Definition
On a sheet metal part, the line marking the boundary where a flat surface begins to curve into a bend. One bend has two tangent lines — one on each side of the bent area — showing where the flat metal stops and the curve starts.
Plain English
It's the line drawn on a piece of sheet metal to show exactly where the flat part ends and the bend begins.
Context Anchor
Seen during aircraft sheet-metal fabrication and repair, especially when marking a part before bending it.
Derivation
Tangent' comes from the Latin 'tangere', meaning 'to touch'. A tangent line touches a curve at the exact point where it meets the straight section — which is precisely what this line marks on the metal.
Why Pilots Care
Mostly relevant to maintenance technicians and builders. Accurate bend tangent lines are essential for laying out a part correctly — get them wrong and the finished piece won't be the right size or shape.
Grounding Statement
On a bent piece of sheet metal, the bend tangent line is the boundary between the flat area and the curved bend area.
Intuition Check
Do not think of the bend tangent line as the center line of the bend. It marks an edge of the bend area, not the middle of it.
Example Sentence 1
The technician marked the bend tangent lines on the aluminum sheet before placing it in the brake.
Example Sentence 2
Measure from the bend tangent line to confirm the flange length after forming.