Sheet Metal Working

SHEET METALWORKING
# Sheet metalworking includes cutting and forming operations
performed on relatively thin sheets of metal.
# 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.
# Most sheet-metal operations are performed on machine
tools called presses.

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 Sheet Metal Working

There are three major categories of sheet-metal processes:

1) Cutting
2) Bending and
3) Drawing
† Cutting is used to separate large sheets into smaller
pieces, to cut out part and to make holes in parts. The
stresses induced in cutting are higher than the
ultimate strength of the metal.
† Bending and drawing are used to form sheet-metal parts
into their required shapes and stresses induced are
higher than the yield strength of the metal.

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 Sheet Metal Working

CUTTING OPERATIONS

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 Sheet Metal Working

Characteristic features of the sheared edge

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 Sheet Metal Working

SHEARING, BLANKING, AND PUNCHING

► Shearing is a sheet-metal cutting operation along a straight line
between two cutting edges.
► typically used to cut large sheets into smaller sections for
subsequent press working operations.
► The upper blade of the power shears is often inclined

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 Sheet Metal Working

Blanking involves cutting of the sheet metal along a closed outline

in a single step to separate the piece from the surrounding stock.
The part that is cut out is the desired product in the operation and is
called the blank.
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.

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 Sheet Metal Working

ANALYSIS OF SHEET-METAL CUTTING

► The punch is of identical shape as that of the die opening except
that it is smaller on each side by an amount called clearance.
► As the punch touches the sheet metal and moves downward, it
pushes the metal into the die opening. The stress will be highest
at the corners of the punch and die and the sheet metal will start
fracturing there.

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 Sheet Metal Working

The various steps in the fracture of material:

► Stressing the material beyond the elastic limit
► Plastic deformation
► Reduction in the area
► Start of fracturing in the reduced area at the punch and die
corners
Process parameters in sheet-metal cutting:
► clearance between punch and die
► stock thickness
► type of metal and its strength
► length of the cut

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 Sheet Metal Working

Clearance (c): The clearance c is the difference in size between the

punch and die.
♦ If the clearance between the punch and the die is correct,
the cracks starting from the punch and the die edges will
meet and the rupture will become complete.
♦ If clearance is too small or too large, the cracks do not meet
and results in ragged edges due to the material being
dragged and torn through the die.

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 Sheet Metal Working

Clearance is too small Clearance is too large

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 Sheet Metal Working

The recommended clearance (c) can be calculated by the following

formula

c = clearance, mm; Ac = clearance allowance; and t = stock thickness, mm

Clearance allowance is determined according to type of metal

Material Ac
2025ST and 6061ST aluminium alloys, brass (all 0.06
tempers), soft cold rolled steel, soft stainless steel
1100S and 5052S aluminium alloys (all tempers) 0.045
Cold rolled steel, half hard, stainless steel, half hard 0.075
and full hard
Aluminium 0.07
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 Sheet Metal Working

Application of clearance in blanking and punching

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 Sheet Metal Working

How to apply Clearance in Blanking and Punching

─ In case of blanking operation, the slug or the blank is the desired part
and hence the die has to be held in size.
─ The die opening size is equal to the blank size and
─ punch size is determined by subtracting the clearance from the die
opening size.
Thus, punch and die sizes for a round blank of diameter Db are
determined as

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 Sheet Metal Working

# In case of punching operation, the hole in the sheet metal is to be

accurate and the slug is to be discarded.
# The punch is to be made to the size of the hole and
# Die opening size is obtained by adding the clearance to the punch
size.
Thus, punch and die sizes for a round blank of diameter Dp are
determined as:

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 Sheet Metal Working

Angular Clearance
In order for the slug or blank to drop through the die, the die opening
must have an angular clearance of 0.25 to 1.5 on each side.

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 Sheet Metal Working

Punch and die clearance after considering the elastic recovery

After cutting operation, the elastic recovery of the sheet metal takes place.
Due to this, the size of the blank increases and that of the hole in the sheet
metal decreases.
The difference in the size due to elastic recovery will depend upon the
blank size, thickness of the sheet metal and material.
Stock thickness (mm) Clearance (mm)
up to 0.25 zero
0.25 to 0.75 0.025
Above 0.75 0.05
► To produce the accurate hole, the punch size should be
increased.
► To Produce accurate blank size, die opening size should be
decreased by an amount given the above table
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 Sheet Metal Working

Penetration
The distance through which the punch enters the stock
thickness to cause rupture is called penetration. Penetration is
usually expresses as percentage of stock thickness and is
called percent penetration.
Depends on the material of the sheet and stock thickness.
Harder materials require smaller penetration as compared
to the softer materials.

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 Sheet Metal Working

Material Percent penetration

Aluminium 0.6
0.15%C steel (annealed) 0.38
0.5%C steel (annealed) 0.24

Stock
thickness 25 20 15 12.5 10 8 6 5 3 2.5 1.6 1.6
(t), mm
%
penetratio 25 31 34 37 44 47 50 56 62 67 70 80
n

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 Sheet Metal Working

Cutting Force

= shear strength of the sheet metal, MPa;

t = stock thickness, mm, and
L = perimeter of the section to be blanked or punched, mm.
The minor effect of clearance in determining the value of L can be
neglected.
If shear strength is unknown
Su = ultimate tensile strength MPa

It is assumed that the entire cutting edge is in contact along the

length L is made at the same time.

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 Sheet Metal Working

Prob: A round disk of 150-mm diameter is to be blanked from a strip of 3.2

mm, half-hard cold rolled steel whose shear strength = 310 MPa.
Determine (a) the appropriate punch and die diameters, and (b) blanking
force. (Estimate above considering without and with elastic recovery)

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 Sheet Metal Working

A compound die will be used to blank and punch a large washer out of
aluminium alloy sheet stock 3.2 mm thick. The outside diameter of the
washer = 65 mm and the inside diameter = 30 mm. Determine: (a) the
punch and die sizes for the blanking operation, and (b) the punch and
die sizes for the punching operation.
Determine also the minimum tonnage press to perform the blanking
and punching operation simultaneously (compound die), if the
aluminium sheet metal has a tensile strength = 290 MPa. Assume that
blanking and punching occur simultaneously. If staggered punches
are used where blanking occurs first, determine the tonnage.

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 Sheet Metal Working

A compound die will be used to blank and punch a large washer out of
aluminium alloy sheet stock 3.2 mm thick. The outside diameter of the
washer = 65 mm and the inside diameter = 30 mm. Determine: (a) the
punch and die sizes for the blanking operation, and (b) the punch and
die sizes for the punching operation.
Determine also the minimum tonnage press to perform the blanking
and punching operation simultaneously (compound die), if the
aluminium sheet metal has a tensile strength = 290 MPa. Assume that
blanking and punching occur simultaneously. If staggered punches
are used where blanking occurs first, determine the tonnage.

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 Sheet Metal Working

SELECTION OF PRESS FOR A GIVENN JOB

FACTORS:
a) Overall work size
b) Stock thickness and material
c) Kind of operation to be performed
d) Power required
e) Speed of operation

For punching/blanking/trimming
Characteristics:
± Small stock thickness
± Small stroke length of punch
± High production rate
± Sudden release of energy/load at the end of stroke
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 Sheet Metal Working

Presses – Crank or eccentric type mechanical presses are preferred.

Sudden release of load is not advisable in hydraulic presses

COINING and Other squeezing operations

Characteristics
# Very large force
PRESS – Knuckle joint mechanical press
Hydraulic press (slower and more powerful) can also be
used.

PRESSING/FORMING
Hydraulic presses are better adapted

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 Sheet Metal Working
CUTTING PUNCH AND DIE

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 Sheet Metal Working
Shear on Punch and Die

When a shear = t/3, the cutting

When shear is zero, the entire
action will start at the leading
perimeter is cut at once
edge and gradually spread. The
resulting in the maximum
maximum load is decreased,
load.
however energy needed remains
unchanged as the punch travel is
larger as compared case (a)
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 Sheet Metal Working

If shear=t, then when the leading edge has travelled through the
stock thickness the trailing edge will start making contact with the
material. The maximum force is about half of that when shear is zero

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 Sheet Metal Working

The provision of shear distorts the material being cut. It is observed

that when the shear is on the punch, the blank cannot be made flat
and when the shear is on the die, the piercing cannot be made flat.

Hence, for blanking operation, shear is provided on the die while for
piercing, shear is provided on the punch.

Whenever possible, double shear should be used so that two shear

faces neutralize the side thrusts.

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 Sheet Metal Working

Punching Blanking

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 Sheet Metal Working

DRAWING (DEEP DRAWING)

Drawing is a sheet-
metal-forming
operation used to
make cup-shaped,
box-shaped, or
other complex-
curved and concave
parts from sheet
metal.

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 Sheet Metal Working

MECHANICS OF DRAWING

Blank diameter, = Db
Punch Diameter = Dp
Punch corner radius = Rp
Die corner radius = Rd
If the punch and die were to have sharp corners (radius = 0), a hole-
punching operation would be accomplished rather than a drawing
operation.

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. The
clearance c is given by

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 Sheet Metal Working

► The punch applies a downward force F and a downward holding

force Fh is applied by the blank holder.
► The work experiences a complex sequence of stresses and
strains
► As the punch first begins to push into the work, the metal is
subjected to a bending operation. The sheet is simply bent over
the corner of the punch and the corner of the die.
► As the punch moves further down, a straightening action occurs
in the metal that was previously bent over the die radius
► The metal in the outer portions of the blank is pulled or drawn
toward the die opening to resupply the previously bent and
straightened metal now forming the cylinder wall

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 Sheet Metal Working

► In addition to friction, compression is also occurring in the outer

edge of the blank.
► As the metal in this portion of the blank is drawn toward the
centre, the outer perimeter becomes smaller.
► As the volume of metal remains constant, the metal is squeezed
and becomes thicker as the perimeter is reduced.
► This often results in wrinkling of the remaining flange of the
blank, especially when thin sheet metal is drawn, or when the
blank holder force is too low. It is a condition which cannot be
corrected once it has occurred.

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 Sheet Metal Working

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 Sheet Metal Working

Drawing Without a Blank holder

Primary functions of the blank holder is to prevent wrinkling of the
flange. The tendency for wrinkling is reduced as the thickness-to-
diameter ratio of the blank increases.
If the t/Db ratio is large enough, drawing can be accomplished without
a blank holder.
The limiting condition for drawing without a blank holder:

The draw die must have the shape of a funnel or cone to permit the
material to be drawn properly.
Drawing without a blank holder has the advantages of lower cost of
tooling.

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 Sheet Metal Working

ANALYSIS OF DRAWING
Drawing ratio (DR)
It is defined for a cylindrical shape as the ratio of blank diameter Db to punch
diameter Dp

The greater is the ratio, the more is the severity of the operation. An
approximate upper limit on the drawing ratio is a value of 2.0. The actual
limiting value for a given operation depends on punch and die corner radii
(Rp and Rd), friction conditions, depth of draw, and characteristics of the
sheet metal

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 Sheet Metal Working
Reduction (r)
The reduction is defined as

It is very closely related to drawing ratio. the value of reduction r should be

less than 0.50.
Thickness to diameter ratio (t/Db)
Often expressed as a percentage, it is desirable for the t/Db ratio to be
greater than 1%. As t/Db decreases, tendency for wrinkling increases.

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 Sheet Metal Working
Drawing Force (F)
The drawing force required to perform a given operation can be estimated
roughly as

F = drawing force, N
t = blank thickness, mm
Su = tensile strength, MPa
Eqn. estimates the maximum force in the operation.
The drawing force varies throughout the downward movement of the punch,
usually reaching its maximum value at about one-third the length of the
punch stroke.

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 Sheet Metal Working
Blank holding Force (Fh)
As a rough approximation, the holding pressure can be set at a value =
0.015 of the yield strength of the sheet metal. This value is then multiplied by
that portion of the starting area of the blank that is to be held by the blank
holder. In equation form,

S0 = yield strength of the sheet metal, Mpa

Rd = die corner radius, mm
The holding force is usually approximately one-third the drawing force

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 Sheet Metal Working
Blank Size Determination (Area Method)
1) Blank diameter D (for sharp corner)
d/rp = 20 and more (for thin gauge sheet with sharp corner)
d = inside diameter of the finished shell
h = height of the finished shell
rp = punch corner radius

2) For a round corner cup (d/rp ratio lies between 15 and 20)

3) When d/rp ratio lies between 10 and 15, D is given by

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 Sheet Metal Working
4) If thick gauge sheet is used, use net or mean diameter (outside diameter –
average thickness) for d in the last equation.
5) On flanged cylindrical shell with relatively sharp corners

d1 = diameter of the cylindrical section

d2 = outer diameter of flange on the finished shell
The above-mentioned equations give theoretical blank size. Small trimming
is necessary to get smooth edges. For this, it is necessary to add extra
metal. Thumb rule is to add about 3.2 mm to the blank diameter for each 25
mm of cup diameter.

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 Sheet Metal Working
Radius of Draw Die or die corner radius (rd)
Range from 4 to 10 times the blank thickness. (Generally average value
of rd is considered)

Punch corner radius (rp)

For single drawing operation, the punch radius should be equal to the corner
radius of the cup to be drawn. When more than one draw is needed to get
the final shape, the minimum punch radius should be 4 times the blank
thickness. On subsequent draw punch radius may be reduced
proportionately.

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 Sheet Metal Working
Draw clearance
The die clearance between the punch and the die should be more than
blank thickness to take into account of the thickening of the blank over the
die radius.
The average draw clearance depends on blank thickness and the
subsequent draw.

Average draw thickness, mm

Blank thickness, mm First draw Redraw
Up to 0.38 1.07t—1.09 t 1.08 t—1.1t
0.4 to 1.27 1.08t—1.1t 1.09t—1.12t
1.3 to 3.18 1.1t—1.12t 1.12t—1.14t
3.2 and above 1.12t—1.14t 1.15t—1.2t

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 Sheet Metal Working
Number of Draws
If the reduction required is large, more than one draw is needed.

♦ As per practice, for first draw, reduction is kept between 45% to 50%.
♦ For subsequent draws, reductions used are 30%, 25%, 16% and 13%.
The material get work hardened due to repeated drawings. To prevent
fracture, the material must be annealed after a total reduction of 60%.

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 Sheet Metal Working
The number of draws can be determined from the radio of inside shell
height (h) to mean shell diameter (d) as given below

h/d ratio Number of draws

Up to 0.7 1
0.7 to 1.5 2
1.5 to 3 3
3.4 to 7 4

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 Sheet Metal Working
Problem:
Determine the blank size and other drawing parameters for a cup made of
cold rolled (CR) steel sheet of 0.8 mm thick. The mean diameter of the cup =
50 mm and inside height = 50 mm. The corner radius of the cup is 1.6 mm.

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 Sheet Metal Working
Redrawing
Direct drawing Reverse drawing

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 Sheet Metal Working
DEFECTS IN DRAWING
Wrinkling in the flange Wrinkling in the wall Tearing in the vertical wall

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Earing (Irregularities) Surface scratches

Occurs in the upper edge of a Occur on the drawn part if the

deep drawn cup, caused by punch and die are not smooth or if
anisotropy in the sheet metal lubrication is insufficient

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