http://thelibraryofmanufacturing.com/rotary_tube_piercing.htmlMetal
Rolling
Metal rollingis one of the most important manufacturing
processes in the modern world. The large majority of all metal
products produced today are subject to metal rolling at one point
in their manufacture. Metal rolling is often the first step in
creating raw metal forms. Theingotorcontinuous castingis hot rolled
into a bloom or a slab, these are the basic structures for the
creation of a wide range of manufactured forms. Blooms typically
have a square cross section of greater than 6x6 inches. Slabs are
rectangular and are usually greater than 10 inches in width and
more than 1.5 inches in thickness. Rolling is most often,
(particularly in the case of the conversion of an ingot or
continuous casting), performed hot.At arolling mill, blooms and
slabs are further rolled down to intermediate parts such as plate,
sheet, strip, coil, billets, bars and rods. Many of these products
will be the starting material for subsequent manufacturing
operations such as forging, sheet metal working, wire drawing,
extrusion, and machining. Blooms are often rolled directly into I
beams, H beams, channel beams, and T sections for structural
applications. Rolled bar, of various shapes and special cross
sections, is used in the machine building industry, as well as for
construction. Rails, for the production of railroad track, are
rolled directly from blooms. Plates and sheets are rolled from
slabs, and are extremely important in the production of a wide
range of manufactured items. Plates are generally considered to be
over 1/4", (6mm), in thickness. Plates are used in heavy
applications like boilers, bridges, nuclear vessels, large
machines, tanks, and ships. Sheet is used for the production of car
bodies, buses, train cars, airplane fuselages, refrigerators,
washers, dryers, other household appliances, office equipment,
containers, and beverage cans, to name a few. It is important to
understand the significance of metal rolling in industry today, as
well as its integration with other manufacturing processes.
Figure:129
Principles Of Metal RollingMost metal rolling operations are
similar in that the work material is plastically deformed by
compressive forces between two constantly spinning rolls. These
forces act to reduce the thickness of the metal and affect its
grain structure. The reduction in thickness can be measured by the
difference in thickness before and after the reduction, this value
is called thedraft. In addition to reducing the thickness of the
work, the rolls also act to feed the material as they spin in
opposite directions to each other. Friction is therefore a
necessary part of the rolling operation, but too much friction can
be detrimental for a variety of reasons. It is essential that in a
metal rolling process the level of friction between the rolls and
work material is controlled, lubricants can help with this. A basic
flat rolling operation is shown in figure:130, this manufacturing
process is being used to reduce the thickness of a work
piece.Figure:130
During a metal rolling operation, the geometric shape of the
work is changed but its volume remains essentially the same. The
roll zone is the area over which the rolls act on the material, it
is here that plastic deformation of the work occurs. An important
factor in metal rolling is that due to the conservation of the
volume of the material with the reduction in thickness, the metal
exiting the roll zone will be moving faster than the metal entering
the roll zone. The rolls themselves rotate at a constant speed,
hence at some point in the roll zone the surface velocity of the
rolls and that of the material are exactly the same. This is termed
theno slip point. Before this point the rolls are moving faster
than the material, after this point the material is moving faster
than the rolls.Figure:131
Sometimes in metal rolling practice, tension, (force), is
applied to a work piece as it is being rolled. This tension may be
applied to the front, (front tension), the back, (back tension), or
both sides. This technique will assist the forces necessary to form
the work, and is usually used on hard to roll
materials.Figure:132
Spreading In Metal RollingIn metal rolling operations, the
plastic deformation causing a reduction in thickness will also
cause an increase in the width of the part, this is
calledspreading.Figure:133
When the work being processed has a high width to thickness
ratio, the increase in width is relatively small and usually of no
concern in industrial manufacturing practice. In cases of low width
to thickness ratios, such as a bar with a square cross section,
spreading can be an issue. Vertical rolls can be employed to edge
the work and maintain a constant width.Figure:134
Grain Structure In Metal RollingIn common industrial
manufacturing industry, the ingot or continuous casting is hot
rolled into a bloom or slab. In addition to producing a useful
shape for further processing, the hot rolling process converts the
cast grain structure into a wrought grain structure. The initial
cast material will possess a non uniform grain structure, typically
large columnar grains that grow in the direction of solidification.
These structures are usually brittle with weak grain boundaries.
Cast structure characteristically contains many defects such as
porosity caused by gases, shrinkage cavities, and solid inclusions
of foreign material that becomes trapped in the metal, such as
metallic oxides.Rolling a metal above its recrystallization
temperature breaks apart the old grain structure and reforms a new
one. Grain boundaries are destroyed and new tougher ones are
formed, along with a more uniform grain structure. Metal rolling
pushes material, closing up vacancies and cavities within the
metal. In addition, hot rolling breaks up inclusions and
distributes their material throughout the work.Figure:135
It should be apparent that the advantages of metal forming are
not just in the creation of useful geometric forms but also in the
creation of desired material properties as well. Cold rolling
processes as discussed earlier, are useful for imparting strength
and favorable grain orientation. Since metal rolling affects grain
orientation, a part can be rolled in a way as to create grains
oriented in a direction such that they give directional strength to
a part useful to that part's specific application. An example of
this can be the difference in grain structure between the threads
of a machined bolt and a rolled bolt. The favorable grain
orientation of the cold rolled bolt will give it directional
strength beneficial to its application.Figure:136
Rolls For Metal RollingMetal rolling manufacturing can produce a
wide range of different products. The width of rolled work can be
as much as several meters, or narrower than a thousandth of an
inch. Metal rolling manufacture also creates rolled work over a
wide range of thicknesses. Metal plates for some boilers may be
rolled to a thickness of 12 inches, while foil for wrapping
cigarettes and candy can be .0003 inches thick. Rolls used in metal
rolling are of various sizes and geometries. In flat rolling
processes, during industrial manufacture, the rolls may typically
be 24 to 54 inches in diameter. In some metal rolling operations,
in the forming of very thin work, the rolls can be as small as 1/4
inch.Rolls are subject to extreme operating conditions during the
metal rolling process. Conditions include, tremendous forces,
bending moments, thermal stresses, and wear. Roll materials are
selected for strength, rigidity, and wear resistance. Roll
materials vary dependent upon the specific metal rolling process.
Common roll materials are cast iron, cast steel, and forged steel.
Forged rolls are stronger and more rigid than cast rolls but are
more difficult to manufacture. In industrial metal manufacturing
processes, rolls are commonly made from nickel steel or molybdenum
steel alloys. With metal rolling operations of certain materials,
rolls made of tungsten carbide can provide extreme resistance to
deflection.
Roll DeflectionsStrength and rigidity are important
characteristics of the rolls used to form product in metal rolling
manufacture. The particular attributes of the rolls will affect
dimensional accuracy as well as other factors in the operation.
During the rolling process great forces act upon the rolls. Rolls
will be subject to different degrees of deflection. In any
particular metal rolling process, it is important to understand how
these deflections will affect the rolls and hence the work being
rolled. The rolls initially start out flat. During a basic flat
rolling operation, it can be observed that the work material will
exert greater force on the rolls towards the center of the material
than at its edges. This will cause the rolls to deflect more at the
center, and hence gives the work a greater thickness in the
middle.Figure:137
To solve this problem in industrial metal rolling manufacture,
the rolls are often ground so that they are thicker towards the
center in such a way as to exactly offset the deflection that will
occur during the process. This extra thickness is
calledcamber.Figure:138
The camber that must be ground into a roll is very specific to a
particular work width, material, and force load. A roll must
usually be manufactured for only one metal rolling process. In some
industrial metal rolling processes, rolls are given temporary
camber by applying forces through their bearings. Another way that
rolls deflect is by the shortening of their radius along the
contact of the work. In other words, they flatten like a tire on a
car might. This type of deflection is important to consider in
manufacturing practice, as it will affect roll radius calculations
and friction.
Defects In Metal RollingA wide variety of defects are possible
in metal rolling manufacture. Surface defects commonly occur due to
impurities in the material, scale, rust, or dirt. Adequate surface
preparation prior to the metal rolling operation can help avoid
these. Most serious internal defects are caused by improper
material distribution in the final product. Defects such as edge
cracks, center cracks, and wavy edges, are all common with this
method of metal manufacturing.Figure:139
Often times a sheet is not defective, it is just not flat
enough. In sheet metal industrial practice, a sheet may be passed
through a series of leveling rolls that flex the sheet in opposite
directions to flatten it. Another interesting defect that can occur
in flat rolling is alligatoring, where the work being rolled
actually splits in two during the process. The two parts of the
work material travel in opposite directions relative to their
respective rolls.Figure:140
In shape rolling manufacture, a work piece will often experience
different amounts of reduction in different areas of its cross
section. One of the goals of roll pass design is to properly design
a series of reductions in such a way as to mitigate the relative
differences in shape change between areas, in order to avoid
material defects. Improper reductions of the product can cause
warping or cracking of the material. Metal rolling practice is not
always the cause of warping or cracking, sometimes defects in the
metal being rolled may be the reason.Figure:141
Rolling MillsIn metal forming industry, rolls themselves do not
function in isolation. In a metal rolling process, rolls, stands,
bearings, housing, motors, and other mechanical equipment are all a
necessary part of the manufacturing operation. The place where all
the equipment for metal rolling manufacture is set up is called a
rolling mill. Rolling mills often vary in the type, number, and
position of rolls. Rolling mill arrangements commonly used in
manufacturing industry today include thetwo high mill, thetwo high
reversing mill, thethree high mill, thefour high mill, thecluster
mill, and thetandem rolling mill.Figure:142
In the two high reversing mill the direction of spin of the
rolls can be reversed. This enables the work to travel through in
one direction, then back through in the other direction. A series
of reductions can be made using the same set of rolls, by passing
the work back and forth. Disadvantages of the two high reversing
mill include the mechanical requirements and power to constantly
overcome and reverse the angular momentum of the
rolls.Figure:143
The three high rolling mill utilizes the principle of passing
the work back and forth to achieve a series of reductions. Unlike
the two high reversing mill, the three high mill has three rolls
that always spin in the same direction. An elevator mechanism lifts
and lowers the work so that it can be passed back and forth through
the rolls.
Figure:144
It is known, in metal rolling practice, that the amount of roll
force is reduced with a smaller radius of the rolls. Smaller radius
rolls, however, deflect easier and must be supported by other
rolls. The four high mill uses this principle with two smaller work
rolls each supported by a larger backing roll.Figure:145
The cluster mill, or Sendzimir mill, uses a small work roll
backed up by many other rolls. This extremely rigid setup is often
used for cold rolling high strength material to a very thin
width.Figure:146
The tandem rolling mill consists of several stands, that the
work material constantly passes through. At each stand the
thickness of the work strip is reduced a certain amount. The total
reduction between the first and last stand can be significant.
There are technical problems associated with tandem rolling, caused
particularly by the fact that the speed of the work material
increases as it passes through each stand. In manufacturing
practice, various control systems are used to keep the entire
operation synchronized. Once the particular technical problems and
initial setup investment is overcome, tandem rolling can provide a
great advantage in the cost and productivity of an industrial metal
rolling process. Tandem rolling can be even more advantageous when
integrated withcontinuous casting.
Figure:147