Definition
In sheet metal layout, the distance from the edge of a bend to the mold line, equal to the sum of the inside bend radius and the metal thickness for a 90-degree bend. It is used to locate the bend tangent lines on a flat pattern before forming.
Plain English
The amount you measure back from the corner of a bent part to find where the bend actually starts on the flat sheet, before you fold it.
Context Anchor
Used during aircraft sheet-metal repair or fabrication, especially when laying out a flat piece of metal before bending it in a brake.
Derivation
From 'set' (to place or position) plus 'back' (away from a reference point). Literally, the distance you 'set back' from the corner of the finished part to mark where the bend will start.
Why Pilots Care
Aviation maintenance technicians use setback to lay out flat metal patterns accurately. If the setback is wrong, the formed part will not match the required dimensions, which can affect fit, strength, and airworthiness of a structural repair.
Intuition Check
Setback does not mean a delay or a problem here. It means a specific layout distance used before bending sheet metal.
Example Sentence 1
Before cutting the flat pattern, the technician calculated the setback for each 90-degree bend using the metal thickness and bend radius.
Example Sentence 2
Using the correct setback kept the finished rib flange at the required length after forming.