ASM Metals HandBook Vol. 14 - Forming and Forging
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Three-Roll Forming
Forming Large Cylinders
Cylinders that are large in diameter or in length can be formed by the three-roll process. Some special procedures may be
required, especially when the sheet is so thin that the cylinder cannot retain a round shape without support. Under these
conditions, overhead cranes or temporary braces or both can be used for support.
When flat blanks of the required length are unavailable, two or more sections can be welded together to obtain the
required length. The following example describes this practice in the forming of large cylinders.
Example 2: Forming 4.72 m (186 in.) OD Cylinders in a Pinch-Type Machine.
Cylinders 4.72 m (186 in.) in diameter and 2.84 m (112 in.) long were formed from blanks prepared by welding together
three sections of copper-clad low-carbon steel. The fabricated blanks were 14.8 m (580¾ in.) long by 2.84 m (112 in.)
wide by 14 mm ( in.) thick. The cylinders were formed cold on a 19 mm (¾ in.) by 3.7 m (12 ft) pinch-type machine.
The major problem in this operation was support because the work was not thick enough to provide natural support.
Overhead cranes were used to hold the formed section of the plate during rolling. The ends of the cylinder were tack
welded before the workpiece was removed from the machine. Temporary braces were then used to hold the cylinders in a
near-round condition while the seam was welded. After welding, outside rounding rings made from 38 × 127 mm (1½ × 5
in.) rectangular steel bars were used to hold the cylinder shape for subsequent attachment to other cylinders. In use, the
welded internal parts stiffened the assembly.
Three-Roll Forming
Forming Truncated Cones
Pyramid-type machines are generally used in the forming of truncated cones. There are two basic limitations on the shape
of conical configurations that can be formed by the three-roll process. First, the smaller diameter (A, Fig. 8) must be large
enough to maintain the established workpiece-to-roll relationship on minimum obtainable diameter, and second, the plate
must be thick enough to be formed by the holdback method shown in Fig. 8 so that the pressure buildup on the holdback
pin does not upset the plate edge or damage the holdback attachment or both.
Fig. 8 Rolling a truncated cone in a three-roll pyramid-type machine from a blank with preformed ends.
The size of cone that can be formed depends largely on the pitch, the spacing and length of rolls, and the diameter of the
upper roll. As the cone height increases, difficulties in getting the large diameter to follow the small diameter accurately
also increase. If the lower roll spacing is too wide, the size of the smaller diameter will be severely limited, because as the
work metal passes the holdback pin (Fig. 8), it will curve into the housing that supports the rolls and rolling will be
impossible.
Procedure. There are several significant differences between forming conical shapes and forming cylinders by the
pyramid-type three-roll process. In rolling cylinders, the blank is rectangular and is rolled in a direction perpendicular to
the rolls. In contrast, curved blanks are used for conical shapes, and they are rolled on a curve. In addition, no pin is
needed in rolling cylinders, and the top roll is pitched for forming cones but is straight and level when forming cylinders.
Before the developed blank is placed in the pyramid-type three-roll machine for forming a cone, the ends of the blank are
preformed in a pinch-type roll from the rear or in a press brake. For forming the cone, the top roll is pitched as shown in
Fig. 8. As the rolls drive the blank, its edge drags around the pin, and the various diameters of the cone are formed. The
blank is then rolled in multiple passes, dragging around the pin until the ends of the blank are closed. If the pitch of the
rolls matches the pitch of the entire length of the cone, a nearly true cone will result.
Truncated cones are sometimes produced by forming two semicircular half-cones and welding them together; the
completed workpiece has two longitudinal seams instead of one. In another method, two or more circular tapered sections
are formed and welded together. When produced by this method, the finished cone has one longitudinal seam and one or
more circumferential seams.
Three-Roll Forming
Forming Bars and Shapes
Pyramid rolls and machines with over-hanging rolls are used to form bars, bar sections, and structural shapes into circles.
Shapes that can be processed by this method include rounds, squares, flats (on the edge or on the flat), I-beams, L-shaped
structurals, and channels. Hardened roll sections that are adjustable to the thickness or cross section of the shape are used
(Fig. 9). Some bars or shapes can be formed with plain, flat rolls, but more often rolls conforming to the shape of the
unformed workpiece are required.
Fig. 9
Roll setup for forming an angle section into a circle. Guide rolls and guide fingers are not required for
this application.
The rolls are adjusted to produce the required workpiece diameter in the same manner as in rolling cylinders from sheet or
plate. Prebending the ends and rolling the workpiece back and forth are also employed in the three-roll forming of bars
and shapes. A significant difference in the rolling of bars and shapes is the frequent use of guide rollers or guide fingers or
both, mounted so as to contact the sides of the workpiece and prevent it from twisting during forming.
One of the most difficult shapes to form by the three-roll process is an angle with one leg inside the circle (Fig. 9). When
forming this shape, spiraling, twisting, buckling of the inside leg, and reduction of the angle between the two legs are
likely to occur. These difficulties can be minimized by using a hardened upper roll having a removable end plate and a
spacer that is not more than 0.81 mm ( in.) thicker than one leg of the angle (Fig. 9). The radius on the upper roll (R,
Fig. 9) must conform to the fillet radius of the angle to be rolled. The other end of this roll (R
1
, Fig. 9) can have the same
radius or a radius conforming to another angle to be rolled. This end of the upper roll can be used by reversing the roll end
for end. In production rolling of the same angle, common practice is to have the same values for R and R
1
(Fig. 9), thus
permitting double the roll life by reversing the roll. Guide rollers and fingers (not required for the section shown in Fig. 9)
also help to produce accurate circles from bar sections.
Three-Roll Forming
Out-of-Roundness
Out-of-roundness, or ovality, of cylinders produced by three-roll forming is caused by one or more of the following
variables:
• Varying thickness of the flat blank
• Varying hardness within the blank
• Overforming or underforming of the ends in the preforming operation
• Springback of the work metal
• Temperature of the metal being formed
• Number of passes
• Condition of the equipment
• Operator skill
The most important of the above factors is operator skill; condition of the equipment is also a major factor.
Variations in the mill product (thickness and hardness within a single sheet or plate) are seldom great enough to warrant
the extra cost that would be necessary for closer-than-normal control of the work metal. Out-of-roundness and other
dimensional variations in the product increase as workpiece diameter increases. Plate thickness above or below the actual
crown thickness of the rolls can also cause dimensional variations. For large or small workpieces, much out-of-roundness
is caused by variations in preforming the ends, regardless of whether pinch rolls or press brakes are used in preforming.
Springback is overcome by forming to a circle smaller than that required for the finished cylinder. However, the
overforming of high-springback material must be done with caution; as the elastic limit is exceeded, metals will take a
permanent set and too much overforming can result.
Hot forming may contribute to out-of-roundness because considerable plastic flow can take place when a steel
workpiece is heated to 870 °C (1600 °F) or higher. The amount of plastic flow varies as the bending load is applied
during forming, and variations are increased by uneven cooling of the work metal. Resulting variations in plate thickness
and curvature contribute to out-of-roundness.
Production Example. Without the use of special techniques or secondary operations, there is likely to be considerable
variation in out-of-roundness among workpieces that are intended to be identical and are produced under the same
conditions. This is demonstrated in the following example.
Example 3: Out-of-Roundness Variations in 44 Cylinders, 432 mm (17 in.) in
Diameter.
Cylinders 432 mm (17 in.) in diameter were produced on a pinch-type machine in one pass. Leading edges of the blanks
were prebent 15° for a distance of 50 mm (2 in.) with a 102 mm (4 in.) radius. After the cylinders were formed and tack
welded, measurements were taken. Out-of-roundness variation in 44 cylinders is plotted in Fig. 10.
Fig. 10 Out-of-roundness variations in 44 cylinders of the same diameter produced in a pinch-
type roll
machine. Dimensions given in inches.
The number of passes used to form a given cylinder can have a significant effect on out-of-roundness. In most
applications, two or more passes (sometimes as many as 12) will produce cylinders that are more nearly true round than
those formed in a single pass.
Methods of Correction. The most effective method for correcting out-of-roundness is to reroll the cylinder carefully
before welding. This operation causes a one-third greater load on the machine than the original rolling.
For cylinders having wall thickness no greater than about 9.5 mm (⅜ in.), a draw-down type of expander applied after
rolling and welding is an effective means of correcting out-of-roundness.
If design permits, beads or flanges can be rolled into the cylinder wall to reinforce it and to help maintain roundness. Oil
drums are examples of the effective use of this technique. When dimensional requirements are plus or minus a few
thousandths of an inch, stock must be allowed for machine boring the cylinder to the specified diameter.
Three-Roll Forming
Forming Speed
The speed of forming is a critical factor in product quality. Low-carbon steel plate up to 7.9 mm ( in.) thick is
sometimes rolled at speeds to 18 m/min (60 ft/min). For a speed this high, however, workpiece diameter is necessarily
medium to large, because it is impractical to control the machine for rolling small diameters at high speed.
The most commonly used speeds for cold forming (particularly for thick plate) range from 3.7 to 6.1 m/min (12 to 20
ft/min). This range is usually maintained in both cold and hot forming; however, to complete hot forming with a
minimum decrease in temperature of the work metal, it is sometimes necessary to increase the speed of the bending roll.
Three-Roll Forming
Roll Deflection
Roll deflection can be calculated by standard formulas, considering the roll as a simple beam supported at both ends. On
pyramid rolls, deflection is often minimized by support rollers applied to the lower rolls. These rolls act as backup rolls.
In forming heavy plate, pressures are high and all three rolls are crowned (made larger at the centers than at the ends).
Crowning is necessary because the rolls deflect under the bending load; if they were straight, all formed cylinders would
bulge somewhat at the center. Because the amount of deflection depends on the bending load, usual practice is to crown
the rolls enough to compensate for the average job in the plant. When forming plate thicker than the actual crown
deflection, the rolls are shimmed by running strips of thin metal (16, 14, 12, or 10 gage) between the rolls and the inside
diameter of the workpiece at the center of the rolls. This shimming compensates for excessive deflection.
When forming metal that is too thin to cause deflection of the rolls, crowning will cause the formed cylinder to be larger
in diameter at the ends than in the center. Correction can be made by shimming the ends of the rolls in a manner similar to
that described above for shimming the centers.
Three-Roll Forming
Alternative Processes
Three-roll forming is the most practical method of producing large cylinders and truncated cones from heavy plate.
Deep drawing is often the most economical method of producing small cylinders from sheet no thicker than 3.18 mm
(0.125 in.). Seamless cylinders or cones can be produced by deep drawing, piercing, and trimming (see the article "Deep
Drawing" in this Volume). However, as cylinder size or wall thickness increases, forming by deep drawing becomes
impracticable. In some applications, three-roll forming and welding are preferred for producing a hollow shape from
stock that is substantially thinner than 3.18 mm (0.125 in.).
Contour Roll Forming. Theoretically, the diameter and length of straight cylinders producible by contour roll forming
are almost unlimited. In practice, however, diameter and wall thickness are limited by the size of available equipment.
Contour roll forming is rarely used for rolling metal thicker than 6.35 mm (0.250 in.) and is most often used for
thicknesses less than 3.18 mm (0.125 in.). Therefore, contour roll forming is impractical for producing large heavy-wall
cylinders (see the article "Contour Roll Forming" in this Volume).
Forming two halves (semicircles) between dies in a press and then welding the two half-cylinders is sometimes
practical. However, even when presses of sufficient size are available for forming large plate, die cost is likely to be
prohibitive. For limited production, a press brake is often used to produce semicircles that can subsequently be joined by
welding into cylinders.
Three-Roll Forming
Safety
Forming rolls move relatively slowly, but require protection for the operator. The most positive method of protection is to
cover the nip point between the feed rolls. One effective guarding device is a solid metal plate covering the nip point
between the feed table and the rolls for the full length of the rolls. This plate, with a stud welded to each end, is attached
to slotted vertical brackets by nuts and washers so that it is adjustable vertically. The brackets are securely fastened to the
feed table.
The height of the feed table can be made adjustable by welding a nut to the lower end of each tubular leg. A long bolt,
with a large washer welded to the top of the head, is screwed into the nut to achieve the desired height. A locknut can be
used to prevent the bolt from turning because of vibration.
Emergency tripping bars connected to electric cutoff switches or, preferably, to reverse electric switches can be used to
stop the rolls. The bars may be at knee level in front of the operator, or directly in front of the bottom feed roll and far
enough below the feed point to avoid accidental tripping.
Feeding guides for narrow workpieces can be made of bar stock or angles that are bolted to the feed table. The guides
should be slotted for ease of adjustment to various widths of workpieces.
Contour Roll Forming
Introduction
CONTOUR ROLL FORMING (also known as roll forming or cold roll forming) is a continuous process for forming
metal from sheet, strip, or coiled stock into desired shapes of uniform cross section by feeding the stock through a series
of roll stations equipped with contoured rolls (sometimes called roller dies). There are two or more rolls per station. Most
contour roll forming is done by working the stock progressively in two or more stations until the finished shape is
produced.
Only bending takes place in contour roll forming; the stock thickness is unchanged except for a slight thinning at bend
radii. The process is particularly suited to the production of large quantities and long lengths to close tolerances and
involves a minimum of handling. Auxiliary operations, such as notching, slotting, punching, embossing, curving, and
coiling, can easily be combined with contour roll forming.
Contour roll forming is used in many diverse industries to produce a variety of shapes and products. The process is also
used for parts that were previously manufactured by extrusion processes. This use is limited, however, to parts that can be
redesigned to have a constant wall thickness. Industries that use roll-formed products include the automotive; building;
office furniture; home appliance and home product; medical; railcar; aircraft; and heating, ventilation, and air
conditioning (HVAC) industries.
Contour roll forming can be divided into two broad categories: a process using precut lengths of work metal (precut or
cut-to-length method), and a process that uses coil stock that is trimmed to size after forming (postcut method).
In precut operations, the work metal is cut to length before entering the forming machine. The precut process usually
employs a stacking and feeding system to move blanks into the machine, a contour roll forming machine operating at a
fixed speed of about 15 to 75 m/min (50 to 250 ft/min), an exit conveyor, and a stacking system. The precut method is
primarily used for low-volume operations and when notching cannot be easily accomplished in a postcut line. Often, the
material is run from a coil to a shear or blanking press and then fed mechanically to the contour roll former.
Tooling for the precut method is relatively inexpensive, because cutting requires only a flat shear die or an end notch die.
End flare is more pronounced than it is with the postcut method, however, and side roll tooling is required to obtain a
good finished shape.
The most efficient, productive, and consistent contour roll forming process is the postcut method. This method requires an
uncoiler, a roll-forming machine, a cutoff machine, and a run-out table (see Fig. 1). Postcut contour roll forming can be
augmented by various auxiliary operations, such as prenotching, punching, embossing, marking, trimming, welding,
curving, coiling, and die forming. These auxiliary operations can be used to eliminate the need for subsequent operations,
resulting in production of a finished product.
Fig. 1 Setup for contour roll forming of coiled stock (postcut method).
Tooling cost and tooling changeover time are greater for the postcut method than is the case with precut operations, but
the increased efficiency of the postcut process balances this limitation.
Contour Roll Forming
Materials
Any material that can withstand bending to the desired radius can be contour roll formed. Thicknesses of 0.13 to 19 mm
(0.005 to ¾in.) and material widths of 3.2 to 1830 mm (⅛ to 72 in.) can be used. Length of the formed part is limited
only by the length that can be conveniently handled after forming.
In some cases, multiple sections can be formed from a single strip; in other cases, several strips can be simultaneously fed
into the machine and combined after forming to produce a composite section. Contour roll forming is almost always
performed at room temperature; however, some materials, such as certain titanium alloys, must be formed at elevated
temperatures. This is done on specially designed machines.
Influence of Work Metal Composition and Condition. The effect of work metal formability on procedures and
results in contour roll forming is generally the same as it is in other forming methods.
Initial yield strength and rate of strain hardening of the work metal affect contour roll forming. One measure for
predicting the formability of a work metal is the Olsen cup test (see the article "Formability Testing of Sheet Metals" in
this Volume).
Work metals of equal thickness, with differing compositions, initial yield strengths, and rates of strain hardening, could
require alterations in the contour rolling operation. Factors to be considered are: power requirements, number of stations,
roll material, lubrication, and speed.
In contour roll forming, it is not uncommon to change the work metal but retain the same shape. In such instances,
changes may be necessary in equipment and tooling.
When work metal thickness and tooling remain unchanged, it is seldom a problem to change to a more formable work
metal. For example, in changing from low-carbon steel to aluminum, major changes would be unlikely. However, it may
be necessary to regrind the finish-station rolls to avoid overbending the section; this would usually be determined by a
trial run.
When the change is to a metal of higher strength, one or more changes in procedure may be required to achieve desired
results. In forming higher-strength metals such as stainless steels, some overforming is usually required to allow for
springback. Residual stress in highly cold-worked metal often causes straightening problems, particularly when forming
asymmetrical shapes. A common remedy is to add roll stations to decrease the amount of forming in a given station.
In most instances, difficulty in maintaining size and angle tolerance increases as the yield strength of the work metal
increases. Optimum rolling speed decreases as yield strength or hardness increases. For example, in changing from carbon
steel to stainless, speed is usually decreased 10 to 25%, mainly to prevent rolls from galling when there is an appreciable
amount of roll sweep. One method of combating roll galling without greatly reducing speed is to use EP (extreme-
pressure) additives in the lubricant; for extreme conditions, pigmented drawing compounds can be added to the lubricant.
The main disadvantage of special lubricants is the difficulty and expense involved in removing them from finished
workpieces.
Aluminum bronze rolls are often an advantage in shaping the difficult-to-form metals because they resist galling;
however, bronze rolls are softer and wear faster than do tool steel rolls. Rolls made from D2 tool steel, hardened and
chromium plated, are usually best for contour roll forming of high-strength metals (see the section "Roll Materials" in this
article).
Contour Roll Forming
Process Variables
In contour roll forming, material is progressively formed as it passes from one station to another. The variable parameters
in a roll forming operation include power requirement, forming speed, and type of lubricant. These parameters are
determined by width, thickness, and type of material; complexity of the cross section to be formed; coating (if any) on the
material; and accuracy required.
The power required by a roll forming machine depends on the torque loss through the drive gearing and the friction
between the material and the rolls as the material is being formed. The particular alloy and its thickness must be taken
into consideration when looking at the effect of material on power requirements. Generally, contour roll forming
machines have motors ranging from 10 to 50 hp on small machines and from 50 to 125 hp on larger machines.
Forming Speed. Speeds used in contour roll forming can range from 0.5 to 245 m/min (1.5 to 800 ft/min), although
this speed range represents unusual extremes. Speeds between 25 and 30 m/min (80 and 100 ft/min) are most widely used.
One or more of the following can influence optimum forming speed:
• Composition of the work metal
• Yield strength or hardness of the work metal
• Thickness of the work metal
• Severity of the forming operation
• Cutting of finished shapes to length
• Number of roll stations
• Required auxiliary operations
• Use of lubricant (coolant)
Lower speeds in the range indicated above (near 0.5 m/min, or 1½ ft/min) are required for contour roll forming titanium
into a relatively complex shape. At the other extreme, a speed of 245 m/min (800 ft/min) has been used in production
operations in which conditions were nearly ideal, that is, for contour rolling low yield strength metal, such as aluminum
or annealed low-carbon steel, in thicknesses less than 0.91 mm (0.0359 in.), in an operation having mild forming severity
and requiring cutoff into relatively long lengths (about 25 m, or 80 ft). To use such high speeds effectively, even though
forming is not severe, more stations are usually required, to minimize the amount of forming in any one station. High
forming speed usually precludes auxiliary operations, such as punching, notching, or welding, and requires a flood of
lubricant at each station.
The first four factors listed above are closely related and influence permissible forming speed. In addition, one or more of
the last four factors may dictate a lower speed regardless of the otherwise permissible speed.
Lubricants prevent metal pickup by the rolls (thus improving finish on the work metal and prolonging roll life) and also
prevent overheating of rolls and work metal. When rolls become overheated, their life is shortened. If work metal is
overheated, it may warp and require straightening. When lubricants can be tolerated, rolling efficiency is usually
increased by their use.
Soluble oils (in a 1-to-12 mixture with water) are the most commonly used lubricants. They are usually applied by a
pumping action from a self-contained sump in the machine base through a manifold having flexible tubes and nozzles that
direct the fluid to the required locations. Gutters are arranged around the top of the machine to catch the fluid and return it
to the sump.
Other lubricants have been used satisfactorily for specific applications. In addition to lubricating and cooling, however, a
lubricant must be nontoxic, noncorrosive to the metal being formed (as well as to rolls and other machine components),
and removable by available shop cleaning facilities. For instance, some silicone-base fluids are excellent lubricants for
roll forming, but they are extremely difficult to remove from metal surfaces. This poses problems in obtaining satisfactory
plating or adherence of organic coatings or adhesives. Extreme-pressure (EP) lubricants are sometimes used in severe roll
forming.
For some applications, no lubricant is permitted (for example, for the forming of painted or otherwise coated metals or for
the forming of complex shapes that would entrap lubricants). The result could be a reduction of rolling speed or lower-
quality finish, or both. However, in some instances, even though flooding with lubricant cannot be tolerated, other means
can be used to supply some lubricant to the rolls. One method is to mount cellulose sponges in constant contact with the
rolls, and to keep them wetted with lubricant by hand or by drip applicators.
Despite the fact that lubrication is helpful and often necessary in contour rolling, application and subsequent removal of
lubricants are significant cost items. When roll forming steel, however, the selection of hot-rolled, pickled, and oiled
grades of work metal has often eliminated the need for an additional lubricant. More information on the role of lubrication
and the types of lubricants used in sheet forming is available in the article "Selection and Use of Lubricants in Forming of
Sheet Metal" in this Volume.
Contour Roll Forming
Machines
The contour roll forming machine most commonly used has a number of individual units, each of which is actually a
dual-spindle roll forming machine, mounted on a suitable baseplate to make a multiple-unit machine. The flexibility of
this construction permits the user to purchase enough units for immediate needs only. The purchase of additional length of
baseplate on the machine allows the addition of units at any time for future needs. Some of these machines are provided
with machined ends on the baseplates, making it possible to couple several machines together, in tandem, to provide
additional units as required.
Screws for making vertical adjustments of the top rolls are designed with dials and scales to provide micrometer
adjustment and a means of recording the position of the top shaft for each roll pass and each shape being formed. Shaft
diameter on most machines ranges from 25 to 102 mm (1 to 4 in.).
Several types of roll forming machines or roll formers are used. They can be classified according to spindle support,
station configuration, and drive system.
Spindle Support. Roll forming machines can be classified according to the method by which the spindles are supported
in the unit. Generally, two types exist: inboard or overhung spindle machines and outboard machines.
Inboard-type machines (Fig. 2 top) have spindle shafts supported on one end that are 25 to 38 mm (1 to 1½ in.) in
diameter and up to 102 mm (4 in.) in length. They are used for forming light-gage moldings, weather strips, and other
simple shapes. Material thickness is limited to about 1 mm (0.040 in.), and the top roll shaft is generally geared directly to
the bottom shaft. This direct-mesh gearing permits only a small amount of roll redressing (no more than the thickness of
the material being formed) on the top and bottom rolls. Tooling changeover is faster on this machine than on the outboard
type of machine.