sheet metal forming

SHEET METAL FORMING

VINAY

Other Cutting OperationsOther Cutting Operations

• Cutoff– Each cut is a new part

• Parting– 2 cutting edges– Less efficient

• scrap

• Slotting– punching a whole

• Perforating– punching of a pattern

• Notching– edge portion removed

• Trimming,Shaving and Fine Blanking– operations to clean up

and smooth out edges

Sheet Metal Forming

• AGENDAAGENDAIntroductionSheet Metal characteristicsShearingPiercing and BlankingDie typesMachines

1.0 Introduction

Sheet metal is simply metal formed into thin and flat pieces. Sheet metal is essentially metal pressed into sheets. These sheets are used at various places.

These sheets can be bent, cut and molded into any shape for use anywhere. Sheet metal is generally produced in sheets by reducing the thickness of work piece by

compressive forces applied through a set of rolls.

Sheets

Sheet Metal Forming

How to differentiate a sheet and a plate ?

• If thickness is less than 6 mm (1/ 4 inches) then it is regarded as sheet.

• If thickness is greater than 6 mm (1/ 4 inches) then it is regarded as plate.

Applications of sheet metals

• Aircraft Bodies

• Automobiles bodies

• Utensils used for domestic purposes

• Beverage cans

Sheet Metal Operations

• Shearing

• Blanking

• Bending

• Stretch forming

• Deep drawing

• Redrawing etc

2.0 Sheet metal characteristics

• Sheet metal is characterized by high ratio of surface area to thickness.

• Forming is generally carried out in tensile forces• Decrease thickness should be avoided as far as

possible as they can lead to necking and failure.• The major factors that contribute significantly

include elongation, anisotropy, grain size, residual stresses, spring back, and wrinkling.

ElongationA test to measure the ductility of a material. When a

material is tested for tensile strength it elongates a certain amount before fracture takes place. The two pieces are placed together and the amount of extension is measured against marks made before starting the test and is expressed as a percentage of the original gauge length.

Materials like low carbon steels exhibit a behavior called yield point elongation, exhibiting upper yield and lower yield points.

Higher elongation leads to Lueder’s band or strain marks

To avoid this reduce yield point elongation by reducing the thickness of sheet by 0.5% to 1.5% by cold rolling, known as temper rolling.

LUEDER’S BAND or STRAIN MARKS

ANISOTROPY

• They may be two types i.e. crystallographic anisotropy (grain orientation) and mechanical fibering (alignment of impurities, voids, inclusions). The anisotropy present in plane of the sheet is called planar anisotropy, and the anisotropy present in thickness direction is called normal anisotropy.

R = εw / εt

normal

anisotropy

Anisotropy

• Planar anisotropy R’ is given by

R’ = (R0 + 2R45 + R90) / 4

where, subscripts 0, 45, 90 refer to angular

orientation of the specimen with respect to rolling

direction of sheet. • Grain size : coarser the grain rougher the surface finish

Generally ASTM grain size of No. 7 is preferred for general sheet metal forming.

Residual stresses :

• Residual stresses can develop in sheet metal forming due to non uniform deformation that take place.

• When disturbed such as by removing a portion of it, the part may distort.

• Tensile residual stresses can lead to stress corrosion cracking of the part unless it is properly relieved.

Spring back

• Because sheet metal generally are thin and are subjected to relatively small strains during forming, sheet metal parts are likely to experience considerable springback.

• This effect is particularly significantly in bending and other forming operations where the bend radius to thickness ratio is high, such as in automotive body parts.

Spring back (wiping die)

Wrinkling

• Compressive stresses are formed in plane of the sheet results in wrinkling (buckling).

The tendency of wrinkling increases with Unsupported length of sheet metal, Decreasing thickness, Non uniformity in thickness, Lubricants trapped can also contribute to wrinkling.

SHEARING

• Shearing is a process for cutting sheet metal to size out of a larger stock such as roll stock.

Shearing Machine

Shearing

• Material thickness ranges from 0.125 mm to 6.35 mm (0.005 to 0.250 in). The dimensional tolerance ranges from ±0.125 mm to ±1.5 mm (±0.005 to ±0.060 in).

• The shearing process produces a shear edge burr, which can be minimized to less than 10% of the material thickness.

• The major variables that affect the shearing are punch force, speed of the punch, lubrication, The hardness of punch and die materials, the corner radii of the punch and the clearance between die and punch.

Variables affecting shearingShear force

• Shear force is basically the product of shear strength of sheet metal and the cross sectional area being sheared

• maximum force can be obtained only when maximum speed is derived or vice versa.

Fmax = 0.7 * UTS * t * L Where, UTS – ultimate tensile strength t – Thickness L – Total length of sheared edge (For round hole of diameter D, L = π * D)

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Engineering Analysis (cont.)Engineering Analysis (cont.)

• Cutting Forces (F)– F = S * t * L

• S - shear strength• t - thickness• L - length of cutting edge

– F = 0.7TS * t *L• TS - Ultimate tensile strength

– Max F is used to determine press for operation

HARDNESS, BURR, RADIUS ON CUTTING SURFACES

• There should be optimum hardness, more than required hardness would make it brittle, and lesser would not withstand higher force .

• Burr are small projection that are formed on outer surface of a formed component. These are undesirable as they increase NVA operations, cost, time.

• Small radius should be given to cutting surfaces to impart strength, it should be optimum

CLEARANCE = THE MEASURED SPACE BETWEEN THE MATING MEMBERS OF A DIE SET ( C )

P

d

d1

C / 2C / 2

D D

Clearance between Punch and Die or Clearance between two cutting blades

Engineering AnalysisEngineering Analysis

• Clearance (C)– Distance between punch and die– 4%-8% of sheet thickness– Small Clearance

• double burnishing• large cutting force

– Large Clearance• Sheet becomes pinched• excessive burr

CLEARANCE = C = d – d1

therefore CLEARANCE PER SIDE = C= C / 2

ALSO CLEARANCE PER SIDE C / 2 IS GIVEN BY

c /2 = 0.01* s* sqr (Tb)S=material thickness Tb= shear stressGenerally clearance is of 5 to 10% of material

thickness.

CLEARANCE

Clearance – its effects

• Greater clearance – rough edges, formation of burr, material is pulled rather being cut

• Smaller clearance – though has good cut edges – forces required is more – also tool withdrawal is difficult .

• Tool and die materials : generally steels are used, ex:- Die steel, HSS for most common operations.

• For higher production carbides are employed.

Scraps : amount of scrap can be a high as upto 30% of original sheet. This can be reduced by proper nesting.

• Die cutting

• Slitting

• Nibbling

• Steel rules

DIE CUTTING

• It is combination of following operations• Perforating.• Parting or shearing a sheet into two.• Notching or removing pieces in edges.• Lancing or leaving a tab with removing any

material

SLITTING

• Shearing operation –using circular blades• These blades follow straight line or circular path or

curved path – depending upon requirement.

NIBBLING• Nibbler (tool) move straight up and moves down

rapidly.• Sheet metal is fed in gap between two cutting

tools and produces overlapping holes• Suitable small runs, several intrcate shapes can

be produced.

http://www.arrl.org/news/features/2004/03/16/1/nibbler-lrg.jpg

PIERCING

• It is a shearing operation.

• Creates open hole in sheet metal by separating the interior section.

• Removed metal is discarded as scrap

PART

slug

BLANKING:

• It is also a shearing operation.

• It enlarges earlier pierced hole.

• Removed metal is desired one.

PARTPART

SLUG

• Very smooth and square edges can be obtained by fine blanking.

DIE TYPES

• Open type

• Compound types

• Progressive types

• Transfer type

Dies and PressesDies and Pressesfor Sheet Metal Processesfor Sheet Metal Processes

• Dies– Components of Dies (picture) – Types

• Simple- single operation with a single stroke• Compound- two operations with a single stroke• Combination- two operations at two stations• Progressive- two or more operations at two or

more stations with each press stroke, creates what is called a strip development

Open type

Compound type

Progressive type

Transfer type

Dies and Presses Dies and Presses for Sheet Metal Processesfor Sheet Metal Processes

• Presses– Definition- In sheet metal working it is a

machine tool with a bed and powered ram that can be driven toward and away from the frame to perform various cutting and forming operations

Dies and PressesDies and Pressesfor Sheet Metal Processesfor Sheet Metal Processes• Solid- one piece construction,up to 1000 tons

• Adjustable bed-accommodates different die sizes

• Open Back-tilted for easy removal of stampings

• Press Brake-wide bed for use of various dies

• Turret-suited for sequence of punching and notching

Straight Sided Frame-high tonnage presses (4000), greater structural stability

Power Systems• Hydraulic-driven by a piston/cylinder system

• Mechanical-eccentric,crankshaft,knuckle joint

CRANK PRESS

FRICTION PRESS

HYDRAULIC PRESS

SHEET METAL FORMING

CONTENTS

BendingStretch formingDeep drawing

ANGAD S NAIK

BENDING

• Most commonly used metal forming process.

• Used – form parts such as flanges curls seams and corrugation

• To impart stiffness by increasing moment of inertia

• Here straight length is transformed into curved length

Bending Processes

ME 4210: Manufacturing Processes and Engineering 4 Prof. J.S. Colton

Terminology used in bending• Bend allowance – length of neutral axis in bend

area.• Used to determine blank length and bend part.

Lb = α (R = kt)

Where = bend angle in radians

R = bend radius

k = constant

t= thickness of sheet

For ideal case k = 0.5

αα

Terminology used in bending

MINIMUM BEND RADIUS

As load is applied compression – inner fiber, tension –outer fiber.

Theoretically strains are equal eo = et

But due to shifting of neutral axis length of bend is smaller in outer surface hence there is difference in strains.

eo = e t = 1 / ((2R /t)+1).

MINIMUM BEND RADIUS• As R/ t increases tensile strain decreases

increases in outer fiber and material may crack a certain strain

• The radius R @ which crack appears on outer surface is called minimum bend radius

• It usually expressed in 2t, 3t, 4t……….

• If R / t approaches 0 then it can completely bendable ex: paper. This characteristic is called bend ability.

Bendability


Factors affecting bend ability

• Bend ability can be increased by increasing tensile stresses

• It can be increased by increasing temperature

• It can be increased by increasing pressure.

• It can be increased by increasing compressive stresses in the plane of the sheet and minimizing tensile stresses in outer stresses.

Bend length L

• As the length increases outer fiber change from uniaxial stresses to biaxial stresses.

• Reason for this: L tends to become smaller due to stretching of outer fiber.

• Biaxial stretching tends to reduce ductility, increases chances of failure.

• As L increases minimum bend radius also increases

Edge condition

• Edges - being rough bend ability decreases.

• Removal of cold work regions such as shaving, machining, or heat treating greatly improves the resistance to edge cracking.

Spring back

• Plastic deformation is followed by elastic recovery upon removal of load this recovery is SPRING BACK.

• This is shown in figure NEXT SLIDE

• Can be observed in short strip metal.

• It can occur in any cross section.

Spring back (wiping die)


Bending Operations


Common bending operations

PRESS BRAKE FORMINGSheet metal can be bent using simple fixtures,

and presses.Press brake is usually machine that is used.This M/C uses mechanical or hydraulic press,

usually suitable for short runs and can be automated.

Die materials can of wood, steels ,carbides .

Press Brake


AIR BENDING• Here only one die is used

• Bending is carried out with a pair of rolls, the larger one is made of polyurethane.

• Upper roll pushes into flexible lower roll.

ROLL BENDING•Here three rolls is used, adjusting distance between three rolls produces various curvatures.

•Here short pieces can also be bent.

ROLL-bending


Air Bending



ROLL BENDING

Common bending operations

• BEADING – here sheet is bent into cavity of the die, improve stiffness, imparts moment of inertia of edges.

• FLANGING – here sheet edges are bent @ 90 deg. This causes hoops stress, if excessive bent then wrinkling chances are more – leads to cracking

STRETCH FORMING

• SHEETS ARE CLAMPED AROUND ITS EDGES AND STRECHTED OVER A DIE.

• Can be moved upwards downwards sideways depending upon requirement.

• Primarily used to make aircraft wing skin, automobile door panels and window frames.

• Although used for low volume prod. It is versatile and economical.

Stretch Forming


STRECTH FORMING

• In most operation blank is rectangular sheet, clamped along narrow edges and stretched length wise.

• Controlling amount of stretch is important to avoid tearing.

• This process cannot produce parts with sharp edges

• Dies- zinc alloys, hard plastics, wood.

DEEP DRAWING

• Used to shaping flat sheets into cup shaped articles.

• This is done by placing blank of appropriate shaped die and pressed into with punch.

Deep Drawing


Cracks

• It is regarded as the ultimate defect.

• Development of cracks destroys its structural integrity.

Buckling or wrinkling• Wrinkling of the edges results in buckling of

the sheet due to high circumferential compressive stresses.

• If a blank diameter is too high punch load will also rise which may exceed critical buckling value. this may lead to failure of sheet .

• To prevent this it is necessary to have sufficient hold pressure to suppress the buckling.

Wrinkling

• Compressive stresses are formed in plane of the sheet results in wrinkling (buckling).

The tendency of wrinkling increases with Unsupported length of sheet metal, Decreasing thickness, Non uniformity in thickness, Lubricants trapped can also contribute to wrinkling.

SURFACE DEFECTS

• SINCE SHEET METAL IS CHARACTERISED BY HIGH SURFACE AREA – SURFACE IS PRONE TO DEFECTS.

• Susceptible to surface blemishes, peeling of surface also known as orange peeling – it is mostly occurs in sheets having large grain size.

• This can be corrects by using sheets having smaller grain size.

Stretcher marks or worms• This is regarded another serious defect.• This defect is characterized by flame like patterns or

depressions on the surface.• These depressions first appear along planes of shear

stresses, they continue growing as they join – they give depression like surface or rough surface.

• This directly related to yield point elongation.• Main difficulty – it appears in regions where strain is less

than yield point• The remedy is to give sheet metal a small cold reduction,

temper rolling, cold works can reduce the defect.To avoid this, reduce yield point elongation by reducing the

thickness of sheet by 0.5% to 1.5% by cold rolling, known as temper rolling.

LUEDER’S BAND or STRAIN MARKS

EARING• Here directional properties are essential.• This usually occurs in deep drawing

processes.• It is formation of wavy edges on a top of a

drawn cup – necessitates extensive trimming.

• It is related to planar isotropy, can be co- related as R =R0 + R90 -2 * R45

• Subscripts represents degree of orientation of fibers.

BURR • This defects is usually seen in shearing and

blanking operations

• These are real productivity killers.

• They not only increase time but cost to deburr.

• Deburring operations add no value to process.

• This is related to clearance between cutting edges

• This can be reduced by decreasing the cutting clearances but note that clearance can be decreased without increasing cutting forces.

sheet metal forming

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