Sheet Metal Forming

4. SHEET METAL FORMING

4.1

INTRODUCTION TO SHEET METAL FORMING

Sheet metal forming is a mechanical working process, by which flat sheets of metal are transformed in to curved shapes, generally by various bending operations. Thus, in sheet metal work bending is known as "forming". However, sheet metal work also includes cutting works like punching, shearing, stretching and drawing. Fundamentals of Sheet Metal Forming Processes Most sheet metal forming operations are cold-working processes, and the formability of the metal depends on their fundamental mechanical properties of malleability, ductility and toughness. Many cold working processes involve deformation of the workpiece by a combination of stretching, compression and bending. Generally, metals and alloys which have good malleability and ductility can be easily deformed by these cold working operations. Formability It can be defined el tive ease ith which a iven metal can be formed. The cold formability of a metal is a function of its tensile stren th kombined with ductility. Alow.tensile stren WIt a high ductility is the most desirable condition for cold forming. FOI-metais with high. tensile strength much work will be required in cold forming, and with low ductility, the • metal flow may not be proper which ma~ lead to cracking under forming loads. Cold forming also leads to work hardening and accumulation of rest ua stresses in the formed part which

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241

make the component unfit for use. Hence, many parts after excessive cold forming are subjected to stress relieving operations. Also, when a metal cannot be formed in one operation, and When excessive forming processes are involved stress-relief annealing is carried out in between the forming operations. ~asic Forming Operations Here, the basic forming operations have been discussed briefly before going in detail to give an idea of the operations. Bending: The bendin 0 erations on metals are mainl de end in on the tou hness of the material under formin . In a sim Ie bendin 0 eration to roduce a strai ht flange, one side of the work Riece is deformed intension and the other side in compression, but in industria prooes§.e.S,bending is generally combined with com~essiQA.aJl(j tf~tchia.g.Thus, in bending a curved flange, the flange may be either in compression or tension, depending upon whether it is on the inside or outside of the curved surface. The elements of bending are also involved in other cold working processes like spinning, cupping, rubber forming and stretch forming.

-

-

Shearing: Shearin is a relimina operation for producing blanks for many processes like pressing, dee drawing, coining, embossing, etc. In addition to roducing blanks, shearinq 15also the basis of other processes like piercing, trimming, notching, and parting. Generally, . hard metals can be sheared more easily than soft and ductile metals. Stretching: Basicall ,stretchin 0 eration is i volved in drawing of wires and tubes In sheet metal forming stretching action is used in stretch forming, spinning and embossing. Stretching is the controlled deformation of a material under tension. This is essential in soft metals, which are weak in tension and lead to rupture in deforming processes. Deep Drawing: Deep drawing is a combination of bendin and stretching operation, with little of compression at the upper rim 0 e cup. eep drawing is mainl used for the roduction of honow,shapes i e cups, cylinders, containers, etc. Coining: It is a cold workin rocess which relies mainl on ure com,.p-ression.A meta) to be formed b coining needs a high malleability. Since malleability generallyJn.cr ases with temperature, the processes wIlich depend on the compression action are hot working in nature like rolling, forging and extrusion. Hence, in cold-forming the use of pure compression actions are very rare. In most cold working operations, the compressive stresses are brought into play indirectly, and are complementary to stretching or bending forces, like in drawing. deep drawing, and many bending operations. Classification of sheet Metal Operations In sheet-metal forming all the operations are carried-out generally in cold conditions. All the sheet-metal operations like cold-cutting, cold-bending, cold-forming., etc, are simply referred to as cutting, bending, forming. etc., respectively. The sheet metal operations can be generally classified as-follows-

242


1) Cutting or Shearing a) Blanking . b) Piercing c) Punching d) Trimming 2) Bendinq

e) Shaving f) Notching g) Slitting

a) Angle Bending 3) Forming

b) Roll Bending

a) Curling b) Wiring c) Flanging 4) Drawing

d) Stretch Forming e) Spinning f) Embossing

a) Deep Drawing b) Redrawing

c) Marforming d) Hydroforming

1:1) Lancing

i) Nibbling j) Oinking

g) Coining h) Ironing

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243

4.2

DIES & PUNCHES

The basic tools used in a metal working press are the punch and the die. The punch is the convex male tool which mates with the concave female die. Generally punch'is the moving part. Die is the fixed part which gives a particular shape to the sheet metal under the pressure of the punch. Hold-down ring is used for applying clamping pressure on the sheet metal to prevent the wrinkling of the sheet in operation. Classification

of Dies

Dies used in sheet metal operations can be classified as followsI.

Based on operations performed 1) Shearing dies a) Piercing/punching d) cutting off 2) Bending dies

b) Blanking e) Parting, etc.

c) Perforating

a) Angle bending c) Forming b) Curling d) Plunging 3) Drawing or cupping dies 4) Squeezing dies

II.

a) Coining b) Embossing c) Planishing Based on Construction 1) Simple or Single operation dies 2) Compound dies 3) Combination dies 4) Progressive dies 5) Gang and follow dies 6) Rubber dies

The complete classifications of dies made under the type of operation are single operation dies. Some of the important single operation dies based on the operation are explained here.

244

Manufacturing Process - I

/"1) Single Operation or Simple Dies As the name indicates these dies perform one operation per stroke and are simple in construction ascompared to other dies. These may be blanking dies, piercing dies or shaping dies like bending die curling die, wiring dies, etc. a) Blanking and Punching/Piercing Dies: Both blanking and punching/piercing are cutti!lg operations. Figure 4-1 shows the basic cons ruction of a blanking and punching/piercing die. Punch is fixed to the upper slide of the press. The sheet is held between the stripper plate and the die block, resting against the stop. As the punch descends it cuts the sheetmetal. The punch fits into the die cavity.

c:=; Cuttingedg==I Metalstrip

+ i +

!,.

f=:::Y-'iI:'~of"::'=

I-

pu~ Stripper

'-----''-----'Stop

,---L~u.L..~--+--:~'Gif=!===~ __

Blankfallingthrough die andbolsterplate

Fig. 4-1. Blanking die

Fig. 4-2. Bending die

Cuttingedge

Manufacturing Precess-

I

245

b) Bending Dies: Figure 4-2 shows a bending die. The contour of the punch and die determines

the final shape of the sheet metal.

.

c) Curling Dies: Figure 4-3 shows a curling die, in which the part of the stock is bend to form circular beads around the edge of drawn cylindrical parts. EJECTOR

EJECTOR

,

" \. " '-, ...,.'" "" '\"" "" " .•. .•. '\ " " ' .•. " ...... '. "

.•.

'"-:-........-r--"T""'"""""" "

" " "

"\

'\

'\

Fig. 4-3. Curling die

Curling handling

Fig. ~.

makes the edges of the formed component

Forming die

strong and also makes it safe for

since the sharp eages will De bent into a circular shape. In the curling operation. as

the punch descends of the operation

into the die cavity, the metal is drawn into the cup shape, and at the end

the edge of the work forms itself into a curl because of the shape of the punch

and the solid wire around the drawn part as shown in figure. The ejectors in the die and the punch eject the component

as the punch ascends along with the formed component.

d) Forming Dies: They are similar to bending dies and th~ form or bend the blank along a curved axis instead of straight axis. In forming there is little or no stretching, hence the t~ickness of the metal remains same. Figure4-4 SiiOWS a simple forming die. As the punch descends, the sheet metal assumes the shape in between the-punch and die. The other single operation

dies like perforating

die, cutting die, coining die, embossing

die, etc., are all similar to the above mentioned dies in basic construction for the punch and die shapes so as to achlevo the required related forming operations

will be explained

and operation, except

forming action.

later in this chapter.

Such dies with

246

Manufacturing Process· i

Acompound

Dies

Com ound dies perform two or more operations in a single stroke. Tliey usually perform blanking and piercing operations. Figure 4-5 shows the operation of a compound die for the production of washers. Both blanking and piercing operation are performed in the same stroke. Upper die

Spring

t

Desired port

Stripper for piercing punch Blanking die Piercing punch Scrap strip

Metal strip Blankmg punch Striper for blanking punch Lower die (a)

(b)

Fig. 4-5. A compound die

/3' Combination Dies A combination die-Rerforms a cuttin action followed by a shaping operation like bending, drawing, etc. FigureA-6 shows a combination die for blanking and drawing operations. -This uses a double action press. The upper platen of the die consists of a blanking punch and a drawing punch as shown in figure. The lower platen consists of die cavities to perform blanking and drawing operation in combination with the corresponding punches. Figure 4-6a shows the initial ready position with the sheet metal on the die. As the upper platen descends, first the blanking punch (the other punch) performs the blanking operation and holds the blank in the die cavity (Figure 4-6b). Then, the drawing punch descends and performs draWIng operation (Figure 4-6c). After the operation, the upper platen ascends and ejects the formed omponent out of the punch. 4) Progressive Dies It is a multiple-station

die. It performs

sheet metal during every stroke. Operation forming". An example is a progressive

two or more operations

done by a progressive

at different

points on a

die is called as "Progressive

blanking and piercing die to make a plain, flat washers

(Figure 4-7). As the strip is fed from right to left, the hole for the washer is first punched, and then the washer is blanked from the strip.

247


BLANKING

DRAWING

PUNCH

PUNCH

(b) (a)

Fig. 4-6. A combination

Blanking punch

die

Piercing punch

Scrap

stripper

IS~

Washer Metal strip

-)00

I

o

Fig. 4-7. A Progreuive die

~

(c)

Manulacturing Process -I

248 5) Gang and Follow Dies

These are also called "multiple-station Dies". They consist a number of punches in a single punching head. In a gang die, shown in figure 4-8a, all the punches descend and punch the holes simultaneously, because all the punches are of same length.

PUNCH HEAD

SIMULTANEOUS CUTIING

\~~--====:::!.-~~ (a) GANG DIE

PUNCH HEAD SUCCESSIVE CUTIING

L-,,-..--r----,--.--,r-'

F-L-.----J (b) FOLLOW DIE Fig. 4-8. Gang and Follow dies

In a follow die, shown in figure 4-8b, several punches are arranged at different levels. Thus, as the punch head descends blanking takes place progressively. The longer punch strikes the strip first, next the shorter one and finally the shortest. Since in follow dies. the blanking is progressive, the press can be of lesser capacity compared to those using gang dies. Blanking operation performed using gang dies is called "multiple blanking" and those with follow dies is called "progressive blanking". 6) Rubber Dies This uses a combination of rubber pad as the die cavity and a metal punch. In these dies, the metal punch is kept stationary on the lower side and the rubber pad is made movable type from the upper end. The metal sheet is kept on the punch and the rubber die descends on the punch. The rubber pad takes the shape of the punch and torms the metal sheet to the punch shape.

249


Upper platen

~-Rubber ---Blank

,--l-----'--....,..--Fonn

pad

block

Press bed

Fig. 4-9. Rubber die

The use of rubber die IS explained schematically in figure 4-9.. The rubber dies are also useful for deep drawing operations. The process of deep drawing \'Ising rubber dies is called "Marforming" and explained later in this chapter. DIE ACCESSORIES ~ To achieve the systematic performance from the dies and punches, many accessories are essential. These accessories are necessary to assist in press works like locating the component. holding the metal. removal of the finished part, etc. The important die accessories are discussed briefly here. 1) Stops

Stops are useful for locating the sheet to a proper operating position on the die cavity. Once the stops are set it makes location easier and the press operation faster and accurate. There are two types of stops - a) button stop and b) lever stop. a) Button Stop: Button stop is the simplest and most common type of stop used. The schematic of a button stop is shown in figure 4-10. In this, a small button or pin is fixed to the die block at a predetermined distance. At the end of each operation the plate IS lifted and pushed till the button stops the sheet at the edge of the slot. Also, the button in the die block can be made spring acted type in which case, at the end of each operation, the button can be made to move in, so that the plate moves and the button rises and stops at the slot edge. This makes the spacing of the sheet metal faster and accurate.

Fig. 4-10. Button stop

250


b) Lever Stop: The schematic of a lever stop is shown in figure 4-11. In this case. a lever is operated in conjunction with the punch. In operation, a lever butts against the slot edge when the punch is descending so as to locate the sheet at a predetermined location. After the operation, when the punch starts ascending the lever also rises, so that the plate can be moved forward. As the punch descends again, the lever moves down and locates the plate in position.

~:~~~~~~~:~:~ Fig. 4-11. Lever.top -,

2) Pilots Pilots are used to locate the blankto a correct position, when it is fed by a mechanical" means. The pilot is a conical shaped part which enters into the previously punched holes and aligns the blank to the correct position. This can be finally set with the help of stops for the next operations. The pilots are fitted into the punch holders as shown in figure 4-12.

Fig. 4-12. Pilot

3) Strippers Strippers mamly used for stripping out i.e., discarding the formed part from the die or punch, in addition to acting as a hold down plate. Strippers are usually attached to the punch platens (figure 4-13). The stripper is a plate attached to the punch holder or platen by means of two helical springs. Provision is made in the stripper plate for the movement of the punch through it. In operation, when the punch platen descends, initially the stripper plate presses against the metal sheet on-the die due to spring pressure and thereby acting as the hold down ri!,g. Then the punch descends through the hole in the stripper plate and performs the forming

Manufacturing Pmeess- I

251

action. As the punch ascends the punch moves back, the metal sheet is stripped off from the punch because of the stripper plate and thus avoiding the plate being moved along with the punch.

Fig. 4-13. Stripper

4) Knockouts Knockout is another type of stripper which ejects the punched part from the punch. The use of a knockout in an inverted blanking die is shown in figure 4-14. In this, as the die platen descends, the metal sheet is sheared and the blank rests on the knockout plate which is moved down under spring compression. Due to the spring action, the knockout plate moves up while the die plate is moving up. Also, the punched part which is held in the die cavity is ejected by an ejector fitted inside the die as shown in the figure.

Fig. 4-14. Knockouts

Manufacturing Process - i

252 5) Pressure Pads

Pressure pads, also called as hold down rings are essential in drawing operations. These are necessary to hold down the metal sheet against the die surface. to avoid the wrinkles that would have been otherwise caused by the descending punch. The hold down pressure should not be high enough to cause friction between the pressure pad and the die surface. -_....,..--Punch

~_-Spring

Pressure pad

Fig. 4-15. Pressure pad

Pressure pads are attached to the punch platen with the assistance of coiled springs as shown in figure 4-15. As the punch descends, the pressure pad (hold down ring) first presses against the metal sheet and holds in position. The punch then performs the drawing operation, and the metal sheet moves between the pressure pad and the die surface and avoids the formation of wrinkles. DIE & PUNCH MATERIALS Different materials are used for the manufacture of dies and punches. Various types of cast steels with suitable hardening and tempering are used. The alloying elements used are refractory (high temperature resistant) metals like chromium, tungsten, molybdenum and vanadium which form hard and stable carbides. Some of the commonly used die and punch matenals. their characteristics and applications are explained briefly here. a) High Speed Steels: This material is commonly used for general purpose applications This material has high hot hardness, which can be used under high velocity operating conditions, where high heat is developed due to the operations. For example, punching of small diameter holes in thick metal sheets and also for progressive dies. The typical composition of a high speed steel is 18% tungsten, 4% chromium, 1% vanadium, upto 1% C and balance iron.

Manulacturing Precess- I

253

b) Shock Resistant Steels: These are used for and coining operations which involve high shocks. c) Water Hardening Steels: This is an economical die material and easy to machine. These are useful for low cost dies. and for dies having simple shapes. d) Oil Hardening Steels: These are slightly costlier than water hardening steels. But have good response to heat treatment and can be machined conveniently. Hence these can be used for making dies which involve high machining to make the shape. This can also be used as a low cost die. e) Tungsten Carbide: This is a costly die material but has excellent properties like high wear resistance and hot hardness. It has a low shock resistance because of its brittle characteristics. These materials are used for high production jobs with minimum shocks.

264


"

.

.;-tIII ,~.,

"

r

-----------------------------------------------------------------SHEARING OPERATIONS

4.4

., Sheafiflg is the separation of metal using two blades. There ar.etW9types of sheanng op ationsa) Simple Shearing and b) Die-shearing In simple shearing, the metal is cut along a-straight line usinga qeneralpurpose sheanng machine. In die-sheannq, punches and dies of various shapes are used t~ cut the metals.

0

.••••••

;"':':~:.; ".

~~~~-. •

.

.. o

"

•••.•

.

.

.0·· .....

>,).,

..

,

.

.

.'

.0··

6

.

'

'.

'0

-r

.

•

,

•••••

'0

•

..

Fig. 4-22. Mechanism of shearing

,. ,

'.

Manufacturing Precess- I

265

SIMPLE SHEARING In shearing operation, a narrow strip of metal is severely plastically deformed to the point where it fractures at the surface in contact with the blades. The fracture then propagates inward to provide complete separation. Mechanism of Shearing Figure 4-22 shows the mechanism of a shearing operation. As pressure is applied to the work-piece by the punch (upper blade). maximum stress concentration occurs where the sharp edges of the tools make contact with the metal. Cracks therefore start from these points. and if clearance between the punch and the die is correct the cracks will meet near the centre of the meta1thickness, thus producing a clean cut with a minimum energy. Clearance in Shearing The cle r !we e blades is an important factor in the shearin pro..Qerclearaace tb k that initiate at the ed es of the blades will p a ate through the metal and meet at the centre of the thickness and give a clean fracture as shown in figure 4-23a. With insufficient clear e, shearing produces a rag ed racture (figure 4-23b) and require'S"more energy. With exc ssive clearance there is greater distortion of the edge which contains burr~ ancL arp projections (figure 4-23c) and this also re~uires more energy since more metal is to be plastically deformed.

-

Moving blade Metal

I

/ I I I I I

:.-

Sheared edge

I I I I I

:.-

Clearance (a)

I

I

I I I I I

~

Clearance (c)

Fig. 4-23. Clearance in shearing

Clearance

(b)

266

Manufacturing Process· i

The Clearance allowed between punch and die is largely dependent on the mechanical properties of the metal to be sheared, but is usually in the region of 6-18% of the metal thickness. Thicker and softer sheet metals require a clearance of 10 to 18% of the thickness. while harder metals require about 6 to 10% of the thickness. Harder metals undergo lesser plastic deformation and have a smaller burnished area than soft metals SHEARING

WITH DIES

Shearing operations shaving, notching, etc.

accomplished

Blanking

using dies include

blanking,

piercing.

trimming,

shapes ca led

"blanks",

which are

...,

It is the 0 eration of cutting-out flat intermediate genera Iy further rocesse to pro uce desired shapes.

Blanking separates the blanks from the metal strip or sheet. In operation, the metal strip is fed on to the die. The die has the same dimension (diameter) as that of the required blank. The upper edges of the die are the cutting edges and should be sharp. The cross-section of the punch is slightly smaller than the shape of the desired blanks, in order to provide the proper clearance between punch and die. A stripper is provided to separate the metal strip from the punch after blanking.

Metal thickness

.L

~-I ~ -/1-

I

0

~ a) Blanking

-11b) Piercing

Fig. 4-24. Use of shear

Piercing

This 0 eration iuerformed to make holes of various shapes' a metal lat:.!'e,-"-""""",_ principal difference between blanking and piercing is that the metal portions that are cut-out from piercing are scrap. In piercing the punch has the cross-section same as that of the ole to be pierced and the die siz..elS-Sligh.tly Lar.gertban the hole required, in order to provide proper clearance between the punch and the die.


267

When two or more piercjng Runches are em 10 ed tog~r in a die, their len ths should differ slightly in order to reduce the force and impact required at a time (i.e., progressive dies sh~ld be used). The diameters of the holes to be pierced should be at least twice the metal thickness in order to avoid excessive punch breakage. I

Punching . Punching and piercing are si ilar 0 erations and a o..pwduc etal sh~. The main difference between punchi Q,iercingis that the punching operation is used to roduce a circular hole, while piercing is used to produce holes of different shapes oth-erthan circular ty e. Except for the dies and punch shapes, all other operation principles in both punching and piercing are same. In single operation dies also. progressive sheanng action can be obtained, which requires smaller cutting force. This is achieved in blanking and piercing tools by providing either the punch or the die with ~m angle. usually referred to as "shear", generally about 10 degrees, or equal to the thickness of the metal being sheared. In a piercing operation. where the resultant perforated strip is required in a flat state, the shear will be on the punch (figure 4-24b). In blanking, where the blank is required to be flat, the shear will be on the die (figure 4-24a). In eithe-r case the work piece will remain flat whilst distortion occurs in that part of the material to be scrapped. For thin materials usually flat punch and dies are used. In all cases, a die relief angie of at least 0.75 degree must be provided so that the blank which is cut-out. drops readily through the die. Trimming It IS a secondary operation in which previously formed parts are finished to size, usually by shearin1l ex metal around the periphery. For example, removal of f (ging flash in a press is a trimming operation. Trimming dies are si' 0 anking dies and the parts are forced through the die by a suitable punc Shaving It is also a secondary operation, in whic 5 all amount of metal is removed to obtain smooth, square an close dimensional edges from sheared parts. A minimum of clearance is used for shayi dies. Parts :0 be shaved are placed in the locating recess in the die cavity and when the punch descends the edges are shaved as the part is forced through the die. Notching Notching is the cutting of relatively small indentations in the edges of work pieces. A nibbler of notching die is used to remove metal from the edge of a work piece by notching action. Lancing Lancin is metal forming operation in which a part of the metal is cut and then bent to some degcee, through the attached portion. This operation is used in makinq venti afion openings, handle openings in a box, etc.

-

268

Manufacturing Process -I

Slitting It is a shearing cut which doesn't remove any metal from tae.sheet, The ortion of the cut materia remain attached to the main work sheet. Nibbling Nibbling is a shearing operation in which the material is cut by unchin metalsheet. The nI er conslsfs OT a step punc ana a die. The ti of the the die and the rest of the punch cuts a slug.

a slot in the

Oinking Oinking is a shearing operation in which thin metal sheets are cut using dinking dies. Oinking orrulerdies are made with steel-cutting flexible rule material. Thin aluminium, stainless steel upto 1 mm thick, -leather, celluloid, and cardboards are blanked by this method. SCRAPLAYOUTANOECONOMY An important aspect to be considered in blanking is to minimise the scrap by optimum layout design. This process is also called as "nesting". The layout for cutting an "L" blank is shown in figure 4-25. Figure 4-25a shows an improper design where a lot of material is scrapped, whereas figure 4-25b shows the optimum design in which a material saving of about 40% is possible depending upon the size of the blank. The minimum gap between the edge of the blank and the side of the strip is given as g = t + 0.015h, where t is the strip thickness and h is the strip width. The gap between the edge of two successive blanks b, depends upon the strip thickness, t. Table 8-1 gives the various values of b.

a) Improper layout

T h

,1~~==~~--~~~==~ 9

~---------------L----------------~ b) Optimum layout

Fig. 4-25. Scrap layout & Economy

269

Manufacturing Process· I Table 8-1. Gap Between Successive Blanks Strip thickness. t. mm

Gap.

t $ 0.8 0.8s + ~ 3.2 t > 3.2

b.

mm

0.8 t

3.2

In certain cases. like when a rolled strip is used as the stock. the relative direction of grain flow with respect t the blank is specified. In such cases. the freedom of nesting is restricted to a greater extent. In the case of circular blanks. saving In scrap matenal is achieved only through multiple rows. Figure 4-26 shows the layout for circular blanks. with single and multiple row layout.

~oooo~ a) Single row

b) Double

row

Fig. 4-26. Layout for circular blanks

270

Manufacturing Process· \

4.5 Definitions

BENDING OPERATIONS

used in Bending

In bending, a straight length of metal is worked into a curved length. The common bending operations are the formation of channels. drums, tanks, etc., using pia In sheets. Figure 4-27 illustrates the terms used in bending

Fig. 4-27. Tenns in bending

Bend Radius, R: It is defined as the radius of curvature inside surface of the bend.

on the concave surface,' i.e ..the

Neutral Axis, NA: When a material is bent. the layers at the outer side of the bend are stretched and lengthened, while those on the inside are compressed and shortened At some intermediate layer there will be neither lengthening nor shortening and the length of this layer will remain unaltered upon bending. This layer is called the "neutral axis". In other words, neutral axis is the circumferencial fibre across the thickness at which the strain experienced is zero. In plastic bending the neutral axis does not remain at the half thickness, as in the case of elastic bending. For a sharp bend the neutral axis is closer to the inside than outside of the bend. Bend Allowance, B: When metal is bent, its final length is increased over its original length because the metal thickness is reduced. As the bend radius becomes smaller, the decrease in thickness increases. The developed length of the centre line of the bent section is called as 'Bend Allowance'.

271

Manufacturing Precess- I

The bend allowance is useful for determininq the length of the blank required for making a bend. If it is assumed that the neutral axis has a radius of curvature equal to R+0.45 h, the end allowance is given by, B

=

(R+0.45h)2rr

CJ.

360 where, '( is the bend angle, in degrees. Minimum Bend Radius: For a given bending operation, the bend radius cannot be below a certain value, at which the metal WIll crack on the outer tensile surface. This minimum value of radius, below which the metal may crack is called 'the minimum bend radius'. It is usually expressed in multiples of the sheet thickness. Thus, 4h bend radius indicates that the metal can be bent without cracking up to a radius equal to four times the sheet thickness. Therefore, the minimum bend radius is a forming limit. Minimum bend radius varies between different metals, and always increases with cold working. Very high ductile metals have a minimum bend radius of zero, i.e., they can be flattened upon themselves. However, generally a minimum bend radius of 0.075 mm is used, , In order to prevent damage to dies and punches. To avoid edge cracking in thin sheet metal forming, the minimum bend radius can be increased by polishing the edges of the sheet. Springback in Bendin~

U is the common forming difficulty encountered in all bending operations Springback is the dimensional change of the formed part after the pressure of the forming tool has been released. It is the effort of metal that is not stressed beyond its elastic limit to reshape itself Into its originai position. If the metal nearest to the neutral axis has been stressed below the elastic limit. the metal will try to return to its preformed shapewhen the arming orces are removed. A metal that has been stressed beyond its elastic limit will resist returning to its original shape, but a certain amount of spring back will occur in a formed part~forces causing the spring back are just reverse of the stresses placed on the metal dunnq forming.

272

Manulacturing Process· I

Harder and thicker metals exhibit higher degree of springback. If the bend raoius IS smaller then also springback will be more. The degree of springback depends on the ratio of the angles ar and the temper of the metal (Figure 4-28). Springback factor K is expressed as0.0

K

=

ar 0.0

= (Ro+hl2) Rtth12

Methods to Combat Springback There are three generally used methods for controlling spring back - over bending. bottoming and stretch forming. Overbending: The commonest method to combat springback is to bend the part to a smaller radius of curvature than is desired, so that when springback occurs the part has the proper radius. Bottoming: It consists of striking the metal severely at the bend area. so that the compressive strains are above yield point. Stretch-forming: In this metal is stretched past the elastic limit. and springback IS avoided. Types of Bending Operations Bending operations can be carried-out either using presses or rollers. Bending the sheet metal using rollers is called as "roll-forming". Press brakes are useful in angle bending of longer work pieces, which use standard air-bend dies and sharp-V dies (see Figure 4-21 a and b). In roll forming. the strip is progressively bent into complex shapes by passing it through a series of driven rolls. The thickness of the sheet is not appreciably reduced. Roll forming can be subdivided into two types: a) Cold-roll forming b) Roll bending a) Cold-roll Forming

-.J...

In cold-roll forming (Figure 4-29a) a flat metal strip is fed lengthwise between one or more pairs of forming rolls. These pairs of rolls are mounted in stands in a straight line like a continuous rolling mill. Only straight sections are formed by cold-roll forming. Cold-roll forming is an economical method for forming straight sections in long lengths and in large quantities. The rolls are specially made for the job and hence the tool cost is relatively high. The diameter of rolls is around 10 to 15 ern. The number of roll stands required depends on the type of section, the metal and the thickness of the metal. In this method the strip is overbent to overcomethe springback.

213

Manuiacturing Process - I Secretion after fonning

\

~

Metal strip before fanning a) Cold-roll fonning "

Upper roll + adjustable .•

Lower rolls adjustable •.• b)

Roll bending

c)

Rearroll + adjustable t

Fig. 4-29. Roll fonning

b) Roll Bending Roll bending (Figure 4-29b and c) uses three rolls to form the metal sheets and plates. To start with, the work piece is bent by some other method so that it is easily gripped by rolls for further forming. Even cylindrical shapes can be produced by this method. One or two rolls are adjustable, so that bending can be achieved to the required degree. The process is very fast and suitable for longer work pieces.

274

Manufacturing Process -I

4.6

FORMING OPERATIONS'

CURLING Curling is forming an edge of ~ircular cross section along a sheet or at the end of a shell or tube. Curling dies are used to form circular edges. The curling operation using a curling die is explained under the curling die in this chapter (Figure 4-3). WIRING It is forming a curled edge on a component with a wire or rod inserted within the curled edge (Figure 4-3). Curling and wiring remove the sharp edges and increase the strength of the edges in the components. FLANGING This is a process of folding the edge of a component by 90 degrees to increase the strength. STRETCH FORMING~ Stretch-forming is the process of forming sheet metal by the application of primarily tensile forces in such a way as to stretch the material over a tool or form block. I.Q.stretch forming a tensile load is applied to the work piece so that elastic limit is exceeded and plastic deformation takes place. Stretch forming equipment consists of a ·hy raulically driven vertical ram which carries the punch or form block and jaws for gripping the ends of the sheet. No female die is used in stretch forming. The grips may be pivoted or fixed depending upon the work requirement (Figure 4-30).

Fig. 4-30. Stretch Fonning

275


S h formin£l is used most extensively in the aircraft industry to produce parts of large radius of curvr ture, frequently with double curvature. It is used in coach-building industry also. Although it is applied mainly to the heat-treatable alloys of aluminium, any ductile metal can be stretch formed. Stainless steel. titanium and magnesium alloys are also stretch formed. Advantages 1) Spring back is largely eliminated, because the stress qradient is relatively uniform. 2) It produces high quality works for aircraft, coach and ship industries. Disadvantages 1) Tearing may occur at the location of the maximum stretch. 2) Wrinkles may be formed in large sheets due to buckling. PINNING S Inning is a process used to produce a.. hollow shape by the application of lateral pressure to a rapidly revolving blank, or causing it to assume the shape of a former which IS rotating with It. Simple hand tools are used in this process. The efficiency of this process depends largely upon the skill of the operator in applying pressure to the rotating blank. Deformation of the metal in spinning is a mixture of bending and stretching. This is suitable for shaping ductile metals and alloys.

FORMING '---TOOL

LATHE REST

o

0

Fig. 4-31. Spinning

276

Manufacturing Process - i

Hot and Cold' Spinning If a metal is s un above its recrystallization temperature it is called.a .hnt.spmmn f spinning is carried out below the recrysTaiiization temperature (generally at room temperature) it is ca e as co spinning. Hot spinning is suitable for thick and large components. while cold spinning is suitable for thin sheet metals. The principle of operation and equipments used are same for both methods. Figure 4-31 shows a simple spinning operation. It consists of a lathe in which the blank is held between a chuck and a tail plate, and simple forming tools, generally bars with rounded ends. Former attached to the chuck corresponds to the intemal shape of the finished component and may be made from a hard wood or metal. For simple' components the former may be solid but for re-entrant shape, the former must be segmented to facilitate withdrawai from the finished product. Adequate lubrication is necessary during spinning operation . For smaller works bees wax. tallow or a mixture of lard oil and white lead are used. whilst for larger works soap is used. When the metal reaches its limit of work hardening, the work piece must be annealed Care must be taken that excess work (pressure) is not put into the blank at any point. or undue thinning may take place and lead to fracture of the material. The spinning speeds vary with the type of material, the diameter and thickness of the blank used. For thick metals with larger diameter the spinning speed is slower (250 rprn); while for thin metals. with smaller diameter and high ductility the spinning speed is very high (2500 rprn).

Advantages of Spinning 1) The process is simple and economical 2) Certain complex

as the former and tools are cheap.

shapes can be produced

only by spinning.

3) Conical shapes, difficult to produce by drawing, can be easily produced by spinning.

Disadvantages 1) Spinning

requires skilled labour.

2) The quality and finish of the product depends

upon the skill of the operator.

3) The process is suitable mainly for simple and circular symmetric

shapes.

Applications Typical spinning products are tank heads, armaments in copper instruments, cooking utensils, dairy and chemical plant parts, etc.

and brass.

musical

Flow-turning The recent development of spinning is called the f1ow-tuming in which, operation is carried-out by the machine itself. Figure 4-32 shows the principle of operation of flow-turning. In this process the material is made to flow plastically by pressure rolling it in the same direction of the roller travel. By this method the wall thickness of the component is made thinner than that of original blank.

277


-

Fig. 4-32. Flow turning

-

Advantages of Flow-turning 1) manual skill required is negligible. 2) The process is very fast compared 3) Thick and less ductile materials EMBOSSING

to simple spinning.

can be formed.

[-

Embossing is a process that produces relatively shallow indentations or raised portions w th no change in metal thickness, and is done on relatively thin sections. ss is carried out by means of a punch and a die (Figure 4-33). The operation is a combination of bending and stretching operations. and t e is no lateral flow of metal as In cotninq. This process is used in forming projections as in the case of 'projection welding'.

-

BLANK

(i)

(ii)

Fig. 4-33. Embossing COINING

X

£oining is a cold-forging operation in which deformation takes place mainly by compression. This process Isused for the manufacture of coins, badges, medals, keys, metal plaques, tokens. etc. The process is carried out in a closed die, and there is no provision forthe extrusion of excess metal (figure 4-34). Care must be taken to see that the gauge of the blanks is not excessive, otherwise which may damage the punch and die.

278

Manufacturing Process ·1

(ii)

(i) Fig. 4-34. Coining

The coining pressure depends mainly on the malleability of the metal and the extent to which it should flow. Because of the metal flow in cold conditions. very high pressures are required (around 1500 N/mm2). For this purpose usually heavy duty hydraulic presses are used. IRONING lronin is a reducin[ operation in which thickness of the shell wall is reduced and Its surface IS made smooth. This is done In redrawmq operations to maintain uniform shell thickness In deep drawing or cupping operations, the top rim of the drawn cup tends to become thicker than the bottom of the cup. To make the wall thickness uniform, in redrawing operations the clearance between the die and the punch is to be reduced to the required wall thickness. This process is called "ironing" .

••

279

Manulacturing Process - I

4.7 - DRAWING OPERATIONS In sheet metal forming drawing is generally referred to as "Deep Drawing". It is a process used for shaping flat sheets into cup-shaped articles such as bathtubs. shell cases and automobile parts. Deep drawing is done by placing a blank of appropriate size over a shaped die and pressing the metal into the die with a punch. End of the punch Flat blank

'----P.-'t'r\-'rW~f-- Partially drawn part

I,---------n II

II

" -+! :.--

Clearance

/,,::::

Balster plate

Side walls

(a)

Fig. 4-35. Deep drawing

= = = - = = = = ~:,~

l

Completely drawn part

(b)

Figure 4-35 shows the general arrangement of deep drawing of a cylindrical cup. Usually, a clamping or hold-down pressure is required to press the blank against the die to prevent wrinkling. This is done by means of a blank holder or hold-down ring. However, the pressure of the blank holder must not be so high as to prevent the movement or flow of the metal, but be sufficient to prevent wrinkling. Since the process forms cup-shaped part, it is also known as "Cupping". Drawing is facilitated when the edges of the punch and die have large radii and generous clearance through which flow of metal can take place.

. Manufacturing Process .]

280 REDRAWING

When depth of draw exceeds 60% of the outside diameter of the cup, one or more redrawing operations may be necessary. But, due to cold-working the metal hardens and thus, before being redrawn the cup must be annealed, then pickled to remove scale and finally washed, dried and lubricated. Most metals permit a total reduction of 50 to 80% before annealing. The redrawing operations may be classified as followsa) Single-action redrawing b) Double-action redrawing c) Reverse redrawing or Indirect redrawing The first two redrawing operations are also called as direct redrawing, as in this case the original outside surface of the cup remains the outside surface of the redrawn cup.

(a)

(b)

(c)

Fig. 4-36. Redrawing operations

Figure 4-36a shows a single action redrawmq operation, in which no blank-holder IS used. It requires a low drawing force and It can be used to produce considerable reduction in thick material, as strain hardening is less in this process. Double-action redrawing is suitable for thin metals which tend to wrinkle. This employs a blank-holder as shown in Figure 4-36b. In this, since the metal bends twice, strain hardening is more.

2B1

Manufacturing Process, I

In a reverse redrawing, the cup is turned inside out so that the outside surface of the drawn cup becomes the inside surface of the redrawn shell (Figure 4-36c). There is no problem of wrinkling iii' th~.~process. The redrawing of rectangular sections can be done by this method as the tenqe'ncy of rectangular sections to wrinkle is greater than the cylindrical shells. Continuous Blanking and Cupping Instead of feeding separate blanks for cupping, either by hand or by a mechanical means, it is advantageous to draw the blanks continuously in a single press. This can be done in either single, double or triple action presses.

(b)

Fig, 4-37, Continuous

blanking & cupping

Figure 4-37 shows such a continuous process of blanking and drawing, using a double action press. In this the metal sheet is fed continuously. The blanking punch first shears out a blank into the die (Figure 4-37a). Then the cupping punch operating concentrically within the blanking punch descends down to form the cupping operation, whilst the blanking punch acts as the hold down ring (Figure 4-37b). This process is suitable for fast rate of production.

284

Manufacturing Process-I

Factors Affecting Drawability Some important

factors which affect the drawability

of a metal are-

a) Die Radius: It should be about 10 times the blank thickness.

b) Punch Radius: A sharp radius leads to local thinning and tearing. This also increases the frictional forces to bend and unbend, thus reducing the drawability. c) Punch & Die Clearance: To achieve a good drawability the clearance between the punch and the die should be about 20 to 40 percent greater than the blank thickness. d) Hold-down Pressure: For better drawability down pressure

and reduce the frictional force, the holdshould be about 2% of the average of the ultimate and the yield stresses.

e) Lubrication: Lubrication at the die side reduces the frictional hence improves the drawability.

forces to a great extent.

DEFECTS IN DEEP DRAWN AND OTHER FORMED PRODUCTS The various follows-

defects

that can arise in deep drawn and other formed .

products are as

1) Separation of Bottom: In this, the cup bottom IS separated ·from the rest of the cup at the location of greatest thinning near the punch radius. This defect may be minimized by reducing the thinning by using a larger punch radius or by decreasing the punch load required for drawing operation. 2) Radial Cracks: In the flange or the edge of the cup cracks may occur in a drawn cup. This defect occurs 'due to a low ductility. This defect is more likely to occur in redrawing without - annealing. 3) Wrinkling

or Puckering:

This defect at the flange or the edges of the cup results from buckling of the sheet due to high circumferencial compressive stresses. This can be prevented by the use of sufficient hold down pressure to suppress buckling.

4) Earing: Earing is the uneven deformation of a blank at its edges. It is caused due to directional properties of the metal, in which the metal deforms more easily in some directions than in others, thus fonning ears on the drawn parts. Earing can be minimized either by avoiding excessive deformation or by varying the shape of the blank. 5) Orange peeling: It is the formation of a rough surface after appreciable deformation. This roughness resembles to some extent the surface of an orange, and hence he name orange-peel effect. This defect can be corrected by using fine grain size sheet metal, which deform easily to give a smooth surface. 6) Stretcher strains or Worms: This is a serious surface-defect found in low carbon sheet steels. This defect shows up as a flame like pattern of depressions in the surface, causing a rough surface. The usual solution to this problem is to give the sheet steel a small cold reduction.

285

Manufacturing Process· I MARFORMING

One of the most expensive items in a deep drawing operation is the cost of the tools and in particular the die. The solution is the use of 'Guerin Process'. originally known as "Marforming". In this the die is replaced by a rubber pad, which under high pressure conforms to the contour of a die block. This process is helpful in the manufacture of shallow forms at cheaper costs. Figure 4-38 illustrates a marforming process. The rubber pad acts as a universal die, which shapes itself to the punch as drawing proceeds and pressure is applied. Thus the tools required are cheap and simple to manufacture. Even a single punch is enough to produce cups of different ()auges of sheet metal, with same internal diameter.

Fig. 4-38. Marfonning

In order to overcome wrinkling or puckering of the metal at the edges of drawn cups due to lack of support, a steel pressure plate is used to surround the punch (see in figure). For marforming operation, a rubber pad and a hydraulic unit can be fitted to any ordinary single acting hydraulic press of adequate duty. HYDROFORMING In this process the solid rubber block of marforming is replaced by a rubber diaphragm supported by hydraulic pressure. This diaphragm is sealed into the' press head in order to withstand high hydraulic pressures. When the diaphragm is not in contact with working surface, then the pressure should be released, otherwise the diaphragm would burst. The hydroforming operation is illustrated in Figure 4-39. In Figure 4-39a the blank to be formed is placed on the blank holder ring and the press head is lowered. The press head with zero fluid pressure grips the blank between the rubber diaphragm and the hold down ring. The hydraulic pressure is then raised (Figure 4-39b) inside the press head by means of pumps to

286


a predetermined value. As soon as this pressure is attained, the punch is driven upwards, forcing the metal against the diaphragm. which acts as a universal die (Figure 4-39c). When the punch reaches its maximum height. it is locked in position, and necessary pressure is maintained in the press head till the blank gains the shape of the punch. The punch is then withdrawn from the finished product. which IS left standing on the blank holder ring.

FINISHED COMPONENT

(b) (a)

Fig. 4-39. Hydroforming

Advantages of Hydroforming 1) The cost of the tool is very less. 2) Local thinning of the work piece is very less, as uniform pressure is applied. 3) Bigger reductions per draw are possible. 4) Since the die (rubber diaphragm) is flexible, returned forms and undercut forms can be produced readily. 5) Two blanks can be fed at a time to the press to produce mating parts with good sliding fit. Applications Hydroforming is generally used for the manufacture of many aircraft parts using aluminium alloys. Mild steel and stainless steel components can also be formed by this process.

Sheet Metal Forming

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