Bending, Folding, Rolling and Press Operations
This domain tests plastic deformation of sheet: bend allowance, bend deduction, springback, minimum bend radius, rolling and press-brake work. Numerical questions on bend allowance are common.
Principles
- During bending, outside fibres stretch and inside fibres compress. The neutral axis stays approximately the same length. The K-factor locates the neutral axis as a fraction of thickness from the inside surface.
- Bend allowance BA = (θ/360) × 2π(R + Kt), where θ is the bend angle, R the inside radius and t the thickness. Minimum bend radius depends on material and temper; too tight a bend cracks the outside.
- Springback is elastic recovery after the load is removed. Overbend or use a compensating die. Rolling uses slip rolls; pre-bend the ends and adjust the roll gap gradually. Press brakes use a punch and V-die; tonnage depends on length, thickness and die opening.
Worked example
A 90° bend has inside radius 4 mm, thickness 2 mm and K = 0.33. BA = (90/360) × 2π(4 + 0.33 × 2) = 0.25 × 2π(4.66) = 7.32 mm. If the two straight legs measured to the tangent points are 50 mm and 80 mm, the flat length is 50 + 80 + 7.32 = 137.32 mm. Reasoning: the blank must contain the neutral-axis arc, not the outside corner.
Traps that cost marks
Using outside dimensions without bend deduction; ignoring springback; setting the roll gap too tight in one pass; and standing in the pinch zone of a press brake or slip roll.