sheets from 0.5 to 25 mm. On recent lasers (4 to 12 kW power) and with well-tuned parameters, typical tolerances are:
For the vast majority of industrial applications, we recommend ISO 2768-m on common dimensions out of the laser. For critical assemblies, ISO 2768-f is achievable on dimensions below 200 mm. Beyond that, specific tolerances (for example ± 0.2 mm on critical center distances) are indicated directly on the drawing.
Plasma cutting is used for medium to heavy thicknesses (6 to 30 mm) when laser is not optimal in terms of cost or throughput. The performance:
High-definition plasma torches (HDP, HPR) achieve precision close to laser on thicknesses of 6 to 15 mm, with significantly lower process cost. For these recent machines, ISO 2768-m is achievable.
For standard plasma, ISO 2768-c remains the reference class. For high-definition plasma, ISO 2768-m is achievable on thicknesses of 6 to 12 mm. Above 20 mm, systematically plan machining rework on critical assembly surfaces.
Oxy-fuel cutting is reserved for heavy and very heavy thicknesses (above 30 mm, up to 300 mm at Baguet). Its precision is lower than laser or plasma, but remains largely sufficient for its intended uses:
ISO 2768-c remains the standard on parts oxy-fuel cut in as-cut condition. For assembly surfaces, chamfer rework or finishing machining is generally specified. For very high-strength steels, plan preheating and controlled cooling to minimize the heat-affected zone.
Bending on CNC press brakes allows reaching high tolerances, but they depend heavily on springback control:
Our operators adjust bending parameters at each material delivery, based on test bends and in-process dimensional measurement. This craft approach achieves ISO 2768-m on common dimensions and ISO 2768-f on critical dimensions below 500 mm, without mechanical rework. For critical angles (sealed welded assemblies for example), a specific tolerance of ± 0.3° is achievable on parts shorter than 2,000 mm.
Cold roll forming offers excellent geometric regularity, but requires a well-set roll tool train:
For the majority of applications (mounting rails, purlins, cladding rails), ISO 2768-m is achievable. For profiles with assembly function (grooves, end-of-profile holes), plan in-line punching or downstream CNC drilling to reach position tolerances of ± 0.2 mm on critical center distances.
Sheet rolling and shell forming have their own specifics:
For welded shells (boilermaking, mechanical-welded structures), ISO 13920 class B is generally appropriate. For precision cones, press calibration rework can be planned to reach radius tolerances of ± 0.5 mm.
Machining (drilling, tapping, milling, boring) remains the reference process for tight tolerances:
On our CNC machining centers, we commonly reach ISO 2768-f on general dimensions and specific half-hundredth tolerances on critical assembly surfaces (interfaces, shafts, bearings). ISO H7/H8 quality classes are our standard for functional holes.
| Process | Dimensional tolerance | Angular tolerance | Typical ISO 2768 class | Surface finish (Ra) |
|---|---|---|---|---|
| Fiber laser thin gauge | ± 0.05 to 0.15 mm | 0.5° to 2° | m / f | 3.2 to 6.3 µm |
| Standard plasma | ± 0.3 to 1 mm | 2° to 5° | c | 6.3 to 25 µm |
| High-definition plasma | ± 0.2 to 0.5 mm | 1° to 3° | m | 6.3 to 12.5 µm |
| Oxy-fuel | ± 1 to 3 mm | 2° to 5° | c / v | 25 to 50 µm |
| CNC bending | ± 0.3 to 0.5 mm | ± 0.5° to 1° | m / f | raw material |
| Cold roll forming | ± 0.3 to 0.8 mm | ± 0.5° to 1.5° | m / c | raw material |
| Rolling / shell forming | ± 1 to 3 mm | - | c (per ISO 13920) | raw material |
| CNC machining | ± 0.02 to 0.2 mm | ± 0.1° | f / H7-H11 | 0.8 to 3.2 µm |
Do not apply the same tolerance to all dimensions. Identify functional interfaces (assembly, sealing, coupling) and apply a specific tolerance to them. Other dimensions remain in general tolerance (ISO 2768).
ISO 2768-m is a good industrial default for the majority of general mechanical parts. ISO 2768-c is suited to standard boilermaking. ISO 2768-f should remain the exception, reserved for precision-machined parts.
A length dimension in tolerance ± 0.5 mm is ambiguous if it does not specify the expected flatness or straightness. Specifying geometric tolerances (ISO 1101) on critical surfaces avoids disputes.
Asking for ± 0.1 mm on a 100 mm dimension out of oxy-fuel cutting is technically impossible, then machining must be planned. The design office must anticipate the manufacturing process from the dimensioning stage.
For welded structures, systematically plan ISO 13920 tolerances and any post-welding straightening or machining. Shrinkage deformations can reach several millimeters on long assemblies.
We address tolerances from the drawing review stage in our engineering office. Our technical analysis cell validates the feasibility of each dimensioning, identifies zones where the planned process does not reach the requested tolerance, and proposes adapted solutions (process change, addition of a machining operation, modification of dimensioning in agreement with the customer).
On each batch produced, dimensional checks are carried out according to a defined quality plan:
Our ISO 9001 quality system and NF EN 15085-2 rail certification guarantee documented tolerance control and full traceability. For nuclear, medical or aerospace applications, we adapt our control plans to specific requirements.
Dimensional tolerances are not a frozen input passed from design to workshop: they are the result of a technical dialogue that must be established from the design phase. Over-tolerancing generates avoidable costs; under-tolerancing generates unacceptable defects.
The expertise of Baguet Métal Parachèvement consists precisely in structuring this dialogue. Our engineering office analyzes your drawings, validates process/tolerance consistency, and proposes dimensional optimizations that reduce your part cost without compromising functionality. This collaborative approach is one of the competitiveness levers that our industrial customers have leveraged for more than forty years.
Do you have a metal transformation project with specific tolerance requirements? Our technical cell analyzes your specification and offers the most economical process / tolerance combination within 24 hours.
