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授課教師：楊宏智教授
1
楊宏智（台大機械系教授）
2
Chapter Outline
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
Introduction
Shearing
Sheet-metal Characteristics and Formability
Formability Tests for Sheet Metals
Bending Sheets, Plates, and Tubes
Miscellaneous Bending and Related Operations
Deep Drawing
Rubber Forming and Hydroforming
Spinning
Superplastic Forming
Specialized Forming Processes
Manufacturing of Metal Honeycomb Structures
Design Considerations in Sheet-metal Forming
Equipment for Sheetmetal Forming
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 Processes
1.Cutting (Shearing)
Shearing to separate large sheets
Blanking to cut part perimeters out of sheet metal
Punching to make holes in sheet metal
2.Bending
Straining sheet around a straight axis
3.Drawing
Forming 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

Shearing
Processing parameters in shearing are
1.
2.
3.
4.



The shape of the punch and die
The speed of punching
Lubrication
The clearance, c, between the punch and the die
When clearance increases, the zone of deformation
becomes larger and the sheared edge becomes
rougher
Extent of the deformation zone depends on the punch
speed
Height, shape, and size of the burr affect forming
operations
Clearance in Sheet Metal Cutting
Distance between punch cutting edge and die cutting edge
Typical values range between 4% and 8% of stock
thickness
If too small, fracture lines pass each other, causing double
burnishing and larger force
If too large, metal is pinched between cutting edges and
excessive burr results
Clearance in Sheet Metal Cutting
Recommended clearance is calculated by:
c = at
where c = clearance;
a = allowance;
and t = stock thickness
Allowance a is determined according to type of metal
Sheet Metal Groups Allowances
Metal group
a
1100S and 5052S aluminum alloys, all tempers
0.045
2024ST and 6061ST aluminum alloys; brass,
soft cold rolled steel, soft stainless steel
0.060
Cold rolled steel, half hard; stainless steel,
half hard and full hard
0.075
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 temper
2. Thickness and size of the blank
3. Proximity to the edges of other sheared edges

When sheared edge is rough it can be subjected to a
process called shaving
Shearing:
Characteristics and Type of Shearing Dies
Punch and Die Shape
 Punch force increases rapidly during shearing
 Location of sheared regions can be controlled by
beveling the punch and die surfaces
Shearing
Punch Force
 Maximum punch force, F, can be estimated from
T = sheet thickness
L = total length sheared
UTS = ultimate tensile strength of the material

Friction between the punch and the workpiece can
increase punch force
Shearing
EXAMPLE 16.1
Calculation of Punch Force
Estimate the force required for punching a 25-mm
diameter hole through a 3.2-mm thick annealed titaniumalloy Ti-6Al-4V sheet at room temperature.
Solution
UTS 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 scrap
Die 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 scrap
2.
Elimination of the need for subsequent spot welding
3.
Better control of dimensions
4.
Improved productivity
Shearing: Tailor-welded Blanks
EXAMPLE 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 Blanks
EXAMPLE 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 Dies
Transfer 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 Dies
Progressive 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 Dies
Compound 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 slow
2. Complex dies is more expensive
Shearing:
Miscellaneous Methods of Cutting Sheet Metal

Other methods of cutting sheets
1. Laser-beam cutting
2. Water-jet cutting
3. Cutting with a band saw
4. Friction sawing
5. Flame cutting
Sheet-metal Characteristics and Formability
Yield-point elongation having both upper and lower yield
points.
This behaviour results in Luder’s bands
Typically
observed with mild-steel sheets
Results in depressions on the sheet surface
Can be eliminated by temple rolling (but sheet must be formed within
a certain time after rolling)
Sheet-metal Characteristics and Formability
Grain 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) drawing
Cupping 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 Metals
Forming-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 Metals
Forming-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

, Plates, and Tubes
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
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 Tubes
Minimum 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 Tubes
Springback
 Plastic deformation is followed by elastic recovery when
the load is removed, called springback
 Springback can be calculated by
Bending Sheets, Plates, and Tubes
Compensation for Springback
 Springback is compensated for by overbending the part
 One method is stretch bending where the part is
subjected to tension while being bent
Bending Force
 Excluding friction, the maximum bending force, P, is

For a V-die, it is modified to
Miscellaneous Bending Operations
Examples of various bending operations
Roll Bending
 Plates 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 production
Performed on a press brake
V-dies are simple and inexpensive
 Edge-Bending:
High production
Pressure pad required
Dies are more complicated and costly
Miscellaneous Bending and Related Operations
Beading
 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 Operations
Flanging
 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
Stretch Forming



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
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 Drawability
Earing
 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 Practice
Earing
 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 Practice
CASE STUDY 16.1
Manufacturing 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 shapes
2. Flexibility and ease of operation
3. Low tooling cost
Tube Hydroforming
CASE STUDY 16.2
Tube 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 Spinning
Tube 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
Spinning
Incremental 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 sheetmetal 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
Explosive Forming

The peak pressure, p, is given by
p = pressure, psi
K = constant that depends on the type of explosive
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

Explosively Formed Part

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)
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




1.
2.
3.
4.
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:
Complex shapes can be formed
Weight and material savings
Little residual stresses
Tooling costs are lower
Superplastic Forming

1.
2.
Limitations of superplastic forming:
Part will undergo shape changes
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 Processes
CASE 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:
Expansion process
Corrugation process
1.
2.
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 Forming
Blank 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 operation
2. Size and shape of workpieces
3. Number of slides
4. Maximum force required
5. Type of mechanical, hydraulic, and computer controls
6. Features for changing dies
7. 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