What is the minimum bend radius for sheet metal?

As a working rule, an inside bend radius of at least one material thickness is safe for most mild steel and aluminium. Tighter than that and the outer fibre starts to crack, and harder tempers crack sooner. If a drawing shows a sharp corner with no radius specified, the shop will apply its own — so specify the radius rather than discovering what you got.

How short can a sheet metal flange be?

The flange has to be long enough for the press brake to grip and form it. A common working minimum is around four times material thickness plus the bend radius. Below that the material slips in the die and the angle becomes unrepeatable — the part is not impossible, it is just no longer consistent, which is worse.

What is a bend relief and why does it matter?

A small notch cut at the end of a bend line where it meets an adjacent edge. Without it the material at that corner is being stretched in two directions at once and tears. It costs nothing to add in CAD and it is the single most common reason a first article comes back with a split corner.

Why does the flat pattern not equal the sum of the sides?

Because material stretches on the outside of a bend and compresses on the inside. The neutral axis shifts, so the developed flat length is shorter than adding the leg dimensions. This is the bend allowance, and it depends on material, thickness, radius and the forming method — which is why the shop should generate the flat pattern from your 3D model rather than you supplying a flat you calculated yourself.

Sheet Metal Design: Bends, Flanges and Reliefs

Most sheet metal problems are three geometry rules being broken.

A radius too tight to bend without cracking. A flange too short for the brake to grip. A corner with no relief, so it tears. None of them are difficult, all of them cost nothing before the flat pattern is cut — and every one of them shows up at first article instead.

The short version

Inside bend radius ≥ 1× material thickness, or the outer fibre cracks.
Flange ≥ 4T + R, or it slips in the die and the angle stops being repeatable.
A missing bend relief is the most common reason a first article comes back with a torn corner.

The three rules, drawn

Every one of these is a CAD edit before the flat pattern is generated, and a scrapped first article afterwards. The relief in particular is the single most common reason a first article comes back with a split corner — and the fix is a notch.

Why the flat pattern is not the sum of the sides

Material stretches on the outside of a bend and compresses on the inside. The neutral axis shifts, so the developed flat length is shorter than adding up the legs. That difference is the bend allowance, and it depends on material, thickness, radius and the forming method.

Practical consequence: send the 3D model and let the shop generate the flat pattern on their own numbers. A flat you calculated yourself, using a K-factor that does not match their tooling, produces parts that are dimensionally wrong in a way nobody notices until assembly.

What else moves the price

Number of bends and setups

Every bend is a handling operation. A part with four bends in one direction is far cheaper than four bends requiring three re-orientations — and the difference does not appear on the drawing.

Tolerance across a bend

Dimensions measured across a formed feature carry the bend's own variation. Datum from a single flat face where you can. Why chains matter →

Hardware and welds

Pressed-in nuts and standoffs are cheap; welding is not. Welding also distorts, which puts your flatness tolerance in danger after the part was already correct.

Finish

Powder coat masks a lot; brushed or anodized finishes show every handling mark, and handling marks happen after forming. Specifying a finish that can be inspected →

When sheet metal is the wrong answer

The same honesty I apply to CNC applies here. Sheet metal is excellent for enclosures, brackets, chassis and panels — low tooling cost, fast iteration, easy to change.

Welding five pieces to imitate a shape a casting makes in one shot.

That is where sheet metal stops being the cheap answer — the assembly labour quietly exceeds the tooling you were avoiding. It happens whenever the part needs thick sections, complex internal geometry, or integrated bosses and ribs.

The equipment, and an honest note on scale

Press brake and shearing equipment — Forming happens here — which is why bend allowance comes off our own tooling

Operator at a production machine — Setups, not just machines — the cost is in the handling

Finished parts ready for inspection — Where formed panels meet moulded and cast parts

Process	Machines	What it means for you
Laser cutting	2	Flat patterns cut in house — no tooling cost, so design iterations are cheap and fast
Press brake forming	3	Bends formed on site, which is why bend allowance comes off our own tooling rather than a textbook K-factor
Punch presses	10	Repeat features and higher-volume blanking, where laser time would dominate the cost

Honest framing: this is a capable sheet metal shop sitting next to a large mould, moulding and casting operation — not a dedicated high-volume stamping house. That is genuinely useful when your product mixes formed panels with moulded or cast parts, because the whole assembly is designed and measured in one place, on the same six CMMs. If your product is only sheet metal and runs into the millions, a specialist stamper will beat us on price and I would say so on the call.