Every press brake has a concentrated load limit calculated in tons per inch in the center of the machine. Exceeding the tons-per-inch limit can damage the machine, the tooling, or the formed part.
For example, I once saw a shop apply the full tonnage of a 600-ton press brake over an 8-in. part in the center of the brake. Big no-no — using the machine in this fashion caused severe ram upset (permanent deformation) in the center of the ram.
To determine the tons per inch load limit, multiply the distance between the side frames by 60 percent, and divide the result into the machine tonnage. For example, if you have a 150-ton brake that and your machine has 10 feet between the side frames, multiply 120 inches by 60 percent; the result is 72 inches. Now divide 150 tons by 72 inches and you get a limit of 2.08 tons per inch for your machine. Therefore, you should not apply more than 25 tons to a 12 in. part.
It follows that the operator must estimate the tonnage required to form a part before making the first bend. Of course, this is critical only if the operator is bottom-bending or coining. You can't exceed the tonnage limit during air bending. Only during bottom bending or coining can the tonnage applied escalate to the rated tonnage of the machine.
Tonnage charts are available from all press brake manufacturers, and charts sometimes are mounted on the press brake itself. Keep in mind, however, that tonnage charts are for air bending only. To use the chart to estimate tonnage for bottoming or coining, simply multiply the air bend tonnage by 4 for bottoming and by 8 for coining.
Sometimes it is better to form a part off-center if your press brake can do this. Check with the manufacturer of your machine before attempting this, though. If a machine is not designed for off-center loading, you can damage it severely if you try it.
Maintenance. In addition, operators should know all of their machine's maintenance requirements. This includes periodic oil and filter changes (for hydraulic machines), regular lubrication, and proper machine level. Even if maintaining the machine isn't the operator's responsibility, he should at least be familiar with the procedures.