Sheet Metal Design Guide: Bend Radius, Tolerances and Materials for Parts Made in China (2026)

1. Five decisions that fix the price before the first quote

Sheet metal is cheap to cut and expensive to redesign. Almost every cost problem on a fabricated part traces back to one of five inputs that were left open on the drawing:

The rest of this guide takes them one at a time.

2. Minimum bend radius: the constraint that drives everything

The rule of thumb

The inside radius of a bend cannot be arbitrarily small: the outer fibre stretches, and past a material-dependent limit it tears. Published industry tables give the following rule of thumb for the minimum inside radius, expressed as a multiple of sheet thickness T:

Treat this as design guidance, not a specification. Two caveats matter more than the numbers:

1. The tooling sets the real floor. Bending works by pressing the sheet into a die with a V-shaped opening. For a given thickness the die opening determines the achievable inside radius and the minimum flange length — so a radius that is geometrically fine may still require custom tooling. Increasing thickness while pushing the radius below the material thickness sharply raises the chance of cracking along the outside of the bend.
2. Material properties are directional and lot-dependent. Formability varies with temper, coating and supplier, and the same nominal grade from two mills is not interchangeable. Test-bend a sample before you commit a production run.

V-opening, flange and force move together

On an air-bend chart, four values are read off together for a given thickness: the die V-opening, the minimum flange length, the achievable inside radius, and the bending force in tonnes per metre. You cannot specify one without the others. As a working relationship:

• A larger V-opening gives a larger inside radius and a longer minimum flange — easier forming, less sharp geometry.
• A smaller radius requires less force but tighter tooling and a shorter flange allowance.
• Bending force scales with material: the same geometry in stainless needs roughly 1.5× the force of steel, while aluminium needs roughly 0.5×.

Design rule. Pick a radius from the table, then check the flange length against what the die can physically reach. A flange that is too short for the tooling is the most common cause of “the drawing looks fine but it cannot be made”.

Bends and holes interact

• Place holes and slots away from the bend line. A hole inside the deformation zone distorts into an oval and loses its datum.
• Where a hole must sit close to a bend, it is normally cheaper to move the hole than to add a relief or a secondary operation.
• Add bend reliefs at the ends of a bend that intersects the part edge, and keep corner radii generous enough for the cutting process.
• Flat-pattern length depends on the K-factor — where the neutral axis sits through the thickness. The K-factor varies with material, radius-to-thickness ratio and tooling, so take it from the fabricator who will run the press, or verify it with a physical test bend. Do not carry a K-factor from one shop’s template into another’s.

3. Tolerances: state the class and the standard number

General dimensional tolerances — ISO 2768-1

ISO 2768-1 exists to simplify drawing indications: instead of tolerancing every dimension, you declare a class in or near the title block and the standard supplies the values. It applies to the dimensions of workpieces produced by metal removal or formed from sheet metal, and defines four tolerance classes:

ISO 2768 defines a general-tolerance system in four lettered classes (fine, medium, coarse, very coarse) for linear and angular dimensions; the class you require, and the exact limits for it, must be taken from the standard itself or from the drawing notes your supplier will work to — confirm with your own copy of the standard or your engineering team.

The standard also covers external radii and chamfer heights, and angular dimensions. Note the direction of the trade-off: moving from class m to class f is not a small step in cost — it changes the process, the fixture and the inspection plan.

General geometrical tolerances — ISO 2768-2

ISO 2768-2 handles general geometrical tolerances rather than dimensions, in three classes — H, K and L — covering straightness and flatness, perpendicularity, symmetry and circular run-out. If flatness or perpendicularity matters on your part, say so explicitly rather than assuming the general class covers it.

Welded assemblies — ISO 13920

If your part is welded, dimensional tolerances are not the right instrument. ISO 13920 specifies general tolerances for linear and angular dimensions and for shape and position of welded structures in four tolerance classes based on customary workshop accuracy, with the class selected from the functional requirements — straightness, flatness, parallelism — rather than from habit. The tolerances apply to weldments, welding assemblies and welded structures, and they are always the ones stated on the drawing. The specifications are based on the independence principle of ISO 8015, under which dimensional and geometrical tolerances apply independently of each other — a point that catches out engineers trained on a different convention.

The standard also notes that special provisions can be necessary for complex structures. If your weldment is complex, agree the deviations explicitly instead of relying on a general class.

Cut-edge quality — ISO 9013

A laser-cut or plasma-cut edge has its own quality classification. ISO 9013 presents geometrical product specifications and quality tolerances for the classification of thermal cuts in materials suitable for oxyfuel flame cutting, plasma cutting and laser cutting, and is applicable to flame cuts from 3 mm to 300 mm. Practical consequence: a part can hold its dimensional tolerance and still have an unacceptable cut edge, or vice versa. Specify both.

How to write it on the drawing

A sheet metal drawing that a factory can quote without asking questions carries:

1. Title-block note: General tolerances ISO 2768-m (or the class you actually need).
2. ISO 2768-K (or H/L) if you are invoking general geometrical tolerances.
3. ISO 13920-B (or the applicable class) on weldments.
4. The cut-edge class per ISO 9013, where the edge quality is functional.
5. Individual tolerances on the few features that matter, with datums.
6. Material designation with its standard, thickness in mm, and the finish with its standard.

Point 6 matters more than it sounds: “stainless 2 mm, brushed” is not a specification.

4. Materials: specify a standard, not an adjective

Chinese sheet metal suppliers work to a mix of ASTM, ISO, EN/German and JIS designations. The names are not interchangeable, and a quote that says only “galvanised” or “304” leaves the mill certificate unspecified.

Two practical notes:

• Cold-rolled steel strip designations such as SPCC and electro-galvanised SECC appear constantly in Chinese quotations. They come from the JIS system and are not the same thing as an ASTM designation. Ask for the standard and the certificate rather than accepting the shorthand.
• Material certificates matter more than material names. The mill certificate is what your customer’s quality department will ask for, and it is what your inspection plan should reference.

5. Surface finishing: match the finish to the environment, not the photo

Rule of thumb for specification: name the finish and its standard and the acceptance test. “Powder coated black” is a colour, not a finish.

6. Eight design rules that prevent expensive rework

1. Set the bend radius from the table, then check the flange against the die. The tooling, not the geometry, is the real limit.
2. Keep holes out of the bend deformation zone. Move the hole before you add an operation.
3. State one general tolerance class for the whole part and reserve individual tolerances for the few features that truly need them.
4. Tolerance the features, not the sheet. Overall blank dimensions carry the general class; hole positions that mate with another part do not.
5. Think about the flat pattern early. K-factor, bend deduction and grain direction all change the blank, and errors compound over multiple bends.
6. Design the finish into the part. Masking, rack contact points and drain paths are drawing decisions, not shop decisions.
7. Give the fabricator 3D data plus 2D drawings. STEP for geometry, drawings for tolerances, datums, finishes and inspection criteria.
8. Agree the inspection basis before the first article. Which features are measured, with what instrument, against which datum — decide it at the quote stage.

7. China factory vs local instant-quote platform: an honest comparison

This is a decision about iteration cost versus unit cost, and it changes as a part matures.

A workable sequence for most programmes: prototype locally to converge the design, then move the frozen part to a Chinese fabricator for production — and re-quote the frozen drawing, not the concept. Sending a drawing that is still changing is the single most reliable way to lose the cost advantage you moved for.

For the compliance and landed-cost side of that move, including certification requirements and the official tariff lookup, see our compliance section rather than relying on a figure quoted in an article — duty rates depend on origin, destination, classification and current trade measures.

9. Request a quote

Commissioning custom sheet metal parts? Send the frozen drawing set — STEP geometry, 2D drawings with the general tolerance class and its standard, material standard, and finish specification — and we will return a manufacturability review before a price.

What to include in your enquiry:

1. Drawing package — 3D geometry (STEP) plus 2D drawings with datums for anything with tight features
2. Material — designation with its standard (e.g. ASTM A1008/A1008M; ASTM A240/A240M) and thickness in mm
3. General tolerance class — the class and standard you are working to (e.g. ISO 2768-m, ISO 13920 class, ISO 9013 cut-edge class)
4. Critical features — individual tolerances, threads and inserts, hole positions that mate with other parts
5. Finish — finish with its standard, masking requirements, and any acceptance test
6. Quantity — prototype, pilot and production volumes separately
7. Inspection basis — which features are measured, with what instrument, against which datum
8. Target market — the destination country determines the documentation you will need

Related guides: all manufacturing guides · architectural door hardware from China · door closers · compliance and standards hub · sourcing and inspection guides.

Sources

All sources retrieved 14 September 2026. Standard numbers are cited to the ISO or ASTM catalogue record or a standards-body summary. Industry rules of thumb are identified as such and are not standard clauses. Standards change; always confirm the current edition of a standard with the issuing body before purchase.