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• High SA:thickness ratio.
• But plates also used.
• E.g. metal desks, appliance bodies, aircraft
panels, beverage cans, car bodies, kitchen
• Sheet forming also called press working.
• Producing a blank from a sheet.
• The thickness or lateral dimensions of the work
piece are intentionally changed in bulk
deformation. In sheet-forming any changes in
thickness is due to stretching of sheet under
tensile stresses (Poisson’s Effect).
• Thickness decreases should generally be avoided
as they lead to necking & failure.
• Sheet forming involves bending & stretching.
Therefore some properties have to be taken care
2. Yield-point elongation
4. Grain size
5. Residual stresses
6. Spring back
YIELD POINT ELONGATION
• Low-Carbon steels exhibit yield point elongation
of the order of a few %.
• This means that when the material yields, it
stretches further in certain regions in the
• Lueder’s bands/stretcher strains/worms are
produced. These are elongated depressions on
the surface of the specimen.
• Objectionable on final part, difficult for coating
• 0.5%-1.5% temper rolling/skin rolling
• Residual stresses are stresses that remain
after the original cause of the stresses (external
forces, heat gradient) has been removed.
• Present in sheet metals parts due to non-
uniform deformation of the sheet during
• By removing a part of it, the part may distort.
• Tensile residual stresses may also cause SCC.
• Spring back may occur in metal parts.
Difference in stresses. Over bending. Stretch
• The thickness of the sheet metal is called its
• The gauge of sheet metal ranges from 30 gauge
to about 8 gauge. The higher the gauge, the
thinner the metal is.
• R/T ratio. The difference between inner & outer
strains increases if R/T ratio decreases i.e.
• Minimum bend radius is expressed in terms of
thickness T. this is the radius at which a crack
will appear on the outer surface.
• Bendability also depends on edge conditions.
• Anisotropy of sheet metal is important in
bendability. Cutting the blank in proper
• Free-bending, where there is only one die. Rolls
are also used to bend sheets. In air bending's truest
form, the material touches the tools at only three
points. used for forming thick gauges (10 gauge and
• Beading is where the edge of the sheet metal can be
into the cavity of a die. Sharp edges are removed &
the overall part becomes stiff.
• Flanging: is a process of bending the edges at 900
(types of flanges in sheet)
• Roll Forming is used for bending continuous
lengths of sheets by passing the sheet through a
series of rolls. Typical products include gutters,
channels, panels, frames etc. Thicknesses used
range from about 0.125mm-20mm.
• Press-Brake Forming: used with metal parts
at least 7m in length & relatively narrow. Dies in
mechanical & hydraulic press.
• Bottoming: there is no air between the punch,
material being formed, and the die. The material
will be bent to 90 degrees at the bottom.
Bottoming as defined will not produce an
accurate bend. Spring back.
• Bottoming With Penetration (Coining).
• A flat sheet metal blank is formed into a cylinder
or box-shaped part by means of a punch that
presses the blank in to the die cavity.
• Typical parts produced are: beverage cans, pots
& pans, containers, sinks etc.
• STRETCHING & pure drawing both take place.
• High blank holder force can cause stretching.
This leads to stretching & necking.
• Low blank holder force. Only blank diameter is
decreased as drawing progresses.
• If the sheet thickness is more than the clearance
between the punch & the die, the thickness will
have to be reduced.
• This effect is known as ironing. Ironing
produces cups with constant wall thickness.
Longer cups are also produced.
• Clearance between the punch & die are usually
maintained at 7-14% more than the original
thickness of sheet. Wall thickness of cup.
• Corner radii of punch & die. Too small causes
fractures at corners. Too big causes wrinkling.
• A punch & a die. Major variables are: punch
force, punch speed, lubrication, surface
condition, & materials used for punch & die.
• Slug & a sheet
• Clearance ;Burr; Burr Height;
• Burr height increases with increasing clearance
& increasing ductility.
• Tools with dull edges are also a major factor in