August 6, 2010
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.
5. So, Which Press Type is Best for the Operation?
Taking into consideration all the variances for material characteristics, part accuracy requirements, part size, speed, and volume, for some applications, general rules of thumb may apply; of course, there are always exceptions. In addition, some combination presses or a combination of different presses in a press line may yield different or better results for these applications than a single press.
Blanking, Piercing, Coining, Shallow Forming—Mechanical. Minster's Cattell said flywheel-drive mechanical presses are suitable for piercing, blanking, bending, and very shallow drawing with progressive dies. "When you have only a blanking or piercing operation, you only have to consider the material thickness in the flat plane," Cattell said.
[Tip] "The best application practice is to keep the press stroke as short as possible. This reduces slide velocity, snap-through, reverse load, and vibration and helps achieve maximum production rates, or SPM," Cattell said.
Because blanking, piercing, coining, and shallow forming are performed very close to the bottom of the stroke, a mechanical press gives better results, Micro Stamping's Vacca said. "A mechanical press makes a very sharp, shallow, and well-defined coin impression better because it stamps faster. Also, for blanking, burr formation is reduced at a mechanical press's higher speeds," Vacca said. "Conversely, burr formation during blanking, punching, and other cutting operations can occur with a hydraulic press."
Eccentric-shaft and crankshaft mechanical presses' small clearance and short stroke make them suitable for blanking and progressive-die applications, AIDA's Boerger added.
"A knuckle drive has low-torque capacity—less than one-half of a crank drive with the same nominal press capacity—and is quite satisfactory for blanking and coining," Pfundtner said.
[Caveat] The highest-quality press will be a poor investment if it is used improperly. No matter which press you bring into your shop, it’s critical that it be used for its best application and within its limitations.
“To blank high-strength steel, standard single-geared presses are being used with long strokes 10 inches or more; this is a poor application for them,” warned Dennis Cattell, senior project manager, The Minster Machine Co. “The slide velocity and punch impact are far too high. Because of this, it is common to enter a facility and see these presses completely destroyed or under major repair.
“In terms of velocity, a 10-inch-stroke press running at 90 SPM with the slide 0.75 inches from BDC has the same velocity as a 1-inch-stroke press running at 1,000 SPM with the slide 0.060 inches from BDC,” Cattell explained.
Blanking—Servo. Servo-mechanical presses can perform short-stroke, high-speed stamping as well as mechanical presses, in addition to long-stroke, slow-speed stamping; however, they cost, on average, 50 percent more than comparable-tonnage presses. Therefore, it is your need for servo's flexibility that is likely to justify its application for blanking and shallow forming.
Boerger said that because a servo-mechanical press's speed can be controlled and reverse-tonnage effects minimized after material fracture, in blanking operations, a greater percentage of a press's overall tonnage rating can be applied than on a mechanical press. For example, on a 250-ton mechanical press, only half the tonnage rating, 125 tons, can be applied. A 250-ton-rated servo-mechanical press can blank up to 220 tons, depending on the application, Boerger said.
Fineblanking—Servo. "Fineblanking takes advantage of the programmable slide motion of a servo press," CPF's Altan said.
Punching, Shearing, Blanking—Pneumatic. Pneumatic presses perform punching, shearing, blanking, forming, piercing, embossing, and complex roll forming, Airam's Meyer said.
Drawing, Deep Drawing—Hydraulic. "It's well-known that in deep-drawing operations, deformation at a low velocity improves metal flow because of the effect of punch velocity on friction conditions," Altan said.
"To draw and deep-draw components such as cookware, gas tanks, and pressure cylinders, strong forces are needed high in the stroke through the BDC. A hydraulic press's control of the force and speed throughout the entire stroke makes it conducive for drawing and deep drawing," Vacca said.
Drawing or Blanking—Servo. Usually a mechanical press is purchased for a specific application, Boerger said. A servo-mechanical press, on the other hand, can be purchased initially for one application, then later used for another. "Because the servo can give stroke lengths from 1 to 12 inches long and apply full tonnage at 1 SPM to maximum speed, blanking can be done one day, deep draws the next," Boerger said.
Deep Drawing—Mechanical Eccentric-gear, Link. Certain drive transmissions empower mechanical presses to perform drawing well too. "Eccentric-gear drives on mechanical presses, designed for longer strokes and deeper draws, make them suitable for stamping a part 6 or 8 inches long," Boerger said. Link drive can halve the contact velocity of crank motion, he said.
[Rule of Thumb] "The press of choice for deep drawing must have enough daylight for part removal." Plus, precise blank-holding pressure enhances formability, Red Stag's Pfundter said. "Blank holding tonnage should be at least 50 percent of the top tonnage. The rate of speed is not important, but controlling the speed is," he said.
Compression Molding—Hydraulic. "One of the growing market segments for hydraulic presses is in compression molding applications," said Darrell Harrelson, lead application and sales engineer, Beckwood Press Co. Some components that historically were fabricated from metals are being made from composites.
"Compression molding requires precise pressure, position, speed control, and parallelism throughout the working stroke," Harrelson continued. "Hydraulic presses can dwell under pressure, vary ram speeds, and provide a constant force over a large area—precisely the press characteristics compression molding requires."
Hydraulic press improvements engineered to optimize the compression molding process include control of ancillary process equipment, actuation of internal die components (such as core knockouts, ejectors, and punches), and up-acting or multiple-action rams, Harrelson said. "Precision-heated platen integration with customer-specified heating tolerances and die controls are common."
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