What Determines the Minimum Distance Between Two Parallel Bend Lines?

The floor is set by the punch tip width plus enough flat sheet between bend lines for the material to seat on the die shoulders without the two forming zones interfering. In practice that means leaving at least the V-die opening's worth of flat material between bend lines, and often more.

Why the die geometry drives the number

Every air bend needs a flat zone on either side of the bend line where the punch nose can press the sheet into the V. If a second bend line sits closer than roughly one die-opening width from the first, the punch can't seat square, because the first bend's radius is already in the way. Or the backgauge can't hold the part flat while the second hit lands.

A wider V-die needed for thicker material pushes the minimum spacing out further too. Not because the material itself demands it, but because the tooling geometry does.

There's a second constraint on top of that. As material gets thicker, the bend radius and the flange attached to it both grow. A flange barely long enough to clear one die can't also serve as the flat run into a second bend a few millimeters away.

What actually goes wrong when you crowd it

The failure mode usually isn't a cracked part. It's a bend that doesn't land where the drawing says. When two bend lines sit too close, the first bend's spring-formed material distorts the flat zone meant for the second bend. The backgauge stop, which should be referencing a flat surface, is actually touching a slightly curved one instead. The part walks off dimension in a way that looks random from job to job, because it depends on exactly how the first bend sprung back.

There's a second common failure. If the punch nose for bend two overlaps the deformed zone of bend one, you get a soft secondary bend where you didn't design one. Usually visible as the part not sitting flat where it's supposed to.

What to do about it

Give each bend line clearance of at least the V-die opening on the side facing the next bend. Check that against your actual tooling, not a rule of thumb pulled from a different shop's chart, since die selection varies enough between operations that spacing which works with one die width can crowd a narrower one.

If two bends genuinely need to sit closer than that, changing the sequence often helps more than fighting the geometry. Or split the part into two pieces joined with PEM hardware.

DigiForge's sheet metal line runs laser cutting, press-brake forming and PEM insertion, holding ±0.5° on bend angle and ±0.13 mm on the cut. Bend spacing that's marginal for the tooling shows up as an out-of-tolerance angle on the second bend, not a clean rejection at the machine. Model the flat pattern with real die dimensions in mind and the spacing problem mostly disappears before the part reaches the brake.