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Karnataka (Govt.) Evening Polytechnic of 13 Unit 5 : Sheet Metal Process

Chapter 5: Sheet Metal Process

Working of sheet metal from 16 gauge down to 19 gauge with the help of hand tools and simple

machines into various forms by cutting, forming into different shapes and joining is called sheet metal

process.

Knowledge of three things are important for sheet metal work

1. Geometry

2. Mensuration (he branch of geometry that deals with the measurement of length, area,

or volume)

3. Properties of Metals

Applications of sheet metal process

1. Production of hoppers and funnels

2. Production of guards, covers and hoods

3. Production of pipes and bends

4. Production of boxes.

5. Used in automobile industries.

6. Used in household equipment.

7. Used in building works.

8. Used in ship building, aircrafts, etc.

Metals used for sheet metal work

Black iron

• It a bluish-black in appearance and it is uncoated Sheet which corrodes rapidly.

• The use of this metal is limited to articles that are to be painted or enameled such as

tanks, pans, stove pipes, etc.

Galvanised iron

• It is a soft steel coated with molten zinc to resist rust and to improve its appearance.

• This metal can be soldered easily.

• It is used for products such as buckets, pans, furnaces, heating ducts, cabinets, gutters,

etc.


Copper

• It is reddish coloured metal which is ductile and malleable.

• This metal does not deteriorate rapidly by the atmosphere,

• This metal can be used in electric field, water pipe, roofing, gutters, etc.

Aluminium :

• It is silvery white in colour with a high strength to weight ratio and good corrosive

resistant qualities.

• It can be easily fabricated.

• It can be used for refrigerator trays, windows, duct work, construction of airplanes.

Stainless steel

• It is an alloy of steel possessing the ability to resist corrosion without any surface

coating.

• It can be used for buildings, food handling equipments, kitchenwares, etc.

Tin plate

• It is a steel sheet coated with pure tin.

• It is having a bright silvery appearance and used for making containers, cans and pans.

Terne plate

• It is a thin steel coated with a mixture of molten tin and lead.

• It is having dull appearance and is used for roofing and tanks

Lead

• It is very soft and heavy metal.

• It is used in highly corrosive acid tanks.

Desirable properties of metals used for sheet metal work

1. Good corrosive resistance.

2. High formability.

3. Good weldability.

4. Highly weather and chemical resistant.

5. Sheets should have high strength.

6. Should have enough ductility.

7. Possess good surface finish.

8. Good machinability.


Sheet Metal Gauges : Standard Gauge Numbers (SWG)

• Use of gauge is to designate the thickness of sheet metal.

• Gauge is defined differently for ferrous (iron-based) and nonferrous metals (e.g.,

aluminum and brass).

Sheet metal operations

Shearing

1. Cutting off

2. Parting

3. Blanking

4. Punching

5. Notching

6. Slitting

7. Lancing


2. Bending

3. Drawing

4. Squeezing.

Shearing

Shearing is a general sheet metal cutting operation and involves three basic steps

• Plastic deformation takes place first

• Fracture takes place as the tool moves further

• Shear takes place finally

Cutting off : It is the process of severing a piece from a strip with a cut along a single line.

Parting: It is the process of removing the scrap between the two pieces to Part them


Blanking: It is the process of cutting a whole piece from sheet metal such the at the piece removed is

the useful part required

Punching: It is the process of producing circular holes in a sheet metal with a punch or die.

Notching : It is the process of removing the metal of desired shape from the side or edge of a sheet

or strip.

Slitting : When shearing is conducted using rotary blades, the process is referred to as slitting. It

cuts the sheet metal length-wise


Lancing : It is the process of making a cut part way across a strip

Bending

Bending is a process by which a straight length is transformed into a curved length to a desired angle.

When the metal bends, the metal is stressed beyond the elastic limit in tension on the outer side and in

compression on the inside of the bend. There is only one line, the neutral line which retains its original

length.


Drawing

• Drawing is the process of producing thin-walled hollow or vessel shaped parts from sheet

metal.

• The drawing process can be divided into two types, deep drawing and shallow drawing

• Deep drawing the length of the object to be drawn is deeper than its width. E.g: Tubes

• In shallow drawing or box drawing : The length of the object to be drawn is less than the width.

For ex : seamless pots, tubs, cans and covers, automobile panels, tops and hoods etc.

Squeezing

• Squeezing is a quick and widely used way of forming ductile metals.

• Common squeezing operations

• Sizing

• coining

• hobbing

• ironing

• riveting


Sizing

• Here the metal is deformed to a particular size by applying force

Coining

• The coining process involves the production of impression and raising of images or characters

from a punch and die into the metal. The metal is made to flow, and design opposite to the one

on the die is formed on the coined piece.

Ironing

• Ironing is the operation in which a hard punch is pressed over a sheet metal cup placed over the

stationary die to reduce its thickness


Hobbing

• Hobbing is the process of deforming the metal to a specific surface shape using the die of

required shape and applying required force

Rivetting

• Riveting operation is carried out to join two metal strips

Press Work

• Press working may be defined as the chipless manufacturing process by which various

components are made from sheet metal.

• This process is also known as cold stamping.

• The press is a metal forming machine tool designed to shape or cut metal by applying

mechanical force or pressure.

• The presses are exclusively used for mass production work and they represent the fastest and

most efficient way to form a sheet metal into finished.


Types of presses

According to the source of power

1. Hand press / ball press / fly press

2. Power Press (Mechanical / Hydraulic / Pneumatic)

A. According to the Design of frame

1. Gap

2. Inclinable

3. Adjustable

4. Horn

5. Straight side

6. Pillar

According to Actions

1. Single Action

2. Double Action

3. Triple Action

According to Mechanism

1. Crank type

2. Eccentric type

3. Cam type

4. Toggle joint

5. Rack and Pinion

6. Gear

Working of press

• Power press ram can be driven by hand or it can be driven by power

• The power press may be operated mechanically (by and electric motor) or by hydraulic or

pneumatic power where compressed fluid gives the necessary power


Mechanical press

• A simple working of mechanical power press driven by crank and connecting rod mechanism as

shown in figure

• The punch is fitted to the ram end and the die is fitted on the bolster plate.

• The fly wheel is mounted at the end of the crank shaft. When the power is switched on, the

motor rotates the flywheel fixed on the crank shaft and this rotary motion is converted into

reciprocating motion by means of crank shaft and ram with punch moves downward to press

the sheet metal between the punch and die.

• The flywheel is used to store the energy for maintaining a constant downward speed of the ram

when the sheet metal is pressed between the punch and the die.

• In this press the motor runs continuously and stores energy in the flywheel. When the operator

presses a foot pedal or actuates a button, the clutch gets engaged, the punch moves towards

the die and sheet kept between the punch and die is deformed


Hydraulic power press

• The operating principle of hydraulic power press is shown in fig. It essentially consists of

double acting cylinder, ram directly attached to the piston in a cylinder, upper die and lower

die.

• Fluid under presser forces the piston downwards which causes the ram and upper die to move

downwards and press against the lower die.

• For return stroke, the fluid enters from the lower side and pushes the piston up.

• These press require greater maintenance than mechanical press.

• The piston movement is quite slow thus the squeezing action is completely carried to the

Centre of the workpiece being pressed.


Ball or fly press

• It consists of a C-shaped rigid casting machine frame to withstand the repeated heavy thrust

exerted by a ram as shown in figure

• The front face of the frame is open to facilitate the feeding of the sheet metal below the ram

from side of the machine.

• The screw of the press operates in a nut by means of handle at the top end of the frame.

• The two heavy iron balls are mounted at the two ends of the arm, which is bolted to the screw

so that when the handle is turned, it causes the screw to rotate within the nut to move the ram

up and down in the guides

• The ram is attached to the lower end of the nut and the punch is attached to the lower end of

the ram. The punch and die constitute the press tool.

• The die is fixed over the press table known as bolster.

• The ram is taken up slowly to the sufficient height and then the sheet metal to be formed is

placed between the punch and die. Now the handle is rotated to store the kinetic energy in

the two heavy balls, which forces the ram to move downward with high pressure.

• The pressing of sheet by the ram against the die with high pressure causes the sheet to form a

desired shape.

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