sheet-metal-forming-processes-130114203855-phpapp02.pdf

7/21/2019 sheet-metal-forming-processes-130114203855-phpapp02.pdf

1/34

Nikhil R. Dhar, Ph. DDepartment of Industrial & Production Engineering

BUET

LECTURE-14

SHEET METAL FORMING PROCESSES


2/34

Lecture-05: Sheet Metal Forming Processes 34/2

Introduction

Sheet metal forming is a grouping of many complementary processes that are used to form

sheet metal parts. One or more of these processes is used to take a flat sheet of ductile

metal, and mechanically apply deformation forces that alter the shape of the material. Before

deciding on the processes), one should determine whether a particular sheet metal can be

formed into the desired shape without failure. The sheet metal operations done on a press

may be grouped into two categories, cutting (shearing) operations and orming

operations.


3/34


Cutting (Shearing) Operations

In this operation, the workpiece is stressed beyond its ultimate strength. The

stresses caused in the metal by the applied forces will be shearing stresses. The

cutting operations include:

unching !iercing)

Blanking

"otching

erforating

Slitting#ancing

arting

Sha$ing

Trimming

%ine blanking


4/34


Shearing Operations

Punching (Piercing): It is a cutting operation by which $arious shaped holes are

made in sheet metal. unching is similar to blanking e&cept that in punching, the

hole is the desired product, the material punched out to form the hole being waste.

!lan"ing: Blanking is the operation of cutting a flat shape sheet metal. The article

punched out is called the blank and is the re'uired product of the operation. The

hole and metal left behind is discarded as waste.


5/34


6/34

Lecture-05: Sheet Metal Forming Processes 34/$

Sha%ing: The edge of blanked parts is generally rough, une$en and uns'uare.

(ccurate dimensions of the part are obtained by remo$ing a thin strip of metal

along the edges.&rimming: This operation consists of cutting unwanted e&cess material from the

periphery of pre$iously formed components.

Fine 'lan"ing: %ine blanking is a operation used to blank sheet metal parts with

close tolerances and smooth, straight edges in one step.

Schematic illustrations of sha$ing on a sheared

edge. !a) Sha$ing a sheared edge. !b) Shearing and

sha$ing, combined in one stroke.%ine blanking


7/34

Lecture-05: Sheet Metal Forming Processes 34/

Shearing Dies

Because the formability of a sheared part can be influenced by the 'uality of its

sheared edges, clearance control is important. In practice, clearances usually

range between * and +* of the sheets thickness- generally, the thicker thesheet, the larger is the clearance !as much as /*). 0owe$er, the smaller the

clearance, the better is the 'uality of the edge. Some common shearing dies are

describe below:

Punch an *ie Shapes: (s the surfaces of the punch and die are flat- thus, the punch

force builds up rapidly during shearing, because the entire thickness of the sheet is

sheared at the same time. 0owe$er, the area being sheared at any moment can be

controlled be be$eling the punch and die surfaces, as shown in the following %igure. Thisgeometry is particularly suitable for shearing thick blanks, because it reduces the total

shearing force.

Examples of the use of shear angles on punches and dies.


8/34

Lecture-05: Sheet Metal Forming Processes 34/+

(a) (b)

Schematic illustrations: (a) before and (b) after blanking a commonwasher in a compound die. Note the separate movements of the die(for blanking) and the punch (for punching the hole in the washer).

,ompoun *ies: Se$eral operations on the same strip may be performed in one stroke

with a compound die in one station. These operations are usually limited to relati$ely

simple shearing because they are somewhat slow and the dies are more e&pensi$e than

those for indi$idual shearing operations.


9/34

Lecture-05: Sheet Metal Forming Processes 34/

(a) Schematic illustration of making a washer in a progressive die. (b) Forming of the top pieceof an aerosol spra can in a progressive die.

Progressi%e *ies: arts re'uiring multiple operations, such as punching, blanking and

notching are made at high production rates in progressi$e dies. The sheet metal is fed

through a coil strip and a different operation is performed at the same station with each

stroke of a series of punches.


10/34

Lecture-05: Sheet Metal Forming Processes 34/.0

&ranser *ies: In a transfer die setup, the sheet metal undergoes different operations at

different stations, which are arranged along a straight line or a circular path. (fter each

operation, the part is transfer to the ne&t operation for additional operations.


11/34

Lecture-05: Sheet Metal Forming Processes 34/..

Forming Operations

In this operation, the stresses are below the ultimate strength of the metal. In this

operation, there is no cutting of the metal but only the contour of the workpiece is

changed to get the desired product. The forming operations include:!ening: In this operation, the material in the form of flat sheet or strip, is uniformly

strained around a linear a&is which lies in the neutral plane and perpendicular to the

lengthwise direction of the sheet or metal. The bending operations include:

*raing: This is a process of a forming a flat workpiece into a hollow shape by means

of a punch, which causes the blank to flow into die ca$ity.

Suee1ing: 1nder this operation, the metal is caused to flow to all portions of a die

ca$ity under the action of compressi$e forces.

23bending

4dge bending

5oll bending(ir bending

%langing

6impling

ress break forming

Beading

5oll formingTube forming

Bulging

Stretch forming


12/34


V-bending Edge bending Roll bending

Bending in -slide machine !ir bending

Bending o" Flat Sheet and #late


13/34


Flanging:%langing is a process of bending the edges of sheet metals to 7/o

Shrink flanging 8 sub9ected to compressi$e hoop stress.

Stretch flanging 8sub9ected to tensile stresses


14/34


*impling:

%irst hole is punched and e&panded into a flange

%langes can be produced by piercing with shaped punchhen bend angle ; 7/ degrees as in fitting conical ends its called flanging


15/34


Schematic illustrations of various bending operations in a press brake

Press 'rea" orming: Sheet metal or plate can be bent easily with simple fi&tures using a

press. #ong and relati$ely narrow pieces are usually bent in a press break. This machine

utili


16/34

Lecture-05: Sheet Metal Forming Processes 34/.$

(a) !ead forming with a single die. (b) !ead forming with two dies" in a press brake.

!eaing: In beading the edge of the sheet metal is bent into the ca$ity of a die.

The bead gi$es stiffness to the part by increasing the moment on inertia of the

edges. (lso, it impro$es the appearance of the part and eliminates e&posed sharpedges


17/34

Lecture-05: Sheet Metal Forming Processes 34/.

#oll$formingprocess

oll orming: %or bending continuous lengths of sheet metal and for large

production runs, roll forming is used. The metal strip is bent in stages by passing it

through a series of rolls.

Stages in roll forming of a sheet3metal door frame. In Stage =, the rolls may be shaped as inAor B.


18/34

Lecture-05: Sheet Metal Forming Processes 34/.+

%ethods of bending tubes. &nternal mandrels"or the filling of tubes with particulatematerials such as sand"are often necessar to prevent collapse of the tubes during bending

.Solid rods and structural shapes can also be bent b these techni'ues

$ube Bending

Bending and forming tubes and other hollow sections re'uire special tooling to a$oid buckling

and folding. The oldest method of bending a tube or pipe is to pack the inside with loose

particles, commonly used sand and bend the part in a suitable fi&ture. This techni'ues

pre$ents the tube from buckling. (fter the tube has been bent, the sand is shaken out. Tubescan also be plugged with $arious fle&ible internal mandrels.


19/34

Lecture-05: Sheet Metal Forming Processes 34/.

Bulging

The basic forming process of bulging in$ol$es placing tabular, conical or cur$ilinear part into

a split3female die and e&panding it with, say, a polyurethane plug. The punch is then

retracted, the plug returns to its original shape and the part is remo$ed by opening the dies.

!a) Bulging of a tubular part with a fle&ible plug. ater pitchers can be made by this method. !b)roduction of fittings for plumbing by e&panding tubular blanks with internal pressure. The bottom ofthe piece is then punched out to produce a >T.? !c) @anufacturing of Bellows.


20/34


Stretch Forming

In stretch forming, the sheet metal is clamped around its edges and stretched o$er

a die or form block, which mo$es upward, downward or sideways, depending on

the particular machine. Stretch forming is used primarily to make aircraft3wing skin

panel, automobile door panels and window frames.

Schematic illustration of a stretch3forming process. (luminum skins for aircraft can be

made by this process.


21/34

Lecture-05: Sheet Metal Forming Processes 34/2.

4&amples of the bending and the embossing of sheet metal with ametal punch and with a fle&ible pad ser$ing as the female die.

Rubber Forming

In rubber forming , one of the dies in a set is made of fle&ible material, such as a

rubber or polyurethane membrane. olyurethanes are used widely because of their

resistance to abrasion, long fatigue life and resistance to damage by burrs or sharp

edges of the sheet blank. In bending and embossing sheet metal by the rubber

forming method, as shown in the following %igure, the female die is replaced with a

rubber pad. arts can also be formed with laminated sheets of $arious nonmetallic

material or coatings.


22/34


The hydroform !or fluid forming) process. "ote that, in contrast to the ordinary deep3drawing process, thepressure in the dome forces the cup walls against the punch. The cup tra$els with the punch- in this way,

deep drawability is impro$ed.

%&dro"orm (or) Fluid Forming #rocess

In hydroforming or fluid forming process, the pressure o$er the rubber membrane

is controlled throughout the forming cycle, with ma&imum pressure reaching //

@a !A/// psi). This procedure allows close control of the part during forming to

pre$ent wrinkling or tearing. hen selected properly, rubber forming and

hydroforming processes ha$e the following ad$antages:

#ow tooling cost

%le&ibility and ease of operation

#ow die wear

"o damage to the surface of the sheet and

apability to form comple& shapes.


23/34


$ube-%&dro"orming #rocess

In tube hydroforming, steel or other metal tubing is formed in a die and pressuri


24/34


(a) explosive forming process. (b) confined method of explosive bulging of tubes.

E'plosie Forming #rocess

4&plosi$e energy used as metal forming

Sheet3metal blank is clamped o$er a die

(ssembly is immersed in a tank with water5apid con$ersion of e&plosi$e charge into gas generates a shock wa$e. The

pressure of this wa$e is sufficient to form sheet metals


25/34


pes of structures made b diffusion bonding and superplastic forming ofsheet metal. Such structures have a high stiffness$to$weight ratio.

Di""usion Bonding and Superplastic Forming

%antages

#ower strength is

re'uired and less tooling

costs

omple& shapes with

close tolerances can be

made

eight and material

sa$ings

#ittle or no residualstress occurs in the

formed parts

*isa%antages

@aterials must not be

super elastic at ser$ice

temperatures

#onger cycle times


26/34


27/34


Dra*ing Operations

4&amples of drawing operations: !a) pure drawing and !b) pure stretching. The bead pre$ents the sheet metalfrom flowing freely into the die ca$ity. !c) ossibility of wrinkling in the unsupported region of a sheet in drawing.


28/34

Lecture-05: Sheet Metal Forming Processes 34/2+

Ironing #rocess

If the thickness of the sheet as it enters the die ca$ity is more than the clearance

between the punch and the die, the thickness will ha$e to be reduced- this effect is

known as ironing. Ironing produces a cup with constant wall thickness thus, the

smaller the clearance, the greater is the amount of ironing.

Schematic illustration of the ironing process. "ote that the cup wall is thinnerthan its bottom. (ll be$erage cans without seams !known as two3piece cans)are ironed, generally in three steps, after being deep drawn into a cup.!ans with separate tops and bottoms are known as three3piece cans.)


29/34


Redra*ing Operations

ontainers or shells that are too difficult to draw in one operation are generally redrawn. In

re$erse redrawing, shown in following %igure, the metal is sub9ected to bending in the

direction opposite to its original bending configuration. This re$ersal in bending results in

strain softening. This operation re'uires lower forces than direct redrawing and the materialbeha$es in a more ductile manner.

5educing the diameter of drawn cupsby redrawing operations: !a)

con$entional redrawing and !b) re$erse

redrawing. Small3diameter deep

containers undergo many drawing and

redrawing operations.

" l i


30/34


$op o" !luminum Can


31/34

!l i $ #i B C


32/34


!luminum $*o-#iece Beerage Cans

(luminum two3piece be$erage cans. "ote the fine surface

finish.

# " Sh t + t l


33/34


Schematic illustration of types of press frames for sheet3 forming operations. 4ach type hasits own characteristics of stiffness, capacity, and accessibility.

#ress "or Sheet +etal

ress selection for sheet metal forming operations depends on se$eral factors:

Type of forming operation, and dies and tooling re'uired

Si


34/34


THANK YOU FOR YOUR ATTENTION

sheet-metal-forming-processes-130114203855-phpapp02.pdf

Documents

Transcript of sheet-metal-forming-processes-130114203855-phpapp02.pdf