Page 22 - MetalForming Magazine September 2022

press, the more severe the effects of snapthrough. Hence, these clearances periodically should be monitored. While minor progressive wear is normal due to continued press usage, a significant increase in clearances between monitoring peri- ods would indicate accel- erated wear, potentially due to snapthrough.
While no industry stan- dards indicate snap- through limits, good prac- tice calls for monitoring and limiting snapthrough as much as possible.
Reverse Tonnage
Reverse tonnage, or tension in the connecting rods, can lead to struc- tural failures of press- drive components such as wrist pins, slide- adjustment mechanisms and connecting rods. Reverse tonnage also can relate to excessive punch penetration.
In this case, a load
monitor represents a good
starting point to capture
the negative tonnages.
However, as reverse tonnage results from tension in the connecting rods while the load monitor actually meas- ures deflections on the frame, lost information may exist in between. In other words, while a load monitor proves a good tool, measured reverse tonnage values may not always be reli- able. Stampers occasionally should use specialized equipment to ensure that reverse tonnages in the load monitor represent reality.
Due to the ability of the press com- ponents to withstand more compres- sion than tension, forward-tonnage capacity measures much greater than reverse-tonnage capacity. In general, presses are designed to tolerate a reverse tonnage to 10 percent of the
Penetration
Contact
Time
Connecting Rods in Compression
Connecting Rods in Tension
Fracture
Typically 200+ ms
Typically 200 ms or less
Snapthrough
Reverse Tonnage
Fig. 2—This load vs. time graph shows snapthrough represented by a sudden drop in load.
Adjustment Screw without Hydraulic Lockup
Adjustment Screw with Hydraulic Lockup
No Hydraulic Preload with Resulting Clearances
Clearance Removed by Hydraulic Preload
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Fig. 3—Reduced clearances can be achieved with mechanisms such as a hydraulic slide lockup, as shown here in a slide-adjustment screw.
press-tonnage capacity. For example, an 800-ton press can handle as much as 80 tons in reverse tonnage. Some press manufacturers offer more robust presses with reverse-tonnage capaci- ties reaching 50 percent of forward tonnage.
Influencing Factors
While many methods exist to esti- mate the forward tonnage in various sheet metal operations, no direct method can estimate reverse tonnage or snapthrough. Nonetheless, consider that reverse tonnage and snapthrough increase with increasing:
• Forming load, influenced by mate- rial thickness and strength, tool geom- etry, and tool timing
• Press inertia
• Press stack-up clearances • Press deflections.
Reducing Snapthrough and Reverse Tonnage
Press design represents a major fac- tor when dealing with reverse tonnage and snapthrough, and as mentioned, reduced clearances in the machine are key here. Reduced clearances can be achieved with mechanisms such as a hydraulic slide lockup, which removes all clearances in the slide adjustment parts (Fig. 3). Other helpful press- design factors include the use of cast- iron components, which dampen vibration, and robust connecting ele- ments such as wristpins.
Reverse Forward Tonnage Tonnage