By Richard Mandel, senior editor

A shopworker assays a metal plate as it runs through a roll bender.

Diagram showing contact points when rolling a cone on a four-roller machine, where the lower central roll can be set with a negative inclination.

This Eagle BA-50's CNC allows for creating a wide array of bend profiles at the machine, while its built-in memory ensures repeatability.

This Bertsch four-roll machine, with CNC, quickly renders a stack of plate into identically bent pieces.

K-Prene urethanecovered lower rollers in Achrotech's tworoller machines permit rolling of perforated or raisedsurface blanks.

Roundo's Model R-21 produces large rings that previously could only be made by forging.

Recent advances in control sophistication, motor design and mechanical technologies are increasing the production speeds, accuracies, capacities and capabilities of today's roll-bending machines. Metal fabricators use such machines to convert sheet metal into cylinders or cylindrical segments, flattened and elliptical cylinders and truncated cones. These machines, with two to four rollers, rotate and bend metal as it passes through them. They handle stock thicknesses from 16-gage (0.061 in.) to 10-in.-thick plate, depending on the power of the machine and the ability of the metal to be curved without damage.

There are three classes of roll-bending machines for different applications:

Initial-pinch machines require inserting the workpiece into the machine twice to pre-bend both ends of a cylinder and ensure closure of the seam. A side roll, moving diagonally toward the top roll, sets the bend radius. The operator pre-bends the first end by inserting the plate into the machine, which clamps the plate and pinches it between the top and bottom rolls. The plate is then removed from the bender and rotated 180 degrees, the second end is inserted into the rolls, and the cylinder is rolled to completion.

Four-roll double-pinch machines roll plate in one pass. Driven top and bottom rolls clamp and pinch the plate, and two side rolls move diagonally toward the top roll. By using first one side roll and then the other, the plate is pre-bent and rolled completely. Four-roll machines also are used to roll cones.

Pinch-pyramid machines, also called three-roll double-pinch machines, operate as either a single-pinch or pyramid machine, allowing fabricators to select the best procedure for a particular jobs. Pinch-pyramid machines drive all three rolls — the top roll is fixed and the two lower rolls move in a straight path or an arc toward the top roll. Like the four-roll double-pinch machine, the pinch-pyramid unit also pre-bends both ends of a plate with one insertion into the bending machine, reducing material-handling time and cost. A wide opening between the rolls allows short, heavy plate to be rolled, and, with accessory tooling, structural sections and barstock on-edge can be rolled.

The single biggest advantage four-roll machines have over three-roll double-pinch types is simplicity. In the four-roll machine, the plate is automatically squared and remains pinched until the pipe is complete. The plate also can be introduced into the machine and parallel to the floor via a powered or free-rolling conveyor, eliminating the need for a second worker to help maneuver the plate, particularly if the machine has both side and vertical roller supports.

Additionally, a four-roll with inclinable side rolls and a hardened contrast die is superior at rolling cones because the lower central pinch roll can be inclined in a negative attitude, allowing the roll to grip the cone only on the large diameter which needs to turn faster. Not all four-roll machines have a lower central pinch roll capable of a negative inclination, so shops should ask about this feature if cone rolling is a priority. Another helpful detail is independently moving side rolls, which allow one to be used as a squaring gage.

When considering a roll bender, shops should review these factors so equipment manufacturers can tailor recommendations to their needs.

• Projected range of thicknesses and widths to roll, including required tolerances.
• Yield point of material, which helps determine required bending force that, in turn, helps determine size of bending rolls.
• Minimum diameter of cylinder to be rolled, as material can work harden when forming small-diameter cylinders in multi-pass rolling.
• Size of production runs and floor-to-floor time of the workpiece.

CNC precision and repeatability
The Model BA-50 CNC profile-bending machine from Eagle Bending (www.eaglebendingmachines.com) combines three rollers with servo-control technology to produce variable radii, spirals, ovals and polycentric forms from most any material type and profile cross section by inputting the desired bend radius into the unit's computer. Bend programs can be built, modified and stored in a graphic format similar to CAD programming. Variable radius and tangent multiple bends can be created by blending or interpolating between the different radii of the workpiece. The computer also provides for auto-diagnostics.

Precise repeat positioning of the bending roll is achieved with a linear transducer. The unit can operate in either vertical or horizontal positions. Additional features include a steel frame, oil-bath speed reducer, electronic motor speed control and a hydraulically powered forming roll. The BA-50 handles flat stock up to 2.75 in., tubing up to 2.5 in. in diameter, and solid stock up to 1.5-in. thick.

Fast four roll
CNC is an option for the Bertsch 87-10 hydraulic four-roll machine distributed by Megafab (www.megafab.com). The 40-hp unit's pre-bending capacity measures 0.875 in. by 10 ft to a 24.5-in. minimum I.D., based on materials having 85,000 psi maximum tensile and 56,000 psi maximum yield, and rolling speed is up to 14 ft. per minute. Top and pinch rollers are 16 in. in diameter with 13-in. side rollers. Direct hydraulic cylinders, one mounted at each end of the machine on all three adjusting rolls, adjust lower pinch rolls and the two side rolls.

Other machine features include a leveling system that ensures parallelism of both side rolls and provides individual tilt control, and sufficient power provided in each side roll adjusting system to full rated capacity plate. Spherical, anti-friction roller bearings are provided on all roll-forging journals. Options include hardened rolls, a cone-rolling attachment and a dedicated overhead support. Other machines in the Bertsch line have handling capacities up to 1.25 in. by 10 in., while special-request rolls have been developed that will bend plate up to 14-in. thick.

Urethane improves two-roller performance
The two-roller machines manufactured by Acrotech Inc. (www.acrotechinc.com) employ lower rolls covered with K-Prene urethane (another product from Acrotech). Under pressure, the steel top roll acts as a rotary punch penetrating the bottom roll. The urethane-coated bottom roll acts as a female die, wrapping the material around the top roll. Rotation of both rolls produces accurate curving and rolling of complete or partial cylinders in one pass. Polished or pre-painted stock can be rolled without marring or damage. The coating also permits these machines to handle perforated, embossed or die-cut stock. The Acrotech machine lineup includes models that handle up to 10-gage mild steel, and one that operates in a vertical orientation.

Bar bender
Roundo AB of Sweden, whose products are distributed in the United States by Comeq (www.comeq.com), has introduced the Model R-21 section-bending machine for pre-bending and rolling 12-in. by 12-in.-square bar to a diameter of 60 in., while leaving a flat end of no more than 20 in. The R-21 also rolls 18-in. by 5-in. bars of high-strength steel to 96-in. diameters, with flat ends of no more than 18 in. This lets fabricators create such articles as wind-tower section-joint rings, which until now could only be done by a forging process. The pinch-pyramid machine curves beams along an X-X axis.

Planetary gears, planetary guides and spherical bearings
Many of the machines in E.G. Heller's Son, Inc's catalog (www.hellerson.com) feature advanced mechanical components such as planetary transmissions, which eliminate the need for clutches and complicated synchronization devices while delivering over 98 percent of the motor's energy to roll surfaces. Planetary guides used in the four-roll hydraulic machines keep approximately 50 percent more area of the plate under bend pressure during the rolling operation, thereby allowing rolls down to 1.1 times upper-roll diameters.

Permanently lubricated spherical bearings on Heller's Son's machines eliminate bushings to reduce required maintenance, and a special torsion-bar system maintains mechanically finite parallelism. Unlike electronic systems or proportional value systems, the machines adjust to full-conical tilt and back to parallel in 5 seconds. Heller's Son also produces the AR Revolution profile/ sectionbending machine, which uses three independent hydraulic planetary drive motors to deliver nearly 100-percent torque to each shaft for maximum traction. Spherical roller bearings reduce friction and aid in the machine's geometry for bending all types of sections, profiles, tube/pipes and aluminum extrusions.