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

B L A N K IN G

P IE R C IN G

N O TC H IN G

TR IM M IN G

L A N C IN G

C U TTIN G

B E N D IN G

F L A N G IN G

H E M M IN G

B E N D IN G

S TR E TC H F L A N G IN G

S H R IN K F L A N G IN G

R E V E R S E F L A N G IN G

H O L E F L A N G IN G

E m b oss in g /F orm B ead s

JO G G L E

F O R M IN G

C Y L IN D R IC A LC U P

D R A W IN G

R E C TA N G U L A RS H E L L

D R A W IN G

IR R E G U L A RS H A P E

D R A W IN G

D R A W IN G

P R E S S W O R K IN G O P E R A TIO N S(S H E E T M E TA L )

THEORY OF CUTTINGTHEORY 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

• Penetration

• Fracture

From both punch side and Die side simultaneously

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

SHEET METAL CUTTING TERMINOLOGY

VARIOUS CUTTING OPERATIONSVARIOUS CUTTING OPERATIONSBLANKING It is the operation of cutting

or shearing a piece out ofsheet to a predeterminedcontour

PIERCING It is the operation ofproducing the holes of roundor contoured shape in theblanks or in formed parts.

NOTCHING In Notching operation a smallpiece or pieces of metal areremoved from the edges/corners of a Strip or a Blank

TRIMMING It is the operation of cuttingoff the excess metal after theDrawing or Formingoperation

LANCING It is the operation of slittingon three sides and openingout by bending the lug aboutan axis on the uncut fourthside with in the blank.

THEORY OF BENDING AND SPRING BACKTHEORY 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 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 as shown in the fig.

• 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 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.

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

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

VARIOUS BENDING OPERATIONSVARIOUS BENDING OPERATIONS

SIMLPEBENDING

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

FLANGING It is similar to above in which the height ofbend is shorter compared to the overall sizeof the part. It strengthens the edges of sheet metal

parts It provides flanges required for

assembling parts by spot welding or anyother joining processes.

HEMMING Hemming is an operation in which the edgeof a component gets folded by 1800 . 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 BENDINGDESIGN CONSIDERATIONS FOR BENDING

Radius of bend T

Minimum height of bend 3T

Relief Notches for bend

DESIGN CONSIDERATIONS FOR HEMMINGDESIGN CONSIDERATIONS FOR HEMMINGWhere appropriate provide an offset To minimise area of fitting To give additional rigidity to inner

panel To reduce potential assembly distortion

Height of flange after

900 flanging = 9.0

+/- 1.0

( except at corners and feature lines)

Maximum permissible radius of bendduring 90

0 flanging = 0.5T

Preferred radius of bend during 900

flanging = 0 (or as minimum aspossible)

THEORY OF FORMINGTHEORY 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.

• 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

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

• 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 OPERATIONSVARIOUS TYPES OF FORMING OPERATIONS

VARIOUS TYPES OF BEADSVARIOUS TYPES OF BEADS

DESIGN CONSIDERATIONS FOR BEADSDESIGN CONSIDERATIONS FOR BEADS

ANGULARBEADS

V-BEADS

BEAKS

THEORY OF DRAWINGTHEORY OF DRAWING• Drawing is a process in which the punch 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.

• 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 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 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 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 one so as to avoid wrinkling, tearing, thinning etc.

DESIGN CONSIDERATIONS FOR CYLINDRICAL CUP DRAWINGDESIGN CONSIDERATIONS FOR CYLINDRICAL CUP DRAWING Empirical Rule to decide required

no. of Draws for Cylindrical cup (Where h = inside height of shell

d = Mean dia. Of shell)

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

Drawing Force for Cylindrical Cup (Fd )

Fd = d t (Su + Sy)/2 Kg.

Where d = Punch diameter (mm.) Su = Ultimate Tensile strength (Kg/mm2) Sy = Yield strength (Kg/mm2) T = Thickness of sheet. (mm.)

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

Empirical Rule for Punch and Die radii.

Punch Radius = 5 t Die radius = 6 t

DESIGN CONSIDERATIONS FOR RECTANGULAR SHELL DRAWINGDESIGN 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)

If =h/r No. of Draws Up to 7 --------- 1 7 -- 13 --------- 2 13 -- 18 --------- 3 18 -- 24 --------- 4

Drawing Force Rectangular Shell (Fr )

Fr = t Su (2r + C1+L C2 ) Kg.

Where Su = Ultimate Tensile strength (Kg/mm2) T = Thickness of sheet. (mm.) r = Corner radius Of shell. (mm.) L = Total length of straight sides ofrectangular 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 DRAWINGDESIGN CONSIDERATIONS FOR IRREGULAR SHAPE DRAWING Empirical Formulae to

analyze the formability of Irregular shaped shells

If e% < 5 , Part can be produced by FormingIf 5 < e% < 30 , Part requires DrawIf e% > 30 , Part cannot be formedIf e% > 5 Wrinkles tend to be shown

Where e% = Percentage of elongation = (L – L0) x 100 L0 = Blank length before forming L = Blank length after forming e% = Difference in percentage of elongation between neighboring sections

General guide lines related to Drawing Operation

Draft angle on draw panel wall = 20 Min. Maximum permissible material thinning = 10% Material utilization Target = 70% and above Drawn corners to be achieved in one Draw