Computer aided analysis and design of sheet metal forming processes:

Abstract

The finite element simulations of a sheet metal forming process help the methods and tooling engineer designing the forming interface for a stamping part by shifting the costly press shop try-outs to the computer aided design environment. The finite element models used in the sheet metal formability and stamping feasibility assessment studies are commonly based on the ideally rigid die-face design. This hypothesis is in general consistent with the present industrial experience even for large draw-dies of conventional steels. Nevertheless, it may not be practicable in the case of the forming high strength steels of moderate thickness because of the comparable higher press loads required to shape the blank. Consequently, an estimation of the die-face deformations during the forming process may be necessary during the evaluation of potential formability and springback problems and possible compensations should be considered in connection with the usual stamping die design and construction routines before submitting to the production. In this part of the study, an engineering methodology is presented for the structural assessment of the stamping tooling and the die-face designs during the sheet metal forming processes. Using the computer aided analysis and design concepts given previously in Part I and in Part II of this study, the proposed approach is employed in the forming interface design of an automotive stamping part including the complete die construction. The results have indicated the relative merits of the die-face distortions on the formability and springback deformations.

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 form within the necessary quality 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 [1]. The methods engineer conducts various try-outs for the forming process design continuing up to the end of workshop try-outs 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 [2]. 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 attainable to estimate the final part geometry after trimming operation and springback deformation. This engineering approach assumes that the die-face deformations 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 [3]. 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 moderate thickness because of the bigger die-face distortions because of the relative high forming forces, which may be not considered insignificant anymore [4]. Hence, the die-face deformations and its implications should be considered in connection with the draw die design before submitting to the production.

In this paper, 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 for a cab body member based on the computer aided design and analysis concepts given in Part I and in Part II 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.