General Tolerances in Sheet Metal: ISO 2768 Classes (m, f, c, v)

Not every dimension on a drawing carries its own tolerance. For untoleranced dimensions the title block states a general tolerance class; in sheet metal the most common reference is ISO 2768. This article summarises the classes and example ranges; which class you choose depends on the part’s function and manufacturing method.

What is ISO 2768?

• ISO 2768-1 defines four classes for untoleranced linear and angular dimensions: f (fine), m (medium), c (coarse), v (very coarse).
• ISO 2768-2 defines three classes for untoleranced geometrical tolerances (flatness, perpendicularity, symmetry, run-out): H, K, L.

“ISO 2768-mK” in the title block means: medium for linear/angular dimensions, class K for geometrical features.

ISO 2768-1 linear dimensions — example ranges

Permissible deviations from the standard’s table (± mm):

Nominal size range (mm)	f	m	c	v
0.5 – 3	±0.05	±0.1	±0.2	—
> 3 – 6	±0.05	±0.1	±0.3	±0.5
> 6 – 30	±0.1	±0.2	±0.5	±1.0
> 30 – 120	±0.15	±0.3	±0.8	±1.5
> 120 – 400	±0.2	±0.5	±1.2	±2.5
> 400 – 1000	±0.3	±0.8	±2.0	±4.0
> 1000 – 2000	±0.5	±1.2	±3.0	±6.0
> 2000 – 4000	—	±2.0	±4.0	±8.0

Separate tables exist for broken edges (chamfers and radii) and for angular dimensions; angular deviations are given per length of the shorter leg, e.g. ±1° for classes f/m up to 10 mm and ±0°30′ for 10–50 mm.

ISO 2768-2 geometrical tolerances (H, K, L)

Classes H (tightest), K and L (widest) apply to straightness/flatness, perpendicularity, symmetry and run-out, read from the table by feature length. For bent sheet parts this class sets the expectation for squareness and flatness after bending.

Which class for sheet metal?

• Laser-cut contours: cut quality itself is classified by ISO 9013 (perpendicularity and roughness of the cut face). For contour dimensions, ISO 2768-m balances function and cost on most sheet parts; class f should be reserved for functional features (bearing seats, fitted holes).
• Press brake bends: bend dimensions depend on thickness, springback and tooling, so m or c is common; individual tolerances on critical dimensions are more economical than a blanket class f.
• Welded assemblies: use EN ISO 13920, the general tolerance standard for welded constructions; ISO 2768 is not intended for welded dimensions.

Preparing the drawing

1. State the general tolerance class in the title block (e.g. ISO 2768-mK).
2. Give individual tolerances to functional dimensions; leave the rest to the general class.
3. Mark bend dimensions clearly as inside or outside dimensions.
4. Add an ISO 9013 class if cut-face quality matters.

The tolerances Pamtek Metal can achieve on its own machines are confirmed per material and part at quotation; this article only reports the general content of the standard.