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ProcessesSheet Metal Bending
Bending of sheet metal is a common and vital process in
manufacturing industry.Sheet metal bendingis 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 punch-die materials varies from hardwood to
carbides. Force for the punch and die action will usually be
provided by a press. 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 ProcessesBending 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 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
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
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
Edge Bending ProcessesSheet 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
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
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 BendingRoll 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
Sheet Metal Roll FormingRoll 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
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 BendingTo 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 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
When the force used to create the bend is removed, the recovery
of the elastic region results in the occurrence ofspringback.
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 SpringbackTechniques 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 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
Sheet Metal BendabilityBendability 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 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 ProcessesSome 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
Metal Tube BulgingTube 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
Metal Tube BendingTubes, 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
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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