Bending all types of material (steel, stainless steel, aluminum, etc.) on a press brake does not date from yesterday.
Press brakes have evolved much since their debut, however, no matter their level of sophistication, the source of different level of errors are still present in everyday life.
The purpose of this article is not to detail in depth each source of error but rather to make a summary list, in order to give you some ideas or simply to inform you. We divided into 4 groups these sources of error: the material to be bent, the construction of the press brake, the positioning of the axes and the tooling used.
Of these four sources, the most difficult to control is certainly the consistency of the material to bend. By simply measuring the thicknesses of the material ordered to each order and request a test mill (“Mill Test”). You will be able to note that the characteristics and dimensions can vary considerably. The less the material is a good quality, the better your chances in one plate or sheet, properties and dimensions will vary. The better the quality, it will be more homogeneous and the properties will be stable, the better the results in bending and you will generate less waste. Sometimes it is better to pay a little more for the material and avoid some unpleasant and costly problems and expense.
The second group relates to the construction of the press brake you use. The new generation presses are generally equipped with a compensating system for bending the lower table deck and mounts. The design and quality of these systems is paramount when it comes time to bending in multi-station mode or when you bend relatively long parts and at a tonnages higher than 30% (sometimes even less) of the nominal capacity of your press brake. For example, if you bend to the left, center or right of the press, the housings of the press being C-frame shaped, open differently, depending on the location of the bending. This flexion will affect the bending of the angle on each side of the piece if it is not compensated correctly. In addition, the flexion of the housings tend to move the tip of the punch slightly toward the front of subject press therefore if there is no mechanism in place to compensate for it, the errors add up.
Also a considerable factor is the flexing of the beams, since the objective is to obtain a constant angle from one end the work piece and since the hydraulic cylinders on the most presses are located at each end, a compensation system on the lower beam (“crowning”) is necessary when you need a minimum of precision. These compensation systems are mainly found in three versions on the market: mechanical, hydraulic-CNC, CNC hydraulic & Dynamic. The first two are based on an estimate of the position by the press controller. The third type, i.e. hydraulic-CNC & Dynamic, self-adjusts itself in real time based on the measured deflection of the upper beam. This type of system is highly effective and greatly increases the quality and consistency of bending, whether for long parts or multi-station mode.
An equally important aspect and rarely considered is the quality and the dimensions of the steel used for the manufacturing of the press. The use of thicker steel plates and with a higher tensile strength will ensure minimum flexion of the beams, which will help improve the straightness and accuracy of the bent parts.
Also part of the construction, precision in the positioning of the different axes (upper beam, back gauge, etc.) is undoubtedly a very important factor as well. Obviously, proper maintenance is required because of the multiple uses will that add up can cause significant errors. The newer press brakes, Synchro type (Y1-Y2), are equipped with back gauges driven by ball screws or rack & pinion and offer a very interesting precision.
A third potential source of error lies in the calculation of the final positioning of the axis. From one CNC controller manufacturer to another, the positioning calculations may differ since they do not all use the same formulas in the calculation. Therefore the operator has to constantly make corrections on the positioning of the axes when a certain precision is desired. Also, the designers of CAD-CAM software conversion can generate a bending program from a 2D or 3D drawing are numerous and again do not all use the same calculations. A good integration of this type of software and controller of your press is paramount. In addition, the bending data (elastic limit, thickness, length, punch, die) need to be entered as precisely as possible in order to minimize sources of error. There are some real time angle measurement systems on the market, which takes some measurements during the bending process to determine the spring back of the material and calculate the exact positioning depending on the desired angle. Although the concept is very interesting at first glance, the solutions offered to date are quite expensive and often have significant drawbacks (interference with parts, frequent calibration, slow process, etc …) and are often left aside after a few uses.
The last source of error identified at this time is associated with the tooling used to bend your parts. Several times before we have installed new press brakes and tried to use the older customers’ tools. The result is more often than not, very disappointing. Worn tooling, damaged or bent, have direct impact on accuracy and quality of your bending operations. Also, if your press brake has several years of usage or has been a victim of inappropriate use, the condition of the attachment surfaces for the punches and the dies can also cause inaccuracies. Bending basic rules still prevail however you must ensure that you use accurate and appropriate tooling.
In conclusion, if you use raw material with uniform and well defined properties, a good quality press brake with compensation of the flexion induced during bending, you have already eliminated a number of sources of errors. The accuracy of the calculated values of axis by the CNC controller being difficult to change, you are left to making sure you use quality and in excellent condition tools.
Yves Garant, eng.