FAA Aviation Mechanic Airframe (AMA)Airframe StructuresMedium
A technician is forming a 90-degree bend in a piece of 0.040-inch thick aluminum sheet using a bend radius of 0.125 inch. What is the setback dimension for this bend?
- A0.125 inch
- B0.205 inch
- C0.165 inch
- D0.040 inch
Show answer & explanationAnswer & explanation
Correct answer: C. 0.165 inch
For a 90-degree bend, setback equals the bend radius plus the material thickness (SB = R + T) since the tangent of half the bend angle (45°) equals 1. Setback = 0.125 + 0.040 = 0.165 inch.
Why the other options are wrong
- A. This is only the bend radius, omitting material thickness.
- B. This value results from an incorrect addition of extra thickness, not the standard formula.
- D. This is only the material thickness, omitting the bend radius.
Setback (Sheet Metal Forming)
The distance the jaws of a brake must be set back from the mold line to form a bend, based on bend radius and material thickness.
- For 90° bend: SB = R + T
- K-factor = tan(bend angle/2)
- Used to locate bend tangent lines on flat pattern layout
Memory trick: Setback = Radius + Thickness, Same Back-up amount.