Designing bent sheet metal parts is one of those tasks where a few millimetres on a drawing decide whether a part forms cleanly on the first try or comes back cracked, distorted, and late. Most of the problems that show up at the press or in the stamping die were already visible in the CAD model before any metal was cut — a hole sitting too close to a bend line, a bend radius tighter than the material can hold, tolerances that stack up across several bends. Designing for manufacturability (DFM) simply means making those choices correctly up front so the part can be produced accurately, repeatably, and economically. This guide walks through the rules that matter most when you design bent sheet metal parts, from material selection to realistic tolerances.

Start with the material and thickness

Nearly every other rule in this guide scales from the material thickness (T). Choose the thinnest, lightest material that still meets the structural and functional needs of the part — thicker stock costs more, cuts slower, and demands larger bend radii. Keep the wall thickness uniform across the part wherever possible; mixing thicknesses forces multiple setups and drives up cost. Common choices for automotive and industrial parts include cold-rolled steel, galvanized steel, stainless steel, and aluminum.

At DIAN STAMPING we regularly process multiphase steel, aluminum, custom rolled and welded plate, and stainless steel, so the material you specify can usually be formed as designed. If you are not sure which grade suits your application, our engineers can recommend a material that balances strength, formability, and cost for your custom sheet metal parts.

Choose the bend radius wisely

The inside bend radius should be at least equal to the material thickness (1T). Radii tighter than this cause cracking on the outside of the bend, especially in harder tempers and when the bend runs parallel to the grain. Softer alloys such as 5052-H32 aluminum tolerate tighter radii, while hard tempers like 6061-T6 often need 1.5T to 2T.

Two habits keep cost down. First, use one consistent bend radius across the whole part so the shop can form it with a single tool set. Second, orient bends across the grain where crack risk is high. Both choices are free on the drawing but save real money in the workshop.

Give flanges enough length and add bend relief

A flange must be long enough for the tooling to grip and form it consistently. As a rule of thumb, the minimum flange length is about 4T plus the bend radius. Flanges shorter than this form inconsistently or need special tooling, which adds cost. Where a bend runs to the edge of a feature, add bend relief — small notches at the ends of the bend line — to prevent tearing and distortion. A good default is a relief width of at least 1T and a relief depth of at least the bend radius plus the material thickness.

Keep holes and features away from bends

Feature placement causes most first-batch rejections. A hole too close to an edge bulges or tears; a hole too close to a bend line distorts into an oval when the part is formed. Keep the edge of a hole at least 2.5T plus the bend radius away from the bend line, and at least 2T from the edge of the part. Punched holes should be at least 1T in diameter, and internal cutout corners should carry a radius of at least 0.5T to avoid stress concentrations. If a feature genuinely has to sit close to a bend, plan to add it after forming as a secondary operation rather than forcing the geometry.

Get the flat pattern right: bend allowance and K-factor

When a sheet is bent, the material stretches on the outside of the bend and compresses on the inside. Somewhere between the two surfaces lies the neutral axis, where the length stays constant. The bend allowance is the arc length of that neutral axis through the bend, and it must be included in the flat pattern or the flanges come out the wrong length.

The K-factor — the ratio of the neutral-axis offset to the material thickness — is not a fixed constant. It changes with material, temper, tooling, and bending method. The default value in many CAD packages was calibrated for mild steel over a standard V-die, so it is worth verifying against the actual material and tooling. A quick test coupon bent on the production tooling tells you the real K-factor for your combination before you commit to a full run.

Plan for springback

After the press releases, the material springs back toward its flat state. Mild steel springs back a few degrees on a 90-degree bend, stainless steel more, and aluminum varies with temper. Modern CNC press brakes compensate automatically by measuring the angle mid-stroke and overbending. As a designer, the practical point is to specify the angle in the formed, sprung-back condition — the condition the part is measured in — rather than the angle at peak pressure.

Set realistic tolerances

The fastest way to inflate the cost of a bent sheet metal part is to put a blanket tight tolerance on every dimension. Tolerances accumulate across bends because of material thickness variation, springback, grain direction, and tooling. A dimension measured across two or four bends carries much more variation than a flat feature. The right approach is to identify the critical-to-function dimensions, tolerance those tightly, and let everything else follow realistic defaults. This is standard practice in automotive stamping, where checking fixtures and CMM inspection verify only the dimensions that matter for fit and function.

Design with the stamping process in mind

For high-volume production, the economics change completely. Instead of forming parts one at a time on a press brake, progressive, transfer, and tandem dies produce complex parts in a single press line with tight, repeatable tolerances. That changes the design conversation: features that are expensive on a press brake — multiple bends, formed flanges, pierced holes — become routine in a well-designed progressive die. If you expect volume, it is worth designing the part with the die in mind and letting an experienced stamping die manufacturer review the geometry before the design is frozen.

At DIAN STAMPING, our engineers design and build progressive, transfer, and tandem dies for automotive body-in-white, door, seating, and chassis components, and we routinely support secondary operations such as laser trimming, hemming, flanging, and spot welding to deliver assembly-ready parts. Getting the sheet metal part design right at this stage is what separates a smooth launch from a costly rework loop.

Work with an experienced partner

Even the best-designed part benefits from a manufacturer that has seen it before. DIAN STAMPING has been building stamping dies and stamped sheet-metal parts since 2003, with more than 20 years of experience serving automotive OEMs including KIA, BYD, Toyota, Honda, Suzuki, and Geely, and exporting to more than 10 countries. The company operates a 50,000 m² facility with a dedicated die workshop, around 110 employees including roughly 35 die designers and technicians, and an annual capacity of about 2,000 sets of medium and small stamping dies. Quality is managed under ISO 9001 with IATF 16949-oriented automotive practices.

Customization is available from 2D drawings, 3D data, or physical samples, and prototype services are offered before mass production. Typical delivery is 30–40 days for steel stamping dies and about 70 days for casting dies. If you are designing bent sheet metal parts for your next program, send your drawings and get a manufacturability review before the design is frozen.