Bending looks like a simple operation, but it is exactly where it most often turns out that a part cannot be made the way it is drawn. Here is what to consider at the design stage — it will save you both time and money.
The Flat Pattern Is Not the Sum of the Sides
The most common misconception. If a part has flanges of 50 and 50 mm, the blank length is not 100 mm. During bending, the metal in the bend zone partially stretches — the outer layer elongates, the inner layer compresses. Because of this, the actual flat pattern length is always less than the sum of the sides.
How much less depends on the metal thickness, the bend radius and the material itself: stainless steel and aluminium behave differently from mild steel. That is why the flat pattern is calculated by a process engineer using the parameters of the specific tooling. If you send a 3D model in STEP, we will build the flat pattern ourselves — more reliable than drafting it by hand.
Minimum Flange Length
To bend metal, something has to hold it down. If the flange is too short, it simply does not reach the die — and the bend cannot be made. The minimum flange length depends on the metal thickness and the tooling size, so a part with a lip of a couple of millimetres on thick metal is physically impossible.
The rule is simple: the thicker the metal, the longer the shortest flange has to be. If the design calls for a very short lip, it is usually easier to make it by welding or to replace it with another solution.
Bend Radius
There is no such thing as a sharp corner in a bent part — there is always a radius on the inside. Trying to make it too small leads to cracks on the outer side of the bend, especially on aluminium and thick steel. As a guideline, the radius is roughly equal to the metal thickness; softer materials can go smaller, harder ones need more.
The orientation of the bend relative to the sheet's rolling direction also matters: a bend across the grain holds up better than one along it. For critical parts we take this into account when nesting on the sheet.
Holes Near the Bend Line
A hole placed too close to a bend deforms — it becomes oval, because the metal in that zone stretches. So holes should be moved away from the bend line, and if that is impossible, they should be drilled after bending as a separate operation. It costs slightly more, but it guarantees the dimension.
Bend Sequence
Each successive bend has to be physically feasible: the part must not collide with the press brake with a section that has already been bent. Because of this, some designs with many bends in different directions turn out to be unmanufacturable, even if everything looks logical on the drawing.
That is exactly why we review drawings before putting a job into production and flag such spots. Often it is enough to change one dimension or replace a bend with a weld seam to make the part manufacturable.
In Short: What to Send
The ideal option is a 3D model in STEP format: it shows the full geometry, and we will build the flat pattern. Acceptable — a DXF drawing with bend lines, thickness and material specified. The minimum — a dimensioned sketch with an explanation of what the finished part should look like.
More about the service itself is on the sheet metal bending page. Send your drawing through the form and we will check the part's manufacturability even before quoting.
