4. Three Major Categories of
Sheet Metal Processes
1. Cutting
Shearing to separate large sheets; or cut
part perimeters or make holes in sheets
1. Bending
Straining sheet around a straight axis
1. Drawing
Forming of sheet into convex or concave
shapes
4
5. Shearing
Sheet metal cutting operation along a straight line
between two cutting edges
Typically used to cut large sheets into smaller
sections for subsequent operations
5
8. SHEAR FORCE
The force required for shearing is:
F = S*t*L; where
S: shear strength of the sheet metal
t: sheet thickness
L: length of the cut edge
The shear strength S can be estimated by:
S = 0.7 * UTS; where UTS: the Ultimate Tensile Strength
The above formula does not consider other factors such as
friction. Friction between the punch and the workpiece
can increase punch force
9. Shearing
Processing parameters
1. The shape of the punch and die
2. The speed of punching
3. Lubrication
4. The clearance, c, between the punch and the die
When clearance increases, the zone of deformation
becomes larger and the sheared edge becomes
rougher
5. Extent of the deformation zone
depends on the punch speed
6. Height, shape, and size of the
burr affect forming operations
10. SHEARING OPERATIONS
Punching is where the sheared slug is scrap
Blanking is where the slug is the part to be used and
the rest is scrap
Die Cutting
Shearing operation consists of:
Perforating: punching holes in a sheet
Parting: shearing sheet into pieces
Notching: removing pieces from the edges
Lancing: leaving a tab without removing any material
11. Shearing: Shearing Operations
Fine Blanking
Very smooth and square edges can be produced by
fine blanking
Fine-blanking process can control small range of
clearances and dimensional tolerances
15. Dies for Sheet Metal Processes
Most pressworking operations performed with conventional
punch and die‑ ‑ tooling
The term stamping die sometimes used for high production
dies
15
17. PROGRESSIVE DIE: Punches and dies are designed so that
successive stages in the forming of the part are carried out
in the same die on each stroke of the press.
The strip is fed from left to
right.
The first punch is to make
the hole of the washer.
The washer is then blanked
from the strip.
The punch A is piercing the
hole for the next washer.
25. BENDING SHEET ANDPLATE
Bending is a common industrial forming operation
Bending imparts stiffness to the part by increasing its
moment of inertia
Outer fibers are in tension, while the inner in
compression
Poisson effect cause the width to be smaller in the outer
region and larger in the inner region
28. 28
Introduction/ Types of Bending
Two common bending methods are:
V-bending
Edge or wipe bending.
In V-bending the sheet metal blank is bent between a V-shaped
punch and die. The figure below shows a front view and
isometric view of a V-bending setup with the arrows indicating
the direction of the applied force:
Figure courtesy of Engineering Research Center for Net Shape Manufacturing
29. 29
Introduction/ Types of Bending
Edge or wipe bending (conducted in lab) involves cantilever loading
of the material. A pressure pad is used to apply a Force to hold the
blank against the die, while the punch forces the workpiece to yield
and bend over the edge of the die. The figure below clearly
illustrates the edge (wipe)-bending setup with the arrows indicating
the direction of the applied force (on the punch):
31. 31
SPRINGBACKIN BENDING
When the bending stress is removed at the end of the deformation
process, elastic energy remains in the bent part causing it to partially
recover to its original shape. In bending, this elastic recovery is called
springback.
It increases with decreasing the modulus of elasticity, E, and
increasing the yield strength, Y, of a material.
Springback is defined as the increase in included angle of the bent part
relative to the included angle of the forming tool after the tool is
removed.
Springback Effects:
The bend angle will decrease (the included angle will increase)
The bend radius will increase
32. Springback in bending
Following is a schematic illustration of springback in bending:
αi: bend angle before springback
αf: bend angle after springback
Ri: bend radius before springback
Rf: bend radius after springback
32
33. Springback in bending
In order to estimate springback, the following
formula
can be used:
Manufacturing processes by S. Kalpakjian and S. Schmid
where:
Ri, Rf: initial and final bend radii respectively
Y: Yield strength
E: Young’s modulus
t: Sheet thickness
33
34. Compensation for Springback
Springback is compensated for by
overbending the part
One method is stretch bending where the
part is subjected to tension while being
bent
35. Bending Sheets, Plates, and Tubes
Bending Force
Excluding friction, the maximum bending force, P, is
For a V-die, it is modified to
K= 1.33 For die opoening of 8T
0.33 for a wiping die
L = Length of the bent part , mm
Y= ultimate Tensile Strength MPa
T= Blank thickness, mm
W= Width between the contact points, mm
W
kYLT
P
2
=
36. Springback in bending
In order to estimate springback, the following
formula
can be used:
where:
Ri, Rf: initial and final bend radii respectively
Y: Yield strength
E: Young’s modulus
t: Sheet thickness
36
49. STRETCH FORMING
Sheet metal is clamped along its edges
and then stretched over a male die
Die moves upward, downward, or
sideways
Used to make aircraft wing-skin panels,
fuselages, and boat hulls
54. Stretch Forming Force
Requirement
F = (YS + UTS)/2 * A
F = stretch forming force (lbs)
YS = material yield strength (psi)
UTS = ultimate tensile strength of the material (psi)
A = Cross-sectional area of the workpiece (in2
)
63. Embossing
Used to create indentations in sheet, such as raised
(or indented) lettering or strengthening ribs
Embossing: (a) cross section of punch and die configuration during‑
pressing; (b) finished part with embossed ribs
63
69. Manufacturing of Metal Honeycomb
Structures
A honeycomb structure consists of a core of
honeycomb bonded to two thin outer skins
Has a high stiffness-to-weight ratio and is used in
packaging for shipping consumer and industrial goods
71. Equipment forSheet-metal Forming
Proper equipment design, is needed to achieve a high
production rate, good dimensional control and high
product quality
Traditional C-frame structure is used for ease of tool
and workpiece accessibility
72. Equipment forSheet-metal Forming
Press selection for sheet-metal forming operations
depends on:
1. Type of forming operation
2. Size and shape of workpieces
3. Number of slides
4. Maximum force required
5. Type of mechanical, hydraulic, and computer controls
6. Features for changing dies
7. Safety features
73. Economics of Sheet-forming Operations
Sheet-forming operations are versatile and can
produce the same part
The costs involved depend on die and equipment costs
and labor
For small and simple sheet-metal parts, die costs and
lead times to make the dies are low
Deep drawing requires
expensive dies and tooling
Equipment costs depend
on the complexity of the
forming operation