Better bend accuracy on the press brake through crowning

Crown system makes things right on press brakes.

In order to guarantee precise, reproducible molding effects, it is important to offset for the deviation that is inherent in the beams (plungers) and the press-brake tables when applying the load. Without compensating for deviation, the work piece is likely to bear certain deformity in the centre when it bends along the entire press brake run. This may be the case for bending presses with a length of 8 ft. or more, 80 or higher tons, while bending longer or larger pieces, though it may happen while forming short work pieces. To maintain a constant bending angle along the entire piece run, the bending press will need a crown technique, in beams, in tables, or both of them.

In the period of continuously renewed series intricate pieces, rapid turnovers croning provides effectiveness, precision and reproducibility. In case of great tonns this technique produces direct bending on challenge offering stuff, that have high extensile force.

Crown Bases

Many press brakes bear servo-hydraulic systems, that allow close-loop technique to control the ram positioning. On the beam ends, beneath the left-side piston and the right-side one, the position measure technique controls the beam movements and precision. Equipment controlling system works out the data and makes use of it according to beam positioning with a precision difference of 0,01 mm. This is possible through making the left-side and rightside valves upon hydraulic pistons active.

 In the bend process servo hydraulic systems ensure the beam pistons reaching the designed positioning in according to the equipment control establishment. This results in direct bending angles. Since pess brake machines have tendency to deflection in the load influence, especially at the centeral part, the centeral angles appear blunter compared to the ones under the plunger. This deviation causes bending angles to differ along the machine length.

For compensation a press brake should bear a crown technique, which allows generating a counter strength, a curving to compensate the beam as well as table deviation. (Fig.1)

This system has been in application from the end of 1970s and the beginning of 1980s. Requirements to have large-capacity press brake machines, operations with steel materials of great extensile force as well as realising long bending have lead to the crown system, since deflections of equipment frames are more possible during operations. Prior to the existence of this method, most producers used to premake curving in the machine tables to compensate approximately 75 % of the deviation. Nowadays technique is even more innovative. Making use of a crown system, which can be programmed an opperator enters thickness of sheets, slit of dies, lengths, extensile force information in the controlling system. The latter is capable of determining the possible table, plunger deflections in an automatic way and it provides preload for every bending. The equipment hydraulic technique as well as crown method obtain precise bending angles almost without any interference of operators.

Options of the crown systems

Crown system is commonly reffered to press-brake machines measuring 80 tons, 8 feet or more. This method is applicable for presses with a smaller burden, though it is dependent on the lengths of the press brake tables as well as the areas of appliance. The larger the space amongst the racks, the greater the deviation of the equipment. Besides greater burden requires higher deviation offset. The crown method is preferable in case of thicker or thinner material.

Hydraulic crown and CNC wedged-style crown systems are the most appliable ones. The latter is possible to purchase as an addition to press-brake eqipment.
A hydraulic crown is integrated in the machines. They both provide precision as well as effectiveness.

The hydraulic crown exerts compression on each cylinder in the bending stroke for compensation of the machine deviation. Usuallly the following methods cannot exert crown all over the equipment base.
Instead, the amounts of inclination for which the crown system is adjusted is dependent on the location of the cylinders. Ceratin hydraulic crown machanisms apply just the centeral cylinders and triple-cylinder mechanism, and others produce crowning more than 60 % of the bar, though not at the entire bar run.

Each hydraulic cylinder uses the one line. Thus, the entire crown strength is impossible to be used by just one cylinder, the strength spreads upon every cylinder. 

The wedged-style crown mechanism bears two wedge lines over the bar, one line is stationary and one is mobile (Fig. 2) the central wedges are more inclined compared to the ones beneath rams. The inclination angle decreases towards the pistons near the bar centre. As soon as they appear in action, mobile wedges create curving with the highest points in the bar middle and lowest points on both bar end under rams creating lath curving into the table. The crown systems with automatic control compensate bar deviation and beams while being exposed to various bend strengths. Wedged style systems allow the table bearing a particular burden. Some Crown mechanisms ensure designing ± 0.01 mm precision and position± 0.005 mm position reproducibility. The wedged systems take into account the crown along the entire length of the frame as it is processed particularly for the frame. It may as well manage with deviation compensation at larger shipload and short formation length. It is possible to realize bending in different positions upon the frame. The setting is done in real terms by the management system, and the process does not lose fabrication time.  

 The multipurpose wedge systems do not refer to press brakes and are made according to standard wedged designs. They are NC controllable, and they take into account deviations, they are not specific to applications or their material. They do not use the main data bank to determine the required number of deviation corrections, thus their crown capability is restricted and complicated.

Multipurpose crown system is possible to upgrade to an existing bending press to improve its abilities. It may as well be mounted on an entry-leveled press of smaller tonnages, offered with no crown enabling operators to have control on the bend process.

Modifying the crowning system is useful and even essential, especially while the applications are changing or evolving.

For example, if a manufacturer of heating, ventilation, and air conditioning systems turns from manufacturing residential elements efficiently shaped without crown system to producing significantly large and heavy elements of industrial installations, the new piece profiles will demand crowning. This mechanism improves productivity excluding adjustment times and settings to provide long bends.

Issues of deviation formation.

 Providing a constant angle over the entire bending length, crown system decreasess tuning procedures, minimizing waste, and excludes any interference for operators.

In case of the system absence, the equipment deviation is overcome manually by adjusting washers or by adjusting dies. Estimating the required number of gaskets takes a long time, often by trial-and-error process, resulting in serious scraps. Even a most skilled tuning operator produces some waste when manufacturing pieces on a hand-adjusted crown-machine.

The NC crown system performs these estimations in an automatiac way. It lowers levels of skills required to drive the equipment, which increases effectiveness as well as productiveness.

Yet, no crown mechanism is capable of compensating for tool troubles like improper aligning of punches and dies, asymmetrical slide of work pieces. Inaccurate or non-uniformly worn tools shape pieces possessing non-constant angles. Tool troubls partly cause errors related with non-uniform angles alongside bending line.

Precisely ground tools help to provide precise bending angles alongside the entire bending lengths. The matrices possess an advanced radius on each side of V-hole, letting the materials to pass through dies more smoothly, with less resistance (Fig. 3).

High-volume operations.

The CNC crown systems allow compensating for the symmetrical deviation of bend presses. If stuff features appear the same upon the piece run, just regular crown is satisfactory. If they are different or complex, especially when working with heavy, longer parts, for material with high tensile like S1.100, S1.300, Hardox®, and Armox®-a progressive CNC-controlled crown mechanism ensures higher compensation of equipment deflections and excessively greater versatility.

Materials with more extensile force demand greater bending strength, possess higher elastic return, and demand a larger rim (see Figure 4). High-strength material usage is expanding, particularly at larger bends (12 feet or more) since producers seek to lower the complete piece heft without compromising constructional integrality.

Crown system is especially essential in case of tandem, tridemal or quadrodrive bending presses combining two, three or four bending brakes, accordingly, of the similar or different tonns in coordinated configurations. The following mechanisms coordinate the equipment cylinders in 5 milli seconds and constantly control to provide consistent direct motion in the equipment. Certain equipment like this bear a different kind of crown systems including double crown tables. In fact, they are two CNC crown mechanisms located above each other with an asymmetrical though additional crown profiles.