Sheet Metal Process

SHEET METAL PROCESS

Now a day the automobile component, aircraft and ship building involves the usage of thin sheets of various metals. If the thickness of the metal is less than six ‘mm’ then it is called as sheet. Hence the knowledge about the sheet metal processing is necessary. This unit deals how the sheet metals are processed for meeting the requirement & Involves methods in which sheet metal is cut into required dimensions and shape; and/or forming by stamping, drawing, or pressing to the final shape. The surface finish of the sheet metals processed by this methods are good when compared to other process like welding, machining etc., A special class of metal forming where the thickness of the piece of material is small compared to the other dimensions. Cutting into shape involve shear forces Forming Processes involve tensile stresses.

The Major operations of sheet Metal are;

(A)Shearing,

(B) Bending,

(C) Drawing  and

(D)Squeezing

Shearing

The mechanical cutting of materials without the information of chips or the use of Burning or melting for straight cutting blades: shearing for curved blades: blanking, piercing, notching, trimming

Classifications of Shearing Processes

Blanking

Punching

Piercing

Trimming

Slitting

Notching

Nibbling

Shaving

Dinking

Perforating

Lancing

Cutoff

Spring

Back

Blanking.

Cutting a piece from the sheet metal by leaving enough scrap around the opening to assure that the punch has enough metal to cut along its entire edge, during which a metal work piece is removed from the primary metal strip or sheet when it is punched.

Punching.

It is the operation to produce circular holes on a sheet metal by a punch and a die. Here the pierced metal is the final product then the operation is called as punching.

Piercing.

It is an operation to produce holes of any desired shape. Piercing produces a raised hole rather than a cut hole. Piercing refers to punching a hole.

Trimming.

It is the operation of cutting and removing unwanted excess metal from the periphery of a previously formed /forged/cast component.

Slitting

Shearing process used to cut rolls of sheet metal into several rolls of narrower width used to cut a wide coil of metal into a number of narrower coils as the main coil is moved through the slitter. Shearing operations can be carried by means of a pair of circular blades.

Notching

Same as piercing- edge of the strip or black forms part of the punch-out perimeter. In this process the metal is removed from the side (OR) EDGE of a sheet to get the desired shape.

Nibbling

Produces a series of overlapping slits/notches. A machine called nibbler moves a small straight punch up and down rapidly into a die.

Shaving

Finishing operation in which a small amount of metal is sheared away from the edge of an already blanked part .can be used to produce a smoother edge. The rough edges of a blanked part are removed by cutting thin strip of metal along the edge on the periphery.

Dinking

Used to blank shapes from low-strength materials such as rubber, fiber and cloth

Perforating.

Process of making multiple holes which are small in diameter and closed together.

Lancing.

Lancing refers to leaving a tab without removing any material. It is an operationof cutting on one side and bending on the other side to form a sort of tab (or) Louver. No metal is removed in this operation. Lancing refers to leaving a tab without removing any material. It is an operationof cutting on one side and bending on the other side to form a sort of tab (or) Louver. No metal is removed in this operation.

Cutting Off.

In this operation a piece is removed from a strip by cutting along a single line.Parting The sheet is sheared into two (Or) more pieces.

Spring back

The elastic recovery of the material after unloading of the tools.

Bending

The plastic deformation of metals about a linear axis with little or no change in the surface area. The purpose of bending is to form sheet metal along a straight line

STRETCH FORMING OPERATIONS

In this process, the sheet metal is clamped along the edges and then stretched over a die (OR) FORM BLOCK, which moves upward, downward (or) side ways, depending on the particular machine.

It is used to make aircraft wing-skin panels, automobile door panels and window frames. The desirable qualities in the metal for maximum strechability are as follows.

1.Fine grain structure. 2.toughness.

3.LARGE SPREAD between the tensile yield and ultimate strength.

Working.

It consists of placing the sheet –metal under a tensile load over a forming block and stretching it beyond its elastic limit and to te plastic range, thus cause a permanent set to take place.

Two  Basic Forms of Stretch forming are,

1.Stretch forming, 2.Stretch – Wrap forming.

Advantages

1.                 In a single operation, blanks can be stretched.

2.                 Heat treatment before and after  stretching process is not required.

3.                 Spring back effect is minimized.

4.                 Tooling cost is low.

5.                 Direct bending is not introduced, and plastic deformation is due to pure tension.

6.                 It is suitable for low volume production.

Disadvantages.

1.uneven thickness of blank cannot be stretched.

2.The maintanence cost of the hydraulic cylinders is high.

SPECIAL FORMING PROCESS.

1.HYDRO FORMING PROCESS

In this process te pressure over the rubber membraneis controlled throughout the forming cycle, with maximum pressure upto- 100 Mpa.This procedure allows close control of the part during forming, to prevent wrinkling (or) tearing. This process is called hydroform or fluif – Forming Process.Hydro forming is a Drawing process.

Advantages of Hydro-formimg Process.

1.It is used for Mass production.

2.Tools can be quickly changed.

3.Complicted shapes , sharp corners can be made by this method.

4.Spring back, Thining off metals are removed.

2.RUBBER PAD FORMING

One of the die material is made up of a flexible material (ex. Rubber) Or (poly-urethane material.In bending and embossing of sheet metal , the female die is replaced with a rubber pad.Pressure in the rubber pad forming is usually in the order of10Mpa.

The blank is placed under the punch called male die.Then the ram (femal part) is moved so that punch touches the top surface of the work. Then the force is appled and gradually. increased on the blank through the rubber pad.

The blank  holder ring is used to distribute uniform pressure throughout the blank.

Thus the required shape is formed on the sheet metal between male and female parts.

Advantages of rubber pad forming.

1.Number of shapes can be formed on one rubber pad.

2.Thining in metal balank does not take place.\

3.setting time of the tool is less.

4.Wrinkle – free , shrink flanges  can be produced.

Disadvantages

1.Rapid wearing of rubber Pads is a problem in this process. 2.Accurate sharp corners cannot be made by this process.

3.Loss of pressure between hydraulic fluid and rubber pad which is a major problem

Aplications.

Flanged Cylinders.

Rectangular cups,

Spherical Domes.

Unsymmetrical shaped components can be made.

3.PEEN FORMING PROCESS.

This process is used to produce curvature on thin sheet metals by shot peening on surface of the sheet. A stream of metal shots is blasted against the surface of the blank.

This process is also called as peen forming technique.

Peening is done with cast- iron (or) Steel shot discharged either from a rotating wheel by an air blast made from a nozzle.

Advantages of Peen forming

Complex shapes can be easily produced . Die and Punch is not used.

Peening is used as a salvage operations for distorted parts (OR)  correcting part.

Disadvantages of peen forming.

This process requires longer time for forming the required shape.

Requires additional devices for forcing out metal shots.

Applications of peen forming.

Specific portions on crankshafts , connecting rods, gears

Honey comb panels like aircraft wings and large tubular shapes can be produced.