Sheet Metal Bending
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3/23/2015
Sheet Metal Bending
Manufacturing Home
Sheet Metal Bending
SHEET METAL
FABRICATION
Sheet Metal Manufacturing
Sheet Metal Cutting
Deep Drawing Sheet Metal
Sheet Metal Ironing
Sheet Metal Spinning
Rubberforming Sheet Metal
High Energy Rate
Forming Of Sheet Metal
MANUFACTURING
PROCESSES
Metal Casting
Metal Forming
Metal Rolling
Metal Forging
Metal Extrusion
Metal Drawing
Powder Processes
Bending of sheet metal is a common and vital process in manufacturing industry. Sheet metal
bending is the plastic deformation of the work over an axis, creating a change in the part's
geometry. Similar to other metal forming processes, bending changes the shape of the work
piece, while the volume of material will remain the same. In some cases bending may produce
a small change in sheet thickness. For most operations, however, bending will produce
essentially no change in the thickness of the sheet metal. In addition to creating a desired
geometric form, bending is also used to impart strength and stiffness to sheet metal, to change
a part's moment of inertia, for cosmetic appearance and to eliminate sharp edges.
Figure:264
Metal bending enacts both tension and compression within the material. Mechanical
principles of metals, particularly with regard to elastic and plastic deformation, are important
to understanding sheet metal bending and are discussed in the fundamentals of metal forming
section. The effect that material properties will have in response to the conditions of
manufacture will be a factor in sheet metal process design. Usually sheet metal bending is
performed cold but sometimes the work may be heated, to either warm or hot working
temperature.
Most sheet metal bending operations involve a punch die type setup, although not always.
There are many different punch die geometries, setups and fixtures. Tooling can be specific to
a bending process and a desired angle of bend. Bending die materials are typically gray iron,
or carbon steel, but depending on the work piece, the range of punchdie materials varies from
hardwood to carbides. Force for the punch and die action will usually be provided by a press.
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Sheet Metal Bending
A work piece may undergo several metal bending processes. Sometimes it will take a series of
different punch and die operations to create a single bend. Or many progressive bending
operations to form a certain geometry.
Sheet metal is referenced with regard to the work piece when bending processes are discussed
in this section. However, many of the processes covered can also be applied to plate metal as
well. References to sheet metal work pieces may often include plate. Some bending operations
are specifically designed for the bending of differently shaped metal pieces, such as for
cabinet handles. Tube and rod bending is also widely performed in modern manufacturing.
Bending Processes
Bending processes differ in the methods they use to plastically deform the sheet or plate.
Work piece material, size and thickness are important factors when deciding on a type of
metal bending process. Also important is the size of the bend, bend radius, angle of bend,
curvature of bend and location of bend in the work piece. Sheet metal process design should
select the most effective type of bending process based on the nature of the desired bend and
the work material. Many bends can be effectively formed by a variety of different processes
and available machinery will often determine the bending method.
One of the most common types of sheet metal manufacturing processes is V bending. The V
shaped punch forces the work into the V shaped die and hence bends it. This type of process
can bend both very acute and very obtuse angles, also anything in between, including 90
degrees.
Figure:265
Edge bending is another very common sheet metal process and is performed with a wiping
die. Edge bending gives a good mechanical advantage when forming a bend. However, angles
greater than 90 degrees will require more complex equipment, capable of some horizontal
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Sheet Metal Bending
force delivery. Also, wiping die employed in edge bending must have a pressure pad. The
action of the pressure pad may be controlled separately than that of the punch. Basically the
pressure pad holds a section of the work in place on the die, the area for the bend is located on
the edge of the die and the rest of the work is held over space like a cantilever beam. The
punch then applies force to the cantilever beam section, causing the work to bend over the
edge of the die.
Figure:266
Rotary bending forms the work by a similar mechanism as edge bending. However, rotary
bending uses a different design than the wiping die. A cylinder, with the desired angle cut out,
serves as the punch. The cylinder can rotate about one axis and is securely constrained in all
other degrees of motion by its attachment to the saddle. The sheet metal is placed cantilevered
over the edge of the lower die, similar to the setup in edge bending. Unlike in edge bending,
with rotary bending, there is no pressure pad. Force is transmitted to the punch causing it to
close with the work. The groove on the cylinder is dimensioned to create the correctly angled
bend. The groove can be less than or greater than 90 degrees allowing for a range of acute and
obtuse bends. The cylinders V groove has two surfaces. One surface contacts the work
transmitting pressure and holding the sheet metal in place on the lower die. As force is
transmitted through the cylinder it rotates, causing the other surface to bend the work over the
edge of the die, while the first surface continues to hold the work in place. Rotary bending
provides a good mechanical advantage.
This process provides benefits over a standard edge bending operation, in that it eliminates the
need for a pressure pad and it is capable of bending over 90 degrees without any horizontally
acting equipment. Rotary bending is relatively new and is gaining popularity in manufacturing
industry.
Figure:267
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Sheet Metal Bending
Air bending is a simple method of creating a bend without the need for lower die geometry.
The sheet metal is supported by two surfaces a certain distance apart. A punch exerts force at
the correct spot, bending the sheet metal between the two surfaces.
Figure:268
Punch and die are manufactured with certain geometries, in order to perform specific bends.
Channel bending uses a shaped punch and die to form a sheet metal channel. A U bend is
made with a U shaped punch of the correct curvature.
Figure:269
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Sheet Metal Bending
Many bending operations have been developed to produce offsets and form the sheet metal for
a variety of different functions.
Figure:270
Some sheet metal bending operations involve the use of more than 2 die. Round tubes, for
example, can be bent from sheet metal using a multiple action machine. The hollow tube can
be seamed or welded for joining.
Figure:271
Corrugating is a type of bending process in which a symmetrical bend is produced across the
width of sheet metal and at a regular interval along its entire length. A variety of shapes are
used for corrugating, but they all have the same purpose, to increase the rigidity of the sheet
metal and increase its resistance to bending moments. This is accomplished by a work
hardening of the metal and a change in the sheet's moment of inertia, caused by the bend's
geometry. Corrugated sheet metal is very useful in structural applications and is widely used
in the construction industry.
Figure:272
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Sheet Metal Bending
Edge Bending Processes
Sheet metal of different sizes can be bent an innumerable amount of ways, at different
locations, to achieve desired part geometries. One of the most important considerations in
sheet metal manufacture is the condition of the sheet metal's edges, particularly with regard to
the part after manufacture. Edge bending operations are commonly used in industrial sheet
metal processing and involve bending a section of the metal that is small relative to the part.
These sections are located at the edges. Edge bending is used to eliminate sharp edges, to
provide geometric surfaces for purposes such as joining, to protect the part, to increase
stiffness and for cosmetic appearance.
Flanging is a process that bends an edge, usually to a 90 degree angle.
Figure:273
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Sheet Metal Bending
Sometimes the sheet metal's material is purposely subjected to tensions or compressions, in
the processes of stretch flanging and shrink flanging respectively. In addition to bending the
edge, these operations also give it a curve.
Figure:274
Beading is common in the edge treatment of sheet metal parts and can also be used to form
the working structure of parts, such as hinges. Beading forms a curl over a part's edge. This
bead can be formed over a straight or curved axis. There are many different techniques for
forming a bead. Some methods form the bead progressively, with multiple stages, using
several different die arrangements. Other sheet metal beading processes produce a bead with a
single die. In a process called wiring, the metal's edge is bent over a wire. How the bead is
formed will depend on the specific requirements of the manufacturing process and sheet metal
part.
Figure:275
Hemming is an edge bending process in which the edge of the sheet is bent completely over
on itself.
Figure:276
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Sheet Metal Bending
Seaming is a sheet metal joining process. Seaming involves bending the edges of two parts
over on each other. The strength of the metal resists breaking the joint, because the material is
plastically deformed into position. As the bends are locked together, each bend helps resist the
deformation of the other bend, providing a well fortified joint structure. Double seaming has
been employed to create watertight or airtight joints between sheet metal parts.
Figure:277
Roll Bending
Roll bending provides a technique that is useful for relatively thick work. Although sheets of
various sizes and thicknesses may be used, this is a major manufacturing process for the metal
bending of large pieces of plate. Roll bending uses three rolls to feed and bend the plate to the
desired curvature. The arrangement of the rolls determines the exact bend of the work.
Different curves are obtained by controlling the distance and angle between the rolls. A
moveable roll provides the ability to control the curve. The work may already have some
curve to it, often it will be straight. Beams, bars and other stock metal is also bent using this
process.
Figure:278
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Sheet Metal Bending
Sheet Metal Roll Forming
Roll forming of sheet metal is a continuous manufacturing process, that uses rolls to bend a
sheet metal cross section of a certain geometry. Often several rolls may be employed, in
series, to continuously bend stock. Similar to shape rolling, but roll forming does not involve
material redistribution of the work, only bending. Like shape rolling, roll forming usually
involves bending of the work in sequential steps. Each roll will form the sheet metal to a
certain degree, in preparation for the next roll. The final roll completes the geometry.
Channels of different types, gutters, siding and panels for structural purposes are common
items manufactured in mass production by roll forming. Rolls are usually fed from a sheet
metal coil. The entry roll is supplied as the coil unwinds during the process. Once formed,
continuous products can be cut to desired lengths to create discrete parts. Closed sections such
as squares and rectangles can be continuously bent from sheet metal coil. Frames for doors
and windows are manufactured by this method. Sheet metal coil is often roll bent into thin
walled pipe that is welded together, at its seam. The welding of the continuous product is
incorporated into the rolling process. Roll forming of channels is a continuous alternative to a
discrete channel bending process, such as the one illustrated in figure 269. Figure 279 shows a
simple sequence used to produce a channel.
Figure:279
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Sheet Metal Bending
This channel could be produced with a punch and die. However, in that case, the length of the
channel would be limited by the length of the punch and die. Roll forming allows for a
continuous part, (limited practically to the length of the sheet metal coil), that can be cut to
whatever size needed. Productivity is also increased, with the elimination of loading and
unloading of work. Rolls for sheet metal roll forming are typically made of grey cast iron or
carbon steel. Lubrication is important and affects forces and surface finish. Sometimes rolls
will be chromium plated to improve surface quality.
Mechanics Of Sheet Metal Bending
To understand the mechanics of sheet metal bending, an understanding of the material
properties, characteristics and behaviors of metal, is necessary. Particularly important is the
topic of elastic and plastic deformation of metal. Information on the properties of metals, with
relation to manufacturing, can be found in an earlier section, (metal forming). It should be
understood also that sheet metal bending produces localized plastic deformation and
essentially no change in sheet thickness, for most operations. It does not create metal flow that
affects regions away from the bend.
The force required to perform a bend is largely dependent upon the bend and the specific
metal bending process, because the mechanics of each process can vary considerably. Proper
lubrication is essential to controlling forces and has an effect on the process. In punch and die
operations, the size of the die opening is a major factor in the force necessary to perform the
bending. Increasing the size of the die opening will decrease the necessary bending force. As
the sheet metal is bent, the force needed will change. Usually it is important to determine the
maximum necessary bending force, to access machine capacity requirements.
The important factors influencing the mechanics of bending are material, sheet thickness,
width over which bend occurs, radius of bend, bend angle, machinery, tooling and specific
metal bending process. Bending a sheet will create forces that act in the bend region and
through the thickness of the sheet. The material towards the outside of the bend is in tension
and the material towards the inside is in compression. Tension and compression are opposite,
therefore when moving from one to the other a zero region must exist. At this zero region no
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Sheet Metal Bending
forces are exerted on the material. When sheet metal bending, this zero region occurs along a
continuous plane within the part's thickness, called the neutral axis. The location of this axis
will depend on the different bending and sheet metal factors. However, a generic
approximation for the location of the axis could be 40 percent of sheet thickness, measured
from the inside of the bend. Another characteristic of the neutral axis is that because of the
lack of forces, the length of the neutral axis remains the same. Fundamentally, to one side of
the neutral axis the material is in tension, to the other side the material is in compression. The
magnitude of the tension or compression increases with increasing distance from the axis.
Figure:280
If a relatively small amount of force is exerted on a metal part, it will deform elastically and
recover its shape, when the force is removed. In order for plastic deformation of metal to
occur, a minimum threshold of force must be reached. The force acting on the neutral axis is
zero and increases with distance from this region. The minimum threshold of force required
for plastic deformation is not reached until a certain distance from the neutral axis in either
direction. The material between these regions is only plastically deformed, due to the low
magnitude of forces. These regions run parallel to, and form an elastic core around, the neutral
axis.
Figure:281
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Sheet Metal Bending
When the force used to create the bend is removed, the recovery of the elastic region results in
the occurrence of springback. Springback is the partial recovery of the work from the bend to
its geometry before the bending force was applied. The magnitude of springback depends
largely on the modulus of elasticity and the yield strength of the material. Typically the results
of springback will only act to increase the bend angle by a few degrees, however, all sheet
metal bending processes must consider the factor of springback.
Figure:282
Methods Of Eliminating Springback
Techniques have been developed, in manufacturing industry, that can eliminate the effects of
springback. One common technique is over bending. The amount of springback is calculated
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Sheet Metal Bending
and the sheet metal is over bent to a smaller bend angle than needed. Recovery of the material
from springback results in a calculated increase in bend angle. This increase makes the
recovered bend angle exactly what was originally planned.
Figure:283
Another method for eliminating springback is by plastically deforming the material in the
bend region. Localized compressive forces between the punch and die in that area will
plastically deform the elastic core, preventing springback. This can be done by applying
additional force through the tip of the punch after completion of bending. A technique known
as bottoming, or bottoming the punch.
Figure:284
Stretch forming is a metal bending technique that eliminates most of the springback in a bend.
Subjecting the work to tensile stress while bending will force the elastic region to be
plastically deformed. Stretch forming can not be performed for some complex bends and for
very sharp angles. The amount of tension must be controlled to avoid cracking of the sheet
metal. Stretch forming is a process often used in the aircraft building industry.
Figure:285
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Sheet Metal Bending
Sheet Metal Bendability
Bendability of sheet metal is the characteristic degree to which a particular sheet metal part
can be bent without failure. Bendability is related to the more general term of formability,
discussed in the sheet metal forming section. The bendability will change for different
materials and sheet thicknesses. Also, the mechanics of the manufacturing process will affect
bendability, since different tooling and sheet geometries will cause different force
distributions.
Metal bending tends to be a less complicated process than deep drawing in the analysis of
forces acting during the operation. One simple method to quantify bendability is to bend a
rectangular sheet metal specimen until it cracks on the outer surface. The radius of bend at
which cracking first occurs is called the minimum bend radius. Minimum bend radius is often
expressed in terms of sheet thickness, (ie. 2T, 4T). The higher the minimum bend radius, the
lower the bendability. A minimum bend radius of 0 indicates that the sheet can be folded over
on itself. Anisotropy of the sheet metal is an important factor in bending. If the sheet is
anisotropic the bending should be performed in the preferred direction. A test to determine
anisotropy is discussed in the sheet metal forming section.
The condition of a sheet metal's edges will influence bendability. Often cracks may propagate
from the edges. Rough edges can decrease the bendability of a sheet metal part. Cold working
at the edges, or within a part, can also reduce bendability. Vacancies within sheet metal can be
another source of material failure while bending. The presence of vacancies will reduce metal
bendability. Impurities in the material, particularly in the form of inclusions, can also
propagate cracks and will decrease bendability. Pointed or sharply shaped inclusions are more
detrimental to bendability than round inclusions. Surface quality of the sheet metal can make a
difference in bending manufacture. Rough surfaces can increase the likelihood of the sheet
cracking under force.
To mitigate these problems, and optimize the bendability of sheet metal, care should be taken
all the way through the manufacturing process. High quality sheet metal comes from high
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Sheet Metal Bending
quality metal. Effective refining techniques, along with a sound sheet metal rolling process
should close up vacancies, break up or eliminate inclusions and provide a sheet metal product
with a smooth surface. Edge treatment such as trimming, or fine blanking, can improve edge
quality. Sometimes cold worked areas can be machined out. Annealing the part to eliminate
regions of cold working and increase ductility also improves metal bendability. Bending
operations are sometimes performed on heated parts, because heating will cause the metal's
bendability to go up. Sheet metal may also, on occasion, be formed in a high pressure
environment, which is another way to make it more bendable.
Cutting And Bending Processes
Some manufacturing processes involve both cutting and bending of the sheet metal. Lancing
is a process that cuts and bends the sheet to create a raised geometry. Lancing may be used to
increase the heat dissipation capacity of sheet metal parts, for example. Another common
process that employs both cutting and bending is piercing. Not to be confused with the forging
process of piercing. Piercing is used to create a hole in a sheet metal part. Unlike blanking,
which creates a slug, piercing does not remove material. The punch is pointed and can pierce
the sheet. As the punch widens the hole the material is bent into an internal flange for the
hole. This flange may be useful for some applications.
Figure:286
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Sheet Metal Bending
Metal Tube Bulging
Tube bulging is a sheet metal manufacturing process in which some part of the internal
geometry of a hollow metal tube is subjected to pressure, causing the tube to bulge outward.
The area being bulged is usually constrained within a die that can control its geometry. Total
length of the tube will be decreased because of the widening of the bulging area. There are
different metal bulging techniques employed in manufacturing industry.
One main group of processes uses an elastomer plug, usually polyurethane. This plug is
placed within the tube. Pressure is applied to the elastomer causing it to bulge. Expanding
outward, the plug bends the sheet metal tube. Upon removal of the force, the elastomer plug
returns to its original shape and can be easily removed. Polyurethane plugs are durable and
will create a good pressure distribution over the surface during bending. Hydraulic pressure
may also be used to produce the same bulging effect. However elastomer plugs are cleaner,
easy to remove and require less complicated tooling. Split dies are used to facilitate the
removal of the part.
Figure:287
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Sheet Metal Bending
Metal Tube Bending
Tubes, rods, bars and other cross sections are also subject to metal bending operations. It
should be remembered that when bending a metal part, springback is always a factor. Several
special manufacturing processes have been developed for the bending of hollow tubes. These
operations can also be used on solid rods. Hollow tubes have the characteristic that they may
collapse when bent. Tubes may also crack or tear, the material's ductility is important when
considering tube failure.
As the bend radius goes down, the tendency to collapse increases. Bend radius in metal tube
bending is measured from the tube's centerline. The other major factor determining collapse is
the wall thickness of the tube. Tubes with a greater wall thickness are less likely to collapse.
Bending a thick walled tube to a large radius is usually not a problem, as far as collapse is
concerned. However, as wall thickness decreases and/or bend radius goes down, solutions
must be found to prevent tube collapse. One solution is to fill the tube with sand before
bending. Another method would be to place a plastic plug of some sort in the tube, then bend
it. Both the sand and the plastic plug act to provide internal structural support, greatly
increasing the ability to bend the tube without collapse.
Stretch bending is a process in which a tube is formed by a stretching force parallel to the
tube's axis and a simultaneous bending force acting to pull the tube over a form block. The
block is fixed and the forces are applied to the ends of the tube.
Figure:288
Draw bending involves clamping the tube near its end to a rotating form block. A pressure pad
is also used to hold the tube stock. As the form block rotates, the tube is bent.
Figure:289
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Sheet Metal Bending
Compression bending is a tube bending process that has some similarities to edge bending of
sheet metal with a wiping die. The tube stock is held by force to a fixed form block. A wiper
like die applies force, bending the tube over the form block.
Figure:290
TOP
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Sheet Metal Bending
Manufacturing Home
Sheet Metal Bending
SHEET METAL
FABRICATION
Sheet Metal Manufacturing
Sheet Metal Cutting
Deep Drawing Sheet Metal
Sheet Metal Ironing
Sheet Metal Spinning
Rubberforming Sheet Metal
High Energy Rate
Forming Of Sheet Metal
MANUFACTURING
PROCESSES
Metal Casting
Metal Forming
Metal Rolling
Metal Forging
Metal Extrusion
Metal Drawing
Powder Processes
Bending of sheet metal is a common and vital process in manufacturing industry. Sheet metal
bending is the plastic deformation of the work over an axis, creating a change in the part's
geometry. Similar to other metal forming processes, bending changes the shape of the work
piece, while the volume of material will remain the same. In some cases bending may produce
a small change in sheet thickness. For most operations, however, bending will produce
essentially no change in the thickness of the sheet metal. In addition to creating a desired
geometric form, bending is also used to impart strength and stiffness to sheet metal, to change
a part's moment of inertia, for cosmetic appearance and to eliminate sharp edges.
Figure:264
Metal bending enacts both tension and compression within the material. Mechanical
principles of metals, particularly with regard to elastic and plastic deformation, are important
to understanding sheet metal bending and are discussed in the fundamentals of metal forming
section. The effect that material properties will have in response to the conditions of
manufacture will be a factor in sheet metal process design. Usually sheet metal bending is
performed cold but sometimes the work may be heated, to either warm or hot working
temperature.
Most sheet metal bending operations involve a punch die type setup, although not always.
There are many different punch die geometries, setups and fixtures. Tooling can be specific to
a bending process and a desired angle of bend. Bending die materials are typically gray iron,
or carbon steel, but depending on the work piece, the range of punchdie materials varies from
hardwood to carbides. Force for the punch and die action will usually be provided by a press.
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Sheet Metal Bending
A work piece may undergo several metal bending processes. Sometimes it will take a series of
different punch and die operations to create a single bend. Or many progressive bending
operations to form a certain geometry.
Sheet metal is referenced with regard to the work piece when bending processes are discussed
in this section. However, many of the processes covered can also be applied to plate metal as
well. References to sheet metal work pieces may often include plate. Some bending operations
are specifically designed for the bending of differently shaped metal pieces, such as for
cabinet handles. Tube and rod bending is also widely performed in modern manufacturing.
Bending Processes
Bending processes differ in the methods they use to plastically deform the sheet or plate.
Work piece material, size and thickness are important factors when deciding on a type of
metal bending process. Also important is the size of the bend, bend radius, angle of bend,
curvature of bend and location of bend in the work piece. Sheet metal process design should
select the most effective type of bending process based on the nature of the desired bend and
the work material. Many bends can be effectively formed by a variety of different processes
and available machinery will often determine the bending method.
One of the most common types of sheet metal manufacturing processes is V bending. The V
shaped punch forces the work into the V shaped die and hence bends it. This type of process
can bend both very acute and very obtuse angles, also anything in between, including 90
degrees.
Figure:265
Edge bending is another very common sheet metal process and is performed with a wiping
die. Edge bending gives a good mechanical advantage when forming a bend. However, angles
greater than 90 degrees will require more complex equipment, capable of some horizontal
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Sheet Metal Bending
force delivery. Also, wiping die employed in edge bending must have a pressure pad. The
action of the pressure pad may be controlled separately than that of the punch. Basically the
pressure pad holds a section of the work in place on the die, the area for the bend is located on
the edge of the die and the rest of the work is held over space like a cantilever beam. The
punch then applies force to the cantilever beam section, causing the work to bend over the
edge of the die.
Figure:266
Rotary bending forms the work by a similar mechanism as edge bending. However, rotary
bending uses a different design than the wiping die. A cylinder, with the desired angle cut out,
serves as the punch. The cylinder can rotate about one axis and is securely constrained in all
other degrees of motion by its attachment to the saddle. The sheet metal is placed cantilevered
over the edge of the lower die, similar to the setup in edge bending. Unlike in edge bending,
with rotary bending, there is no pressure pad. Force is transmitted to the punch causing it to
close with the work. The groove on the cylinder is dimensioned to create the correctly angled
bend. The groove can be less than or greater than 90 degrees allowing for a range of acute and
obtuse bends. The cylinders V groove has two surfaces. One surface contacts the work
transmitting pressure and holding the sheet metal in place on the lower die. As force is
transmitted through the cylinder it rotates, causing the other surface to bend the work over the
edge of the die, while the first surface continues to hold the work in place. Rotary bending
provides a good mechanical advantage.
This process provides benefits over a standard edge bending operation, in that it eliminates the
need for a pressure pad and it is capable of bending over 90 degrees without any horizontally
acting equipment. Rotary bending is relatively new and is gaining popularity in manufacturing
industry.
Figure:267
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Sheet Metal Bending
Air bending is a simple method of creating a bend without the need for lower die geometry.
The sheet metal is supported by two surfaces a certain distance apart. A punch exerts force at
the correct spot, bending the sheet metal between the two surfaces.
Figure:268
Punch and die are manufactured with certain geometries, in order to perform specific bends.
Channel bending uses a shaped punch and die to form a sheet metal channel. A U bend is
made with a U shaped punch of the correct curvature.
Figure:269
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Sheet Metal Bending
Many bending operations have been developed to produce offsets and form the sheet metal for
a variety of different functions.
Figure:270
Some sheet metal bending operations involve the use of more than 2 die. Round tubes, for
example, can be bent from sheet metal using a multiple action machine. The hollow tube can
be seamed or welded for joining.
Figure:271
Corrugating is a type of bending process in which a symmetrical bend is produced across the
width of sheet metal and at a regular interval along its entire length. A variety of shapes are
used for corrugating, but they all have the same purpose, to increase the rigidity of the sheet
metal and increase its resistance to bending moments. This is accomplished by a work
hardening of the metal and a change in the sheet's moment of inertia, caused by the bend's
geometry. Corrugated sheet metal is very useful in structural applications and is widely used
in the construction industry.
Figure:272
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Sheet Metal Bending
Edge Bending Processes
Sheet metal of different sizes can be bent an innumerable amount of ways, at different
locations, to achieve desired part geometries. One of the most important considerations in
sheet metal manufacture is the condition of the sheet metal's edges, particularly with regard to
the part after manufacture. Edge bending operations are commonly used in industrial sheet
metal processing and involve bending a section of the metal that is small relative to the part.
These sections are located at the edges. Edge bending is used to eliminate sharp edges, to
provide geometric surfaces for purposes such as joining, to protect the part, to increase
stiffness and for cosmetic appearance.
Flanging is a process that bends an edge, usually to a 90 degree angle.
Figure:273
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Sometimes the sheet metal's material is purposely subjected to tensions or compressions, in
the processes of stretch flanging and shrink flanging respectively. In addition to bending the
edge, these operations also give it a curve.
Figure:274
Beading is common in the edge treatment of sheet metal parts and can also be used to form
the working structure of parts, such as hinges. Beading forms a curl over a part's edge. This
bead can be formed over a straight or curved axis. There are many different techniques for
forming a bead. Some methods form the bead progressively, with multiple stages, using
several different die arrangements. Other sheet metal beading processes produce a bead with a
single die. In a process called wiring, the metal's edge is bent over a wire. How the bead is
formed will depend on the specific requirements of the manufacturing process and sheet metal
part.
Figure:275
Hemming is an edge bending process in which the edge of the sheet is bent completely over
on itself.
Figure:276
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Seaming is a sheet metal joining process. Seaming involves bending the edges of two parts
over on each other. The strength of the metal resists breaking the joint, because the material is
plastically deformed into position. As the bends are locked together, each bend helps resist the
deformation of the other bend, providing a well fortified joint structure. Double seaming has
been employed to create watertight or airtight joints between sheet metal parts.
Figure:277
Roll Bending
Roll bending provides a technique that is useful for relatively thick work. Although sheets of
various sizes and thicknesses may be used, this is a major manufacturing process for the metal
bending of large pieces of plate. Roll bending uses three rolls to feed and bend the plate to the
desired curvature. The arrangement of the rolls determines the exact bend of the work.
Different curves are obtained by controlling the distance and angle between the rolls. A
moveable roll provides the ability to control the curve. The work may already have some
curve to it, often it will be straight. Beams, bars and other stock metal is also bent using this
process.
Figure:278
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Sheet Metal Roll Forming
Roll forming of sheet metal is a continuous manufacturing process, that uses rolls to bend a
sheet metal cross section of a certain geometry. Often several rolls may be employed, in
series, to continuously bend stock. Similar to shape rolling, but roll forming does not involve
material redistribution of the work, only bending. Like shape rolling, roll forming usually
involves bending of the work in sequential steps. Each roll will form the sheet metal to a
certain degree, in preparation for the next roll. The final roll completes the geometry.
Channels of different types, gutters, siding and panels for structural purposes are common
items manufactured in mass production by roll forming. Rolls are usually fed from a sheet
metal coil. The entry roll is supplied as the coil unwinds during the process. Once formed,
continuous products can be cut to desired lengths to create discrete parts. Closed sections such
as squares and rectangles can be continuously bent from sheet metal coil. Frames for doors
and windows are manufactured by this method. Sheet metal coil is often roll bent into thin
walled pipe that is welded together, at its seam. The welding of the continuous product is
incorporated into the rolling process. Roll forming of channels is a continuous alternative to a
discrete channel bending process, such as the one illustrated in figure 269. Figure 279 shows a
simple sequence used to produce a channel.
Figure:279
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This channel could be produced with a punch and die. However, in that case, the length of the
channel would be limited by the length of the punch and die. Roll forming allows for a
continuous part, (limited practically to the length of the sheet metal coil), that can be cut to
whatever size needed. Productivity is also increased, with the elimination of loading and
unloading of work. Rolls for sheet metal roll forming are typically made of grey cast iron or
carbon steel. Lubrication is important and affects forces and surface finish. Sometimes rolls
will be chromium plated to improve surface quality.
Mechanics Of Sheet Metal Bending
To understand the mechanics of sheet metal bending, an understanding of the material
properties, characteristics and behaviors of metal, is necessary. Particularly important is the
topic of elastic and plastic deformation of metal. Information on the properties of metals, with
relation to manufacturing, can be found in an earlier section, (metal forming). It should be
understood also that sheet metal bending produces localized plastic deformation and
essentially no change in sheet thickness, for most operations. It does not create metal flow that
affects regions away from the bend.
The force required to perform a bend is largely dependent upon the bend and the specific
metal bending process, because the mechanics of each process can vary considerably. Proper
lubrication is essential to controlling forces and has an effect on the process. In punch and die
operations, the size of the die opening is a major factor in the force necessary to perform the
bending. Increasing the size of the die opening will decrease the necessary bending force. As
the sheet metal is bent, the force needed will change. Usually it is important to determine the
maximum necessary bending force, to access machine capacity requirements.
The important factors influencing the mechanics of bending are material, sheet thickness,
width over which bend occurs, radius of bend, bend angle, machinery, tooling and specific
metal bending process. Bending a sheet will create forces that act in the bend region and
through the thickness of the sheet. The material towards the outside of the bend is in tension
and the material towards the inside is in compression. Tension and compression are opposite,
therefore when moving from one to the other a zero region must exist. At this zero region no
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forces are exerted on the material. When sheet metal bending, this zero region occurs along a
continuous plane within the part's thickness, called the neutral axis. The location of this axis
will depend on the different bending and sheet metal factors. However, a generic
approximation for the location of the axis could be 40 percent of sheet thickness, measured
from the inside of the bend. Another characteristic of the neutral axis is that because of the
lack of forces, the length of the neutral axis remains the same. Fundamentally, to one side of
the neutral axis the material is in tension, to the other side the material is in compression. The
magnitude of the tension or compression increases with increasing distance from the axis.
Figure:280
If a relatively small amount of force is exerted on a metal part, it will deform elastically and
recover its shape, when the force is removed. In order for plastic deformation of metal to
occur, a minimum threshold of force must be reached. The force acting on the neutral axis is
zero and increases with distance from this region. The minimum threshold of force required
for plastic deformation is not reached until a certain distance from the neutral axis in either
direction. The material between these regions is only plastically deformed, due to the low
magnitude of forces. These regions run parallel to, and form an elastic core around, the neutral
axis.
Figure:281
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When the force used to create the bend is removed, the recovery of the elastic region results in
the occurrence of springback. Springback is the partial recovery of the work from the bend to
its geometry before the bending force was applied. The magnitude of springback depends
largely on the modulus of elasticity and the yield strength of the material. Typically the results
of springback will only act to increase the bend angle by a few degrees, however, all sheet
metal bending processes must consider the factor of springback.
Figure:282
Methods Of Eliminating Springback
Techniques have been developed, in manufacturing industry, that can eliminate the effects of
springback. One common technique is over bending. The amount of springback is calculated
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and the sheet metal is over bent to a smaller bend angle than needed. Recovery of the material
from springback results in a calculated increase in bend angle. This increase makes the
recovered bend angle exactly what was originally planned.
Figure:283
Another method for eliminating springback is by plastically deforming the material in the
bend region. Localized compressive forces between the punch and die in that area will
plastically deform the elastic core, preventing springback. This can be done by applying
additional force through the tip of the punch after completion of bending. A technique known
as bottoming, or bottoming the punch.
Figure:284
Stretch forming is a metal bending technique that eliminates most of the springback in a bend.
Subjecting the work to tensile stress while bending will force the elastic region to be
plastically deformed. Stretch forming can not be performed for some complex bends and for
very sharp angles. The amount of tension must be controlled to avoid cracking of the sheet
metal. Stretch forming is a process often used in the aircraft building industry.
Figure:285
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Sheet Metal Bendability
Bendability of sheet metal is the characteristic degree to which a particular sheet metal part
can be bent without failure. Bendability is related to the more general term of formability,
discussed in the sheet metal forming section. The bendability will change for different
materials and sheet thicknesses. Also, the mechanics of the manufacturing process will affect
bendability, since different tooling and sheet geometries will cause different force
distributions.
Metal bending tends to be a less complicated process than deep drawing in the analysis of
forces acting during the operation. One simple method to quantify bendability is to bend a
rectangular sheet metal specimen until it cracks on the outer surface. The radius of bend at
which cracking first occurs is called the minimum bend radius. Minimum bend radius is often
expressed in terms of sheet thickness, (ie. 2T, 4T). The higher the minimum bend radius, the
lower the bendability. A minimum bend radius of 0 indicates that the sheet can be folded over
on itself. Anisotropy of the sheet metal is an important factor in bending. If the sheet is
anisotropic the bending should be performed in the preferred direction. A test to determine
anisotropy is discussed in the sheet metal forming section.
The condition of a sheet metal's edges will influence bendability. Often cracks may propagate
from the edges. Rough edges can decrease the bendability of a sheet metal part. Cold working
at the edges, or within a part, can also reduce bendability. Vacancies within sheet metal can be
another source of material failure while bending. The presence of vacancies will reduce metal
bendability. Impurities in the material, particularly in the form of inclusions, can also
propagate cracks and will decrease bendability. Pointed or sharply shaped inclusions are more
detrimental to bendability than round inclusions. Surface quality of the sheet metal can make a
difference in bending manufacture. Rough surfaces can increase the likelihood of the sheet
cracking under force.
To mitigate these problems, and optimize the bendability of sheet metal, care should be taken
all the way through the manufacturing process. High quality sheet metal comes from high
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quality metal. Effective refining techniques, along with a sound sheet metal rolling process
should close up vacancies, break up or eliminate inclusions and provide a sheet metal product
with a smooth surface. Edge treatment such as trimming, or fine blanking, can improve edge
quality. Sometimes cold worked areas can be machined out. Annealing the part to eliminate
regions of cold working and increase ductility also improves metal bendability. Bending
operations are sometimes performed on heated parts, because heating will cause the metal's
bendability to go up. Sheet metal may also, on occasion, be formed in a high pressure
environment, which is another way to make it more bendable.
Cutting And Bending Processes
Some manufacturing processes involve both cutting and bending of the sheet metal. Lancing
is a process that cuts and bends the sheet to create a raised geometry. Lancing may be used to
increase the heat dissipation capacity of sheet metal parts, for example. Another common
process that employs both cutting and bending is piercing. Not to be confused with the forging
process of piercing. Piercing is used to create a hole in a sheet metal part. Unlike blanking,
which creates a slug, piercing does not remove material. The punch is pointed and can pierce
the sheet. As the punch widens the hole the material is bent into an internal flange for the
hole. This flange may be useful for some applications.
Figure:286
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Metal Tube Bulging
Tube bulging is a sheet metal manufacturing process in which some part of the internal
geometry of a hollow metal tube is subjected to pressure, causing the tube to bulge outward.
The area being bulged is usually constrained within a die that can control its geometry. Total
length of the tube will be decreased because of the widening of the bulging area. There are
different metal bulging techniques employed in manufacturing industry.
One main group of processes uses an elastomer plug, usually polyurethane. This plug is
placed within the tube. Pressure is applied to the elastomer causing it to bulge. Expanding
outward, the plug bends the sheet metal tube. Upon removal of the force, the elastomer plug
returns to its original shape and can be easily removed. Polyurethane plugs are durable and
will create a good pressure distribution over the surface during bending. Hydraulic pressure
may also be used to produce the same bulging effect. However elastomer plugs are cleaner,
easy to remove and require less complicated tooling. Split dies are used to facilitate the
removal of the part.
Figure:287
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Metal Tube Bending
Tubes, rods, bars and other cross sections are also subject to metal bending operations. It
should be remembered that when bending a metal part, springback is always a factor. Several
special manufacturing processes have been developed for the bending of hollow tubes. These
operations can also be used on solid rods. Hollow tubes have the characteristic that they may
collapse when bent. Tubes may also crack or tear, the material's ductility is important when
considering tube failure.
As the bend radius goes down, the tendency to collapse increases. Bend radius in metal tube
bending is measured from the tube's centerline. The other major factor determining collapse is
the wall thickness of the tube. Tubes with a greater wall thickness are less likely to collapse.
Bending a thick walled tube to a large radius is usually not a problem, as far as collapse is
concerned. However, as wall thickness decreases and/or bend radius goes down, solutions
must be found to prevent tube collapse. One solution is to fill the tube with sand before
bending. Another method would be to place a plastic plug of some sort in the tube, then bend
it. Both the sand and the plastic plug act to provide internal structural support, greatly
increasing the ability to bend the tube without collapse.
Stretch bending is a process in which a tube is formed by a stretching force parallel to the
tube's axis and a simultaneous bending force acting to pull the tube over a form block. The
block is fixed and the forces are applied to the ends of the tube.
Figure:288
Draw bending involves clamping the tube near its end to a rotating form block. A pressure pad
is also used to hold the tube stock. As the form block rotates, the tube is bent.
Figure:289
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Compression bending is a tube bending process that has some similarities to edge bending of
sheet metal with a wiping die. The tube stock is held by force to a fixed form block. A wiper
like die applies force, bending the tube over the form block.
Figure:290
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