SHEET METAL

FORMING
PROCESSES
Prepared By:-

• Varmora Raxit V.
 Sheet Metal Working

1. Cutting Operations
2. Bending Operations
3. Drawing
4. Other Sheet Metal Forming Operations
5. Dies and Presses for Sheet Metal Processes
6. Sheet Metal Operations Not Performed on Presses
 Sheet Metal Working
Defined
Cutting and forming operations performed on relatively
thin sheets of metal
 Thickness of sheet metal = 0.4 mm (1/64 in) to 6 mm
(1/4 in)
 Operations usually performed as cold working
 Sheet and Plate Metal
Products
■ Sheet and plate metal parts for consumer and
industrial products such as
– Automobiles and trucks
– Airplanes
– Railway cars and locomotives
– Farm and construction equipment
– Small and large appliances
– Office furniture
– Computers and office equipment
 Advantages of Sheet Metal
Parts
 High strength
 Good dimensional accuracy
 Good surface finish
 Relatively low cost
 Economical mass production for large quantities
 Sheet Metalworking
Terminology

 Punch-and-die - tooling to perform cutting, bending,

and drawing
 Stamping press - machine tool that performs most
sheet metal operations
 Stampings - sheet metal products
 Basic Types of Sheet Metal
Processes
1. Cutting
– Shearing to separate large sheets
– Blanking to cut part perimeters out of sheet metal
– Punching to make holes in sheet metal
2. Bending
– Straining sheet around a straight axis
3. Drawing
– Forming of sheet into convex or concave shapes
 Sheet Metal Cutting

Shearing of sheet metal between two cutting edges:

(1) Just before the punch contacts work
(2) Punch begins to push into work, causing plastic deformation
 Sheet Metal Cutting

Shearing of sheet metal between two cutting edges:

(3) Punch compresses and penetrates into work causing a smooth cut surface
(4) Fracture is initiated at the opposing cutting edges which separates the sheet.
 Shearing
Sheet metal cutting operation along a straight line between
two cutting edges
 Typically used to cut large sheets

Shearing operation: (a) side view of the shearing operation; (b) front
view of power shears equipped with inclined upper cutting blade.
 Blanking and Punching
Blanking - sheet metal cutting to separate piece (called a
blank) from surrounding stock
Punching - similar to blanking except cut piece is scrap,
called a slug

(a) Blanking and (b) punching.

 Clearance in Sheet Metal
Cutting
Distance between punch cutting edge and die cutting
edge
 Typical values range between 4% and 8% of stock
thickness
– If too small, fracture lines pass each other,
causing double burnishing and larger force
– If too large, metal is pinched between cutting
edges and excessive burr results
 Clearance in Sheet Metal
Cutting
 Recommended clearance is calculated by:
c = at
where,
c = clearance;
a = allowance; and
t = stock thickness
 Allowance a is determined according to type of metal
 Punch and Die Sizes

■ For a round blank of diameter Db:

– Blanking punch diameter = Db - 2c
– Blanking die diameter = Db
Where, c = clearance

■ For a round hole of diameter Dh:

– Hole punch diameter = Dh
– Hole die diameter = Dh + 2c
Where, c = clearance
 Punch and Die Sizes

Die size determines blank size Db;

punch size determines hole size Dh.;
c = clearance
 Angular Clearance
Purpose: allows slug or blank to drop through die
■ Typical values: 0.25 to 1.5 on each side
 Cutting Forces

Important for determining press size (tonnage)

F=StL
where
S = shear strength of metal;
t = stock thickness, and
L = length of cut edge
 Sheet Metal Bending
Metal on inside of neutral plane is compressed, while metal on
outside of neutral plane is stretched

both compression and tensile elongation of the metal

occur in bending.
 V-Bending
 For low production
 Performed on a press brake
 V-dies are simple and inexpensive
 Edge Bending
 For high production
 Pressure pad required
 Dies are more complicated and costly
 Bend Allowance Formula
α
Ab = 2π ( R + K bat )
360
Where,
Ab = bend allowance;
 = bend angle;
R= bend radius;
t = stock thickness; and
Kba is factor to estimate stretching
 If R < 2t, Kba = 0.33
 If R  2t, Kba = 0.50
 Spring back

Increase in included angle of bent part relative to included

angle of forming tool after tool is removed
■ Reason for spring back:
– When bending pressure is removed, elastic energy
remains in bent part, causing it to recover partially
toward its original shape
 Spring back

 

Spring back in bending is seen as a decrease in bend angle and an increase in

bend radius: (1) during bending, the work is forced to take radius Rb and
included angle b' of the bending tool, (2) after punch is removed, the work
springs back to radius R and angle ‘.
 Bending Force
Maximum bending force estimated as follows:
K bf TSwt 2
F
D
Where, F = bending force;
TS = tensile strength of sheet metal;
w = part width in direction of bend axis; and
t = stock thickness.
For V- bending,
Kbf = 1.33; for edge bending,
Kbf = 0.33
 Drawing

Sheet metal forming to make cup-shaped, box-shaped, or

other complex-curved, hollow-shaped parts
■ Sheet metal blank is positioned over die cavity and
then punch pushes metal into opening
■ Products: beverage cans, ammunition shells,
automobile body panels
■ Also known as deep drawing (to distinguish it from
wire and bar drawing)
 Drawing

■ Clearance in Drawing
Sides of punch and die separated by a clearance c given by:
c = 1.1 t
where, t = stock thickness
 In other words, clearance is about 10% greater than stock
thickness
 Ironing
■ Makes wall thickness of cylindrical cup more uniform

Ironing to achieve more uniform wall thickness in a drawn cup:

(1) start of process; (2) during process. Note thinning and
elongation of walls.
 Embossing
Creates 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.
 Guerin Process

Guerin process: (1) before and (2) after. Symbols v and F indicate
motion and applied force respectively.
Dies for Sheet Metal
Processes

Punch and Die Components

Compound Die
 Progressive Die

(a) Progressive die;

(b) associated strip development
Stamping Press
 Power and Drive Systems

■ Hydraulic presses - use a large piston and cylinder to

drive the ram
– Longer ram stroke than mechanical types
– Suited to deep drawing
– Slower than mechanical drives
■ Mechanical presses – convert rotation of motor to
linear motion of ram
– High forces at bottom of stroke
– Suited to blanking and punching
 Stretch Forming
Sheet metal is stretched and simultaneously bent to achieve
shape change

Stretch forming: (1) start of process; (2) form die is pressed into the
work with force Fdie, causing it to be stretched and bent over the
form. F = stretching force.
 Roll Bending
Large metal sheets and plates are formed into curved
sections using rolls
 Roll Forming
Continuous bending process in which opposing rolls produce
long sections of formed shapes from coil or strip stock

Roll forming of a
continuous
channel section:
(1) straight rolls,
(2) partial form,
(3) final form.
 Spinning
Metal forming process in which an axially symmetric
part is gradually shaped over a rotating mandrel using
a rounded tool or roller
■Three types: 1.Conventional spinning
2.Shear spinning
3.Tube spinning

Conventional spinning: (1) setup at start of process; (2) during spinning;

and (3) completion of process.