Sheet Forming Techniques

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SHEET FORMING TECHNIQUES


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INTRODUCTION

High SA:thickness ratio.

But plates also used.

E.g. metal desks, appliance bodies, aircraftpanels, beverage cans, car bodies, kitchenutensils.

Sheet forming also called press working. Producing a blank from a sheet.

shearing


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INTRODUCTION

The thickness or lateral dimensions of the workpiece are intentionally changed in bulkdeformation. In sheet-forming any changes inthickness is due to stretching of sheet undertensile stresses (Poissons Effect).

Thickness decreases should generally be avoidedas they lead to necking & failure.


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FACTORS

Sheet forming involves bending & stretching.Therefore some properties have to be taken careof:

1. Elongation2. Yield-point elongation

3. Anisotropy

4. Grain size

5. Residual stresses

6. Spring back

7. Wrinkling


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YIELD POINT ELONGATION Low-Carbon steels exhibit yield point elongation

of the order of a few %.

This means that when the material yields, it

stretches further in certain regions in thespecimen.

Lueders bands/stretcher strains/worms areproduced. These are elongated depressions on

the surface of the specimen. Objectionable on final part, difficult for coating

operations

0.5%-1.5% temper rolling/skin rolling


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RESIDUAL STRESSES

Residual stresses are stresses that remainafter the original cause of the stresses (externalforces, heat gradient) has been removed.

Present in sheet metals parts due to non-uniform deformation of the sheet duringforming.

By removing a part of it, the part may distort.

Tensile residual stresses may also cause SCC. Spring backmay occur in metal parts.

Difference in stresses. Over bending. Stretchbending.


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RESIDUAL STRESSES

SPRINGBACK


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GAUGE

The thickness of the sheet metal is called itsgauge.

The gauge of sheet metal ranges from 30 gaugeto about 8 gauge. The higher the gauge, thethinner the metal is.


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BENDABILITY

R/T ratio. The difference between inner & outerstrains increases if R/T ratio decreases i.e.sharper bends.

Minimum bend radius is expressed in terms ofthickness T. this is the radius at which a crackwill appear on the outer surface.

Bendability also depends on edge conditions. Anisotropy of sheet metal is important in

bendability. Cutting the blank in properdirection.


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BENDING OPERATIONS Free-bending, where there is only one die. Rolls

are also used to bend sheets. In air bending's truestform, the material touches the tools at only threepoints. used for forming thick gauges (10 gauge andthicker)

Beading is where the edge of the sheet metal can beinto the cavity of a die. Sharp edges are removed &the overall part becomes stiff.

Flanging: is a process of bending the edges at 900(types of flanges in sheet)

Roll Forming is used for bending continuouslengths of sheets by passing the sheet through aseries of rolls. Typical products include gutters,channels, panels, frames etc. Thicknesses usedrange from about 0.125mm-20mm.


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PRESS BRAKE


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BENDING OPERATIONS-TUBES

Press-Brake Forming: used with metal partsat least 7m in length & relatively narrow. Dies inmechanical & hydraulic press.

Bottoming: there is no air between the punch,material being formed, and the die. The materialwill be bent to 90 degrees at the bottom.

Bottoming as defined will not produce anaccurate bend. Spring back.

Bottoming With Penetration (Coining).


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DEEP DRAWING

A flat sheet metal blank is formed into a cylinderor box-shaped part by means of a punch thatpresses the blank in to the die cavity.

Typical parts produced are: beverage cans, pots& pans, containers, sinks etc.

STRETCHING & pure drawing both take place.

High blank holder force can cause stretching.This leads to stretching & necking.

Low blank holder force. Only blank diameter isdecreased as drawing progresses.


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DEEP DRAWING

If the sheet thickness is more than the clearancebetween the punch & the die, the thickness willhave to be reduced.

This effect is known as ironing. Ironingproduces cups with constant wall thickness.Longer cups are also produced.

Clearance between the punch & die are usually

maintained at 7-14% more than the originalthickness of sheet. Wall thickness of cup.

Corner radii of punch & die. Too small causesfractures at corners. Too big causes wrinkling.


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SHEARING MECHANISM A punch & a die. Major variables are: punch

force, punch speed, lubrication, surfacecondition, & materials used for punch & die.

Slug & a sheet Clearance ;Burr; Burr Height;

Burr height increases with increasing clearance& increasing ductility.

Tools with dull edges are also a major factor inburr formation.


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SHEARING

Punching/Piercing

Blanking

Perforating

Slitting

Parting

Notching

Lancing Nibbling

Documents

Sheet Forming Techniques