Manufacturing Technology II(ME-202)

Sheet Metal Operations

Dr. Chaitanya Sharma

PhD. IIT Roorkee

Title of slide

Lesson ObjectivesIn this chapter we shall discuss the following:

Learning Activities1. Look up

Keywords2. View Slides; 3. Read Notes, 4. Listen to

lecture

Keywords:

Sheet-Metal Forming (SMF) Processes SMF, also called press working, press forming or stamping, is

among the most important of metal working processes.

This processes was known to human as early as 5000 B.C. whenhousehold utensils, jewelry and other objects were made byhammering and stamping metal such as gold silver and copper.

SMF operations are cold working operations which produce awide range of light weight consumer and industrial products oflow cost parts with very high volume and at a fast rate using plate.

SMF involves workpiece with a high ratio of surface area tothickness i.e. plate of thickness less than 5 mm.

SMF products include metal desks, aircraft fuselages, beveragecans, car bodies and kitchen utensils.

Rectangular, large sheets of include black iron, galvanized iron,copper, aluminium, tin, stainless steel, brass, lead, zinc etc.

Various SMF operations include: Shearing, Punching, Piercing,Blanking, Bending, shaving, Deep drawing etc.

Sheet-Metal Parts

(a) (b)

Figure : Examples of sheet-metal parts.

(a) Die-formed and cut stamped parts. (b) Parts produced by spinning.

Characteristics of Sheet-Metal Forming Processes

Stresses In Sheet Metal Operations

Stress Induced Operations

Shearing Shearing, blanking, piercing,trimming, shaving, notching,nibbling.

Tension Stretch forming

Compression Coining, sizing, ironing, hobbing

Tension & Compression

Drawing, spinning, bending,forming and embossing

Shearing Process Shearing process involves cutting sheet metal and other objects

into individual pieces by subjecting it to shear stress in thethickness direction, typically using a punch and a die.

This is similar to paper punch in action.

Shearing usually starts with formation of cracks on both the topand bottom edges of the work piece. These cracks meet eachother and separation occurs with rough fracture surface.

The punch and die may be of any shape circular, straight bladeetc.

The important process variables are Punch force, Speed ofpunching, edge conditions of the sheet, punch and diematerials, corner radii of punch and die, lubrication andclearance.

Clearance determine the shape and quality of sheared edge.

Shearing with a Punch & Die

Figure 16.2 (a) Schematic illustration of

shearing with a punch and die, indicating

some of the process variables.

Characteristic features of (b) a punched

hole and (c) the slug

Punch force, F 0.7TL UTS

Shearing is a sheet metal cutting

operation along a straight line

between two cut-ting edges.

Metal is brought to plastic stage

by pressing between two shearing

blades which initiates fracture at

cutting points.

The fracture progresses on either

side of the sheet further progress

downward resulting in separation.

Clearance & Its Effect Clearance is the gap between die and punch as indicated by “c” in fig.

Clearance determine the shape and quality of sheared edge. Asclearance increases the edges becomes rougher and deformation zonebecomes larger.

Furthermore, metal is pulled into the clearance area, and the shearededges become more and more rounded.

In fact. If the clearance is too large the sheet metal is bent and thussubjected to tensile stresses.

Generally, clearance range between 2 to 8 % of the sheet thickness butmay be as small as 1% in fine blanking.

Clearances are smaller for soft metals and they are higher as the sheetthickness increases

Clearance & Its Effect

Effect of the clearance on Shearing

Figure : (a) Effect of the clearance, c, between punch and die on the deformation zone in

shearing. As the clearance increases, the material tends to be pulled into the die rather

than be sheared

(b) Microhardness (HV) contours for a 6.4-mm (0.25-in.) thick AISI 1020 hot-rolled steel

in the sheared region.

Shearing operations In punching the sheared slug is discarded while in

blanking the slug is the part itself and rest is the scrap.

Following are the operations based on shearing process:

Die cutting is used for producing parts for various usesby perforating, parting, notching, slitting, lancing.

Fine blanking

Slitting

Nibbling

SMF Operations Shaving is removing of thin strip of metal along edges to

obtain smooth and straight edges of accurate dimensions.

Perforating is punching a number of small holes in a sheet.

Parting or shearing sheet into two or more pieces usuallywhen the adjacent blanks do not have matching contour.

Slitting is a shearing operation carried out with a pair ofcircular blades

Notching is cutting the metal by punch from the edges

SMF Operations Trimming is removing of unwanted excess material from

the periphery of previously formed component.

Lancing is cutting operation in which a hole is partially cutand then one side is bent down to form a sort of tab.

Fine blanking is used to produce very smooth and squareedges in gears, cams etc.

Nibbling is the operation of making a number ofoverlapping holes using nibbler which moves a straightpunch up and down rapidly into a die. This is used tomake large elongated hole.

The Shaving Process

Figure 16.9 Schematic illustrations of the shaving process. (a) Shaving a sheared edge. (b)

Shearing and shaving combined in one stroke.

Shaving is removing of thin strip of metal along edges toobtain smooth and straight edges of accurate dimensions.

Slitting with Rotary Knives

Figure 16.6 Slitting with rotary knives. This process is similar to opening cans.

Slitting is a shearing operation carried out with a pair ofcircular blades

Shear Angles

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Figure 16.10 Examples of the use of shear angles on punches and dies.

SMF Equipments

SMF equipments includes:

• Forming presses

• Dies

• Punches/Tools

Figure: Equipments in SMF Process

Classification of Presses for SMF

1. Source of power

a) Mechanical Presses

b) Hydraulic Presses

2. Method of actuation of ram (slides)

a) Crank shaft driven presses

b) Eccentric driven presses

c) Toggle driven presses

d) Piston operated presses

e) Cam operated presses

f) Rack & pinion driven presses

g) Screw presses

h) Knuckle joint driven presses

a) Single action presses

b) Double action presses

c) Triple action presses

4. Type of frame

a) Open or C frame presses

b) Closed frame presses

5. Type of work

a) Punching presses

b) Blanking presses

c) Drawing presses

d) Bending presses

e) Forming presses etc.

Presses used in sheet metal forming may be classified according to:

Press Frames

Figure 16.56 (a)-(f) Schematic illustrations of types of press frames for sheet-forming

operations. (g) A large stamping press.

SMF Machines

SMF is carried out either mechanical or hydraulic machines

Mechanical Presses

Energy stored in a flywheel is

transferred to the movable slide onthe down stroke of the press.

Quick action, short stroke.

Hydraulic Presses

Hydraulic energy moves the slides.

Slower action, longer stroke, chance of leakage

Shearing Mechanical M/c

Hydraulic deep drawing press

Presses For SMF Operation

According to number of slides, which can be operated independently of each other action of presses may be:

1. Single action presses One slide

Vertical direction

2. Double action Presses Two slides

The second action is used to operated the hold down, which prevents wrinkling in deep drawing.

3. Triple action presses Two action above the die, one action below the die

Single Action Press

Press brake is single action press, has very long narrow bed.

Used to form long, straight bend in pieces such as channels and corrugated sheets

Tooling For SMFBasic tools used in SMF are punch and die.

Punch : A convex tool for making holes by shearing, or makingsurface or displacing metal with a hammer.

Die: A concave die, which is the female part as opposed to punchwhich is the male part.

Punch and dies are generally made from heat treated high alloy steels.

Punches and DiesPunch and die in stamping

Classification of SMF Dies

In, practice components are produced essentially bycombinations of blanking, piercing, bending or drawingoperation in a certain order, requiring different dies.

Dies may be classified according to:

1. Type of press operationa) Cutting dies

b) Forming dies

2. Method of operationa) Simple die

b) Compound die

c) Combination die

d) Progressive die

e) Transfer die

f) Multiple dies

Types of DiesCompound Dies:

• Several operations can be performed on thesame piece in one stroke of the press.

• Combined processes and create a complexproduct in one shot.

• Used in metal stamping processes of thin sheets.

Transfer Dies:

• Transfer dies are also called compounding type ofdies.

• The part is moved from station to station withinthe press for each operation.

Compound die

Transfer die

Progressive die

• Progressive die optimise the material use

• Determining factor are: Volume of production and complexity of the shape

Simple Die

Simple dies: Perform a single operation (for example,cutting, blanking, or punching) with each stroke of thepress.

Compound Die Compound dies: Perform two or more operations at a single

position of the metal strip. To do more than one set ofoperations, a compound die consists necessary sets of punchesand dies.

During the part of stroke , piercing of holes is done in the stockand further travel, the blanking operation is done.

These are slower than progressive dies in operation. But highertolerance can be achieved than progressive dies. Small strip canbe used .

Figure : Schematic illustrations: (a) before and (b) after blanking a common washer in a

compound die. Note the separate movements of the die (for blanking) and the punch (for

punching the hole in the washer).

Combination Dies

Combination dies: It is similar to a compound die and can perform

more than one operation at one station.

The main difference between combination and compound die is

that that a cutting operation is combine with here non-cutting

operations such as bending and forming.

Progressive Dies Progressive dies: Perform two or more operations simultaneously in

single stroke of punch at two or more positions of the metal strip.

The places where each operations are carried out are calledstations.

These are suited for mass production so that handling cost isreduced.

Figure : (c) Schematic illustration of making a washer in a progressive die. (d) Forming of

the top piece of an aerosol spray can in a progressive die. Note that the part is attached to

the strip until the last operation is completed.

Characteristics of Metals Used in Sheet-Forming

Sheet Metal

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Figure 16.12 (a) Yield-point elongation in a sheet-metal specimen. (b) Luder’s bands in a low-

carbon steel sheet. (c) Stretcher strains at the bottom of a steel can for household products.

Source: (b) Courtesy of Caterpillar Inc.

Cupping Test and Bulge-Test

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Figure 16.13 (a) A cupping test (the Erichsen test) to determine the formability of sheet metals.

(b) Bulge-test results on steel sheets of various widths. The specimen farthest left is subjected

to, basically, simple tension. The specimen farthest right is subjected to equal biaxial stretching.

Source: Courtesy of Inland Steel Company.

Bending Bending is the operation of transforming a straight length into

curved length.

It is used for changing sheets and plated into channels, drums,tanks etc.

During bending outer surface of material is subjected to tensionand inside surface is in compression.

Strain in bent material increases with decreasing radius ofcurvature.

Stretching of the bend causes the neutral axis of the section tomove towards the inner surface

Methods of BendingFollowing are the commonly used method of bending:

V Bending: A wedge shaped punch forces the metal sheet or strip intoa wedge shape die cavity. The bend angle may be acute, 900 orobtuse.

Edge Bending: A flat punch forces the stock against the vertical face ofthe die. Bend axis is parallel to the edge of the die and the stock issubjected to cantilever loading.

U Bending: It is similar to V bending in operation. Punch for U bendingis rectangular.

Spring back in Bending

Figure : Spring back in bending.

RiR f

4RiY

ET

3

3RiY

ET

1

Spring back is the elastic recovery by bent material on the removal ofapplied force.

The part tends to recover elastically after bending, and its bend radiusbecomes larger.

Under certain conditions, it is possible for the final bend angle to besmaller than the original angle (negative spring back).

The spring back for low carbon steel is 1-20 , for medium carbon steel is3-40 and for phosphor bronze and spring steel is 10-150 .

Methods of Reducing or Eliminating Springback

Figure : Methods of reducing or eliminating spring back in bending operations.

Spring back in bending operations may be eliminated by overbending the part by an angle equal to spring back angle.

This can be done by having face of the punch undercut or relieved .

Other Bending Operations

Figure : Examples of various bending operations.

Press Brake

Figure 16.23 (a) through (e) Schematic illustrations of various bending operations in a press brake.

(f) Schematic illustration of a press brake

Bead Forming

Figure 16.24 (a) Bead forming with a single die. (b) and (c) Bead forming

with two dies in a press brake.

Flanging OperationsFigure 16.25 Various

flanging operations. (a)

Flanges on a flat sheet. (b)

Dimpling. (c) The piercing

of sheet metal to form a

flange. In this operation, a

hole does not have to be

pre-punched before the

punch descends. Note,

however, the rough edges

along the circumference of

the flange. (d) The

flanging of a tube. Note the

thinning of the edges of the

flange.

Roll-Forming Process

Figure 16.26 (a) Schematic illustration of the roll-forming process. (b) Examples of

roll-formed cross-sections.

Methods of Bending Tubes

Figure 16.27 Methods of bending tubes. Internal mandrels or filling of tubes with particulate

materials such as sand are often necessary to prevent collapse of the tubes during bending. Tubes

also can be bent by a technique consisting if a stiff, helical tension spring slipped over the tube.

The clearance between the OD of the tube and the ID of the spring is small, thus the tube cannot

kick and the bend is uniform.

Tubular Parts

Figure 16.28 (a) The bulging of a tubular part with a flexible plug. Water pitchers can be

made by this method. (b) Production of fittings for plumbing by expanding tubular

blanks under internal pressure. The bottom of the piece is then punched out to produce a

“T.”

Stretch-Forming Process

Figure : Schematic illustration of a stretch-forming process. Aluminum skins for aircraft can

be made by this method.

Stretch forming uses only male die or form block to produce largecontour sheets accurately for aircrafts wings and fuselage.

The sheet metal is stretched to yield point in tension, and thenwrapped over and around the form block.

This greatly eliminate spring back and require low tooling cost.

Deep-Drawing

Figure : (a) Schematic illustration of the deep-drawing process on a circular sheet-metal blank. The stripper

ring facilitates the removal of the formed cup from the punch. (b) Process variables in deep drawing.

Fmax DpT UTS DoDp

0.7

Drawing is the process of forming a flat piece of material (blank) into hollow cup like

shape by means of punch which causes blank to flow into die cavity.

If the depth of formed cup exceeds the diameter then it is termed as deep drawing.

This is used for making various geometries and sizes such as bottle caps and

automobile panels.

Conventional Spinning

Figure :(a) Schematic illustration of the conventional spinning process.

(b) Types of parts conventionally spun. All parts are axisymmetric.

Shear-Spinning and Tube-Spinning

Figure 16.43 (a) Schematic illustration of the shear-spinning process for making conical

parts. The mandrel can be shaped so that curvilinear parts can be spun. (b) and (c) Schematic

illustrations of the tube-spinning process

Hydroform Process

Figure : The hydro form (or fluid-forming) process

In hydro form (or fluid-forming) process a pressurized liquid behind therubber pad is used to exert force required for forming. The rubber pad acts asseal at the end of liquid container.

When container descends over the punch hydrostatic pressure acts over thesheet equally from all sides forcing the part to be formed to wrapped on tothe punch.

Note that in contrast to theordinary deep-drawingprocess, the pressure in thedome forces the cup wallsagainst the punch. The cuptravels with the punch; inthis way, deep draw abilityis improved.

This is used for deeper cupsof complex shapes withsharp details

Tube-Hydro-forming

Figure : 1 (a) Tube-hydro forming process. (b) Example of tube-hydro formed parts.

(b)

Automotive exhaust and structural components, bicycle frames, and hydraulic and pneumatic fittings

are produced through tube hydro-forming.

Explosive Forming

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Figure 16.45 (a) Schematic illustration of the explosive forming process. (b)

Illustration of the confined method of the explosive bulging of tubes.

Embossing with Two Dies

Figure 16.37 An embossing operation with two dies. Letters, numbers, and

designs on sheet-metal parts can be produced by this process.

Bending and Embossing of Sheet Metal

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Figure 16.39 Examples of the bending and embossing of sheet metal with a metal punch

and with a flexible pad serving as the female die. Source: Courtesy of Polyurethane

Products Corporation.

Aluminum Beverage Cans

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Figure 16.38 (a) Aluminum beverage cans. Note the excellent surface finish. (b)

Detail of the can lid showing integral rivet and scored edges for the pop-top.

(a)

Figure : MFP involved in manufacturing a two-piece aluminum beverage can.

Can Manufacture

Efficient Part Nesting for Optimum Material Utilization

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Figure 16.51 Efficient nesting of parts for optimum material utilization in blanking.

Source: Courtesy of Society of Manufacturing Engineers.

Cost of Conventional Spinning Versus Cost of Deep Drawing

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Figure 16.57 Cost comparison for manufacturing a round sheet-metal container either by

conventional spinning or by deep drawing. Note that for small quantities, spinning is more

economical.