Introduction

It is known that a crucial part of the production of a sheet metal stamping die is essentially the development of a die-face design aiming a tooling surface geometry that gives a fully developed blank shape a defect-free stamping from within the necessary constraints. The design of stamping tooling elements starts with the part geometry as the basic input data and the methods engineers try to determine the minimum number of operations for a given stamping form in order to reduce the forming tooling costs while satisfying the objective stamping criteria. The methods engineer conducts various try-outs for the forming process design continuing up to the end of workshop try-out until to the mass production phase of the stamping part. Since both the stamping die-face design and the plastic workability of the sheet metal determine the characteristics of blank deformations, additional care should be paid in the forming of high strength steels to adapt to the lower formability and higher springback deformations.

In line with the advancements in the computer aided design and analysis tools the die try-out phase may be carried out reliably in computer generated virtual design environment, and the methods and tooling engineering takes the advantage of the finite element method based simulation in the prediction of the probable formability problems, such as cracks, wrinkles or excessive thinning, related to the die-face designed for a given stamping form. It is also stainable to estimate the final part geometry after trimming operation and springback deformation. This engineering approach assumes that the die-face deformation during the drawing process are negligible and the industrial practice has proved the validity of this assumption for even large inner panel draw-dies in the case of conventional draw-quality steels. The notion of an ideally rigid draw-die construction, nevertheless, becomes arguable when it comes to the forming of new class high strength steels of comes to the forming of new class high strength steels of moderate thickness because of the bigger die-face distortions because of the relative high forming forces, which may be not considered insignificant anymore. Hence, the die-face deformations and its implications should be considered in connection with the draw die design before submitting to the production.

Following a short review of the stamping die design practice; a computational methodology is presented for the assessment and control of die-face deformations during the sheet metal forming processes. The proposed approach is employed in the forming process design and analysis concepts given in Part1 and in Part 2 of this study. The die-face deformations are taken into account in the computer aided design of the process tooling. The part formability analysis and springback deformations are conducted including the tooling deformations. The relative differences between the ideally rigid and deformable forming interfaces are discussed, and the assumption of an ideally rigid die-face design is fulfilled by increasing the punch casting wall thickness.

Die-face design concepts

The die-face design for a sheet metal forming die may be defined as the composition of a complete surface geometry that deforms a sheet metal blank plastically into a desired stamping shape by ensuring a rigid tooling construction. The design process starts with the part geometry as the basic input data, the methods engineer firstly decides on the drawing direction by tipping the part to the most favorable axis, and eliminating the risk of an undercut. Then, using the material formability and minimum allowable thickness, the amount of stretching deformation is determined and the number of stretch-draw operations is estimated. Using the half-thickness offset geometry of the sheet metal part the designer sets additional surfaces for the punch face by extending the part edges, filleting the sharp edges and by unfolding the flange-type of geometry in the CAD environment. Using the material properties and the amount of maximum stretching deformation, the maximum achievable drawing depth is estimated, and a set of drawbar and counter bar surfaces may be added to both punch and die in order to minimize the deformation gradient during the initial stage of the forming process. After deciding on the press operation type, the binder geometry is generated using a set of flat or developable surfaces, and usually integrating with the draw-bead and contra-bead elements in order to restraint the material flow over the punch in an controlled manner. After the creation of punch and binder interface, their geometric counterparts are developed usually by offsetting the surfaces with a clearance amount that is typically a few percent larger than the sheet metal thickness using the CAD software. At this stage using the allowable thinning of the part, the amount of stretch, and the blank size estimate may be done using the volume constancy assumption.

Once the methods engineer has created a entire geometric description of the blank and die faces in a CAD environment, the finite element analyses may be performed in order to investigate the process feasibility in terms of the formability, part geometry after springback and forming loads by assuming and ideally rigid die construction. The finite element simulation of the stamping process is done usually in two steps. A forming analysis is conducted to determine the metal deformation for a given punch and binder loading and, secondly, the springback deformation following the removal of the tooling is computed with the forming stress distribution and the deformed geometry from the forming step as the inputs along with material thickness distributions. Depending on the relative qualities of the process and material parameters, several virtual try-outs may be necessary in order to reach the optimum tooling geometry and forming elements. At this point, the forming loads and the type of draw action is determined in accordance with the available press line specifications. Finally, the complete stamping tooling surface is approved and submitted to the draw die construction and manufacturing department.