A sheet metal part is designed around four decisions: the inside bend radius as a multiple of thickness, the K-factor that converts that radius into a flat-pattern length, the distance of holes and flanges from each bend, and the tolerance class that governs cut and bent dimensions. This guide gives the formulas, a minimum bend radius table taken from mill data, and the standards behind the tolerances, namely ISO 2768-1, ISO 9013, DIN 6930-2 and DIN 6935.
What is the K-factor and how does it set the flat pattern?
Bending stretches the outer fibres and compresses the inner ones. Somewhere between them lies a neutral surface whose length does not change, and the flat length of a bend equals the length of that surface. The K-factor locates it: K = t / T, where t is the distance from the inside face to the neutral surface and T is the sheet thickness. The bend allowance for a bend of angle A (degrees) and inside radius R is
BA = A × π / 180 × (R + K × T)
Take T = 2 mm, R = 2 mm, A = 90° and K = 0.4, a value chosen here only to illustrate the arithmetic. Then BA = 1.5708 × 2.8 = 4.40 mm. If the two outer leg lengths are 30 mm and 50 mm, the outside setback is R + T = 4 mm per leg, the bend deduction is 2 × 4 − 4.40 = 3.60 mm, and the flat length is 80 − 3.60 = 76.40 mm. The same number follows from the straight portions: 26 + 46 + 4.40.
K is not a material constant. It depends on grade, thickness, radius, bend angle and bending method, which is why every CAD system lets you override it and every shop keeps its own values. A change of 0.1 in K moves the allowance of this 90° bend by 0.31 mm. On six bends with the same error, the part length is off by up to 1.9 mm, several times the ±0.3 mm that ISO 2768-m allows on a length of 30 to 120 mm. Calibrate K with a test bend in the grade and thickness you buy, or let the supplier send its bend table.
DIN 6935 takes a different route for steel. It calculates the flat length with a compensation value υ (Ausgleichswert) instead of K, and its supplement 2 tabulates calculated υ values for bend angles, radii and thicknesses.
What is the minimum bend radius for steel and aluminium?
The minimum inside radius depends on grade, temper, thickness and tooling, so it is quoted as a multiple of the sheet thickness t. Higher strength and thicker sheet both raise the multiple. Table 1 gives published values for four grades; every row comes from a producer or distributor, not from a design handbook.
Table 1. Minimum inside bend radius for a 90° bend, as a multiple of thickness t
| Material | 1.5 to 3 mm | over 3 to 6 mm | over 6 to 10 mm |
|---|---|---|---|
| Domex 355MC (SSAB, meets S355MC of EN 10149-2) | 0.2 | 0.3 | 0.5 |
| Strenx 700MC D/E (SSAB, meets S700MC of EN 10149-2) | 0.8 | 1.2 | 1.6 |
| EN AW-5754, O/H111 | 1.0 | 1.0 | 2.0 |
| EN AW-5754, H22/H32 | 1.0 | 1.5 | 1.5 |
| EN AW-5083, O/H111 | 1.0 | 1.5 | 2.5 |
Source: SSAB product pages for Domex 355MC (published bands 1.8–3.0, 3.01–6.0 and 6.01–16.0 mm) and Strenx 700MC D/E (bands 2.0–3.0, 3.01–6.0 and 6.01–10.0 mm), both for longitudinal and transverse bends; Metallservice, factor F for aluminium (radius = F × thickness).
Three cautions apply. First, the SSAB values hold for both longitudinal and transverse bends. For other products the rolling direction can matter, so state it on the drawing. Second, we found no producer table for austenitic stainless steels such as 1.4301, so this article gives no figure for them; ask the supplier for the radius it has qualified. Third, a radius equal to or above the table value is not a guarantee: tooling, springback and the edge condition left by cutting still decide whether the bend cracks.
How close can a hole or flange be to a bend?
A flange must be long enough to rest on the die, and a hole too close to a bend stretches into an oval. Protolabs gives both limits as the same multiple of thickness T plus the inside bend radius R, by material and thickness band.
Table 2. Minimum flange length and feature distance from a bend, Protolabs tooling
| Thickness T | Aluminium, copper | Cold-rolled and galvanised steel | Stainless steel |
|---|---|---|---|
| 0.024–0.050 in. (0.6–1.3 mm) | 3.0 × T + R | 3.5 × T + R | 3.5 × T + R |
| 0.060–0.090 in. (1.5–2.3 mm) | 2.5 × T + R | 2.75 × T + R | 2.75 × T + R |
| 0.100–0.134 in. (2.5–3.4 mm) | 2.5 × T + R | 2.5 × T + R | 2.5 × T + R |
| 0.160 in. and over (4.1 mm and over) | 3.0 × T + R | 3.5 × T + R | 4.0 × T + R |
Source: Protolabs, sheet metal design toolkit; metric conversions rounded.
For a 2 mm steel sheet with R = 2 mm the minimum flange is 2.75 × 2 + 2 = 7.5 mm. These are one supplier's tooling limits, not a standard, and a shop with other dies will differ. The limit grows with both thickness and radius, so a larger radius lengthens every flange and every keep-out zone around a bend.
Which tolerances apply to laser-cut and bent parts?
In this supplier's table, bent features carry wider limits than cut ones. Table 3 compares what one supplier publishes with the class m limits of ISO 2768-1.
Table 3. Published sheet metal tolerances (Protolabs, 2023) against ISO 2768-m
| Feature | Protolabs | ISO 2768-m |
|---|---|---|
| Edge to edge, edge to hole, hole to hole, one surface | ±0.127 mm | ±0.2 mm (6–30 mm), ±0.3 mm (over 30–120 mm) |
| Hole diameter | ±0.127 mm | ±0.2 mm (6–30 mm) |
| Bend to edge, one surface | ±0.254 mm | ±0.2 mm (6–30 mm), ±0.3 mm (over 30–120 mm) |
| Bend to hole, bend to bend, one surface | ±0.381 mm | ±0.2 mm (6–30 mm), ±0.3 mm (over 30–120 mm) |
| Bend to bend, three or more bends | ±0.762 mm | ±0.3 mm (over 30–120 mm), ±0.5 mm (over 120–400 mm) |
| Bend angle | ±1° | ±0°30′ (shorter leg over 10–50 mm) |
Source: Protolabs, Sheet Metal Tolerances (2023); ISO 2768-1:1989, Tables 1 and 3 (class m, values as in our ISO 2768 article). The Protolabs figures describe that supplier, not a general rule.
The comparison shows why a general tolerance on a bent part needs a conversation before the order. ISO 2768-m asks ±0°30′ on a bend whose shorter leg is 10 to 50 mm long, while this supplier states ±1° for all bends. A feature positioned from a bend can exceed ±0.3 mm without any error on the supplier's side. Dimension functional features from a single datum edge, preferably a cut edge, and keep bend-to-bend dimensions off the critical list. Our network works to ISO 2768-m as standard and holds tighter limits where the drawing specifies them; see sheet metal for the process.
Which standards cover cut edges, bending and general tolerances?
ISO 2768-1:1989 covers linear and angular dimensions without individual tolerances. Its scope names workpieces produced by metal removal or formed from sheet metal. The standard is still published, and a revision as a single-part ISO 2768 is listed as a project. ISO 2768-2 on geometrical tolerances was withdrawn in 2021, so the old general flatness classes are gone for sheet parts; state flatness on the drawing where it matters.
DIN 6930-2:2011-10 gives general tolerances for stamped steel parts made from flat products, in classes f, m, g and sg for linear and angular dimensions, coaxiality and symmetry. It applies only if the drawing or a related document refers to it.
ISO 9013:2017 classifies the quality of thermally cut edges and gives tolerances for oxyfuel (3 to 300 mm), plasma (0.5 to 150 mm) and laser cuts (0.5 to 32 mm). Amendment 1 followed in 2024, and BSI lists BS EN ISO 9013:2017+A1:2024 as current. The standard states that flatness defects are not addressed, so warping of a laser-cut plate needs its own requirement. Quote the standard and the class you need on the drawing.
DIN 6935:2011-10, cold bending of flat steel products, and its supplement 2 are listed as current.
What should the drawing and the flat-pattern file state?
Check these points before the data goes out for quotation:
- Material grade and temper, not only "steel" or "aluminium", because bend radius and K-factor follow the grade.
- One general tolerance reference with year, for example ISO 2768-m, and an ISO 9013 class if a cut edge is functional.
- Inside bend radius per bend, and the grain or rolling direction where it matters.
- Dimensions taken from one datum edge, with bend positions given as the inside or outside mould line, stated.
- Flatness, if required, as an individual tolerance.
- A 3D model and a flat pattern as STEP or DXF alongside the PDF; the CAD data package article lists what a supplier needs, and the technical drawing checklist covers the title block.
Frequently asked questions
- What is the K-factor in sheet metal?
- K is the distance t from the inside face of the bend to the neutral surface, divided by the sheet thickness T. It is 0.5 when the neutral surface stays at mid-thickness and falls as the surface shifts toward the inside of the bend. CAD systems use it to compute the bend allowance.
- What is the minimum bend radius for steel sheet?
- There is no single value. SSAB publishes 0.2 × thickness for Domex 355MC at 1.8 to 3.0 mm and 1.6 × thickness for Strenx 700MC D/E at 6 to 10 mm, both for a 90° bend. The radius depends on grade, thickness and bending method, so take it from the mill data of the grade you buy.
- Which general tolerance applies to sheet metal parts?
- ISO 2768-1 states that it applies to workpieces made by metal removal or formed from sheet metal, with classes f, m, c and v. DIN 6930-2:2011-10 covers stamped steel parts in classes f, m, g and sg, but only when the drawing refers to it.
- Does ISO 9013 apply to laser cutting?
- Yes, for laser cuts from 0.5 mm to 32 mm in material thickness. ISO 9013:2017, amended in 2024, classifies thermal cuts by oxyfuel, plasma and laser process. It does not address flatness defects of the cut part.
- Is DIN 6935 still valid?
- Yes. DIN 6935:2011-10, cold bending of flat steel products, is listed as current by DIN Media. Its supplement 2 of 2011-10 gives calculated compensation values for the flat length.
Sources
- ISO 2768-1:1989 life cycle and scope, workpieces made by metal removal or formed from sheet metal (ISS, national ISO member)
- DIN ISO 2768-1:1991-06 (DIN Media)
- ISO 9013:2017 scope and status (ISS, national ISO member)
- BS EN ISO 9013:2017+A1:2024, current, published 2024-10-31 (BSI)
- DIN 6930-2:2011-10, Stanzteile aus Stahl, Allgemeintoleranzen (DIN Media)
- DIN 6935:2011-10, Kaltbiegen von Flacherzeugnissen aus Stahl (DIN Media)
- DIN 6935 Beiblatt 2:2011-10, calculated compensating values (DIN)
- SSAB Domex 355MC, minimum inner bending radius for a 90° bend
- SSAB Strenx 700MC D/E, minimum inner bending radius for a 90° bend
- Metallservice, mechanical values of mill-finish aluminium semi-finished products, minimum bend radius factor F
- Protolabs, sheet metal design toolkit: minimum flange length and features near bends
- Protolabs, sheet metal tolerances (2023)
- Bending (metalworking): bend allowance and K-factor definition (Wikipedia)
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