Unit-5 Sheet Metal Process
Content
• Rolling process
• Classification of Sheet metal
• Processes
• Shearing
• Punching and Blanking
• Bending machines – Manual
• Hydraulic
• Electric
• Turret punch
• Laser cutting
• Sheet metal calculations
• Layouts and case studies
Introduction to Sheet metal
• The raw material for sheet metal manufacturing processes is the output of
the rolling process. Typically, sheets of metal are sold as flat, rectangular
sheets of standard size. Therefore the first step in any sheet metal process
is to cut the correct shape and sized ‘blank’ from larger sheet.
• The operations are performed on relatively thin sheets of metal:
• Thickness of sheet metal = 0.4 mm to 6 mm
• Thickness of plate stock > 6 mm
• Operations usually performed as cold working
Rolling process
• Rolling is a deformation process in which the thickness of the work is reduced by
compressive forces exerted by two opposing rolls
• Rolling is the plastic deformation of materials caused by compressive force applied
through a set of rolls.
• In rolling it is classified into two types :
1. Hot rolling process
2. Cold rolling process.
Rolling process
• Rolling power is directly proportional to roll diameter
• Planetary mill has a pair of large heavy rolls surrounded by number of smaller
rolls.
Rolling process
Grain Structure in rolling
• The wrought or cast product gets hot rolled, the gain structure, which is
coarse gained becomes finer in size, elongation along the direction of
rolling
Rolling process
Thread and gear rolling: Threads on cylindrical work pieces can be cold formed
using a pair of flat dies or cylindrical rolls under reciprocating or rotary motion.
Example : Screws, Bolts and Externally threaded fasteners.
• It is a high productivity process involving no loss of material.
• Due to grain flow in thread rolling strength is increased.
Shape rolling: In shape rolling structural sections such as I-section, rails,
channels can be rolled using shape rolling.
• Blooms are usually taken as raw materials for shape rolling
Ring rolling: In this process, two circular rolls, one of which is idler roll and the
other is driven roll are used.
• A pair of edging rollers are used for maintaining the height constant.
• Examples : Large rings of turbines, roller bearing races, flanges and rings for
pipes
Tube piercing: Rotary tube piercing is used for producing long thick walled
tubes.
• Cavity forms at the center due to tensile stress, in a round rod when
subjected to external compressive stress – especially cyclic
compressive stress.
Rolling process
Flat Rolling and its Analysis :In flat rolling, the work is
squeezed between the rolls so that its thickness is reduced by an amount
called the draft.
Where d = draft , mm and = final thickness,
mm
• Draft is sometimes expressed as a fraction of the starting stock thickness
called the reduction.
where r is reduction
Rolling process
• In rolling work width increases this is known as spreading and it tends to be
most pronounced with width to thickness ratio and low coefficient of friction.
The equation is given by
Where and are the before and after work widths ,mm and and are
the before and after work lengths, mm
The above equation in terms of volume is :
where and are the entering and exiting velocities of the work.
• The rolls contact the work along an arc defined by the angle . Each roll has
radius R, and its rotational speed gives it a surface velocity This velocity is
greater than the entering speed of the work and less than its exiting speed
.
• When the work velocity equals roll velocity this is known as no-slip point or
neutral point
• The amount of slip between the rolls and work can be measured by means
of the forward slip
Rolling process
Forward Slip:
where s = forward slip, vf = final (exiting) work velocity, m/s , and vr = roll
speed ,m/s.
True strain experienced by the work in rolling is based on before and after
stock thickness.
Using true strain the average flow stress applied to the work material in
flat rolling is given by
=
Coefficient of friction in rolling depends upon
Rolling process
• Coefficient of friction sufficient to perform rolling and roll force F required
to maintain separation between the two rolls can be computed by
integrating the unit roll pressure over the roll –work contact area. This can
be expressed as
F=w
where F = rolling force, N, w = the width of the work being rolled, mm , p=roll
pressure, MPa, and L =length of contact between rolls and work, mm.
Rolling process
Contacting length L can be approximated by :
The Torque in rolling can be given by : T=0.5FL
The Power required to drive each roll is given by P =2
where P is Power J/s N= rotational speed (rev/min) F = rolling force , N and
L =contact length, m
Problem: A 300-mm-wide strip 25-mm thick is fed through a rolling mill with
two powered rolls each of radius ¼ 250 mm. The work thickness is to be
reduced to 22 mm in one pass at a roll speed of 50 rev/min. The work
material has a flow curve defined by K = 275 MPa and n = 0.15, and the
coefficient of friction between the rolls and the work is assumed to be 0.12.
Determine if the friction is sufficient to permit the rolling operation to be
accomplished. If so, calculate the roll force, torque, and horsepower.
Rolling process
• Solution : The draft attempted in this rolling operation is
d =25 -22 = 3mm
The friction coefficient of friction is = =(
contacting Length L= =27.4 mm
True strain: = 0.128
( . ) .
Average Flow stress : = = 175.7 MPa
.
Rolling force : F 175.7(300)(27.4) = 144786 N
Torque : T=0.5 (144786)(0.0274)
Power : P = 2(pi) (50) (144786)(0.0274)=12432086 N-m/min=207201 N-m/s
(W)
Horse power = HP= =278 hp
.
Sheet metal Cutting Operation
Shearing: It is a sheet-metal cutting operation along a straight line between
two cutting edges. Shearing is typically used to cut large sheets into smaller
sections for subsequent press working operations. It is performed on a
machine called a power shears, or squaring shears. The upper blade of the
power shears is often inclined, to reduce the required cutting force.
Sheet metal Cutting Operation
Blanking :It 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: It 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.
fig (a)is blanking process and fig (b) is punching process
