Most sheet metal rework is not a manufacturing problem — it is a drawing problem. A hole placed 3 mm from a bend line, a 5 mm flange on a 3 mm plate, a countersunk M6 in a 1.5 mm panel: each of these describes a part that cannot be made the way it is dimensioned, and the shop has three options — ask, guess or scrap. None of them is free. The DFM rules below are the ones we apply when reviewing customer drawings before an order is released.
The five details that cause most rework
- Holes too close to a bend. Anything within 2 × t + R of a bend line pulls into an oval when the flange is formed.
- Flanges too short for the tooling. Below roughly 4 × t the flange slips into the die and the bend radius collapses.
- Dimensions referenced to a bend. Every bend adds tolerance; chain-dimensioning across four bends guarantees a build-up you cannot hold.
- Hardware without clearance. Self-clinching fasteners need a flat, undistorted land, which a bend region cannot provide.
- Finishing conflicts. Powder coat over a ground stud or a gasket land means someone masks it by hand, or scrapes it afterwards.
Bend radius, flange length and K-factor
The inside bend radius is set by the tooling unless you specify a radius that tooling can hold. Forcing a tighter radius than the material allows cracks the outside of the bend, and the crack usually appears in a repeat batch rather than in the first part.
Minimum inside radius by material
- Cold-rolled / mild steel, 1–4 mm: 0.8–1.0 × t. At 1.0 × t the bend is stable and repeatable on standard tooling.
- Stainless 304/316, annealed: 1.0 × t minimum, 1.5 × t recommended. Stainless work-hardens during forming, so the risk is a later batch, not the first part.
- Aluminium 5052-H32: 0.8–1.0 × t — the forming grade; use it wherever the part has bends.
- Aluminium 6061-T6: 3–4 × t, and still worth discussing. In practice, bend 5052 and machine 6061.
- Hot-dip galvanized: 1.0 × t with the coating intact, but the zinc layer can craze. Where protection matters, form first and galvanize after — and design drain and vent holes for the galvanizing bath.
Minimum flange length
A 90° air bend uses a V-die opening of roughly 6–8 × t, and the flange must be long enough to be held by the die shoulders. The working rules are 4 × t for a 90° bend and 6 × t for bends over 90°. On 2 mm steel that means a minimum flange of about 8 mm: a 5 mm flange is not manufacturable without a special die, which becomes a one-time tooling charge. Take the K-factor behind your flat pattern — roughly 0.33 when the inside radius equals the thickness — from the shop that will run the part, not from a CAD default.
Hole placement, minimum diameter and relief features
- Hole to bend line: minimum 2 × t + R, measured from the inside bend line to the nearest edge of the hole; 3 × t is preferred. Below that the hole distorts into a teardrop — cut it after forming, or convert it to a slot with a relief.
- Hole to outside edge: minimum 1 × t, preferred 2 × t. Punching closer tears the edge or bulges the material. Laser cutting can go thinner, but the bearing area around a fastener is the real limit.
- Web between holes: minimum 1 × t. Below one thickness both processes become unreliable and tooling wears fast, which shows up in your price rather than in a rejection notice.
On minimum hole size: punching needs a hole of at least 1 × t, so a 2 mm hole in 3 mm steel cannot be punched at all. Laser cutting floors at about 0.5 × t or 1.0 mm. For a small hole in thick material the honest options are drilling or EDM, and both change the price by a factor rather than a percentage.
Bend relief and corner relief
Reliefs are the cheapest insurance in sheet metal design and the most frequently omitted. A bend relief is a small cut at the end of a bend line that stops the tear; without it, forming a flange next to a perpendicular wall tears the material or distorts the adjacent feature, and the shop will either add the relief itself or refuse the part.
- Width: at least 1 × t. Depth: at least R + t, with 1.5 × t as a safe default.
- Corner relief on box parts: radius at least R + 0.5 × t; a sharp internal corner at a formed junction is a crack initiation point.
- Where a relief is not possible, move the bend line so the two features no longer meet.
Hardware: fasteners, studs and threads
Hardware is where a design stops being a plate and becomes an assembly, and where DFM discipline saves the most money per part.
- Self-clinching fasteners: specify the mounting hole at the manufacturer's tolerance, typically +0.08 / 0 mm. Position and roundness both matter, because the fastener is swaged in by displacement.
- Clearance from a bend: the fastener needs a flat land of its full head diameter, and its nearest edge should sit at least 3 × t + R from the inside bend line — the same logic as for holes.
- Threads in sheet: cut taps in material under 2 × the nominal thread diameter give one or two usable threads and strip in service. Use a forming tap, a clinch nut or a welded nut.
- Countersinks: a machine countersink removes material and needs about 3 × the head depth in thickness — roughly 7 mm for an M4 flat head. In a 2 mm panel specify a dimpled countersink, where material is displaced rather than removed, so the head sits flush at full thickness.
- Weld studs versus clinch studs: weld studs need access for the gun and mark the visible face. Clinch studs leave the front face clean, which is usually what an enclosure needs.
Tolerance: how stack-up actually accumulates
Sheet metal is not a machined part, and drawings that tolerate it like one generate scrap. The practical shop standard for a formed enclosure is ±0.2 mm on hole position across a flat blank (±0.1 mm achievable on laser-cut parts), ±0.25–0.5 mm bend to bend, ±1° on bend angles, and material thickness as supplied — commonly ±0.1 mm. Do not tolerance a finished part to a value the raw coil cannot hold.
The consequence is that a six-bend box can drift up to ±1.5 mm across the weld seam when every feature is dimensioned from a different bend. The fix is not a tighter tolerance but a datum strategy: nominate one primary datum — usually the first bend or a formed feature that appears in every view — and dimension critical interfaces from it. Chain dimensions are acceptable only for non-critical features such as cable entries and label positions. Mark which dimensions are genuinely critical rather than leaving the shop to assume everything is, and where two parts mate, tolerance the interface rather than the part.
Welding, finishing and closure details
Welding is a heat process and heat moves metal, so design for the distortion rather than assuming the welder will control it.
- Stitch over seam by default. A continuous seam on a thin panel pulls the assembly out of flat. Specify continuous welds only where water ingress, EMI or structural load demands them.
- Weld access and sequence. A weld symbol in a corner the torch cannot reach gets ignored or made badly, so say so in a note. Where a critical dimension crosses a weld, arrange operations so final forming happens after the heat.
- Powder coat thickness: 60–100 µm of exterior-grade polyester is an insulator. It will not hold a ground path by bolt torque alone, so ground studs, threaded inserts and mating electrical surfaces must be masked before coating and noted on the drawing.
- Drain and vent holes: an enclosure that can trap water needs a drain at its lowest point, and one with a temperature cycle needs a pressure-equalizing vent. A fully sealed cabinet with no vent draws moisture in through its own seals as it breathes.
- Edge condition: laser-cut edges are sharp. Specify deburring or a small radius on any edge a person will touch, and state whether visible faces are grain-brushed or as-formed.
DFM rules at a glance
| Feature | Practical rule | If you break it |
|---|---|---|
| Inside bend radius | 1 × t steel and 5052; 1.5 × t stainless | Cracking on the outside of the bend |
| Flange length | 4 × t at 90°, 6 × t for acute bends | Flange slips, radius opens, angle drifts |
| Hole to bend line | 2 × t + R minimum, 3 × t preferred | Hole becomes a teardrop, fastener will not seat |
| Hole to outside edge | 1 × t minimum, 2 × t preferred | Edge tear-out, bulge, weak bearing area |
| Clinch fastener clearance | Flat land, 3 × t + R from the bend | Fastener will not seat, panel will not lie flat |
| Threaded hole in sheet | Material at least 2 × nominal thread diameter | One usable thread, strips during assembly |
| Countersunk flat head | 3 × head depth, or specify a dimpled countersink | Panel thinned under the head, head protrudes |
| Coated electrical surfaces | Mask ground studs and mating faces before coating | No reliable ground path, or manual masking cost |
Where a design decision shows up in the price
| Design decision | Typical cost effect | Comment |
|---|---|---|
| Combining three brackets into one formed part | 15–30% lower for the assembly | Removes set-up, handling and fastening |
| Stainless where coated steel would do | Material cost 3–4× higher | Justify it with the environment, not preference |
| Tolerance tighter than ±0.1 mm on a formed part | Inspection and scrap added | Tolerance the interface, not the part |
| Masking several features before powder coat | Manual labour per mask point, per part | Group masked features into one operation |
One closing point: review the drawing before the tooling exists. Every rule above costs nothing to fix on a screen and costs a die, a fixture or a scrap batch once the part is in production.
Xingyuan Electric (Suzhou) Co., Ltd. is based in Lili Town, Wujiang District, Suzhou, China, and manufactures sheet metal enclosures and electrical assemblies for energy storage, EV charging, advanced manufacturing and power distribution customers worldwide. The company runs a 52-person team in a 5,000+ square metre production facility with more than 20 laser cutting, CNC bending and welding machines, delivering standard orders in 7–15 days and building every order to drawing or to sample. DFM review, cutting, bending, welding, hardware insertion and finishing are handled in house, so manufacturability is settled before the order is released rather than after the first part is scrapped.