A Practical Guide to Press Brake Bending

The press brake is where a flat blank becomes a part, and where most dimensional problems in sheet metal are created or avoided. It is a simple machine doing a subtle job: a punch pushes material into a die, the material stretches on the outside of the bend and compresses on the inside, and where those two effects balance decides how long the blank had to be in the first place. This guide covers the mechanics, the numbers, and the design rules that keep a part cheap.

How does a press brake work?

A press brake clamps a flat sheet between a punch above and a die below, then drives the punch down to force the material into the die opening. Almost all modern sheet metal work is air bending, where the punch stops short of the bottom of the die and the sheet touches at only three points. The bend angle is set by how far the punch descends, not by the tool geometry, which is why one tool set covers a wide range of angles.

The two alternatives matter less but are worth knowing. Bottoming drives the punch until the sheet conforms to the die, giving a tighter angle tolerance and less springback at the cost of much higher tonnage. Coining goes further still, plastically deforming the material at the bend to almost eliminate springback, and needs several times the tonnage again. In an enclosure shop, air bending does close to all of the work.

How much tonnage does a bend need?

Tonnage scales with material strength and thickness squared, and falls as the die opening widens. That relationship is why a wider vee costs less force but gives a larger inside radius. As a working guide for mild steel in a vee eight times the thickness:

Thickness	Vee opening	Approx. tonnage per metre	Typical inside radius
1.0 mm	8 mm	~10 t	~1.3 mm
1.5 mm	12 mm	~15 t	~2.0 mm
2.0 mm	16 mm	~21 t	~2.6 mm
3.0 mm	24 mm	~31 t	~4.0 mm
5.0 mm	40 mm	~52 t	~6.5 mm

Stainless needs roughly 1.5 times these figures and aluminium roughly half. Our bending capacity runs to 165 tonnes, which covers heavy enclosure and frame work in a single hit rather than in stages.

What is bend allowance and why does the K factor matter?

When a sheet bends, the outer surface stretches and the inner surface compresses. Somewhere between them is a neutral axis whose length does not change. The K factor expresses where that neutral axis sits, as a fraction of the material thickness measured from the inside surface. It typically falls between 0.33 and 0.45 depending on material, thickness and inside radius.

The K factor is what turns a folded model into a correct flat blank. Get it wrong and every bent part comes out short or long by the same amount, which looks like a machine problem and is actually an arithmetic one. A shop that bends the same materials on the same tooling every day builds an empirical K factor table from measured parts, which beats any textbook value.

This is also the reason a flat pattern generated at one shop cannot be assumed correct at another. Send the folded 3D model and let the fabricator unfold it against their own tooling.

What is springback, and how is it handled?

Metal bent past its elastic limit still contains an elastic component, so it opens slightly when the punch retracts. Higher strength material springs back more; a larger inside radius relative to thickness springs back more. Stainless is noticeably worse than mild steel and high strength grades worse again.

Modern machines handle it with angle measurement during the stroke, adjusting depth in real time to land on the target angle rather than relying on a fixed overbend. Where that is not available, the compensation is empirical: bend, measure, correct the program, and record it against that material and tooling combination.

What design rules keep bending cheap?

Six rules cover most of what makes a part economical to bend, and all six are decided in CAD.

• Minimum flange length. A flange must be long enough to sit on the die, roughly four times the material thickness plus the bend radius. Shorter flanges need special tooling or a secondary operation.
• Hole to bend distance. Keep holes at least two and a half times the thickness plus the bend radius away from the bend line, or they deform into ovals.
• Bend relief. Where a bend ends part way across a part, cut a relief notch. Without one the material tears at the transition.
• Consistent radii. Using one inside radius throughout means one tool set and no tool changes mid part. Every tool change is setup time on the quote.
• Bend direction. Where possible, keep all bends in the same direction so the operator does not have to flip the part between hits.
• Grain direction. Bending across the rolling grain resists cracking; bending along it invites it, especially in aluminium and higher strength steel.

What tolerances are realistic?

Bending tolerance is not one number, it accumulates. A single bend on a well set machine holds an angle within about half a degree and a flange dimension within a few tenths of a millimetre. By the fourth or fifth bend on a large panel, the stack up is what governs, and where you place the datum decides which dimension absorbs it.

The practical instruction to give a fabricator is which dimensions are critical and which are reference. If every dimension on a drawing carries a tight tolerance, the part is quoted as though every one matters, and that is expensive. Identify the two or three that actually control fit and let the rest run to a general tolerance.

When does panel bending beat a press brake?

For tall four sided enclosure bodies with many short flanges, a panel bender forms the whole perimeter in one automated cycle without an operator manipulating a heavy part through eight separate hits. It is faster, more repeatable and easier on both the part surface and the operator. The press brake remains better for long single bends, unusual profiles, heavy plate and low volume work. Having both is why an enclosure body and its bracket can be made economically on the same day.

The full forming envelope is on the capabilities page. If you have a folded model and want to know what it costs to bend, send the file and we will unfold it against our own tooling and come back with a number.