1. Sheetmetal
Bending

2. 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

3. Bending automation Robots used to tend the press brake

4. Bending types V-bending Edge 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

5. 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

6. 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 s e
Elastic Recovery

7. 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.

8. Bending model – force ranges
Examples in text show that punch forces are in the thousands of pounds rather than hundreds of thousands!

9. Sheet metal bending video clip

10. Sheet metal bending
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