So, Which Press Type is Best for the Part?
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.
6. So, Which Press Type is Best for the Part?
Small, Simple, High-volume Part—Mechanical. The flywheel-driven mechanical press still can do high-volume, relatively straightforward work faster and cheaper than a servo-mechanical or hydraulic press, equipment manufacturers concurred. "Eccentric-shaft, or crankshaft, presses generally have a small clearance and a shorter stroke, suitable for simple part stamping in a progressive-die application," Boerger said.
Large Part—Mechanical. "Many large parts have off-center loading and springback issues," Pfundtner said. "The cushion should be approximately 50 percent of the top tonnage and controllable for pre-acceleration plus speed and stroke height. Parallelism and deflection need to be very tight throughout the working stroke."
"A four-point mechanical press with a controllable hydraulic cushion is the best combination for the die designs and high-strength steel often used in large automotive body panel stamping," Pfundtner continued. "A four-point press can reduce the shimming required, and may even reduce draw beads. However, a mechanical press's fixed daylight and fixed stroke may limit flexibility."
Large Part—Hydraulic. A strong hydraulic press or a transfer press equipped with parallelism control parts can handle large parts with off-center loads. In addition, they do not have the fixed-daylight and fixed-stroke limitations, Pfundtner said.
Large Part—Automation. Automation can improve productivity and safety in moving large parts, manufacturers say. Transfer presses are suitable for this reason also.
Complex Part. To produce a complex part in a single press, stampers use multitool dies, such as progressive dies, and in-press transfer processes, but the press must have the capacity—bed length, tonnage, and work energy—to accommodate up to 20 die stations, not just four to five, manufacturers said
Complex Part—Link-drive Mechanical. "A mechanical link-drive press uses various link mechanisms to drive the slide, enhancing formability versatility," Pfundtner said. "For example, when stamping a complex part with drawn sections, such as a lawnmower component, the link drive allows you to reduce cycle time during the drawing of the mower deck while maintaining the drawing speed within the steel's material limits.
"Minimizing the change in drawing speed and increasing the drawing speed during the approach stroke and the return stroke reduce cycle time, compared to a crank-drive mechanical press," Pfundtner said.
Complex Part—Servo. A servo-mechanical press's greatest impact may be on complex forming and exotic-material applications in which parts cannot be formed any other way, equipment manufacturers agreed. "A servo press's ability to reach full torque at low speeds allows stampers to perform forming operations at slide velocities that cannot be achieved with conventional press drives," Boerger said.
Schuler's Kinzyk added, "In general, the relative increase in output by using a servo press is greatest when applied to parts that are complex to form or where complex automation processes are currently operating at a low or medium process speed or stroke rate."
"For example, a typical 400-ton press can operate up to 100 SPM. The relative productivity gained by increasing productivity from 80 to 100 SPM [25 percent] is not as dramatic as increasing from 20 to 40 SPM [100 percent] or from 40 to 60 SPM [66 percent]," Kinzyk said.
Large, Complex Part—Transfer Press. Greg Cornett, operations manager, Tennessee Stampings, said his company purchased a transfer press because of its intricate parts forming and scrap reduction capabilities.
"In a typical prog die, the part stays intact with the web as it travels through the die. A transfer press actually allows you to cut the part loose from the web and manipulate it. If you need to turn it at a 90-degree angle, you can make that happen during the transfer," Cornett said.
Cornett pointed to a large subpanel for a floor pan. "This is a left-hand part. There is a right-hand part as well, a mirror image. We stamp out the blanks [left-hand and right-hand] in one die to obtain maximum material optimization. If you were to run this through a progressive die, you'd just have a huge waste in scrap because a progressive die can produce only one part," Cornett said.
Complex Part—Press Line. Some press manufacturers maintained that best results for complex parts may be achieved by using a combination of different presses in a press line, with the lead press having one capability, the others another.
A dual-slide press—a press with two individually controllable slides—allows the closing forces to be tailored to exact specifications to produce complex parts, Schuler's Haller said. For example, to produce a complex part with difficult draws, such as a pickup door panel, a deep draw can be performed in the first high-tonnage drawing station, and the second station can be equipped with a die for embossing operations. Or, to form a dishwasher inner door, a slide cushion integrated into the first slide can form the inner door; a second slide can be used for the edge beading and finishing of housing parts.
From a safety standpoint, each press must be looked at as an individual system, said Adam Allansson, president, AP&T, North America Inc. "The safety system is designed to ensure maximum protection for the operator and those around the machine.
"Additional system safety features may include a ram-locking device to secure the ram in the upper position, a service platform for working on top of the press, and even adequate lighting within the press bed area," Allansson said.
"The press should have a redundant safety circuit on all safeguarding. The hydraulic system itself should be equipped for safety, with a redundant double-channel hydraulic safety valve configuration." If one channel fails, the press stops and the system controller will notify the operator through an alarm on the HMI," Allansson said.
Die handling equipment designed to facilitate quick die change, such as rolling bolsters, doubles as personal safety equipment to protect operators from injury.
Emergency-stop devices, electrosensitive and optoelectronic devices, and sensors can be incorporated into the press system to prevent accidents.
Overload Protection. "If a mechanical press applies more pressure than it is rated for, the press can get stuck at BDC of the stroke, causing very costly tooling and press breakage," said Tom Vacca, engineering manager, Micro Stamping Corp.
A mechanical press equipped with a hydraulic overload protection (HOLP) system prevents that, explained Dennis Boerger, AIDA America "When an overload occurs, an oil-filled chamber trips, enacting our patented oil escape system that delivers a response in 10 milliseconds, ensuring that your press is not stuck at BDC or damaged. This millisecond reaction is possible only with a patented ball socket-type HOLP system," he said. The company's servo-mechanical presses are equipped with the overload protection system also.
"Many press manufacturers offer optional hydraulic overload protection built into the ram connections that can trip very fast with oil evacuation into a chamber, " Cattell said. "However, they only protect the press. The initiation of the device occurs when the overload is detected at the point of overload, not before. The inertia of the slide and die cause die damage."
A hydraulic press has built-in overload protection to protect the die, hydraulic press manufacturers said. "Hydraulic force is easily limited in a hydraulic press by means of a relief valve," said Darrell Harrelson, lead application and sales engineer, Beckwood Press Co. Pressure relief valves can be set at predetermined limits so that if part ejection fails, the ram simply stops moving.
In the event of loss of air pressure or electrical power, a pneumatic press will not double-trip or mis-stroke, commented Rick Meyer, sales manager, Airam Press Co. Ltd.