March 11, 2008
This multi-source article offers readers advice on the criteria to consider when buying a press. The article examines application suitability, drives, and controls as well as other considerations such as tonnage, frame construction, speed, and horsepower.
The following questions–and their answers, provided by industry equipment manufacturers and experts–are intended as a general guide to help you simplify the daunting task of selecting a press or press system.
2. Beyond Part Characteristics, Material Characteristics, and Frame Design, What Else Do I Need to Know About Selecting Tonnage?
[Rule of Thumb] Tonnage Ratings. "Every press in the world is rated by the tonnage [force in tons] that it can apply at its rated distance from BDC of the press cycle to BDC of the same press cycle," Minster's Cattell said.
Tonnage Ranges. Metalworking press tonnages are from 3 to 5,000 (cold forging press tonnage is as high as 50,000). The average working range for a mechanical press is between 30 and 1,000 tons, Cattell said.
Tonnage—Bed Size. High-strength steel (HSS) is more difficult to form than other materials, and requires more tooling stations and more press tonnage; therefore, presses with higher tonnage capacity and larger beds are needed, Boerger said.
High tonnage and large beds also are required to stamp multiple parts with each press stroke using multitool dies, such as progressive dies, manufacturers said (see Frame Design, Load, Deflection, Bed Size, Part Size sidebar).
Tonnage—Hydraulic. "The tonnage required to do a job is the same for a hydraulic press as it is for a mechanical press," said Greenerd's Dining. "There may be certain applications, such as deep drawing, where the full power stroke characteristic of a hydraulic press reduces the tonnage, but there are no known instances where using a hydraulic press requires more tonnage."
Tonnage—Pnuematic. Typically, pneumatic press tonnages are from 3 to 210, said Rick Meyer, sales manager, Airam Press Co. "Full rated tonnage is assured throughout the stroke because air pressure remains constant."
[Tip] Less Tonnage Is Required for Hot Forming. You may be able to purchase a lower-tonnage press for hot forming than for cold forming. Swamy Kotagiri, director of research and development, Cosma Intl., said that the tonnage requirement to hot-form a HSS part is substantially lower than is needed to cold-form it. "For example, hot-forming the draw portion of a B pillar requires only 30 percent to 40 percent of the tonnage needed to cold-form it using a similar-strength material," he said. "So the part, which requires 1,000 tons to be cold-stamped, needs only 300 to 400 tons to be hot-formed," he said.
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