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Press selection—sorting it out Part II
Part II: Beyond Part Characteristics, Material Characteristics, and Frame Design, What Else Do I Need to Know About Selecting Tonnage?
- By Kate Bachman
- March 11, 2008
- Article
- Bending and Forming
Matching Press Characteristics to Your Applications
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.
Frame Design, Load and Deflection
A gap-frame, or C-frame, press comprise designs from benchtop to floor models and can be used within a limited tonnage and bed size range. "Limitations for C-frame design are mostly due to its proclivity to 'yawn' or lean back when stamping large components or performing heavy-duty applications," said Darrell Harrelson, lead application and sales engineer, Beckwood Press Co. "This can make higher-tonnage machines cost-prohibitive, because a significant increase in structural steel is required to overcome a cantilevered load."
Harrelson said unguided rams are suitable for work over a concentrated or center load, while guided rams offer better resistance to deflection in moderately off-center-loaded applications.
"A two-post, straight-side hydraulic press is suitable for heavy-duty use of narrow to wide dies with bed sizes up to 24 inches deep from front to back. Enhanced ram rigidity minimizes deflection for a more consistent die alignment than C-frames. Two-post designs are limited to applications that require die access only from the front and rear, but they can be more economical than traditional gap-frame designs," Harrelson said.
"A four-post press is similar to a two-post press, but it is designed for larger dies with increased bed size requirements," Harrelson said. "It is well-suited for jobs requiring superior rigidity and precise ram-to-bed parallelism. The design allows die access from all four sides. These presses often are either of a modular construction using tie-rods or a monolithic construction (all-welded).
The best way to analyze the integrity of a press's structural frame is through engineering design testing, using 3-D modeling and comprehensive finite element analysis (FEA) testing before it has been built, Harrelson said.
Other frame designs include side slab, and multiframe.
About the Author
Kate Bachman
815-381-1302
Kate Bachman is a contributing editor for The FABRICATOR editor. Bachman has more than 20 years of experience as a writer and editor in the manufacturing and other industries.
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The Fabricator is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The Fabricator has served the industry since 1970.
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