A bend in sheet metal is not a fold; it is a controlled stretch of the outside surface and a compression of the inside. That is why every material and thickness has a radius below which the outside fibre cracks, and why the flat pattern is never simply the sum of the finished dimensions. Both facts have to be designed for, not discovered at the press brake.
Why there is a minimum at all
As the radius tightens, strain on the outer surface rises. Past the material's limit it tears, and just short of it the bend develops microcracks that fail later in service. Ductile aluminium and mild steel tolerate tight bends; harder tempers and higher strength alloys need much more generous radii. The rule of thumb is expressed as a multiple of material thickness, and the multiple comes from the alloy and temper.
Grain direction changes the answer
Rolled sheet has a grain, and bending across the grain is far more forgiving than bending along it. A radius that works in one orientation can crack in the other on the same material. If a part has bends in two directions, either specify the generous radius for both, or specify the orientation on the drawing so the fabricator can nest the blank accordingly.
What a bend calculator is really doing
It computes bend allowance or bend deduction: the difference between the sum of the outside dimensions and the length of the flat blank. That depends on radius, thickness, angle and where the neutral axis sits, expressed as a K factor. Shops carry their own tested values for their tooling and material, which is why an in house calculator will usually beat a generic one.
Practical rules that keep parts cheap
Use one radius throughout the part so one tool set does the job. Keep holes and slots far enough from a bend that they do not distort. Keep flanges long enough to be gripped by the tooling. And accept the shop's standard radius unless there is a reason not to, because a non standard radius may mean a punch nobody has.
Questions people ask about bend radius
What is the minimum bend radius for sheet metal?
It is expressed as a multiple of thickness and depends on the alloy and temper. Soft aluminium and mild steel bend tightly; hard tempers and high strength alloys need considerably more. Ask the fabricator for their tested values for the exact material.
Why is my flat pattern longer than the sum of the dimensions?
Because material stretches and compresses through the bend, so the flat length differs from the folded dimensions by the bend allowance. That is what a bend calculator computes.
Does grain direction matter?
Yes. Bending across the grain is more forgiving than bending along it, and the same radius can be fine in one orientation and crack in the other.
Should I use the same radius everywhere?
Where function allows, yes. One radius means one punch and fewer tool changes, which shortens setup and lowers the price of every part.