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SHEET METAL WORKING

Unit-III

By:Zoha NasirAssistant ProfessorJIT, Barabanki

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Products made of sheet metals are all around us. They include a very wide range of consumer and industrial

products, such as beverage cans, cookware, file cabinets, metal desks, appliances, car bodies, trailers, and aircraft fuselages Sheet forming dates back to about 5000 B.C., when household utensils and jewelry were made by hammering and stamping gold, silver, and copper.

Compared to those made by casting and by forging, sheet-metal parts offer the advantages of light weight and versatile shape.

However, the term press working or press forming is used commonly in industry to describe general sheet-forming operations, because they typically are performed on presses.

Sheet metal working

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The basic mechanism of sheet metal processes are stretching and bending. The major factor that influence the overall operation are:ElongationYield point elongationAnisotropyGrain sizeResidual stressesSpring back andWrinkling

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It is a mass production process. Low labour cost. Production rates are rapid. Very low production cost.

Advantages

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Generally, production requirements in excess.Set up times are lengthy.

Disadvantages

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Presses shape work piece without cutting away material, that is, without making chips.

Presses are equipped with dies and punches designed for such operations as forming, punching and shearing.

Presses

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Proper selection of a press is necessary for successful and economical operation.

Press is a costly machine, and the return on investment depends upon how well it performs the job.

There is no press that can provide maximum productively and economy for all application so, when a press is required to be used for several widely varying jobs, compromise is generally made between economy and productivity.

 Important factors affecting the selection of a press are size, force, energy and speed requirements.

Selection of presses

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Types of presses for sheet metal working can be classified by one or a combination of characteristics, such as source of power, number of slides, type of frame and construction, type of drive, and intended applications.

Types of presses

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Hand pressPower pressMechanical pressHydraulic pressCrankCam etc…

Classification of presses

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In a typical die and punch set used for blanking operation the sheet metal used is called strip or stock.

The punch which is held in the punch holder is bolted to the press ram while die is bolted on the press table.

During the working stroke, the punch penetrates the strip, and on the return stroke of the press ram the strip is lifted with the punch, but it is removed from the punch by the stripper plate.

The stop pin is a gage and it sets the advance of the strip stock within the punch and die.

The strip stock is butted against the back stop acting as a datum location for the center of the blank.

DIE AND PUNCH ASSEMBLY

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The clearance angle provided on the die depends on the material of stock, as well as its thickness. For thicker and softer materials generally higher angular clearance is given.

In most cases, 2 degree of angular clearance is sufficient. The height of cutting land of about 3 mm is generally sufficient.

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The three methods of working sheet metal in presses are:

ShearingDeep drawingBending

Press work methods

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This process involves cutting sheet metal by subjecting it to shear stress, usually between punch and die.

The major variables in this process are:Punch forceSpeed of punch, lubricationsEdge condition of the sheetPunch and die materials.

Shearing

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BlankingPiercingSlittingNotchingPerforatingNibblingShavingSlottingTrimming lancing

Shearing operations

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Progressive diesCompound diesCombination dies

Shearing dies

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Parts requiring multiple operations to produce can be made at high production rates in progressive dies.

The sheet metal is fed through as a coil strip, and a different operation (such as punching, blanking, and notching) is performed at the same station of the machine with each stroke of a series of punches.

Progressive die

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Progressive dies are made with two or more stations arranged in a sequence. Each station performs an operation on the work piece, or provides an idler station, so that the work piece is completed when the last operation has been accomplished.

Thereafter each stroke of the ram produces a finished part. Thus after the fourth stroke of a four – station die, each successive stroke will produce a finished part. Operations which may be carried out in a progressive die are piercing, blanking, forming, drawing, cut – off, etc.

  Progressive dies are used where higher production rates are desired and the material is neither too thick nor too thin. Their use helps in cutting down the material handling costs.

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Several operations on the same sheet may be performed in one stroke at one station with a compound die.

Such combined operations usually are limited to relatively simple shapes, because

(a) the process is somewhat slow and (b) the dies rapidly become much more expensive to produce than those for individual shearing operations, especially for complex dies.

Compound die

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 Compound die combines the principles of the conventional and inverted dies in one station. This type of die may produce a work piece which is pierced and blanked at one station and in one operation. The piercing punch is fastened in the conventional position to the punch holder. Its matching die opening for piercing is machined into the blanking punch. The blanking punch and blanking die opening are mounted in an inverted position. The blanking punch is fastened to the die shoe and the blanking die opening is fastened to the punch holder.

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This type of die is similar to that of a compound die except that here non cutting operations such as bending and forming are also included as part of the operation.

Combination die

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 It is designed with the die block fastened to the punch holder and the punch fastened to the die shoe. During the downward stroke of ram, the blank is sheared from the strip. The blank and shedder are forced back into the die opening, which loads a compression spring in the die opening . At the same time the punch is forced through the scrap strip and a spring attached to the stripper is compressed and loaded

Inverted die

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Numerous parts made of sheet metal are cylindrical or box shaped, such as pots and pans, all types of containers for food and beverages, stainless-steel kitchen sinks, canisters, and automotive fuel tanks.

Such parts usually are made by process in which a punch forces a flat sheet-metal blank into a die cavity.

Although the process generally is called deep drawing (because of its capability for producing deep parts), it also is used to make parts that are shallow or have moderate depth.

It is one of the most important metalworking processes because of its widespread use.

Deep drawing

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In the basic deep-drawing process, a round sheet-metal blank is placed over a circular die opening and is held in place with a blank holder, or bold-down ring.

The punch travels downward and forces the blank into the die cavity, forming a cup.

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In deep drawing process, various types of forces operate simultaneously, which are:

The annular portion of the sheet metal work piece between the blank holder and the die is subjected to a pure radial drawing.

The portion of the work piece around the corners of punch and die are subjected to bending operation.

The portion of the job between the punch and die walls undergoes a longitudinal drawing.

Analysis of Deep drawing

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Neglecting friction between job and die wall, drawing force F can be obtained from :

F=2ptσ

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Bending is one of the most common industrial forming operations. We merely have to look at an automobile body, appliance, paper clip, or file cabinet to appreciate how many parts are shaped by bending. Furthermore, bending also imparts stiffness to the part by increasing its moment of inertia

Bending

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Analysis of bending

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It is the length of the neutral axis in the bend, . This determines the blank length needed for a bent part. It can be approximately estimated from the relation

Lb = a ( R + k t )where, Lb = bend allowance (mm)a = bend angle (radian)R = bend radius (mm)t = thickness of sheet (mm), andk = constant, whose value may be taken as 1/3 when R < 2t, and as 1/2 when R ³ 2t.

Bend Allowance

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There are two general types of die bending : V – die bending and wiping die bending. V – die bending is used expensively in brake die operations and stamping die operations. The bending force can be estimated from the following simple relation.

P = k.Y.L.t2 / Dwhere P is bending force, g is the yield stress of the material, L is the bend length ( bend allowance ), t is the sheet thickness, D is the die opening and k is a constant 

Bending Force

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 Punching or blanking is a process in which the punch removes a portion of material from the larger piece or a strip of sheet metal.

If the small removed piece is discarded, the operation is called punching,

whereas if the small removed piece is the useful part and the rest is scrap, the operation is called blanking

Blanking/Punching

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 The clearance between the die and punch can be determined as c = 0.003 t. t where t is the sheet thickness and t is the shear strength of sheet material. For blanking operation, die size = blank size, and the punch is made smaller, by considering the clearance.

          The maximum force, P required to be exerted by the punch to shear out a blank from the sheet can be estimated as

P = t. L. t          where t is the sheet thickness, L is the total length

sheared (such as the perimeter of hole), and t is the shear strength of the sheet material.

CONTINUED…

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It is a process by which a hole is cut (or torn) in metal. It is different from punching in that piercing does not generate

a slug. Instead, the metal is pushed back to form a jagged flange on the back side of the hole.

A pierced hole looks somewhat like a bullet hole in a sheet of metal.

Piercing

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