授課教師：楊宏智教授

1

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楊宏智（台大機械系教授）2

Chapter Outline1. Introduction2. Shearing3. Sheet-metal Characteristics and Formability4. Formability Tests for Sheet Metals5. Bending Sheets, Plates, and Tubes6. Miscellaneous Bending and Related Operations7. Deep Drawing8. Rubber Forming and Hydroforming9. Spinning10. Superplastic Forming11. Specialized Forming Processes12. Manufacturing of Metal Honeycomb Structures13. Design Considerations in Sheet-metal Forming14. Equipment for Sheetmetal Forming15. Economics of Sheetforming Operations

Introduction Products made of sheet metals are common Pressworking or press forming is used for general

sheet-forming operations, as they are performed on presses using a set of dies

Introduction A sheet-metal part produced in presses is called a

stamping Low-carbon steel has low cost and good strength and

formability characteristics Manufacturing processes involving sheet metal are

performed at room temperature

Sheet Metalworking Defined Cutting and forming operations performed on relatively

thin sheets of metal Thickness of sheet metal = 0.4 mm (1/64 in) to 6 mm

(1/4 in) Thickness of plate stock > 6 mm Operations usually performed as cold working

Sheet and Plate Metal Products Sheet and plate metal parts for consumer and industrial

products such as Automobiles and trucks Airplanes Railway cars and locomotives Farm and construction equipment Small and large appliances Office furniture Computers and office equipment

Advantages of Sheet Metal Parts High strength Good dimensional accuracy Good surface finish Relatively low cost Economical mass production for large quantities

Sheet Metalworking Terminology Punch-and-Die - tooling to perform cutting, bending,

and drawing Stamping press - machine tool that performs most

sheet metal operations Stampings - sheet metal products

Three Basic Types of Sheet Metal Processes1.Cutting (Shearing)Shearing to separate large sheetsBlanking to cut part perimeters out of sheet metalPunching to make holes in sheet metal2.BendingStraining sheet around a straight axis3.DrawingForming of sheet into convex or concave shapes

Sheet Metal Cutting (Shearing)

(1) Just before punch contacts work; (2) punch pushes into work, causing plastic deformation; (3) punch penetrates into work causing a smooth cut surface; and (4) fracture is initiated at opposing cutting edges to separate the sheet

Shearing Before a sheet-metal part is made, a blank is removed

from a large sheet by shearing The edges are not smooth and perpendicular to the

plane of the sheet

Punch and Die Sizes Die size determines blank size Db Punch size determines hole size Dh c = clearance

ShearingProcessing parameters in shearing are1.The shape of the punch and die2.The speed of punching3.Lubrication4.The clearance, c, between the punch and the dieWhen clearance increases, the zone of deformation becomes larger and the sheared edge becomes rougherExtent of the deformation zone depends on the punch speedHeight, shape, and size of the burr affect forming operations

Clearance in Sheet Metal CuttingDistance between punch cutting edge and die cutting edgeTypical values range between 4% and 8% of stock thicknessIf too small, fracture lines pass each other, causing double burnishing and larger forceIf too large, metal is pinched between cutting edges and excessive burr results

Clearance in Sheet Metal CuttingRecommended clearance is calculated by:

c = atwhere c = clearance; a = allowance; and t = stock thickness

Allowance a is determined according to type of metal

Sheet Metal Groups AllowancesMetal group a

1100S and 5052S aluminum alloys, all tempers 0.045

2024ST and 6061ST aluminum alloys; brass, 0.060 soft cold rolled steel, soft stainless steel

Cold rolled steel, half hard; stainless steel, 0.075 half hard and full hard

Shearing Clearance Clearance control determine quality of its sheared edges which influence formability of the sheared part Appropriate clearance depends on:1. Type of material and temper2. Thickness and size of the blank3. Proximity to the edges of other sheared edges

When sheared edge is rough it can be subjected to aprocess called shaving

Shearing: Characteristics and Type of Shearing DiesPunch and Die Shape Punch force increases rapidly during shearing Location of sheared regions can be controlled by

beveling the punch and die surfaces

ShearingPunch Force Maximum punch force, F, can be estimated from

Friction between the punch and the workpiece can increase punch force

T = sheet thicknessL = total length sheared UTS = ultimate tensile strength of the material

ShearingEXAMPLE 16.1 Calculation of Punch ForceEstimate the force required for punching a 25-mm diameter hole through a 3.2-mm thick annealed titanium- alloy Ti-6Al-4V sheet at room temperature.

SolutionUTS for this alloy is 1000 MPa, thus

Blanking and Punching Blanking - sheet metal cutting to separate piece

(called a blank) from surrounding stock Punching - similar to blanking except cut piece is

scrap, called a slug

Shearing: Shearing Operations Punching is where the sheared slug is scrap Blanking is where the slug is the part to be used and the

rest is scrapDie Cutting Shearing operation consists of: Perforating: punching holes in a sheet Parting: shearing sheet into pieces Notching: removing pieces from the edges Lancing: leaving a tab without removing any material

Shearing: Fine Blanking smaller clearance used pressure pad w/ v-shaped stringer locks the sheet

metal movement of punch, pressure pad, cushion are

controlled by triple-action hydraulic presses

Shearing Operations: Slitting Shearing operations are through a pair of circular

blades, follow either a straight line, a circular path, or a curved path

Shearing Operations: Nibbling A contour is cut by a series of overlapping slits or

notches Simple tools can be used to produce complex shape

Shearing: Tailor-welded Blanks Laser-beam butt welding involves two or more pieces of sheet

metal with different shapes and thicknesses The strips are welded to obtain a locally thicker sheet and

then coiled The welded assembly is then formed into a final shape.

Resulting in:1. Reduction in scrap2. Elimination of the need for subsequent spot welding 3. Better control of dimensions4. Improved productivity

Shearing: Tailor-welded BlanksEXAMPLE 16.2 Tailor-welded Sheet Metal for Automotive Applications Production of an outer side panel of a car body is by

laser butt welding and stamping

Shearing: Tailor-welded BlanksEXAMPLE 16.2 Tailor-welded Sheet Metal for Automotive Applications Some of the examples of laser butt-welded and

stamped automotive-body components.

Characteristics and Type of Shearing DiesTransfer Dies Sheet metal undergoes different operations arranged

along a straight line or a circular path

Tool and Die Materials Tool and die materials for shearing are tool steels and

carbides Lubrication is needed for reducing tool and die wear,

and improving edge quality

Characteristics and Type of Shearing DiesProgressive Dies a different operation is performed at the same station with each stroke of a series of punches

Characteristics and Type of Shearing Dies For high product production rates The part shown below is the small round piece that

supports the plastic tip in spray cans

Characteristics and Type of Shearing DiesCompound Dies Operations on the same sheet may be performed in

one stroke with a compound die Limited to simple shapes due to:1. Process is slow2. Complex dies is more expensive

Shearing: Miscellaneous Methods of Cutting Sheet Metal Other methods of cutting sheets1. Laser-beam cutting2. Water-jet cutting3. Cutting with a band saw4. Friction sawing5. Flame cutting

Sheet-metal Characteristics and FormabilityYield-point elongation having both upper and lower yield points.This behaviour results in Luder’s bands

Typically observed with mild-steel sheetsResults in depressions on the sheet surfaceCan be eliminated by temple rolling (but sheet must be formed within a certain time after rolling)

Sheet-metal Characteristics and FormabilityGrain Size Affects mechanical properties and surface appearance Smaller the grain size, stronger is the metal

Dent Resistance of Sheet Metals Dents caused by dynamic forces from moving objects that

hit the sheet metal Dynamic yield stress, instead of static yield stress, should

be the significant strength parameter

Formability Tests for Sheet Metals Sheet-metal formability is the ability of the sheet metal to

undergo the desired shape change without failure Sheet metals may undergo 2 basic modes of deformation: (1)

stretching and (2) drawingCupping Tests In the Erichsen test, the sheet specimen is clamped and round

punch is forced into the sheet until a crack appears The punch depth is a measure of formability of the sheet Easy to perform, but does not simulate exact conditions of actual

forming, and not reliable for complex parts.

Formability Tests for Sheet MetalsForming-limit Diagrams To develop a forming-limit diagram, the major and minor

engineering strains are obtained Major axis of the ellipse represents the major direction and

magnitude of stretching Major strain is the engineering strain and is always positive Minor strain can be positive or negative Curves represent the boundaries between failure and safe

zones

Formability Tests for Sheet MetalsForming-limit Diagrams Forming-limit diagrams is to determine the formability of

sheet metals

Forming-Limit Diagram (FLD) Blank stretched over a punch, deformation observed and

measured in the region where failure has occurred The curves represent the boundaries between failure and safe

zones Different mat'ls have different FLDs, and the higher the curve,

the better is the formability Compressive minor strain is associated with a higher major

strain than a tensile minor strain of the same magnitude The effect of sheet-metal thickness is to raise the curves Friction, lubrication at punch/sheet-metal interface, and surface

scratches, are important factors

Bending Sheets

Bending is a common industrial forming operation Bending imparts stiffness to the part by increasing its

moment of inertia Outer fibers are in tension, while the inner in

compression Poisson effect cause the width to be smaller in the

outer region and larger in the inner region

, Plates, and Tubes

Bending Sheets, Plates, and Tubes Approximate bend allowance is

For ideal case, k = 0.5,

Minimum Bend Radius Engineering strain during bending is

Minimum bend radius, R, is

Bending Sheets, Plates, and TubesMinimum Bend Radius Increase the bendability by increase their tensile

reduction of area Bendability also depends on the edge condition of the

sheet Improve resistance to edge cracking by removing the

cold-worked regions Cold rolling results in anisotropy by preferred

orientation or mechanical fibering

Bending Sheets, Plates, and TubesSpringback Plastic deformation is followed by elastic recovery when

the load is removed, called springback Springback can be calculated by

Bending Sheets, Plates, and TubesCompensation for Springback Springback is compensated for by overbending the part One method is stretch bending where the part is subjected

to tension while being bentBending Force Excluding friction, the maximum bending force, P, is

For a V-die, it is modified to

Miscellaneous Bending OperationsExamples of various bending operationsRoll BendingPlates are bent using a set of rolls.Curvatures can be obtained by adjusting the distance between the three rolls

V-Bending and Edge Bending V-Bending:Low productionPerformed on a press brakeV-dies are simple and inexpensive Edge-Bending:High productionPressure pad requiredDies are more complicated and costly

Miscellaneous Bending and Related OperationsBeading Periphery of the sheet metal is bent into the cavity of a

die The bead imparts stiffness to the part by increasing the

moment of inertia of that section

Miscellaneous Bending and Related OperationsFlanging In shrink flanging, the flange is subjected to

compressive hoop stresses and cause the flange periphery to wrinkle

Bending Operations

most common forming operation paper clip, file cabinet etc

Press Brakes – Bending Equipment

Sheet metal or plate can be bent easily with simple fixtures using a press

complex bends with long narrow bed and short adjustable strokes metal bent between interchangeable dies

Roll Forming Also called contour-roll forming or cold-roll forming Used for forming continuous lengths of sheet metal and

for large production runs Dimensional tolerances, springback, tearing and

buckling of the strip have to be considered

Bending of Tube Stock Stretch bending of tube: (1) start of process and (2)

during bending

Tube Bending Large dia. thin wall, with small bend radius tend to

cause wrinkle at inner side of the tube Oldest method of bending a tube is to first pack its

inside with loose particles and then bend it into a suitable fixture

Thick tube can be formed to a large bend radius without the use of fillers or plugs

Tube Forming tubular part placed in a split-female die and

expanded with a polyurethane or rubber plug punch retracted; plug returns to its original

shape and removed by knockout rod finished part removed by opening the split die

(water pitcher) production of fitting for plumbing

Manufacturing of Bellows(a) Bulged tube :Tube bulged at several equidistant locations

(b) Compressed tube :Bulged tube compressed axially to collapse bulged regions, thus forming bellows

Stretch Forming Sheet metal is gripped by two sets of jaws The jaws stretch the metal sheet and wrap it around

a form block (die) Most of deformation is induced by tensile stretching,

and forces on the form block are far less Very little springback results Form block often made of wood or low-melting metal Produce large parts in low or limited quantity

Sheet metal is clamped along its edges and then stretched over a male die

Die moves upward, downward, or sideways Used to make aircraft wing-skin panels, fuselages, and

boat hulls

Stretch Forming

Deep Drawing Sheet metal forming to make cup-shaped, box-shaped,

or other complex-curved, hollow-shaped parts Sheet metal blank is positioned over die cavity and then

punch pushes metal into opening Products: beverage cans, ammunition shells,

automobile body panels Also known as deep drawing (to distinguish it from wire

and bar drawing)

Deep Drawing- a punch forces a flat sheet metal into a die cavity- depth greater than diameter

Deep Drawing Material beneath punch remains unaffected and

becomes cup bottom Cup wall is formed by pulling the remainder of disk

inward the radius of die - Hoop stress tends to cause buckling or wrinkling - Pressure ring is used to suppress wrinkle

Deep Drawing Wrinkling can be reduced if a blankholder is loaded by

maximum punch force

The force increases with increasing blank diameter, thickness, strength and the ratio

Redrawing Cup becomes longer as it is redrawn to smaller

diameters since volume of the metal is constant If the shape change is too severe, more than one

drawing step is required. The second drawing step, and any further drawing step

is referred as redrawing

Ironing If the clearance between the punch and the die is large,

the drawn cup will have thicker walls Thickness of the cup wall can be controlled by ironing,

where drawn cup is pushed through one or more ironing rings

Deep Drawing: Deep DrawabilityEaring In deep drawing, the edges of cups may become wavy

and the phenomenon is called earing Earing is caused by the planar anisotropy Planar anisotropy of the sheet is indicated by

Deep-drawing PracticeEaring Too high a blankholder force increases the punch force

and causes the cup wall to tear Draw beads are needed to control the flow of the blank

into the die cavity and reduce the blankholder forces

Deep-drawing PracticeCASE STUDY 16.1Manufacturing of Food and Beverage Cans Aluminum beverage cans has excellent surface finish Detail of the can lid is shown

Rubber Forming Dies are made of solid materials, such as steels and

carbides The dies in rubber forming is made of a flexible material

(polyurethane membrane) In the bending and embossing of sheet metal, the

female die is replaced with a rubber pad

Hydroforming In the hydroform, or fluid-forming process, the

pressure over the rubber membrane is controlled throughout the forming cycle

Control of frictional conditions in rubber forming is a factor in making parts successfully

Rubber Forming and Hydroforming In tube hydroforming metal tubing is formed in a die

and pressurized internally by a fluid, usually water Rubber-forming and hydroforming processes have the

advantages of:1. Capability to form complex shapes2. Flexibility and ease of operation3. Low tooling cost

Tube HydroformingCASE STUDY 16.2Tube Hydroforming of an Automotive Radiator Closure Figure shows a hydroformed automotive radiator

closure Sequence of operations: (1) tube as cut to length; (2)afterbending; (3) after hydroforming

Spinning Spinning is a process that involves the forming of

axisymmetric parts over a mandrel A circular blank of flat sheet metal is held against a

mandrel (form block of desired shape) and rotated while a rigid tool deforms and shapes the material over the mandrel; Disk of sheet metal progressively shaped by localized pressure with small roller

Suitable for conical and curvilinear shapes

Shear Spinning (Forming) A simplified version of the spinning process in which

each element of the blank maintains its distance from the axis of rotation.

Metal flow is entirely in shear and no radial stretch has to take place to compensate for the circumferencial shrinkage

Shear Spinning Also known as power spinning, flow turning,

hydrospinning, and spin forging Use to produce an axisymmetric conical or curvilinear

shape while reducing the sheet’s thickness and maintaining its maximum (blank) diameter

Tube SpinningTube Spinning The thickness of hollow, cylindrical blanks is reduced

by spinning them on a solid, round mandrel using rollers

Can be carried out externally or internally Various external and internal profiles can be produced

from cylindrical blanks with constant wall thickness

SpinningIncremental Forming Simplest version is incremental stretch expanding A rotating blank is deformed by a steel rod with a

smooth hemispherical tip to produce axisymmetric parts CNC incremental forming uses a CNC machine tool to

follow contours at different depths across the sheet-metal surface

Advantages are low tooling costs and high flexibility in the product shapes

Explosive Forming Use of explosive charge to form sheet (or plate) metal

into a die cavity Explosive charge causes a shock wave whose energy

is transmitted to force part into cavity Applications: large parts, typical of aerospace industry

(1) Setup, (2) explosive is detonated, and (3) shock wave forms part and plume escapes water surface

The peak pressure, p, is given by

The mechanical properties of parts similar to those made by conventional forming methods

The dies may be made of aluminum alloys, steel, ductile iron or zinc alloys

p = pressure, psiK = constant that depends on the type of explosive

Explosive Forming

Explosive used as a source of energy Rapid conversion of explosive charge into gas

generates a shock wave Pressure of shock wave is sufficient to form sheet

metal no limit to the size of the workpiece (suitable for low

quantity of large parts, ie aerospace application)

Explosively Formed Part

Electromagnetic Forming coil current rapidly discharged from capacitor bank eddy current generated in the tube (workpiece) repelling force between the coil and the tube forces generated collapse the tube Higher the electrical conductivity of the workpiece, the

higher the magnetic forces

Peen Forming Used to produce curvatures on thin sheet metals by

shot peening one surface of the sheet Surface of the sheet is subjected to compressive

stresses The process also induces compressive surface residual

stresses, which improve the fatigue strength of the sheet

Laser Forming Involves the application of laser beams as a heat

source in specific regions of the sheet metal Process produce thermal stresses, which can cause

localized plastic deformation of the sheet In laser-assisted forming, the laser acts as a localized

heat source, thus reducing the strength of the sheet metal at specific locations

Improve formability and increasing process flexibility

Superplastic Forming The behavior of superplastic are where tensile elongations were

obtained within certain temperature ranges Superplastic alloys can be formed into complex shapes by

superplastic forming Have high ductility but low strength Advantages:1. Complex shapes can be formed2. Weight and material savings3. Little residual stresses4. Tooling costs are lower

Superplastic Forming Limitations of superplastic forming:1. Part will undergo shape changes2. Must be formed at sufficiently low strain rates

Diffusion Bonding/Superplastic Forming Fabricating of complex sheet-metal structures by combining

diffusion bonding with superplastic forming (SPF/DB) Application for aerospace industry Improves productivity and produces parts with good dimensional

accuracy and low residual stresses

Specialized Forming ProcessesCASE STUDY 16.3 Cymbal Manufacture

Manufacturing of Metal Honeycomb Structures A honeycomb structure has light weight and high

resistance to bending forces, used for aircraft and aerospace components

2 methods of manufacturing honeycomb materials:1. Expansion process 2. Corrugation process

Manufacturing of Metal Honeycomb Structures A honeycomb structure consists of a core of

honeycomb bonded to two thin outer skins Has a high stiffness-to-weight ratio and is used in

packaging for shipping consumer and industrial goods

Design Considerations in Sheet-metal FormingBlank Design Poorly designed parts will not nest properly Blanks should be designed to reduce scrap to a

minimum

Equipment for Sheet-metal Forming Press selection for sheet-metal forming operations

depends on:1. Type of forming operation2. Size and shape of workpieces3. Number of slides4. Maximum force required5. Type of mechanical, hydraulic, and computer controls6. Features for changing dies7. Safety features

CNC Turret Press Parts Sheet metal parts produced on a turret press, showing

variety of hole shapes possible (photo courtesy of Strippet Inc.)

Economics of Sheet-forming Operations Sheet-forming operations are versatile and can

produce the same part The costs involved depend on die and equipment costs

and labor For small and simple sheet-metal parts, die costs and

lead times to make the dies are low Deep drawing requires expensive dies and tooling Equipment costs depend on the complexity of the

forming operation