CHAPTER 7: SHEET-METAL FORMING

PROCESSES AND EQUIPMENT

第 7 章：钣金成型工艺和设备
Dì 7 zhāng: Bǎn jīn chéngxíng gōngyì hé shèbèi

References:
1) ©2013 John Wiley & Sons, Inc. M P Groover, Principles of Modern Manufacturing 5 edition
2) @2020 Pearson, Serope Kalpakjian, Manufacturing Engineering and Technology 8 edition
Chapter Outline
1. Introduction
2. Cutting Operations (Shearing, Blanking, Punching)
3. Sheet-metal Characteristics and Formability
4. Formability Tests for Sheet Metals
5. Bending Sheets, Plates, and Tubes
6. Miscellaneous Bending and Related Operations
7. Deep Drawing
8. Rubber Forming and Hydroforming
9. Spinning
10. Superplastic Forming
11. Specialized Forming Processes
12. Manufacturing of Metal Honeycomb Structures
13. Design Considerations in Sheet-metal Forming
14. Equipment for Sheetmetal Forming
15. Economics of Sheetforming Operations
Introduction

◻ Pressworking or press forming is used for general

sheet-forming operations, as they are performed on
presses using a set of dies.
◻ A sheet-metal part produced in presses is called a
stamping (to force downward).
◻ 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)
Sheet Metalworking Terminology

◻ Stampings - sheet metal products

◻ Punch-and-die - tooling to perform cutting,
bending, and deep drawing
◻ Stamping press - machine tool that performs
most sheet metal operations
 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

◻ Low-carbon steel – common used sheet metal has low cost and
good strength and formability characteristics
◻ High strength steel used for automotive application.
◻ Aluminium for beverage cans, kitchen utensils, etc.
◻ Most manufacturing processes involving sheet metal are performed
at room temperature (cold working).
◻ Hot stamping titanium alloys and various high-strength steels to
increase formability and decrease forming loads on machinery.
Basic Types of Sheet Metal Processes

1. Cutting
🞑 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. Deep Drawing
🞑 Forming of sheet into convex or concave shapes
CUTTING
剪切
Jiǎn qiè
Shearing, Blanking, and Punching

◻ Three principal operations in pressworking that

cut sheet metal:
🞑 Shearing
🞑 Blanking
🞑 Punching
 Cutting Operation

Shearing - cutting sheet Blanking - sheet metal Punching - similar to

metal from a large sheet cutting to separate piece blanking except cut piece is
(usually from a coil) into (called a blank) from scrap, called a slug
blanks of desired shaped surrounding stock
(a) Side view of the
operation;
(b) Front view of power
shears equipped with
inclined upper cutting
blade
 Punch and Die
Schematic illustration of shearing with a punch and die

➢ Formation on cracks on both the top and bottom edges of the workpiece.
➢ These cracks eventually meet each other and complete separation
occurs.
 The slug

A punched hole

➢ The rough fracture surfaces are due to the cracks.

➢ The smooth shiny burnished surfaces are from the contact and
rubbing of the sheared edge against the walls of the punch and die.
➢ Burr is a thin edge. Burr height increase with increasing clearance and
ductility of the sheet metal
 Effect of the Clearance, c

• 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. The zone of deformation becomes larger
and the sheared edge becomes rougher
• In practice, clearances usually range between 2% and 8% of the
thickness of the sheet.
Effect of Punching Speed

❖ Edge quality can be improved by increasing punch

speed; speed may be as high as 10 to 12 m/s.

❖ With increasing speed, the heat generated by plastic

deformation is confined to a smaller and smaller zone.

❖ Consequently, the sheared zone is narrower, and the

sheared surface is smoother and exhibits less burr
formation.
Punch Force
◻ Important for determining press size (tonnage)
◻ The force required to punch out a blank, basically the product of
the shear strength of the sheet metal and the total area being
sheared along the periphery.
◻ Maximum punch force, F, can be estimated from:

F = 0.7TLS
T = sheet thickness
L = total length sheared (perimeter of the shape)
S = shear strength of the material

◻ The coefficient 0.7 is an empirical factor that takes into account

various factors such as the material properties, the geometry of
the punch, and the nature of the punching operation.
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 titanium-
alloy Ti-6Al-4V sheet at room temperature. Given the S for
this alloy is 1000 MPa.

Solution
CUTTING MECHANISMS
 Die Cutting 模切
❖ Die cutting basic process consists of:
❖ Perforating 射孔 : punching holes in a sheet
❖ Parting 离别 : shearing sheet into pieces
❖ Notching 开槽 : removing pieces from the edges
❖ Lancing 刺 : leaving a tab without removing any material

Have various uses, particularly in assembly with other components.

 Fine Blanking
◻ Very smooth and square edges can be produced by fine blanking.
◻ Fine-blanking process can control small range of clearances and
dimensional tolerances.
◻ A V-shaped stinger or impingement mechanically locks the sheet tightly
in place and prevents the type of distortion of the material.

Comparison of sheared edges, Schematic illustration of fine blanking operation.

conventional cs. Fine blanking
 Slitting
◻ The operation use a pair of circular blades, follow either a
straight line, a circular path, or a curved path.
◻ A slit edge normally has a burr, which may be folded over
the sheet surface by rolling it (flattening) between two
cylindrical rolls.
◻ If not performed properly, slitting operations can cause
various distortions of sheared edges.
 Steel Rules
❖ Suitable for soft metals, paper, leather, and rubber.
❖ The die is pressed against the sheet (a concept similar to cookie cutter),
which rests on the flat surface, and it shears the sheet along the shape of
the steel rule.

Nibbling
❖ In this process, a machine called a nibbler,
moves a small straight punch up and down
rapidly into a die.
❖ A sheet is fed through the gap and several
overlapping holes are made.
❖ High flexibility, able to make intricate slots
and notches
❖ Nibbling is economical for small production
runs because no special dies are required.
 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
◻ Resulting in:
1. Reduction in scrap
2. Elimination of the need for subsequent spot welding
3. Better control of dimensions
4. Improved productivity
 Tailor-welded Blanks
Tailor-welded Sheet Metal for Automotive Applications
◻ Production of an outer side panel of a car body is by

laser butt welding and stamping

Tailor-welded Blanks
Tailor-welded Sheet Metal for Automotive Applications
◻ Some of the examples of laser butt-welded and

stamped automotive-body components.

CHARACTERISTICS AND TYPE OF CUTTING DIES
切削模具的特点和类型
Qiēxiāo mújù de tèdiǎn hé lèixíng
Characteristics and Type of Cutting Dies

Clearance
◻ Distance between punch cutting edge and die cutting edge
◻ Typical values range between 2% 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
◻ When sheared edge is rough it can be subjected to a process
called shaving (trimming process).

Shearing and
Shaving
processes
Shaving
combined in
process
one stroke
Characteristics and Type of Cutting Dies

Punch and Die Shape

◻ Location of sheared regions can be controlled by

beveling the punch and die surfaces

◻ Beveling is suitable for shearing thick sheets – reduce

the force at the beginning of the stroke.

Example of the use of shear angles on punches and dies

Characteristics and Type of Cutting Dies

Compound Dies
◻ Several operations on the same sheet may be performed in one
stroke at one station with a compound die
◻ Limited to simple shapes due to:
1. Process is slow
2. Complex dies is more expensive
Characteristics and Type of Cutting Dies

Progressive Dies
◻ Multiple operations on the sheet metal. Can be made at high
production rates
◻ The sheet metal is fed through as a coil strip, and a different
operation (such as punching, blanking, and notching) is performed
at the same station of the machine with each stroke and using a
series of punches
◻ The part shown below is the small round metal tip that supports
the plastic nozzle in spray can.
 Characteristics and Type of Cutting 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.

Miscellaneous Methods of Cutting Sheet Metal

◻ Other methods of cutting sheets

1. Laser-beam cutting - typically used with computercontrolled
2. equipment to cut (a) any shape consistently, (b) various
thicknesses, and (c) without the use of any punches or dies.
3. Water-jet cutting - effective on metallic as well as nonmetallic
materials
4. Cutting with a band saw – a chip-removal process
5. Friction sawing - which involves a disk or a blade that rubs
against the sheet or plate at high surface speeds, thus raising the
temperature and separating the sheet
6. Flame cutting - a common method, particularly for thick
plates, and used widely in shipbuilding and on heavy structural
components
SHEET METAL CHARACTERISTICS AND FORMABILITY
钣金特性和成形性
Bǎn jīn tèxìng hé chéngxíng xìng
Sheet-metal Characteristics and Formability
 Sheet-metal Characteristics and Formability

Elongation
◻ A specimen subjected to tension undergoes uniform elongation
◻ Because in sheet forming the material usually is being stretched, high
uniform elongation is essential for good formability

Yield-point Elongation
◻ Yield-point elongation results in L¨uder’s bands (also called stretcher-
strain marks or worms) on the sheet.
◻ Lüder’s bands has elongated depressions on the surface of the sheet,
causing coarseness on the surface degrades appearance and may also
cause difficulties in subsequent coating and painting operations.
◻ Reduce the effect by an additional reduction in the sheet thickness of
0.5% to 1.5% by cold rolling, known as temper or skin rolling.
 Sheet-metal Characteristics and Formability

Yield-point Elongation

Anisotropy
❑ Anisotropy (directionality) of the sheet. Obtained during the thermo-
mechanical processing
❑ There are two types: crystallographic anisotropy, also called preferred
orientation of the grains) and mechanical fibering, which is the
alignment of impurities, inclusions, and voids throughout the thickness
of the sheet.
 Sheet-metal Characteristics and Formability

Grain Size
◻ Affects mechanical properties and surface appearance.
◻ Smaller the grain size, stronger is the metal. and the coarser the grain,
the rougher is the surface appearance. An ASTM grain size of 7 or
finer is preferred for general sheet-forming operations.

Dent Resistance of Sheet Metals

◻ Dents caused by dynamic forces from moving objects that hit the
sheet metal. In typical automotive panels, for example, impact
velocities range up to 45 m/s.
◻ The flatter the panel, the higher is dent resistance, because of the
sheet’s flexibility.
FORMABILITY TESTS FOR SHEET METALS
金属板材的成形性测试
Jīnshǔ bǎncái de chéngxíng xìng cèshì
 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 Cupping test (Erichsen test), the sheet specimen is clamped and
round punch is forced into the sheet until a crack appears.
◻ The punch depth, d, at which a crack appears is a measure of
the formability of the sheet.
 Formability Tests for Sheet Metals

Forming-limit Diagrams (FLD)

◻ Forming-limit diagrams is to determine the formability of sheet

metals and predict the likelihood of failure (usually in terms of

necking or thinning) during deformation.
◻ A series a of such tests is performed on a particular type of

sheet metal, a forming-limit diagram is constructed,

identifying the boundaries between failure and safe zones
Safe Zone and Failure Zone: The area below the FLCs represents the
safe zone, where the material is expected to undergo uniform
deformation without failure. The area above the FLCs represents the
failure zone, where localized thinning or necking is expected to occur.
BENDING
弯曲
Wānqū
 Bending Sheets, Plates, and Tubes

◻ Bending is a common industrial forming operation

◻ The outer fibers of the material are in tension, while the

inner fibers are in compression.

(a) and (b) The effect of elongated inclusions (stringers) on cracking as a
function of the direction of bending with respect to the original rolling
direction of the sheet.
(c) Cracks on the outer surface of an aluminum strip bent to an angle of 90◦.
 Bending Terminology

Note: the bend radius is measured to the inner surface of the bent part.

Because of the Poisson effect, the width of the part (bend length, L) becomes smaller in
the outer region, and larger in the inner region compared to the original width,
Types of Sheet Metal Bending

◻ V-bending - performed with a V-shaped die

◻ Edge bending - performed with a wiping die

◻ Application notes: ◻ Application notes:

🞑 Low production 🞑 High production
🞑 V-dies are simple 🞑 Dies are more complicated
and inexpensive and costly
Stretching during Bending

◻ If bend radius is small relative to stock thickness,

metal tends to stretch during bending
◻ Important to estimate amount of stretching, so

final part length = specified dimension

◻ Problem: to determine the length of neutral axis of

the part before bending

Bend Allowance Formula

Approximate bend allowance is:

where Lb = bend allowance;  = bend angle; R=

bend radius; T = sheet thickness; and k is is a
constant, which in practice typically from:
■ If R < 2T, k = 0.33
■ If R  2T, k = 0.50
 Bend Allowance Formula
Minimum Bend Radius
◻ Refers to the radius at which a crack first appears at the outer fibers
of a sheet being bent. It can be shown that the engineering strain on
the outer and inner fibers of a sheet during bending is given by the
expression:

◻ as R/T decreases (i.e., as the ratio of the bend radius to the

thickness becomes smaller), the tensile strain at the outer fiber
increases, and the material eventually develops cracks

◻ The bend radius usually is expressed in terms of the thickness,

such as 2T, 3T, and 4T
A 3T minimum bend radius indicates that the smallest radius to which the sheet can be bent,
without cracking, is three times its thickness.

It has been shown that there is an inverse

relationship between bendability and the tensile
reduction of the area, r, of the material.

Minimum bend radius, R, is

approximately:
 SPRINGBACK

◻ When bending pressure is removed, elastic energy remains in

bent part, causing it to recover partially toward its original shape
◻ Springback results in a decrease in bend angle and an increase

in bend radius: (1) during bending, work is forced to take radius

Rt and angle b' of the bending tool, (2) after punch is removed,
work springs back to R and ‘
◻
Compensation for Springback

◻ Springback is compensated for by

overbending the part. Several trial may be
necessary to obtain the desired part.

◻ Another method is to coin the bend area, by

subjecting it to highly localized compressive
stresses between the tip of the punch and
the die surface. The technique is also called
bottoming the punch.

◻ In another method, the part is subjected to

stretch bending, in which the part is under
external tension while being bent.
 BENDING FORCE
➢ The bending force for sheets and plates can be estimated by assuming that the
process is one of simple bending of a rectangular beam.
➢ The bending force is a function of the yield strength of the material, Sy, the length
of the bend, L, the thickness of the sheet, T, and the die opening, W.

◻ Excluding friction, the maximum bending force, P, is

where the factor k ranges from about 0.3 for a wiping die, to about 0.7 for a
U-die, to about 1.3 for a V-die

◻ For a V-die, it is often modified to

◻ where the punch-tip radius and
the sheet thickness are relatively small compared to the die opening
 MISCELLANEOUS BENDING AND
RELATED OPERATIONS
各种弯曲及相关操作
Gè zhǒng wānqū jí xiāngguān cāozuò
Miscellaneous Bending and Related
Operations
◻ Examples of various bending operations
Miscellaneous Bending and Related
Operations
◻ Press-brake Forming
◻ Sheet metal or plate can be bent easily with simple

fixtures using a press

◻ The machine uses long dies in a mechanical / hydraulic
press suitable for small production runs
 Miscellaneous Bending and Related
Operations

Bending in a Four-slide Machine

◻ Relatively can bent short pieces

◻ Lateral movements are synchronized with vertical die

movement to form the part into desired shapes

Roll Bending
◻ Plates are bent using a set of rolls.

◻ Curvatures can be obtained by adjusting

◻ the distance between the three rolls

Miscellaneous Bending and Related
Operations
Beading
◻ Periphery of the sheet metal is bent into the cavity of a

die
◻ The beads improve the appearance of parts and

eliminate exposed sharp edges.

Miscellaneous Bending and Related
Operations
Flanging
◻ A process of bending the edges of
sheet metals, usually to 90◦

◻ In shrink flanging, the flange is

subjected to compressive hoop
stresses; if excessive, however, the
stresses can cause the flange
periphery to wrinkle. The wrinkling
tendency increases with decreasing
radius of curvature of the flange.
◻ In stretch flanging, the flange
periphery is subjected to tensile
stresses; if excessive, however, they
can lead to cracking along the
periphery of the flange.
Miscellaneous Bending and Related
Operations
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

Miscellaneous Bending and Related
Operations
Tube Bending and Forming
◻ 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

Miscellaneous Bending and Related
Operations
Dimpling, Piercing, and Flaring
◻ In dimpling, a hole first is punched and then expanded
into a flange
◻ Flanges and tube ends may be produced by piercing
with a shaped punch
◻ When the bend angle is less than 90°, the process is
called flaring

Hemming and Seaming

◻ Hemming increases the stiffness and appearance of the
part
◻ Seaming is joining 2 edges of sheet metal by hemming
Miscellaneous Bending and Related
Operations
Segmented Dies
◻ Dies consist of individual segments placed inside the

part and expanded mechanically in a radial direction

◻ Inexpensive and used for large production runs

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
Miscellaneous Bending and Related
Operations
Stretch Forming