Why Does a Bracket's Hole Pattern Come Out Shifted Relative to a Formed Edge Even Though the Flat Pattern Looked Correct?

Because the flat pattern is checked flat, and the part isn't flat once you bend it. The hole positions are correct in the CAD flat pattern, but the bend line itself moves during forming. The neutral axis shifts toward the inside of the bend, the material stretches slightly on the outside, and the actual location of the bend relative to the flange edge ends up a little different from where the math said it would land. Any hole near that bend rides along with the error.

Where the shift actually comes from

The flat pattern software calculates a theoretical bend line based on K-factor and material thickness. That's a good estimate, not a measurement.

The real bend line on the part depends on the actual die opening used, the actual radius the material takes, and how much springback the operator dialed out. If any of those differ slightly from what the flat pattern assumed, a die swap, a thicker than nominal sheet, more springback in one batch of material than another, the formed edge lands a few tenths off from where the flat pattern predicted. Every hole referenced from that edge inherits the offset.

Holes far from the bend barely notice. A hole close to the bend line shows the shift clearly, because any bend-line error is a larger fraction of the distance being toleranced.

Why it looks worse than a flat-pattern mistake

The natural instinct is to blame the CAD model or the nesting software. Usually neither is wrong. The flat pattern is dimensionally correct for the assumptions it was built on. The part just didn't form to those exact assumptions.

That's the nature of forming. It's a plastic deformation process with some real-world variance, not a subtractive process where the tool goes exactly where it's told. This is also why a part that measured within tolerance on the flat blank can measure out of tolerance after bending. The flat dimensions were never wrong. The bend introduced the deviation.

What to do about it on the print

If a hole's location relative to a formed feature actually matters, for a mating connector, a fastener that lines up with another part, anything with a tight positional callout across a bend, don't just dimension it off the theoretical bend line in the flat pattern. Give the hole enough clearance to absorb realistic bend-line movement, or hold the position with a datum scheme that references the formed feature directly rather than the flat blank.

For DigiForge parts, our sheet metal bend tolerance is ±0.5° on angle and ±0.13 mm on the cut. If a hole near a bend has to land within a tighter window than that stacked with the cut tolerance allows, that's a design conversation worth having before the part goes to the brake, not after the first article comes back off.

Moving the hole a bit farther from the bend line is usually the simpler fix. Distance is the cheap way to improve tolerance.