Active leveling control offsets press, tooling problems
June 17, 2008
Active leveling control (ALC) counteracts the negative effects of off-center loading; significant breakthrough shock common with high-strength steels (HSS); and reverse shock loading associated with nitrogen springs in dies.
Active leveling control counteracts the negative effects of off-center loading, including premature tool and press wear, breakthrough shock, and reverse shock loading.
Stamping shops are driven to offset today's increasing competition and decreasing margins by consolidating operations and processes, minimizing operator error, and reducing material scrap. To achieve this and thrive, they often must access advanced technology.
One development that equips hydraulic presses to meet these challenging demands is active leveling control (ALC) (see lead image).
This technology counteracts the negative effects of off-center loading; significant breakthrough shock common with high-strength steels (HSS); as well as reverse shock loading associated with nitrogen springs in dies.
ALC uses closed-loop control for each corner of the press ram, keeping the press ram parallel while also reducing shock. Presses equipped with ALC apply a high-speed motion controller, linear transducers, and proportional valves to synchronize the press actuators. The 32-bit, multiaxis, high-speed motion controller loops in less than 1 millisecond.
Either two- or four-axis ALC is used to achieve bed-to-ram parallelism, depending on the application. Two-axis control provides either left-to-right or front-to-rear parallelism. For applications requiring both left-to-right and front-to-rear parallelism, four-axis control is recommended.
Stamping using a small die on one side of a press with a large bed can result in off-center loading problems.
While off-center loading may be unavoidable, it can wreak havoc on the press, dies, and overall application. Poor part quality and inconsistency, premature tool wear, and harmful stress on the press structure are likely to occur if the press has not been properly designed to handle an unbalanced load.
Off-center loading has many possible causes. The most common cause simply is that the geometry of the part formed is not conducive to an even load. Many parts do not form evenly over the press bed. Also, consolidating multiple operations, such as punching and forming with multiple dies or progressive dies in a single machine, can create off-center loading.
Using some tooling that is designed to limit tonnage, such as scissor-cut perimeter blanking tools and staggered punches, can result in off-center loading as well. Operators may load a press improperly, creating misalignment. This can be especially problematic for presses with big beds and small tooling.
A properly engineered ALC system counteracts the negative effects of off-center loading in several ways: It protects the press structure and tooling, thereby decreasing maintenance costs. It allows many processes that historically could not be combined in one press now to be combined efficiently. The control can replace the use of multiple presses, which reduces labor costs as well as saves floor space and creates room for more capacity. To some degree, servo-mechanical press controls parallel ALC technology, but at a higher capital expenditure.
Tremendous breakthrough shock can occur during the blanking of advanced high-strength steels (AHSS) and HSS. In these instances, an ALC's high-speed controller senses the ram acceleration during breakthrough and adjusts the axis control valves to restrain the ram while maintaining parallelism. This greatly reduces the effects of the breakthrough shock in these very hard metals, thereby helping to improve the life of the press and die.
An ALC system also counters reverse shock loading of dies equipped with nitrogen springs. Although nitrogen springs are critical die components in some applications, they can cause problems that need to be compensated for in the press system. Typically, nitrogen springs are engineered into dies for blank holding, shock control, or load distribution compensation. As a nitrogen spring is compressed, a reverse force builds inside the spring as it tries to extend to the initial position. New nitrogen spring systems can be engineered with dampening controls to minimize the effects of extension forces, once they are compressed.
In traditional applications, as the press retracts, the up-acting force from nitrogen springs can cause reverse shock loading. In many applications, nitrogen springs are used to balance off-center loads. While this works well in the press ram's down motion, it can wreak havoc in the ram's retract motion.
An ALC system controls the retract portion of the press cycle as well as the down motion. The controlled retraction maintains bed-to-ram parallelism in unbalanced nitrogen spring systems and uses programmable distance on the upstroke to allow for smoother decompression. Once the ram has retracted past the nitrogen spring's stroke, it can be placed safely into a traditional open-loop, fast-retract mode.
Stampers taking advantage of hydraulic design technology such as ALC are likely to experience increased efficiency of their production processes, increased part quality, reduced die wear, and reduced complex setup time.