A hole near a bend comes out as an oval
Distortion near bends, and how far features must stay away.
A perfectly round hole punched too close to a bend line does not stay round once that bend is actually formed, because the same local stretching that lets the bend's outer surface curve at all reaches sideways into whatever material is nearby, pulling a nearby circle out of shape into something closer to an oval, elongated in the direction the bend itself is stretching.
Why a bend distorts more than just the metal at its own crease
The earlier articles in this set have already described how a bend's outer surface stretches to accommodate the curve it is being forced through, but that stretching does not stay neatly confined to the exact width of the bend itself, it fades gradually into the surrounding flat material rather than stopping abruptly at some clean boundary. A feature cut into that surrounding material close enough to the bend line sits inside this fading zone of partial stretch, and whatever shape that feature started out as gets pulled along with the material around it, distorted by exactly the same elongation that is simultaneously letting the bend itself curve properly. A hole positioned well clear of any bend line sits entirely outside this zone and comes through the bending process completely unaffected, its shape identical before and after, while the identical hole positioned too close does not have that luxury, since it is caught directly inside the region the bend is actively stretching. The distortion is not limited to round holes either, a slot, a rectangular cutout or any other feature caught inside the same stretching zone warps along exactly the same lines, the round hole simply making the effect easiest to see because any deviation from a perfect circle is immediately obvious to the eye in a way an already-rectangular slot's slight elongation is not.
The stretched-fabric comparison
A circular polka dot printed on a stretchy cotton t-shirt, sitting on a flat, unstretched section of the fabric over the chest, stays a perfect circle through ordinary wear, since nothing in that particular patch of fabric is being pulled in any one direction more than another. The identical dot printed close to the shoulder seam, where the fabric stretches noticeably every time an arm lifts or reaches forward, gradually distorts into a visible oval over repeated wear, elongated specifically in the direction the fabric is being pulled at that exact spot, while a dot only a few centimetres further from the seam, outside the region that actually stretches, stays round the whole time. A hole punched into sheet metal too close to a bend line is caught in exactly the same kind of localised stretch zone the shoulder-seam dot is, distorted from a circle into an oval by the same directional elongation the bend needs in order to curve at all, and a hole positioned further from that same bend, like the dot further from the seam, comes through entirely unaffected.
Why the safe distance depends on both thickness and radius
How far a hole actually needs to sit from a bend line to avoid this distortion is not a single fixed number, since it depends on how far the bend's own stretching zone actually reaches into the surrounding flat material, and that reach is itself governed by the material's thickness and the radius the bend is being formed around. A thicker material or a tighter bend radius concentrates more of the demanded stretch into a narrower band immediately around the bend, which can, somewhat counterintuitively, mean a tight, sharp-looking bend needs less clearance around it than a wide, gentle one, since the gentler bend spreads its stretching gradually across a wider region of surrounding material rather than concentrating it narrowly. A designer placing a hole close to a bend has to check the specific combination of thickness and radius in use rather than reaching for a single memorised safe distance that happened to work for a different material or a different bend on a previous job, and a rule of thumb copied unchanged from one project to the next, without rechecking it against the new project's own thickness and radius, is exactly the kind of shortcut that quietly stops being safe the moment either of those two figures changes.
One figure worth keeping in mind
A common design guideline for holes near bends places the safe minimum distance from a hole's edge to a bend line at somewhere around three to four times the material's thickness added to the bend radius itself, a figure that grows with both a thicker material and a more generous bend, and a hole placed any closer than that carries a real, visible risk of coming out of the press brake measurably out of round.
What follows from this
Checking a drawing for this specific clearance, hole to bend line, is a cheap thing to verify before a part is ever cut, and considerably cheaper than discovering the distortion after a batch of parts has already been bent and every single hole meant to take a bolt or a pin has come out slightly elliptical instead of round. A hole distorted this way does not fail gracefully either, since a bolt that fit the nominal round hole comfortably can bind or refuse to seat cleanly in the oval version, and the fastening problem this creates arrives at final assembly, often the furthest point downstream from where the actual mistake, placing a hole too close to a bend line, was originally made on the drawing. Where a feature genuinely has to sit close to a bend for functional reasons and no clearance can be spared, the more reliable fix is often to punch the hole after bending rather than before, cutting it directly into the already-formed part where no further stretching will ever reach it, trading a slightly more complex process sequence for a hole that starts and stays exactly the shape it was actually meant to be.