Sheet Metalworking
Cutting Operations
Shearing by two sharp cutting edges.
Plastic deformation penetration fracture
Shearing, Blanking and Punching
Shearing - Cutting on sheet-metal with
straight edges.
Blanking - Cutting sheet-metal with a closed
contour.
Punching - Making holes on sheet-metal.
Sheet-metal Cutting
c = at
c = clearance
a = allowance
t = thickness
Blanking:
Blanking punch dia. = Db- 2c
Blanking die dia. = Db
0.25 to 1.5
Punch
Hole punch dia. = Db
Hole die dia. = Db+ 2c
Sheet-metal Cutting
Small clearance
Cutting force = StL
S = Shear strength of sheet-metal
t = Sheet-metal thickness
L = Length of cutting edge
or, Cutting force = 0.7TtL
T = Ultimate tensile strength of sheet-metal
Large clearance
Sheet-metal Cutting Operations
Cut-off
Parting
Slotting
Perforating
Notching / Seminotching
Sheet-metal Cutting Operations
Fine blanking close tolerances
and smooth
edges in one
step.
Trimming - Cutting operation to remove excess metal
Shaving - Shearing with very small clearance to obtain
accurate dimensions., secondary or finishing operation.
Bending
V- bending
Edge - bending
Bending Analysis
BA = 2  A( R + Kbat ) / 360
where BA = bending allowance, A = bend angle, R = bend
radius, t = stock thickness, Kba = stretch factor.
For R< 2t, Kba= 0.33 ; for R 2t, Kba= 0.5.
A  Ab
SB 
Ab
where A’ = include angle of sheet-metal part
A’b = include angle of bending tool
w = width of sheet-metal
2
K bf Twt
Bending force F 
D = die opening dimension
D
Kbf = 1.33 V-bending
Kbf = 0.33 edge bending
Spring back,
Bending Analysis
Spring back
Die-opening
dimension
Bending Operations
Straight flanging
Hemming
Stretch flanging
Shrink flanging
Seaming
Curling
Bending Operations
Channel bending
Offset-bending
U-bending
Corrugating
air-bending
Tube forming
Drawing
c = Clearance
Db = blank diameter
Dp = Punch diameter
Rd = die corner
radius
Rp = Punch corner
radius
F = drawing force
Fh = holding force
Deep Drawing
Initial step
bending of edge
Thinning and drawing
straightening of side wall
final cup shape
Drawing Analysis
Db
Drawing ratio, DR 
 2.0
Dp
where Db = blank diameter
Dp = punch diameter
t = thickness of the starting blank
Db  D p
Reduction, r 
 0.5
Dp
Thickness to diameter ratio, t / Db  1%
D
Drawing force, F  DptT ( b  0.7), max. at 1/3 stroke.
Dp
Holding force, Fh  0.015Y D2  ( Dp  2.2t  2Rd )2

b

Holding pressure may be set at 0.015 of the yield strength
T = Tensile strength, Y = Yield strength, Rd = die corner radius
Drawing Analysis
• Blank diameter can be calculated from the conservation of
volume based on the final volume of the part.
• If the limits on the drawing ratio, reduction and thicknessto-diameter ratio are exceeded, the blank must be drawn in
steps or having annealing between the steps.
• Process optimization:
–
–
–
–
Punch and die corner radii
friction
depth of draw (per step)
material characteristics
Other Drawing Operations
Redrawing
Reverse drawing
– r, 40%-45% first draw
– r, 30% second draw
– r, 16% third draw
Drawing without blank
holder, Db  D p  5t
Drawing Defects
a) Wrinkling in flange - small holding force
b) Wrinkling in the wall - insufficient holding force,
wrinkling initially occurring on the flange.
c) Tearing - high stress, sharp die radius
d) Earing - anisotropy of the material
e) Surface scratches - Die or punch not having a smooth
surface, insufficient lubrication
Other Sheet-metal Operations
Ironing - squeezing and
drawing in conjunction
Corning & Embossing - to
produce surface details
Lancing - combination of
cutting and forming
Other Sheet-metal Operations
Rubber forming processes:
Guerin process - low cost,
small volume, pressure
up 10 MPa.
Hydroforming - Higher
pressure, up to 100 MPa.
Dies
Components of a punch and die for blanking operation
Dies
Progressive dies
and final part
Press
crankshaft
Knuckle joint
eccentric
Press are driven mechanically or
hydraulically.
Stretch Forming
Stretching and forming with a die at the same time.
Roll Forming
Similar to the
the rolling
process but
working on
sheet-metal or
tubes.
Spinning
Conventional
spinning
Shear spinning - thinning
and bending occur at the
same time.
Spinning
Tube spinning - similar to shear spinning but working on a
tube.
High Energy Forming
Explosive forming
Electrohydraulic forming
Electromagnetic forming
Tube Bending
Stretch bending
Drawing bending
Compression bending
R/D  3.0 with mandrel
R/D  1.5 without mandrel