August 8, 2002
Giving operators the know-how they need to produce parts efficiently and cost-effectively can be as easy as 1-2-3-4.
Many factors contribute to good part formation on a press brake. But every operator must understand four simple but crucial elements of the process for that to happen:
Foremost are the operators, who need to be well-trained and offered regular training updates on the proper use of the shop's brakes.
Operators also should know the importance of the other three factors listed previously and be able to use those elements to the fullest to produce the most accurate parts possible in the shortest amount of time. The goal of operators is not only to make good parts but also to keep setup time to a minimum within the confines of a given press brake.
Remember, time spent in setup is time not spent making parts, so reducing setup time is very important. Only the operator's expertise can reduce setup time on a given machine.
Operators also should remember the correlation between setup time and the number of parts made so that they can factor setup time into the time per part produced.
For short runs the setup time must be minimized as compared to longer runs of complicated parts. It follows, then, that simple parts requiring little setup time can and should be made in small quantities economically, while complicated parts with longer setup times should be made in larger quantities. Press brake operators are responsible for helping to make that decision, and they need the expertise to make the right decision.
Matching Expertise With Machine. Many years ago shops had only two types of press brakes to choose from — mechanical and hydraulic. Since both of these required long setup times, it was practical to make many parts resulting in large inventories.
With the subsequent introduction of retrofitted CNCs and then full CNC brakes, setup time was reduced greatly, and the expertise the operator needed changed. Setup and operation of a retrofit CNC or a full CNC brake require a different level of experience and knowledge than manual (non-CNC ) brake setup and operation.
Therefore, the type of brake available determines the required level of operator knowledge, as well as the outflow of formed parts. An operator with extensive experience on a non-CNC brake must be completely retrained for a CNC brake; operators must become intimately familiar with their machines and know those machines' physical limitations.
Limits. 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.
The operator should know and understand the terms air bending, bottom bending, and coining. Each type of bending requires different tooling, and while there can be some overlap, it just isn't practical to use the same set of tools for all types of bends.
Air bending is always the preferred method of bending, but the angle tolerance and the required inside radius sometimes make it necessary to bottom-bend or coin.
The formed inside radius in air bending is related to the die opening (0.156 multiplied by the die opening) or 1.25 times the material thickness. For bottom bending, the inside radius is approximately equal to the material thickness; for coining, the inside radius is equal to the radius of the punch. Angle tolerance of the part is determined by the accuracy of the machine and the accuracy of the tooling.
Operators must be familiar with all available tooling. In addition, they really should check tooling tolerances each time a punch and die is pulled from storage. It is the responsibility of the operator to make sure that the tooling always is within the manufacturer's specifications; if it is not, the tooling must be remachined or replaced. It is almost impossible to make good parts with bad tooling.
All pertinent information for making the part should be on the drawing.
This includes the dimensions of the part, bend angle, inside radius, and the blank size, complete with acceptable tolerances for each. The drawing should be based on the capability of the press brake and tooling that are available for the job. If any information is omitted from the drawing, an operator faces the problem of filling in the blanks. This takes time away from making parts and more often than not leads to a high percentage of scrap.
Operators need training in all aspects of a drawing's information and how this information is related to the tooling and the machine.
It is the knowledge and experience of press brake operators that help to guarantee part accuracy and economy; it is management's responsibility to provide the training necessary to give that expertise to the operators in the first place.