Bending process • Points: • Sheet thickness (t) in range 1/64” £ t £ 1/4” • If t > 1/4” then stock called plate • Usually cold-worked, but warm worked if brittle material or thicker stock • Tooling is punch and die • Products are called stampings • Machines are called presses • Plastic deformation
Bending types • V-bending • Large range of angles • Dies expensive • Low production operations • Edge bending • Cantilever bending • Pressure pad to hold part from slipping • For angles £ 90° • More expensive
Bending model • Bending force estimated from bending force equation for simply supported beam: • F = Kbf TS w t2/D • where • TS = tensile strength • w = plate width • t = plate thickness • D = die opening dimension or cantilever length • Kbf = bending factor, where for • V-bending Kbf = 1.33 Edge bending Kbf = 0.33 A = bend angle
s Elastic Recovery e Bending model - springback • Springback is a measure of elastic recovery after plastic load released • SB = (A’ – Ab’)/ Ab’ • To compensate: • Overbending – increase punch angle and decrease punch radius • Bottoming – plastically deform with additional punch pressure
Bending model – bend allowance If R/t is small (R is punch radius), the sheet metal will stretch some, changing length of stock required to complete final shape. In effect, the neutral axis displaces from the center of the stock. The bend allowance (BA) accounts for this from the equation: BA = 2 p A (R + Kbat)/ 360 where if R/t < 2, Kba = 0.33 R/t ³ 2, Kba = 0.50 and Kba is a stretching factor.
Bending model – force ranges Examples in text show that punch forces are in the thousands of pounds rather than hundreds of thousands!
Sheet metal bending What have we learned?