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Bending: taking the guesswork out of material variations with closed loop feedback.

Press brake manufacturers have wrestled with various approaches to producing accurate angles during the bending operation. Methods offered thus far have been dependent on such devices as crowning systems, displacement measuring systems with strain gages, and CNC controls.

When bending, five parameters or variables must be addressed. They are bend angle, consistency of the bend angle, flange length, parallelism along the bends, and hole position relative to the bend line or the edge of the plate. With press brakes capable of achieving repeatabihty in the range of four tenths today, the difficulty in controlling these parameters comes into play when dealing with material variations. Making a bend in a workpiece within tolerance requires that all of the parameters be achieved while addressing material thickness variation and bending loss (springback).

Producing acceptable parts from the very first bend, without time-consuming and scrap-producing trial bends, requires a combination of precision tooling, CNC control, and real-time monitoring of the bending process.

Typically, the theoretical bending formulas for determining the depth adjustment of the ram do not account for such variations in materials as tensile strength, rolling direction, and thickness. Moreover, the calculation depends on the exact knowledge of tooling geometry.

Development of CNC controls has proven especially valuable because corrections in setups can be readily made. The operator simply has to perform several bending tests and checks after each new setup until he has achieved the correct angle for the material condition and tooling. Once the operator has optimized the required bend or bends, batch production can begin.

The assumption, of course, is that all parts from the batch are so alike that when corrections are applied to optimize the first part they apply to all parts produced.

In conventional bending, variation in material characteristics causes deviation of the bend angle. Even if the ram reaches the same position within four tenths, a different bend an e will inevitably be produced because of the difference in thickness and subsequent ram position relative to the change in thickness. For example, DIN standards (German) allow a plate to have a thickness tolerance of 8%. Converting this tolerance into a bending angle can mean a difference of 2 deg in the bend. Similarly, differences in rolling direction of the material will produce a variation of up to 1 deg. Until now, it has generally been assumed that this variation is caused by a different springback, an assumption that has proved to be incorrect. The springback is generally the same in both rolling directions; it is the resultant bend radius that is different, causing a change in bend angle.

Closed loop adaptive forming works because the bend angle can be measured from the outside of the part, in real time, using a combination of a mechanical sensor and an "expert" database of information. The database is developed based on experience and the inter-dependency of precision tooling, material characteristics, and press brake geometry. The longer the user works with the system, the more intelligent it becomes.

LVD Corp has developed its Easy-Form system to measure and control the bending angle in-process in a real-time closed loop process. It consists of a mechanically activated sensor, which is a linear encoder located at the front of the press brake table, and an expert system that is stored in the control unit. The Easy-form determines the position of the ram based on the actual bend angle being produced with respect to the differences in tensile strength, rolling direction, and thickness of material.

The Easy-Form system is activated as the sensor approaches the die. Its precise positioning is guaranteed by the stopping device that contacts the die. The sensor measures and monitors position from the bottom side of the material relative to the inside of the V-die as it is being formed.

As the measurement is carried out on the outside of the angle on a straight section of material, the measurement is not affected by material thickness or bend radius. All methods that measure from within the die are necessarily wrong, since the bend radius and material thickness may change.

The Easy-form system works in two phases. In the first phase, the sensor is pushed automatically against the workpiece to be bent, close to the die. During the bending operation, the sensor follows the plate constantly, transmitting the digital information in real time 200 times/sec to the CNC control unit that processes it and subsequently monitors the position of the ram until the required angle is reached. Then the ram halts for a moment to allow the material to flow in a static condition. During the whole bending operation, the curve of the angle development in comparison to the ram position has been accurately measured.

In the second phase, the ram is positioned to reduce the bending angle by the angle correction stored in the expert system. After springback, the obtained angle will be the one required. The expert system obtains its information as follows. By means of the Easy-form system a bend is made with a regulation of 0 deg angle correction. When the programmed angle is achieved, the control applies an angle correction factor stored in the database to allow for springback compensation. The angle correction depends on the tensile strength of the material and on the friction between the material and the die. A different roughness or an oil film on the surface will cause similar effects. Trials have shown that the rolling direction has a negligible influence on the springback of the material, (the resultant difference in bend radius does effect the angle).

The Easy-form system doesn't use the geometrical tooling data because the measured bending angle is the guiding parameter for positioning the ram. This means that precise data for tooling geometry are unnecessary. As the bending system positions the ram to the required angle, and not to a calculated depth adjustment of the ram, the influence of all uncontrollable or uncalculable factors is neutralized. The integrated expert system is transparent to the user and automatically stores "experience" information for later use.

Used with precision tooling and in combination with a crowning table (V-axis), the Easy-form system produces a correct bending angle along the entire length of the sheet within 1/4 deg.
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Copyright 1996 Gale, Cengage Learning. All rights reserved.

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Author:Daugherty, Rod
Publication:Tooling & Production
Date:Feb 1, 1996
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