ISO 2768 for formed parts: scope, limits and what to ask for

A drawing arrives with ISO 2768-m in the title block and no tolerance on a 100 mm formed flange. Class m allows ±0.3 mm over that range. The first article measures 0.42 mm long, the part is rejected, and both sides are technically correct: the drawing asked for something the process stack does not deliver, and the standard never promised it would.

The standard is a drawing-shortening convention. It fills in the tolerances you did not write, so a drawing does not carry a number on every dimension. It is not a capability statement, it does not distinguish a cut edge from a formed one, and it does not cover welded assemblies.

The sections below set out what the standard covers, what class m allows, why it fails in both directions on a formed part, and what to put on the drawing instead. The other standards that sit beside it are collected in the standards and compliance hub.

What does ISO 2768 actually cover?

ISO 2768-1:1989 sets general tolerances for linear and angular dimensions that carry no individual tolerance indication. It defines four classes, f (fine), m (medium), c (coarse) and v (very coarse), and applies them to the dimensions of metal and plastic parts. ISO 2768-2 covers geometrical tolerances for features without individual indications, in classes H, K and L.

The purpose is administrative. Without it, every dimension on a drawing needs its own tolerance, and a bracket with 40 dimensions becomes unreadable. With it, the title block carries one class and the exceptions are marked individually.

The scope has hard edges that are worth knowing before the first article argument:

• It covers dimensions without individual tolerance indications. A dimension that carries its own tolerance is not affected by the class, and neither is a reference, basic or theoretical dimension.
• It covers nominal sizes from 0.5 mm to 4,000 mm. Below 0.5 mm the deviation must be indicated next to the dimension, and above 4,000 mm the standard has no table.
• It does not cover welded assemblies. A welded frame is a different structure with a different error budget, and ISO 13920 exists for it.
• It does not cover features governed by another standard, such as threads, gears and splines.
• It says nothing about surface texture, edge condition or material thickness. Those come from other callouts or from the material specification.

What class m allows, by size

The table below is the part 1 linear table. Read it as the tolerance an unmarked dimension inherits, not as the tolerance the shop can hold.

Class f has no entry above 2,000 mm, and class v has none below 3 mm, which is why a title block should name a class the part actually spans.

Choosing the class is a separate decision from the table. Class m is the usual callout for sheet metal because it matches general fabrication without inviting a tolerance nobody measures. Class f belongs on a drawing where most dimensions are machined and the sheet metal parts are small; applying f to a 400 mm formed panel asks for ±0.2 mm across a bend, which is a machining number. Class c suits heavy fabrication and large weldments where the section is stiff and the interface is not critical. Class v is a rough-works callout and should not appear on a part that meets another part.

Part 2 is invoked separately and often forgotten. A title block of ISO 2768-m gives linear and angular defaults only; adding the K class to the designation, as in 2768-mK, brings in part 2 and with it flatness and perpendicularity. Without the second letter, a panel that must sit flat has no flatness requirement at all.

The angular table runs on the length of the shorter side of the angle: class m allows ±1° up to 10 mm, ±0°30′ from 10 to 50 mm, ±0°20′ from 50 to 120 mm, ±0°10′ from 120 to 400 mm and ±0°5′ above 400 mm. Those look tight in degrees and are not tight at the end of a long flange. A ±0°5′ band over 1,000 mm puts the free edge 1,000 × tan 5′ = 1.45 mm off nominal, which is looser than the ±1.2 mm linear tolerance on the same dimension.

Why the title block fails on a bent part

A cut dimension and a formed dimension are not the same measurement, even when the standard treats them the same way.

A cut feature inherits the laser or punch error, which on a fibre laser is about ±0.10 mm for a profile up to 3 mm and ±0.15 mm by agreement at 2–3 mm. Class m allows ±0.2 mm at 6–30 mm, so the general tolerance is looser than the process by a factor of two and hides nothing.

A formed dimension inherits more. Take a 100 mm flange on 2 mm CRCA formed on a press brake:

• cut profile and blank length: ±0.15 mm
• bend deduction, including a thickness band of ±0.08 mm: ±0.11 mm
• springback and ram repeatability: ±0.20 mm

Worst case is 0.46 mm and the root-sum-square is 0.30 mm. Class m allows ±0.3 mm, which sits exactly on the RSS and well inside the worst case. In production the part will sometimes comply and sometimes not, and the reject rate depends on which coil arrived.

That is the failure in one direction. The other direction is just as common: on a 6–30 mm cut feature class m allows ±0.2 mm when the laser holds ±0.1 mm, so the drawing invites a part that the shop would not have shipped if the tolerance had been written properly. The general class is simultaneously too tight for the bend and too loose for the cut.

Broken edges are not linear dimensions

A chamfer or an external radius is a broken edge under ISO 2768-1, and it has a separate table with much wider values. Under class m and class c, a broken edge from 0.5 to 3 mm allows ±0.4 mm. A 1 × 45° chamfer on a 2 mm edge can therefore measure anywhere from 0.6 mm to 1.4 mm and still conform.

That matters when the chamfer is doing a job. A weld prep, a lead-in for a mating part, a deburred edge on a part that will be handled, or a chamfer that controls a coating edge needs its own dimension. Leaving it to the general class is how a weld prep ends up at 0.6 mm on a 3 mm fillet.

The same applies to internal radii. A general class does not control the radius at the inside corner of a formed part, and a corner that is too tight for the material is a crack risk rather than a dimensional one. Minimum bend radius by material covers that limit separately.

Where ISO 2768 stops, and what to use instead

The standard is a default for unmarked dimensions. Three cases need a different document.

Welded assemblies. ISO 13920:2023 sets general tolerances for welded constructions in four length and angle classes, A to D, plus shape and position classes. A welded frame called out to ISO 2768-m is called out to nothing, because part 1 does not cover welded constructions at all. The welded assembly tolerance guide works through the stack that sits underneath the class.

Cut edges. ISO 9013:2017 classifies thermal cut edges by perpendicularity and roughness into quality ranges. It describes the edge, not the part dimension, so a drawing that needs a particular edge condition needs an ISO 9013 range as well as a dimensional tolerance.

Stamped parts. A press tool holds its own tolerances, and the general class often does not reflect what a progressive die can do on a small feature. Where a national standard exists for stampings, such as DIN 6930-2, it is a better base than a general table written for machined parts.

None of these replaces an explicit tolerance. A specific dimension always overrules the general class, and a supplier who is told class m and nothing else will apply it literally.

Coating is the case the standard does not mention and every coated part runs into. Powder at 60–80 µm adds 0.06–0.08 mm per coated face. A 10 mm hole becomes 9.86 mm after a 70 µm film, and a 100 mm outside dimension becomes 100.14 mm. Both are still inside class m at that size, so the dimension report passes while the bolt fit changes. Where a hole is a clearance fit or a thread is masked, say so; the general class will not protect it.

How Ganesh applies the class on a formed part

Ganesh quotes ±0.5 mm typical on formed parts, with tighter critical features agreed on the drawing per feature rather than assumed across the part. Laser-cut profiles and hole positions are held to about ±0.10 mm at sheet thicknesses up to 3 mm, and the 200 T, 5-axis brake with a 4,450 mm bed holds the bend angle within the tooling and material allow.

In practice the general class is used for the dimensions that do not control anything: overall sizes, non-functional cutouts, the position of a label pocket. The dimensions that control assembly are marked individually and checked at first article, with PPAP where the customer requires it. The inspection record then measures the dimensions the drawing actually controls, not every dimension at the class value.

That approach is cheaper for both sides. A drawing that marks six controlling dimensions and leaves the rest to class m gives the inspector a short list and the shop a clear target. A drawing that relies on the title block for everything gives both sides a long argument.

What to put on the drawing

1. The class, with the part number it applies to, for example ISO 2768-m on the sheet that carries the formed geometry.
2. A short list of controlling dimensions with their own tolerances: mounting hole pattern, interface heights, locating features and anything that meets another part.
3. A datum scheme for the formed part, so the inspector measures from the same face the fixture used.
4. The angular tolerance per bend where the bend angle controls assembly, rather than relying on the angular table.
5. A flatness or straightness callout where the panel must sit flat, either through part 2 class K or an explicit figure in millimetres per metre.
6. The edge condition, including which edges are deburred, which are weld prep and which carry a chamfer that matters.

Send the drawing with the controlling dimensions marked to our engineering team. We will tell you which of them the general class cannot carry and what each one needs instead, before the quotation fixes a route.

When the general class is the wrong tool

A sealing face, a bearing fit, a hole pattern that locates a motor, a welded frame and a panel that must be flat are all outside what a title-block class can deliver. Those need an explicit tolerance, a datum and an inspection method, and usually a different process after forming: machining, reaming, drilling from the fixture or a straightening check.

For the measurement side of the same question, bending tolerances and inspection covers how a formed dimension is checked, and laser cutting tolerances for OEM parts covers the cut figures that feed the stack.