November 20, 2009
Stamping high-strength steel, rather than mild steel, requires a different press system design approach. Standard presses are not designed to withstand the forces associated with HSS. A link drive can reduce the impact when the upper die touches the blank holder,and a heavier-than-normal press frame minimizes deflection.
Editor's Note: This is Part II of a two-part article. Part I focuses on the press technology available to form and cut high-strength steel. Considering the higher capital investment of servo presses compared to mechanical presses, stampers are advised to take advantage of in-die capabilities for streamlining postprocessing and handling to achieve the cost efficiencies that are needed to meet cost pressures. Part II outlines cost efficiencies that stampers can realize with presses that are equipped to stamp in-die, thereby meeting cost pressures and component cost-down requirements.
The servo press can increase output by customizing the stroke profile for forming with improved quality. The capabilities of a servo drive also can lead to major changes in overall value-added part production. The freely programmable slide motion (see Figure 1) allows the press to integrate additional manufacturing functions, eliminating downstream work commonly performed outside of the press and, as a result, the need to palletize parts and transfer them to additional machines.
Some additional functions that can be carried out in the die with optimized servo-drive slide control are:
These functions can be performed within the die in one integrated process during or directly after forming in the press cycle, whether the slide speed is reduced (for threading while forming before bottom dead center [BDC]); the slide stops at BDC (for horizontal piercing); or the slide speed is reduced after forming and BDC (for joining or welding functions).
Figure 2 shows an example of a spot welding operation in a 1,100-ton servo transfer press. Using a dynamic pendulum stroke that does not require a full rotation of the drive gears, the servo press line doubled the output of a conventional mechanical press without increasing the forming speed.
Using cams in dies for high-strength material requires special design considerations such as dampening and enforcement. Servo presses allow simplified die design and optimization according to part and material requirements (see Figure 3), and the piercing operation becomes independent from the vertical slide movement.
As the automotive industry demands more high-strength steel (HSS) parts and further cost reductions, stamping departments must take a holistic view of overall part and assembly production and also be involved early in the design process. Press requirements for processing HSS include higher tonnages, CNC cushions, optimized forming curves from link-drive or servo-drive presses, and overall robust press components and design.
Beyond press output, servo-drive presses also allow manufacturing facilities to produce HSS parts with high quality and less stress on the press while combining downstream processes into the stamping die.
Stamping is a capital-intensive manufacturing operation—both in terms of the cost of the press and the operating costs. Considering that press life can be more than 30 years, the short-term savings that can be realized from buying the least expensive press could result in long-term higher operational costs. In addition, a less expensive press can cost opportunities if the system is not equipped for future production tasks.
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