Even the most advanced press brakes and tools have to handle variations of materials.

Question: Our company has acquired 2 new press brake machines. Despite the fact that they offer many options, do not actually meet our need. We still face problems, such as the inability to perform angle repetition. The tools we have are new, each precision punch helps to reduce the set-up time. Yet, gaining consistency in angularities remains a big challenge.

We mainly produce mild steels with a thickness of 10-0.25 inches, so I wonder whether the material we use is related to this. If yes, what type or class is preferable? How else could we fix the problems with angle incompatibility having maintained the whole stream?

Answer: Each of us has their own preferences about the brands and models of press brakes. The driving system can be electrical, hydraulic or servo-hydraulic, and each one works differently. However, all the types provide reproducibility in micrometers. 

Each manufacturer of press brakes offers patented versions capable of creating or breaking specific applications. This is just finessed. In terms of fundamental options, basically, all the branded press brakes areas suitable and accurate. This is the case for the tools as well. Staying in the specific tooling, like the precision ground or planer ones, and matching your task with the tool will lead you to perfect outcomes. 

To gain the utmost excellence through an upgraded press brake, precision ground tools are the best. They mostly bear a fabrication tolerance of ± 0.0004- ± 0.0008 inches and total heights and tooling core.  The planner tool has a tolerance of ± 0.005 inches per 10 feet, making it hard to apply in a phased installation of press brakes. 

You had better not use your press and tools fully. Avoid using your machine at its utmost or least capacities. Use it in optimal capacity. In terms of security, provide 20 % more load capacities for the machine and tool than required by your application. 

Change in Thickness and Tension

Regardless of the press brake and tool capability to make a highly accurate performance, the change of materials has to be taken into consideration, particularly while implementing a task of tough tolerances.

For instance, let us observe 10-ga. Hot-roll steels. They can nominally be as thick as 0.1345 inches, still, may vary from 0.1285-0.1404 inches. Materials may be differently thick over the breadth of sheets. At the mills, the roller deflects at the core, which brings on more thickness in the central area and less at the ridges for sheets. Thus, the one-piece sheet may be of different thicknesses. Suppose the difference is 0.007 inches from ridges to the central area. This will suffice to produce a bending angle change of 5 degrees within workpieces.    

Each material has an utmost tensile strength (UTS). If you deal with 10-ga. ASTM A36 steels, the UTS forbearance area is from 58 KSI to 80 KSI.

Hot-roll Vs. Cold-roll

Hot-roll and cold-roll steels differ in certain fundamental aspects. Hot-roll steel is exposed to rolling at elevated grades of temperature, they may bear plenty of surplus tense brought about by unevenly occurring cooling. The surplus tense worsens the variation from part to part.

Fig. 2

The steels then get transformed through re-rolling at cold mills, mainly under room temperatures. Next, they get annealed and tempered. All this provides more excellent finishing for cold-roll steels than the hot-roll pickled and greased steels. Cold-roll steels are less carbonic and after annealing, they possess more softness compared to the hot-roll ones, that are featured with higher strength.

Influence on Radius and Bending Deduction

Suppose we form an excellent bending radius, which implies that the sheet is as thick as the bending radius. You may gain stability and repeatability for bending. Yet, the changeability of materials has to be considered. Now let us observe the variations in thicknesses. 10-ga materials range within 0.1285 and 0.1404 inches, which differs with 0.0119 inches. While performing the bending calculation, each thickness provides us with a specific bending deduction (BD). The calculation for materials of 0.1285-inch thickness gives a 0.222-inch bending deduction. The same calculation for materials of 0.1404-inch thickness gives a 0.243-inch bending deduction. As you see, the variation is 0.021 inches.

The change in ductility strengths observed before affects the inner bending radii as well. In the case of air formation, the high ductility strength leads to large floating radii. Changing of the radii will cause changes in bending deduction.

Smaller Radius and Acute Bend

You mentioned about the excellent condition of your tools, however, nothing was noted about the particular punching nose radius and the part eventual bending radius.

In case of extremely small radii of punching noses for your applications, make use of acute ones. They are capable of applying a great strength to the limited size of an area. Dependent on the bending lengths, the punching nose can start piercing materials. 

Actually bending becomes acute while achieving inner bending radii 63 % below the material thicknesses. It is possible to determine acute bending from the control fold at the center of the inside bending radii (Fig.1) No matter how hard you try, it is impossible to place a small inner radius in this acute bends; the narrow punches go deep into the bends.

Acute bends enhance the impacts of the bend variables, which include the bending angles, and consequently influence the lineal measurements of the bending. Thus, improper bending angularity is formed, and the situation even worsens by measuring out the improper bending angles.

The material grade of sheets affects the consistency and finished look of your products. Grains of materials take the course of rolling while being rolled at mills. The grain effect on sheet behavior is significant. Cheaper materials tend to contain more admixture, are large-grained and sensitive to changes of bending angles. Features of grains range within sheets and batches. (Fig. 2)

While bending a small bend radius in the longitudinal direction to grains (when the tool and grain directions are parallel), the probability of cracks upon the outer face of bends increases., which in turn brings on angle variations, particularly when performing acute bends.

Nest Complication 

Any sheet steel has a certain course of grains. Cold-roll steels show relatively more expressed preferences in direction compared to the hot-roll ones. The difference in bending angularity impacts depends on the relation between bending lines and grains.

You may be dealing with a part carved out of optimal nest layouts, where it is possible to put parts despite grain directions. In case of failure to keep the same ratio between grains and bending lines for 2 parts, bend performance becomes impossible.

Consideration of Small-batch Models 

Bend complications arise from a range of variables. It seems challenging to identify the roots of the problems without the entire image. The matter has perhaps to do with the material, which includes the sequence of the material flow into the machine. It was noted that you needed to sustain a single-part flow or one-part pulling.

One-part pulling is applied vastly in the production. However, this may not be the preferable option for your workshop press-brake department. Regardless of the excellent condition of your tooling and press brake described by you, there is a need for removing and replacing the tooling quite often. All the handheld settings bring about an additional variation factor. The one-part pulling may be replaced with small-batch models. This can reduce the number of installations and more consistency with the least complication and costs of large-scale production.