Sheet Metal operation

1. 1Mahar Tahir Sattar (MTS)

2. 2Mahar Tahir Sattar (MTS)

3. Sheet Metal Bending Operations
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4. Sheet Metal Bending
• It is the operation in which the material in the form of
flat sheet or strip is strained perpendicular to the
lengthwise direction.
• The only movement of the grains in the material takes
place at the point where the bend takes place
• The outer fibers in the external radius are stretched because
they are under tension
• The fibers at the inner radius are under compression
• At some point within the material the forces are neither
compressive nor tensile
• This plane of the material is called the neutral bending line
(or neutral axis)
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5. • Straining sheet metal around a straight axis to take a
permanent bend
5(a) Bending of sheet metal
Sheet Metal Bending
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6. • Metal on inside of neutral plane is compressed,
while metal on outside of neutral plane is
stretched
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(b) both compression and
tensile elongation of the
metal occur in bending.
Sheet Metal Bending
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7. • Bending should take place at right angle to the grain direction
• If the bending operation takes place parallel to the grain
direction, cracking will develop
Sheet Metal Bending
Effect of grain direction
on sheet metal bending
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8. Developed Length
• The material before bending is flat stock
• The question arises as to how long the flat stock is to be cut so
that, once the stock is bent, the finished part will meet the print
requirements
• The length of the neutral bending line gives the true length of
the piece after it has been bent
• Its length is the same after bending as before bending
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9. Developed Length
NOTE:
1. All bending equations that follow are based on the
length of the neutral bending line and internal
dimensions
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10. Bend Lines and Layout Line
• The lines where the internal radii are tangent to the legs
of a bend are called bend lines
 A line drawn parallel to, but halfway between, both bend
lines is the layout line at which the punch contacts the
workpiece
• If the internal radius = 0˚, the layout line and bend lines
coincide
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11. Bend Lines And Layout Line
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12. Types of Sheet Metal Bending
• V-bending - performed with a V-shaped die, wedge-shaped
punch to force the metal into the die
• The included angle of the V-bend can be changed by varying
the distance the punch forces the sheet-metal into the V-die
• Characteristics
• Used for low production
• Performed on a press brake
• V-dies are simple and inexpensive
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13. V-bending
• When the punch does not ‘bottom out’, the processes are
referred to as air bending
• In air bending the material being formed contacts only at the
point of the punch and two edges of the V-die opening
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V-bending (air bending)
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14. V-bending
• When the punch is allowed to bottom out in the V-bend
die, it is called the bottoming die
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V-bending (bottoming)
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15. • Also called ‘cantilever’ bend dies
• The workpiece is clamped to a die block by a spring-
loaded pressure pad
• The punch wipes the extended material over one edge
of the die
• For high production
• Dies are more complicated and costly
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Edge Bending
Edge bending
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16. TYPES OF BENDING DIES
3. U-dies or Channel Dies:
• The workpiece produced resembles the
letter ‘U’ or channel
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17. V-Bending Dies
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18. Bend Allowance
• If the bend radius is small relative to stock thickness, the
metal tends to stretch during bending. It is important to
be able to estimate the amount of stretching that occurs,
if any, so that the final part length will match the
specified dimension.
• The problem is to determine the length of the neutral
axis before bending to account for stretching of the final
bent section. This length is called the bend allowance,
and it can be estimated as follows:
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19. Bend Allowance
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20. Springback
• Increase in included angle of bent part relative to
included angle of forming tool after tool is removed
• Reason for spring back:
– When bending pressure is removed, elastic energy remains in
bent part, causing it to recover partially toward its original
shape
• Over bending is the simplest way of combating spring
back problems
• The workpiece is bent through a greater angle than
required and it springs back to the required angle
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21. Springback in bending is seen as a decrease in bend angle and an increase in
bend radius: (1) during bending, the work is forced to take radius Rb and
included angle b' of the bending tool, (2) after punch is removed, the work
springs back to radius R and angle ‘.
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Springback

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22. Bending Force
Maximum bending force is estimated as follows:
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where F = bending force; TS = tensile strength of sheet
metal; w = part width in direction of bend axis; and t =
stock thickness, D = die opening dimension.
For V- bending, Kbf = 1.33; for edge bending, Kbf = 0.33
D
wtTSK
F
bf
2
)(

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23. Die opening dimension D: (a) V-die, (b) wiping die.
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Die Opening Dimension
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24. Other Bending Operations
• Flanging is a bending operation in which the edge of a
sheet metal part is bent at 90°angle
• It is often used to strengthen or stiffen sheet metal
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Flanging: (a) straight flanging, (b) stretch flanging, and (c) shrink flanging
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25. Other Bending Operations
• Hemming (also called flattening) involves bending the edge of
the sheet over on itself
• Usually performed in more than one bending step
• Used to eliminate the sharp edge on the workpiece, to
increase stiffness, and to improve appearance
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Hemming
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26. Other Bending Operations
• Curling, also called beading, forms the edges of the part into
a roll or curl
• Done for purposes of safety, strength, and aesthetics
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Curling
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27. Curling
• It rolls a raw edge of sheet metal into a roll
or curl
• The metal needs to be ductile so that it rolls
easily
TYPES OF FORMING DIES
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28. Other Bending Operations
• Seaming is a related operation in which two sheet metal
edges are assembled obtained after the hemming.
• Staking is a method of fastening sheet metal by squeezing
protrusion formed in one part inside a hole in the second
part, and then deforming the protrusion
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Staking
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