Transcript HABEEB HATTAB HABEEB Office: BN-Block, Level-3, Room-088 Email: [email protected] Ext. No.: 7292 H/P No.: 0126610058 PRESSWORKING & OPERATION PRESSWORKING AND OPERATION Definitions: Press working: is cutting, forming and drawing of sheet.

HABEEB HATTAB HABEEB
Office: BN-Block, Level-3,
Room-088
Email: [email protected]
Ext. No.: 7292
H/P No.: 0126610058
PRESSWORKING
& OPERATION
PRESSWORKING AND OPERATION
Definitions:
Press working: is cutting, forming and drawing of
sheet metal materials using punches and dies.
Shear forces: are the forces applied to the metal
to be cut, formed or drawn.
Shear strength: is the resistance of the metal to
shear forces. In metal press working the shear forces
must be greater than the shear strength.
A press: is a machine that is used in pressworking
operations. Presses use tools called dies and
punches to perform pressworking.
PRESSES
Advantages and features of presses:
1. Unlike machine tools, presses are flexible and
multipurpose machines. This is because one press
can employ many interchangeable.
Tools (dies and punches), so a standard press can
perform different types of operations (forming,
shearing and punching).
2. Presses are productive machines (used in mass
production) because for one stroke one part is
produced. That is why presses have found wide
applications in automotive, aircraft and appliances
industries.
Factors of Selection of Presses
1. Operation type
2. Size of part
3. Required power
4. Speed of operation
Parts of Presses:
Frame, Bed, Ram, Ram slides, Ram drive,
Flywheel, Transmission, Bolster plate,
Power source
Presses rating:
Press rating is given in tons and it is also called
tonnage rating. It is the energy spent on the work at
each stroke. This energy is obtained from that stored
in the flywheel:
N2D2W
E 
5.25* 109
Where E- Energy, (ton-in) available at 10% slow down
from normal rpm of the flywheel.
N- rotary speed of flywheel, rpm
D- diameter of flywheel, in
W- weight of flywheel, lb
Classification of Presses
A. (According to the power source):
1. Manual-operated presses: for press working of
thin sheets.
2. Power-operated presses: mechanical and
hydraulic.
B. (According to the operation type):
1. Crank or eccentric types presses (for punching and trimming)
2. Knuckle joint presses (for coining and embossing)
3. Hydraulic presses (for drawing)
C. (According to the frame type):
Types of frames: { inclined frame, gap frame, arch frame,
straight side frame, horn frame}
1. Inclined open back press:
This press has a gap frame. The press can be inclined to permit the parts to
slide off the back side (That is why it is called open back).
2. Gap press:
It has gap or C-frame. Adv.-Excellent access to the tools, can be use fro
long and wide parts.
3. Arch Press:
It has an arch frame and used for light work.
4. Straight side press:
This is powerful because it has a vertical straight side frame which takes the
heavy loads.
5. Horn press:
The press has a heavy shaft (horn) instead of bed. It is used to presswork
cylinder parts.
Other Types of Presses
1. Press Brake:
It uses long dies to presswork long pieces. The press is suitable for small
production runs. The tools applied are very simple.
2. Turret Press:
This press consist of turrets that contain dies and punches of different sizes
and shapes. It produces holes by selecting the required punch from the
rotating the turret. (65 holes/ min)
Features Of Hydraulic Presses
1. Adapted to drawing operation, because of the slow
uniform motion of the ram.
2. Hydraulic presses push or squeeze the work while
mechanical presses (previous types) actually strike
the work.
3. Their frames are very strong.
4. Adjustable power of the press because oil (working
fluid) can be trotted (controlled) (adv)
5. Less productive than mechanical presses because of its
slow motion (disadv)
6. Require high maintenance level (disadv)
Drive Mechanisms For Presses
1. Single crank drive: It gives harmonic motion. It reaches its
maximum speed at midstroke.
2. Eccentric drive: It gives shorter strokes than the crank
drive.
3. Rack and Gear : It gives long and length controlled strokes
4. Hydraulic drive: It gives slow powerful motion of the ram for
forming and drawing operations.
5. Knuckle joint drive: It is used for coining because of its high
load capacity.
6. Toggle drive
7. Screw drive
Feed Mechanisms of Presses
Purpose: Feed mechanisms introduce stocks to the dies
without endangering the operator’s hands. In addition, they
provide uniform and rapid feeding, which is important to high
production.
PRESSWORKING OPERATIONS
Presses employ tools called punches and dies. Punches are
attached to the ram, so a punch together with the ram and
enters the die cavity. The die is usually stationary and rests
on the press bed or a bolster plate.
PRESSWORKING OPERATIONS Cont.----------
a – Shearing:
Shearing is the process of cut along a straight line across a
strip to produce small pieces of sheet metal.
Shearing applies shearing forces that must be greater than
the ultimate strength of the metal. So the metal passes from
elastic and plastic deformation to the state of the ultimate
strength. At this moment fractures start to take place and
they progress with more shearing forces.
Types of Shearing:
1. Blanking : It refers to the operation of cutting out sheet
metal “blanks” or flat shapes from strips. The remaining
part of the strip is waste metal and called “skeleton”.
2. Piercing or punching = Cutting or enlarging a hole.
3. Nibbling: A machine called nibbler moves a punch while the
sheet is fed. This operation is similar to the making of an
elongated slot in a sheet of paper by the successive
punching of holes with a paper punch.
Adv.-Economical process for small production runs because
it does not employ special dies.
4. Shaving or sizing: the extra material from a rough-sheared
(previously sheared) edges is trimmed.
5. Slotting: is the cutting of elongated holes (slots).
6. Notching: Removing pieces or various shapes from
the edges.
7. Perforating: Punching a number of holes in a sheet.
8. Slitting: Longitudinal cutting of a strip.
9. Lancing: Leaving a tap without removing any material.
Difference between blanking and punching :
Importance of Clearance:
Clearance is the gap per side between the die and punch.
Improper clearance leads to excessive deformation because
fractures between the top and bottom of the stock do not
meet.
Factors that influence the clearance definition:
1. Thickness,
2. Hardness
3. Strength of the stock material.
Calculation of punch force:
F = π D tS
- Punch force depends on:
D- Diameter (in or mm)
t – thickness (in)
S – Shear strength (psi)
With correct clearance, the material breaks suddenly so the force
decreases suddenly.
Force
Force
With improper clearance the material breaks slowly so the forces do
not dissipate quickly.
Punch travel
Punch travel
b – Bending and Forming:
In bending the metal experiences both compression (at the inner
surface) and tension (at the outer surface) at the same time.
Bending is performed in V or U shaped dies.
Length of bend: is equal of the length of arc produced after bending:
L = 2Π(r +0.4t)α / 3600
Where
r- Inside radius of bend, in or mm
α- Angle of bend
t – Thickness, in or mm
Springback and its compensation:
After bending and removing the bending forces, the radius of bend
increases because the metal retains some elasticity that causes elastic
recovery (springback). The springback can be corrected by over
bending an amount such that when the bending force is removed, the
part will return to the design dimensions.
c – Drawing:
Drawing consists of pressing or stretching a flat blank or sheet over
a die having the interior shape of the workpiece. During drawing,
sever tensile stresses are induced into the sheet.
Types of Drawing:
1. Single-action drawing (simple drawing): It is performed on singleaction presses (only on movement of the ram).
2. Double-action drawing: It is performed on double action presses
that have two slides one within the other (one for moving the ram
with the punch and the other for holding the blank edges using a
ring)
Why are hydraulic presses suitable for drawing?
1. Because of the slow action
2. Because the speed can be controlled.
3. Because of uniform squeezing pressure.
4. Because of strength of the press
Rubber –Pad Forming (Guerin Process)
Instead of employing a punch, a boxlike container filled with a rubber
pad is mounted on the ram. Only male die is applied. As the ram
descends, it forces the rubber to exert forces on the plate and as a
result, the plate will flow around the die and thus forming the part.
Advantages
1. Good surface of the part because friction with hard tool materials
is eliminated.
2. Low cost of tools
Deep Drawing (Hydroforming or Fluid Forming)
It uses the same principle of rubber-pad forming but the differences
are:
• The rubber pad has a cavity filled with a fluid. The fluid is
introduced to the cavity under pressure through a valve.
• Instead of the male die, a punch is used. When the oil is
introduced to the cavity under a certain pressure, the punch
moves up pressing the plate against the rubber.
Efficient Use of Materials in Stamping: ( Refer to fig. 16.28)
It involves the principle of minimum waste . This means skeleton or
the leftover after blanking should be reduced to minimum.
Types of Material losses during stamping:
1. Waste = Skeleton or leftover after blanking
2. Scrap = Loss of materials because of mistakes (worn punches)
3. Shrinkage = Loss of material because of changes in its quality.
THANK YOU