Metal deforming – With use of control means energized in response to activator... – Including plural sensors or sensor responsive to comparison...
Patent
1996-06-24
1998-11-03
Jones, David
Metal deforming
With use of control means energized in response to activator...
Including plural sensors or sensor responsive to comparison...
72 198, 72 201, 72 214, 723893, 723896, 72702, B21D 501
Patent
active
058292888
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a method for folding a metal sheet to a well-defined angle.
In a known method, a metal sheet is deformed in a recess of a die through a pressure means being moved above the recess, pressing on the sheet, towards the die until it reaches a well-defined end position.
The punch is then raised again, and the sheet springs back elastically a little. The difference in angle between the situation in which the sheet is clamped between the punch in its minimal position and the vee-block and the situation in which the sheet is entirely clear of the tools is known as the spring-back.
In order to form a fold with a desired angle, the Y-position which the punch must assume is determined by means of a folding formula or table. Folding formulae and tables only take account of the sheet thickness and the V-opening. They generally do not take account of material properties such as tensile strength and rolling direction, but the latter also influence the folding angle obtained. The folding angle actually obtained will therefore be only approximately the same as the desired folding angle.
After the sheet is clear of the tools, the angle obtained is measured. On the basis of the deviation in angle, a calculation of the correction necessary on the Y-position of the punch is made. A fold is made in a new sheet taking into account the correction in Y, and the result is measured again. This is iterated until the fold angle obtained lies within the desired accuracy limits.
It is important to point out that the folding formula which gives the Y-position as a function of the desired angle is only an approximation. For, the angle obtained greatly depends on the material used, the composition of the material, the texture of the material, the direction of the fold relative to the rolling direction, and the thickness of the material. All these parameters which influence the angle obtained ensure that, for each fold, the correction must be established experimentally in order to obtain a fold within the desired accuracy limits.
The angle can also be measured in each case only after the sheet is clear of the tools, which is when the spring-back has been completed. It goes without saying that this experimental optimization of the folding angle is laborious and time-consuming. Moreover, it is always necessary to make a number of test pieces, which are wasted.
In the case of modern folding presses, the folding formula is incorporated in the control system. The operator thus merely has to enter his experimentally measured angle in the control system. The control system then automatically calculates the correction on the Y-position for the next test piece, but here again the angle is still measured manually, the optimization is still laborious, and worthless test pieces have to be made.
This has to be carried out for each fold of a particular workpiece. Once all folds of a particular workpiece have been optimized, production of a series of the same pieces can be commenced. It is then assumed that all pieces of the range are so similar that the corrections applied, which were optimal for the first piece, are applicable to all pieces. Needless to say, thickness variations, variations in material properties and a different orientation relative to the rolling direction will cause a deviation in the angle obtained.
The method according to the invention is a method by means of which the folding to the desired folding angle is optimized during the making of the fold, i.e. it is an adaptive folding method.
A number of adaptive folding methods are known. Such methods are: Measured
Methods which measure the force as a function of the position of the punch aim to derive a number of material properties and thickness deviations therefrom. A mathematical model is then used with such data to forecast up to what position (lowering) the punch must go in order to obtain the desired angle.
The force is measured by a force transducer (for example, strain gauges) which is disposed along the force path. T
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Jones David
L.V.D. Company N.V.
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