Sheet metal working
By
Dr. V Phanindra Bogu
Dept. of Mech. Engg.
Sheet Metal Operation
• Sheet metalworking includes cutting and forming
operations performed on relatively thin sheets of
metal.
• Typical sheet-metal thicknesses are between 0.4
mm and 6 mm. When thickness exceeds about 6
mm, the stock is usually referred to as plate rather
than sheet.
• The sheet or plate stock used in sheet
metalworking is produced by flat rolling.
• The most commonly used sheet metal is low
carbon steel (0.06%–0.15% C is typical).
Terms in sheet metal operation
• Sheet-metal processing is usually performed at
room temperature (cold working).
• The exceptions are when the stock is thick, the
metal is brittle, or the deformation is significant.
These are usually cases of warm working rather
than hot working.
• The tooling that performs sheet metal-work is
called a punch-and-die; the term stamping die is
also used.
• The sheet-metal products are called stampings.
Categories of sheet-metal processes
The three major categories of sheet-metal processes are
1. Cutting
2. Bending
3. Drawing
Cutting is used to separate large sheets into smaller
pieces, to cut out part perimeters, and to make holes in
parts.
Bending and drawing are used to form sheet-metal parts
into their required shapes.
• Cutting of sheet metal is accomplished by a
shearing action between two sharp cutting edges.
• The shearing action is depicted in the four stop-
action sketches of Figure, in which the upper
cutting edge (the punch) sweeps down past a
stationary lower cutting edge (the die).
• As the punch begins to push into the work, plastic deformation
occurs in the surfaces of the sheet.
• As the punch moves downward, penetration occurs in which the
punch compresses the sheet and cuts into the metal.
• This penetration zone is generally about one-third the thickness
of the sheet.
• As the punch continues to travel into the work, fracture is
initiated in the work at the two cutting edges.
• If the clearance between the punch and die is correct, the two
fracture lines meet, resulting in a clean separation of the work
into two pieces.
Symbols v and F indicate motion and applied force, respectively,
t = stock thickness, c = clearance.
• Rollover: The depression made by the punch in the work
prior to cutting. It is where initial plastic deformation
occurred in the work.
• Burnish: Just below the rollover is a relatively smooth
region. This results from penetration of the punch into the
work before fracture began.
• Fractured zone: A relatively rough surface of the cut edge
where continued downward movement of the punch
caused fracture of the metal.
• Burr: A sharp corner on the edge caused by elongation of
the metal during final separation of the two pieces.
SHEARING, BLANKING, AND PUNCHING
• Shearing: is a sheet-metal cutting operation along a straight
line between two cut-ting edges.
• It is performed on a machine called a power shears, or
squaring shears.
• The upper blade of the power shears is often inclined, as
shown in Figure (b), to reduce the required cutting force.
BLANKING, AND PUNCHING
• Blanking: The part that is cut out is the desired product in
the operation and is called the blank. As shown in Figure (a).
• Punching: is similar to blanking except that it produces a
hole, and the separated piece is scrap, called the slug. The
remaining stock is the desired part. The distinction is
illustrated in Figure (b).
ANALYSIS OF SHEET-METAL CUTTING
• Clearance: The clearance “c” in a shearing operation is the
distance between the punch and die, as shown in Figure (a).
Typical clearances in conventional press-working range
between 4% and 8% of the sheet-metal thickness “t”.
C = Act
where c = clearance in mm, Ac=clearance allowance;
and t =stock thickness in mm.
The clearance allowance is determined according to type of
metal.
The punch and die sizes for a round Punch and die sizes for a round hole
blank of diameter Db are determined as of diameter Dh are determined as:
• In order for the slug or blank to drop through the
die, the die opening must have an angular
clearance (see Figure) of 0.25° to 1.5° on each side.
Cutting Forces
• Estimates of cutting force are important because this force
determines the size (tonnage) of the press needed.
• Cutting force F in sheet metalworking can be determined by
F = S*t*L
where S = shear strength of the sheet metal, Mpa.
t = stock thickness, mm and
L = length of the cut edge, mm.
In blanking, punching operations,
L is the perimeter length of the blank or hole being cut.
If shear strength is unknown
F = 0.7(TS)*t*L
where TS = ultimate tensile strength MPa
Components of a punch and die
for a blanking operation.
Compound die
• More complicated press
working dies include
compound dies, combination
dies, and progressive dies.
• A compound die performs
two operations at a single
station, such as blanking and
punching, or blanking and
drawing.
• A good example is a
compound die that blanks
and punches a washer.
Progressive die
• A progressive die
performs two or
more operations
on a sheet-metal
coil at two or
more stations
with each press
stroke.
Progressive die
• The part is fabricated
progressively. The coil is
fed from one station to
the next and different
operations (e.g., punching,
notching, bending, and
blanking) are performed
at each station.
• When the part exits the
final station it has been
completed and separated
(cut) from the remaining
coil
Bending Operation
• Bending in sheet-metal work is defined as the straining of the
metal around a straight axis, as shown in Figure (a).
• During the bending operation, the metal on the inside of the
neutral plane is compressed, while the metal on the outside of
the neutral plane is stretched.
• These strain conditions can be seen in Figure (b). The metal is
plastically deformed so that the bend takes a permanent set upon
removal of the stresses that caused it.
• Bending produces little or no change in the thickness of the sheet
metal.
Types of bending
The two common bending methods and associated tooling
are V-bending, performed with a V-die; and edge bending,
performed with a wiping die
V-bending: the sheet metal is bent
between a V-shaped punch and
die.
Included angles ranging from very
obtuse to very acute can be made
with V-dies.
V-bending is generally used for
low-production operations
Fig. Two common bending methods: (a) V-bending and (b) edge bending; (1) before and (2)
after bending. Symbols: v=motion, F = applied bending force, Fh = blank
Types of bending
Edge bending: involves
cantilever loading of the sheet
metal. A pressure pad is used to
apply a force Fh to hold the base
of the part against the die, while
the punch forces the part to
yield and bend over the edge of
the die. In the setup shown in
Figure(b), edge bending is
limited to bends of 90° or less.
Fig. Two common bending methods: (a) V-bending and (b) edge bending;
(1) before and (2) after bending. Symbols: v=motion, F = applied bending
force, Fh = blank
Drawing
• Drawing is a sheet-metal-forming operation used to
make cup-shaped, box-shaped, or other complex-
curved and concave parts.
• It is performed by placing a piece of sheet metal
over a die cavity and then pushing the metal into
the opening with a punch, as in Figure. The blank
must usually be held down flat against the die by a
blankholder.
• Common parts made by drawing include beverage
cans, sinks, cooking pots, and automobile body
panels.
MECHANICS OF DRAWING
• Drawing of a cup-shaped part is the
basic drawing operation, with
dimensions and parameters as
pictured in Figure.
• A blank of diameter Db is drawn
into a die cavity by means of a
punch with diameter Dp .
• The punch and die must have
corner radii, given by Rp and Rd .
• The sides of the punch and die are
separated by a clearance c.
• This clearance in drawing is about
10% greater than the stock
thickness: c = 1.1 t
Stages in DRAWING
FIGURE. Stages in deformation of the work in deep drawing: (1) punch just before
contact with work, (2) bending, (3) straightening, (4) friction and compression, and
(5) final cup shape showing effects of thinning in the cup walls. Symbols: v=motion
of punch, F = punch force, Fh = blankholder force.
Thank you
