Why rectangular elbow rework happens — and how to kill it
By Field to Fab Team
If you keep a tally of why a piece comes back to the brake for a redo, "the elbow is mirrored" sits somewhere near the top. The Field Foreman measured throat and back correctly, the shop fabricated exactly what was on the sketch, but the elbow turns the wrong way once it is hung. Both sides did their job — the handoff was the problem.
The root cause is that "throat" and "back" on a flat sketch do not encode handedness. A 12 x 8 elbow with a 6 in throat and 18 in back fits four physical pieces — left-up, left-down, right-up, right-down — and the sketch only differentiates two of them at best.
Adding a written note ("turns left") helps but does not scale. Notes get missed, rotated, or lost between the truck and the brake. The reliable fix is a 3D preview the Field Foreman rotates with their thumb before submitting. Once they have looked at the geometry from the angle the duct will actually hang, they catch the wrong-handed orientation in seconds — typically before the order even leaves the truck.
We built Field to Fab's live 3D preview specifically because of this failure mode. Within a month of turning it on at our pilot shop, mirrored-elbow rework dropped from roughly 1 in 12 fittings to under 1 in 80. Same shop, same crew, same brake — different handoff.