Press Working Operations - Sheet Metal

PRESS WORKING OPERATIONS(SHEET METAL) PRESS WORKING OPERATIONS (SHEET METAL)

CUTTING

BENDING

FORMING

DRAWING

BLANKING

BENDING

STRETCH STRETCH FLA NGING

CYLINDRICAL CUP DRAWING

PIERCING

FLANGING

SHRINK FLANGING

RECTANGULAR SHELL DRAWING

NOTCHING

HEMMING

REVERSE FLANGING

IRREGULAR SHAPE DRAWING

TRIMMING

HOLE FLANGING

LANCING

Embossing/ Form Beads

JOGGLE

THEORY OF CUTTING When cutting sheet metal in a die the forces applied to the sheet by the punch and die are basically shear forces, that is, equal and opposite forces spaced at a small distance apart on the metal and produce the cutting. The cutting or separation of the sheet metal is effected through the following stages of shearing •

Roll over

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Penetration

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Fracture

From both punch side and Die side simultaneously The spacing or the small distance of the two shearing planes is called clearance.It clearance. It will be generally about 8 - 10% of metal thickness for MS Sheets.

SHEET METAL CUTTING TERMINOLOGY

VARIOUS CUTTING OPERATIONS BLAN BLANK KING ING

It is is the the oper operat atio ion n of cutt cuttin ing g or shearing a piece out of  sheet to a predetermined contour

PIERCING

It is the operation of  producing the holes of round or contoured shape in the blanks or in formed parts.

NOTCHING

In No Notchin o eration a sm small piece or pieces of metal are removed from the edges/  corners of a Strip or a Blank 

TRIM TRIMMI MING NG

It is the the ope opera rati tion on of cutt cuttin ing g off the excess metal after the Drawing or Forming operation

LANCING

It is is th the op operation ion of of sl slitti tting on three sides and opening out by bending the lug about an axis on the uncut fourth side with in the blank.

THEORY OF BENDING AND SPRING BACK •

BENDING

Bending is the process of folding a sheet about a straight line axis which lies in the neutral plane. Bends are made in sheet metal to gain rigidity, to produce a part of  o f  desired shape & perform a particular function etc. The cross section of the bend inward from neutral axis is in compression, outward from neutral axis is in tension tens ion as shown in the fig.

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SPRING BACK

During bending the metal nearer to the neutral axis is stressed to the values below the elastic limit.This phenomenon creates a narrow elastic band on both sides of o f the neutral axis.The metal further away from the neutral axis is stressed beyond the yield strength and is plastically deformed and permanently set. When the bending force on the metal is released the elastic band tries to return to the original flat condition but cannot return fully due to the restrictions of the plastically deformed zones . Some slight return does occur as the elastic and plastic zones come to an equilibrium and this return is known as spring back.

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The amount of spring back mainly depends on the ratio of bending radius to stock thickness.

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Spring back can be reduced by over bending, Bottoming or stretch bending.

VARIOUS BENDING OPERATIONS SIMLPE BENDING

The operation of Folding the sheet about a straight-line axis is called simple bending

FLANGING

It is is si similar to to abo abov ve in in whi whicch th the he height of  bend is shorter compared to the overall size of the part.  It strengthens the edges of sheet metal parts  It provides flanges required for assembling parts by spot welding or any other joining processes.

HEMMING

Hemming is an operatio tion in which the edge 0 of a component gets folded by 180 .  It improves the rigidity of the edge  It facilitates joining of two parts as in the case of Bonnet assy. /Door assy.

DESIGN CONSIDERATIONS FOR BENDING Radius of bend



T

Mini Minimu mum m hei hei ht of bend bend

Relief Notches for bend



3T

DESIGN CONSIDERATIONS FOR HEMMING Where appropriate provide an offset  To minimise area of fitting  To give additional rigidity to inner panel  To red reduce uce otential otential assembl assembl distorti distortion on

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Height of flange after 0 +/- 1.0 90 flanging = 9.0 ( except at corners and feature lines)

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Maximum permissible radius of bend 0 during 90 flanging = 0.5T 0 Preferred radius of bend during 90 flanging = 0 (or as minimum as possible)

THEORY OF FORMING •

Forming is the process in which the shape of the punch and die is directly reproduced in the metal with little or no metal flow.

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Forming, Bending or drawing actions may be combined in a die and is classified as Form die or Draw die depending on the dominant action of  process

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The decision to use a form die instead of a Draw die will depend largely on the complexity of the shape & geometrical criteria.

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The use of draw die may be indicated, if the form die would cause the metal to tear because of excessive tensile strain(stretch) or form objectionable wrinkles because of excess crowding of metal etc defects.

VARIOUS TYPES OF FORMING OPERATIONS 

STRETCH FLANGING

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In stretch flanging tensile strain increases from zero at the flange break line (axis of bending) to a maximum at the flange edge. For that reason any tearing will start from the flange edge. The amount of tensile strain increases with increasing forming angles and increasing flanging heights

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In shrink flanging, the tendency to wrinkle increases from zero at the flange break line to a maximum at the flange edge.

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Reverse flanging is the combination of stretch flanging and shrink flanging

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It is the process of forming a flange around a hole

BEADS

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Beads are long narrow depressions embossed in the sheet metal for stiffening purposes

JOGGLE

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It is a stepped surface provided at lap joint.

SHRINK FLANGING

REVERSE FLANGING HOLE FLANGING

VARIOUS TYPES OF BEADS ANGULAR These are used to stiffen the flat BEADS areas in large automotive panels V-BE V-BEAD ADS S

These hese are used used to sti stiffen fen the flat areas of small and medium size automotive panels

FLAT V-BEADS

This is similar to above v-bead but flat at the tip These are commonly used for small parts These are generally used in draw dies for controlli controllin n flow of mater material ial..

ROUND BEADS

BEAKS

These are used to provide rigidity and maintain accuracy in angle of  bend.

DESIGN CONSIDERATIONS FOR BEADS

ANGULAR BEADS

V-BEADS

BEAKS

THEORY OF DRAWING •

Drawing is a process in which the punch p unch causes a flat , precut metal blank in to the die cavity to assume the shape of seamless hollow vessel without excessive wrinkling, thinning, or fracturing.

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METAL FLOW IN DRAWING PROCESS when the punch of draw die forces a portion of metal blank through the bore of draw ring, different forces such as radial tension, circumfrential compression and bending & frictional forces come in to action as shown in the fig.These forces cause a complicated complicated plastic flow of the material in the blank. The volume and the thickness of the component remain constant and the final shape of  the cup will be similar to contour of the punch. The progressive stages of cupping are schematically shown in fig. After a small stroke of the punch, cupping stage A, the metal elements 2, 3, 4, & 5 of the blank move radially toward toward the center of the blank. The flow of these elements go on till the final stage C of the cup is reached. Thus by the end of the draw of the cup area 1 is unchanged in shape and size in the bottom of the cup. The areas 2,3 & 4 change from the shape of angular segments to longer parallel-sided shapes and area 5 also changed in it’s size and shape. Due to this metal flow phenomena ph enomena in Drawing operation, Blank  holding pressure is to be applied on the blank in such a way that the metal flow will be a controlled controlled one so as to avoid wrinkling, tearing, thinning etc.

DESIGN CONSIDERATIONS FOR CYLINDRICAL CUP DRAWING 

Empirical Rule to decide required no. of Draws for Cylindrical Cylindrical cup (Where h = inside height of shell d = Mean dia. Of shell)

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Drawing Force for Cylindrical Cup (Fd)

If   =h/d No. of Draws Up to 0.75 --------- 1 0.75 -- 1.5 --------- 2 1.5 -- 3.0 --------3 3.0 -- 4.5 --------4 Fd =



d t (Su + Sy)/2

Kg.

Where d = Punch P unch diameter (mm.) 2 Su = Ulti Ultima mate te Tens Tensil ilee stre stren n th K /mm /mm 2 Sy = Yield strength (Kg/mm ) T = Thickness of sheet. (mm.) 

Blank Holder Force(F b)

Fb = 1/3 of Drawing force(Approx.) force(Approx.)

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Empirical Rule for Punch and Die radii.

Punch Radius = Die radius =

5t 6t

DESIGN CONSIDERATIONS FOR RECTANGULAR SHELL DRAWING 

Empirical Formulae to decide required no. of Draws for Rectangular shell (Where h = inside height shell r = Corner radius Of shell)

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Dra rawi win n For Force Rect Rectan an ular ular She Shell (Fr)

If   =h/r No. of Draws Up to 7 --------1 7 -13 --------- 2 13 -- 18 --------- 3 18 -- 24 --------- 4 Fr = t Su (2r + C1+L C2 )

Kg.

Where 2 Su = Ulti Ultima mate te Tens Tensil ilee stre stren n th (K /mm /mm ) T = Thickness of sheet. (mm.) r = Corner radius Of shell. (mm.) L = Total length of straight sides of  rectangular shell C1 = 0.5 – 2(lower values for shallow draw Higher values for h/r > 0 C2 = 0.2 – 0.3 ( For easy to sever Draw Conditions)

DESIGN CONSIDERATIONS FOR IRREGULAR SHAPE DRAWING 

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Empirical Formulae to If e% < 5 , Part can be produced by Forming analyze the formability If 5 < e% < 30 , Part requires Draw of Irregular shaped If e% > 30 , Part cannot be formed shells If  e% > 5 Wrinkles tend to be shown

General guide lines related to Drawing Operation

Where e% = Percentage of elongation = (L – L 0) x 100 L0 = Blank length before forming L = Blank length after after forming forming e% = Difference Difference in percentage of  elongation between neighboring sections  Draft angle on draw panel wall = 2 0 Min.  Maximum permissible material thinning = 10%  Material utilization Target = 70% and above  Drawn corners to be achieved in one Draw