Modifying A Metal Stamping Die To Compensate For Springback

Method for modifying a metal stamping die to compensate for springback includes the steps of metal stamping a first part using a base, or pre-existing die set, and creating a surrogate die having the shape of the first stamped part. These steps are followed by simulated metal stamping of a blank with the surrogate die, where the blank is shaped according to the desired finished part. 

Then, the base die is modified by mapping the forming stresses, from the stamped blank, to an indicator blank, which is allowed to relax and then is employed as a template for modification of the base die. Following this, the accuracy of the newly modified base die is determined by metal stamping a second part in the modified base die and by comparing a number of dimensions of the second part with a number of corresponding dimensions of the desired finished part.

"Springback" is a phenomenon characterized by the regressive movement of stamped materials, particularly metals, following removal of a metal stamping from a press. In general, dies must be constructed to overbend materials, so as to produce finishedparts meeting all required dimensional specifications. Springback compensation presents a major obstacle during metal stamping tool development, especially for lightweight materials such as aluminum and high strength steels. If not properly accommodated,springback may cause the shape of stamped panels or structures to deviate unacceptably from the intended design, rendering the stamped part unusable. As manufacturers of motor vehicles, in particular, move to the use of more aluminum, as well as higherstrength steels, to save vehicle weight, the more pronounced springback tendencies affecting these materials adds as much as six months to the standard tooling development time of a vehicle manufacturer. This timing is incompatible with current marketdemands.

Known practices for compensating for springback primarily employ manual correction through an iterative, or trial-and-error, process. Thus, long years of experience are needed for an engineer to properly predict and correct for springback. Thisitself is a problem because materials such as aluminum and high strength steels have springback characteristics which are significantly different from those of mild steel--the staple material of vehicle bodies for more than one hundred years. Thepresent invention provides a method for tooling design to accommodate shape distortions introduced by springback. Because the method may be completely computer based, it may be applied during early draw die development stages, without the necessity of aphysical prototype.

A method for modifying a metal stamping die includes the steps of metal stamping a first part using a base die, creating a surrogate die having the shape of the first part, and using the surrogate die to simulate metal stamping of a blank which is shaped accordingto the desired finished part. Then, the base die is modified by mapping the forming stresses from an indicator blank which is dimensionally identical to the base die. The indicator blank is allowed to relax and deform, and is then used as a template tomodify the base die. A second part is then stamped with the modified base die and compared with the desired finished part. This comparison includes a plurality of corresponding dimensions from the second part and the desired finished part.

The present method further includes additional steps in the event that the previously described dimensional comparison indicates that the second part is not sufficiently comparable to the desired finished part. These additional steps includesimulating metal stamping of another blank shaped according to the desired finished part, using the surrogate die which has been modified to the shape of the second part, and further modifying the base die by mapping the forming stresses from the restampedblank. Then, a third part will be stamped using the further modified base die, and compared with the desired finished part. This process continues iteratively until the stamped part meets all of the required dimensional criteria of the desired finishedpart.

According to another aspect of the present invention, the first part which is stamped in the base die may be stamped either conceptually in a software simulation, or in a physical, pre-existing die set. Similarly, the surrogate die may becreated as part of a software simulation or as a physical die set. The indicator blank is also contained in software.

It is an advantage of a method according to the present invention that the time required to make changes in metal stamping dies is greatly reduced, and this is particularly so in the case of dies used to form materials such as aluminum and highstrength steels.