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PRESSWORKING PRESSWORKING & OPERATION& OPERATION

PRESSWORKING PRESSWORKING & OPERATION& 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.

PRESSWORKING AND OPERATION

PRESSESAdvantages and features of presses:1. Unlike machine tools, presses are flexible and multipurpose machines. This is because one press can employ many interchangeable.

A press: is a machine that is used in pressworking operations. Presses use tools called dies and punches to perform pressworking.

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.

1. Operation type2. Size of part3. Required power4. Speed of operation

Factors of Selection of Presses

Parts of Presses:

Frame, Bed, Ram, Ram slides, Ram drive, Flywheel, Transmission, Bolster plate, Power source

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:

Where E- Energy, (ton-in) available at 10% slow down from normal rpm of the flywheel.

N- rotary speed of flywheel, rpmD- diameter of flywheel, inW- weight of flywheel, lb

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Presses rating:

1. Manual-operated presses: for press working of thin sheets.

2. Power-operated presses: mechanical and hydraulic.

Classification of Presses

A. (According to the power source):

1. Crank or eccentric types presses (for punching and trimming)2. Knuckle joint presses (for coining and embossing)3. Hydraulic presses (for drawing)

B. (According to the operation type):

Types of frames: { inclined frame, gap frame, arch frame, straight side frame, horn frame}

C. (According to the frame type):

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.

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)

Other Types of 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)

Features Of Hydraulic 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 strokes4. 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 drive7. Screw drive

Drive Mechanisms For 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.

Feed Mechanisms of Presses

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

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.

PRESSWORKING OPERATIONS Cont.----------

a – 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.

Types of Shearing:

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.

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.

Importance of Clearance:

Difference between blanking and punching :

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)

Factors that influence the clearance definition:

With correct clearance, the material breaks suddenly so the force decreases suddenly.

With improper clearance the material breaks slowly so the forces do not dissipate quickly.

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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.

b – Bending and Forming:

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

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.

Springback and its compensation:

Types of Drawing:

1. Single-action drawing (simple drawing): It is performed on single-action presses (only on movement of the ram).

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.

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)

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

Why are hydraulic presses suitable for drawing?

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.

Rubber –Pad Forming (Guerin Process)

1. Good surface of the part because friction with hard tool materials is eliminated.

2. Low cost of tools

Advantages

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.

Deep Drawing (Hydroforming or Fluid Forming)

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.

Efficient Use of Materials in Stamping: ( Refer to fig. 16.28)

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