Conformado de Planchas Italiano

Sheet Metal Forming Processes

Deep Drawing

Deep Drawing• Like shearing but with

clearance > material thickness• The Process

– Place round blank over die– Clamp in place– Punch travels downward into die

• metal deforms by stretching into the die

Deep Drawing

• Advantages– Very high production rates (e.g. cans)– Can make parts no other process can

make• Containers of all shapes and sizes

• Disadvantages– Limited shapes– Material quality critical to obtaining good

parts– Dies are expensive

Deep Drawing

c = ClearanceDb = blank

diameterDp = Punch

diameterRd = die corner

radiusRp = Punch corner

radiusF = drawing forceFh = holding force

Deep Drawing

Initial step bending of edge straightening of side wall

Thinning and drawing final cup shape

Deep Drawing Parameters and Practices

• Blankholder force– Too high - workpiece tears– Too low - - cup gets wrinkles– Blankholder force is 0.7 to 1 % of the total

of the yield and ultimate tensile strength of the workpiece

• Radius/diameter of punch• Diameter of blank• Thickness of blank

Deep Drawing Parameters and Practices

• Clearance between punch and die – Clearances usually 1.07 to 1.14 thickness– Too low - blank pierced

• Friction/lubrication• Radius of entry into die• Radius on end of punch

– too small - cracking at corners– Too large - wrinkling

• Punch force

Deep Drawing Forces and Stresses

• Stress states are complex, force difficult to compute– Under blankholder

• Compressive through thickness• Tensile along radius• Compressive (poisson's effect along

circumference– In die

• Mostly tensile in punch direction• Also tensile at right angles to punch direction• Stresses cause material to thin in punch

direction• and shrink around die in circumferential

direction

Deep Drawability

• Deep drawability is a complex phenomenon

• An important empirical metric is– Limiting Drawing Ratio (LDR) given by

LDR = Maximum blank diameterPunch diameter

• LDR is related to fundamental properties of the material being drawn through a mechanical property called:– Normal or Plastic Anisotropy

Normal Anisotropy

• Normal Anisotropy, – R = width strain/thickness strain– LDR is proportional to the average normal

anisotropy• It is measured by measuring the

deformations in length on width of a normal tensile specimen

LDR and Anisotropy Deep draw cup with ears

Metal Flow• Between die and punch flow is similar to

that in drawing• Under the blankholder much smaller metal

movement and is uniform• In non-circular products flow becomes non-

uniform• To control flow in non--circular products use

– Drawbeads• The bending and straightening of the metal as it

flows provides greater resistance to metal movement

• Design and location important to good products– Design blank appropriately

• Cut corners of square blanks at 45o

Metal Flow

Drawing Defects

a) Wrinkling in flange - small holding forceb) Wrinkling in the wall - insufficient holding force,

wrinkling initially occurring on the flange.c) Tearing - high stress, sharp die radiusd) Earing - anisotropy of the materiale) Surface scratches - Die or punch not having a

smooth surface, insufficient lubrication


Ironing & Redrawing

Ironing

– Reduces thickness variation from top to bottom of cup

– Like drawing a sheet where the die is formed by the clearance between the punch and its die

Redrawing

– Used for complex shapes– Multistep dies– Reverse redrawing– Allows very deep cups to be formed (Beverage

cans)

Redrawing Reverse Redrawing

Deep Drawing of Cans

• 7 Sheet metal operations

1 - Blanking


2 - Deep Drawing

3 - Redrawing


5 - Doming

4 - Ironing


7 - Seaming

6 - Necking


Stretch forming

Stretch Forming• Used predominantly to make aircraft parts• Like stretch bending for tubes, but uses

flat sheet• Material is pulled across a tool while

being subjected to tensile stress

Stretch Forming

• Advantages – Suitable for low volume production– Versatile– Economical, inexpensive dies

• Disadvantages– Cannot form parts with sharp corners or

depressions

Stretch forming

Other Related Processes

• Rubber forming– Large flexible rubber base acts as a die

• The rubber acts like a semi fluid medium pushing the sheet against the die shape

– Advantages• Only one side of the die needs to be made

– Disadvantages• Shapes are limited

Rubber Forming


• Hydroforming or fluid forming– Uses a fluid medium separated from the

workpiece by a rubber membrane– Fluid kept under high pressure (100 MPA,

15,000 psi) during the forming cycle

Other Related Processes• Spinning

– Similar to forming pots with clay on a potters wheel

– Only for axisymmetric parts– Usually involves multiple passes– May require elevated temperatures for thick

parts or low ductility materials

Types of Spinning• Conventional

– thickness retained, outside diameter typically reduced

• Shear spinning/power spinning/flow turning/hydrospinning/spin forging– Thickness reduced, outside diameter retained

Conventional Spinning

Shear spinning• Also known as power spinning, flow turning,

spin forging • Related to Ironing

Types of Spinning

• Tube Spinning– Wall thickness reduced– Can be executed

• internally or externally• forward or backward

– Used to make • Pressure vessels• Car and truck wheels• Rocket and missile parts


• Shock Wave Forming– Shock created by

• Explosives (Explosive Forming)• Electric spark ( Electrohydraulic forming)• Gas

– Uses a hydrostatic shock wave to push material into a die.– Still need to clamp the workpiece to the die


• Magnetic pulse forming– Pulsed magnetic forces move the metal

against a die– Often used to seal a tube against a

cylindrical part

Forming Equipment• Like forging presses• Important parameters for forming

equipment– Stiffness – Rigidity

• Several types– Mechanical– Hydraulic– Pneumatic– Pneumatic/hydraulic

Forming Equipment• Selection parameters include:

– Type of forming– Size and shape of the workpiece– Length of stroke of the slides– Number of strokes per minute– Speed and shut height– Number of slides– Maximum force– Type of controls– Die change mechanism

Forming Equipment Shapes

• C-Frame most common– Easy access

• O or box- type stiffer• Can have several presses linked by

automated transfer equipment

A Large Stamping Press

Break Press

Die Design

Die DesignSummary

• Die set components• Types of die sets• Strippers• Knockouts• Shedders• Compound dies• Progressive dies

Die DesignIntroduction

• Die sets are tools to hold and aid in the alignment of..– cutting– forming– bending etc.



• Guide bushings and guide pins– align the punch and die holders

• Shank– fits into the press ram

• Flange– to fix the die holder to the press


• Stamping tools are mounted between the punch and die holders.

• Accurate slip fit is maintained between the guide pins and the guide bushings.

• Precision die sets– ball bearing bushing are used

Die DesignDie Sets

• Selection– feeding method– rigidity required– viewing access– alignment accuracy

Die DesignDie Sets

Die DesignDie Sets

Die DesignStrippers

• Applies pressure to the stock strip• sticking occurs

Die DesignPunches

• Designed to..– not to deflect– withstand stripping forces– not to rotate with cutting action

• life extended by heat treatment– strength– wear resistance

Die DesignPunches

• Punch deflection avoided by making the body diameter of the punch larger than the cutting edge.

Die DesignPunches

Die DesignPiercing Punches

• Smaller and longer than blanking punches• Designed to withstand shock, deflection, and

buckling• Higher likelihood of damage

– easy removal/ replacement

Die DesignPunches

• Smaller diameter punches need support for most of their length

Die DesignPunches

• Thin short punches - a quill provides support

Die DesignPunches

• Thin punches -a quill provides support

Die DesignPunches

• To prevent the punch from turning in the punch plates.

Die DesignDie Blocks

Die DesignDie Blocks

• Sectioned blocks– large die blocks– difficult machining– intricate shapes

Die DesignCompound dies

• Combine one or more operations in one station during one stroke of the press.


• Advantages– Pierced holes are held to close tolerances and

concentricity with edges.– Larger parts can be blanked in a small press– Compound dies are economical

• one press• one operator


• Advantages– Besides cost savings– relocation errors can be minimized.– Elimination of inaccuracy


• Disadvantages– Certain parts or blank shapes and sizes,

compound dies are not suitable.– Too much die detail crowded into one area,

• Example,– Punched hole too close to the edge of the

blank => failure

Die DesignCompound Dies

• Multiple operations within same stroke

Die DesignProgressive Dies

• Made with two or more stations in a row.• As the press ram descends

– each station performs an operation on the workpiece

– idler station• Stock strip advances

– series of stations– one or more die operation


• The part remains attached to the scrap skeleton until the last station.– Cut-off

• Each press ram stroke– complete workpiece

• Scrap skeleton– cut-up by a scrap cutter

Die DesignProgressive Dies : Operations

• Several stamping operations are combined and performed in successive stations.


• Other names– Cut & carry die– Follow die– Gang die

• Operations which may be included in a progressive die:– Piercing, blanking, forming, drawing, and cut-

off

Die DesignProgressive Dies : Pilots

• Parts must be fed and located accurately in a progressive die.

• Stock positioning• Direct pilots

– holes punched in the part• Indirect pilots

– holes pierced in scrap-strip.

Die DesignProgressive Dies : Pilots

• Advantages– Not affected by workpiece change– Size and location not limited (as with direct

pilots)• Disadvantages

– Material width and lead may increase– Drawing affects scrap-strip

Die DesignProgressive Dies : Stations

• Distance from one station to the next must be the same.

• Station-to-station distance = The distance the strip moves to relocate at each station

Die DesignProgressive Dies : Costs

• Die cost– high

• Production runs– high volume

• Press cost– high : automatic press with cut-off, feed,

straightener etc.


• Operator requirement– reduced : main task is to replace the coil of

stock.• Scrap

– high : nesting is limited






Die DesignSimple Die : Trimming

Conformado de Planchas Italiano

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