What's a Standard Hole-to-Bend-Line Relief Distance, Expressed as a Multiple of Material Thickness?

The common rule of thumb is four times material thickness from the edge of the hole to the inside of the bend, measured to the bend tangent line. Below that, the hole starts to distort as the bend forms; above it, you're just leaving flat pattern real estate on the table for no reason.

Why four times thickness

When a press brake forms a bend, the material inside the bend radius stretches on the outside face and compresses on the inside face, and that strain doesn't stop cleanly at the bend line. It bleeds outward into the flat section on either side. A hole sitting too close to that zone gets pulled out of round as the surrounding material moves. Four times thickness gives the strain field enough distance to settle back to zero before it reaches the hole. On thin gauge material that's a small number and easy to satisfy. On heavier plate it adds up fast, and that's usually where a design gets flagged during review.

The number isn't a hard physical constant. It's a conservative rule that covers most bend angles and most of the materials we run. Tighter bend radii, higher bend angles, and stiffer alloys all push the safe distance up a little. If a part is tight on space and the hole absolutely has to sit closer, the real answer is to model it or run a first-off and measure, not to shave the rule of thumb down from memory.

What "measured to" actually means

This trips people up on drawings more than the ratio itself. The four-times-thickness distance is measured from the edge of the hole to the bend tangent line — the point where the flat section ends and the radius begins — not to the centerline of the bend and not to the apex on the outside face. If you measure to the wrong reference you'll either over-constrain a part that would have been fine or under-constrain one that won't be. Whoever's laying out the flat pattern needs to know which reference the rule is using, and it should be spelled out on the drawing rather than assumed.

What happens if you ignore it

A hole that's too close to a bend line comes out oval instead of round, or it comes out round but shifted from nominal position because the material around it moved during forming. Neither shows up until the part is already bent, which means it shows up on a first article, not in review. On a small hole meant for a fastener or a PEM insert, a few tenths of distortion is the difference between a clean assembly and a part that gets kicked back.

The fix once you're in this position is usually not re-tooling the bend. It's moving the hole out, or in cases where the hole has to stay put, laser cutting it undersized and reaming or drilling it to final size after forming. That adds an operation, so it's worth catching before the DXF goes out the door.

The practical takeaway

Keep holes at least four times material thickness from the bend tangent line unless you've got a specific reason and a verified test part to go tighter. Call out the reference point on the drawing, not just the number. It's a five-second check on a flat pattern that saves a scrapped first article.