New materials create a need for high-tonnage presses, larger beds
July 10, 2007
Metal stampers are using more high-strength steel, which is more difficult to form, requires more tooling stations, and more press tonnage. As a result, stampers are moving away from traditional production methods and investing in presses with higher tonnage capacity and larger beds.
Metal stampers are using more high-strength steel, which is more difficult to form than mild steel and requires more tooling stations and press tonnage to produce parts. As a result, stampers are moving away from traditional production methods and investing in presses with higher tonnage capacity and larger beds.
In response to global competition, stampers are trying to reduce labor-intensive processes by automating single-press transfer systems and press-to-press transfer lines, including gap transfer systems. Automation also can be a factor in the need for increased press bed area.
Die design and material requirements also are changing because of the negative effect high-strength steel and tighter part tolerances have on die life. The need to ensure longer die life and improve part accuracy is placing greater importance on mechanical press features such as high rigidity, low bearing clearance, close tolerances, slide guide systems, and wide-spaced connections.
High-tonnage transfer and progressive straight-side presses offer a range of flexible options that can be used as standalone work centers or with automation. Transfer systems continue to meet the automotive industry's specialized requirements for large, complex parts from thinner, stronger materials, as well as complete assemblies and production from tailor welded blanks.
A larger bed size—from 12 feet to as much as 24 feet in length—allows stampers to process wide materials, and high off-center load-bearing capacity provides flexible die layouts. High-accuracy and rigid, low-deflection press components help ensure consistent part quality and improved tolerances.
Automating a progressive-die press system in conjunction with coil feeders or transfer-die operations that use transfer feeds typically requires a straight-side press. Double-crank straight-side presses can produce large-scale parts requiring corresponding stroke lengths. When used in a tandem line, automation can deliver improved productivity as parts are moved from press to press. If an automated progressive-die press system is to be used to form high-strength material, the presses should have minimum press tonnages of 800 to 1,000.
In a multipress system, a straight-side press can run individually; when extra tooling is required, it can run as a single large transfer press by initiating the intermediate stage between the presses and running them in a continuous, synchronized operation. This multipress system can be used to form high-strength steel. The tonnage to do the job would depend on the parts to be produced.
Tie-rod straight-side presses can provide long or short strokes for transfer or progressive-die operations. However, unitized-frame straight-side presses also can meet the same production requirements. Two unitized-frame straight-side presses can provide up to 12 tooling stations, with material feeding left to right through both presses.
When used individually, the first press feeds left to right with up to six tooling stations. The second press can stamp a different part, feeding material right to left with up to six tooling stations. This combination can provide more tonnage capacity without a dedicated transfer system. In addition, the one-piece frame can reduce manufacturing costs—through elimination of some machining and assembly processes downstream—without sacrificing production speed and press rigidity. Installation costs also are low because the press frame requires no assembly.
A gap-frame press transfer system is flexible and is a smaller investment than a dedicated transfer press system. Many stampings produced in transfer dies can be run successfully on gap presses. The ability to connect as many gap presses as needed for higher tonnage requirements makes this transfer system suitable for a range of stampings. Because the system does not require large accessory equipment or a pit, press configurations can be easily changed.
A nine-point suspension can eliminate the shimmy motion of the slide, which creates greater part accuracy and extended die life.
Mechanical press accuracy, especially in high-strength-material applications, has improved with the introduction of lubrication-free, preloaded roller slide guide systems (see Figure 1). Because preloaded roller slide guides are lubrication-free, the potential for a part to become contaminated with press oil is eliminated. A dry slide guide uses a roller bearing mount with the ability to swivel. This capability helps the roller maintain contact with the guide surface on the column during off-center load situations.
By using this type of roller bearing slide guides, stampers can use a straight-side press to produce a variety of stampings, from high-volume, thick progressive-die parts to larger, lower-volume, high-cosmetic pieces. This dry slide guide also can be used for larger dies and transfer-die applications.
A preloaded, zero-clearance slide guide system with high-pressure oil lubrication has been developed to help improve press accuracy further. The guides consist of semispherical shoes operating against flat guide ways attached to a massive frame structure. The press slide is tall, which enables guide points to be spaced far apart, providing long slide guides. The surfaces of the mating components are made of materials that possess natural lubricity. Oil is forced between these preloaded surfaces. These slide guide systems have helped make the straight-side press suitable for the automotive, appliance, and HVAC markets.
Increasing pressure to form parts from high-strength materials has prompted press builders to develop technology like servo forming and new press models. Patented servo forming technology that can be added to gap and straight-side presses allows stampers to produce parts from aluminum, magnesium, high-strength steel, and titanium.
In addition to infinite slide position control, the high-torque, low-RPM motor in this advanced technology eliminates the flywheel, clutch/brake, and main drive motor found on a standard mechanical press. Direct drive offers the same maximum stroke length and torque rating as on a conventional mechanical press, while allowing full torque at as little as five strokes per minute. Full torque at low speeds allows stampers to perform forming operations at slide velocities that cannot be achieved with conventional press drives (see Figure 2).
Servo technology allows successful forming of high-strength materials such as high-carbon steel.
A new design introduced this year is built with a single-point center suspension and a no con-rod design. This press is able to support automotive, flow control forming, fineblanking, and cold forming applications and has the ability to form high-strength materials. Eight zero-clearance, spherical, shoe-type gibs on the slide allow high-precision forming while delivering low deflection and control of lateral slide movement caused by off-center or unbalanced loads.
Though the basic structure and components of mechanical presses remain intact, technology advances and design innovations allow the mechanical press to evolve to more effectively meet the needs of the metal forming industry.