Determination of spring-back of stainless steel sheet metal in “V” bending dies

Abstract

It is known that when bent on dies, sheet metals are prone to some amount of spring-back depending on elastic deformation. Obtaining the size desired and design of die depends on the knowledge of the amount of this spring-back. This research has been conducted to determine experimentally spring-back of sheet metals on bending dies. The amount of spring-back in sheet metals at different bending angles has been obtained by designing a modular “V” bending die. Furthermore, a contribution to the field literature is aimed through spring-back graphics. Spring-back graphics for three kinds of materials of different thickness have been obtained by using four different bending methods on eighteen different modular dies and a total of 720, 10 at least belonging to each of the kinds, stainless steel sheet metal samples are bent. The results have shown that of the four different bending methods used in the field most, two cannot be employed for spring-back, and that holding the punch longer on the material bent reduces spring-back whereas an increase in the thickness of the material, and bending angle increase spring-back values. Spring-back values vary between 0.5° and 5°.

Introduction

Stainless steel sheet metals have a wide range of application in industry and commonly used for automobiles, household goods, electronics and medical devices. Changes made in the composition of stainless steel to obtain the required mechanical and chemical properties according to the fields used influence their cold shaping. High tensile strength, resistance to corrosion, low thermal conductivity, and ductility that stainless steels possess and presence of a great amount of chrome–nickel and some amount of molybdenum, strength enhancing elements, are the primary factors that complicate cold shaping compared to other materials. Majority of these goods around us are formed by means of a bending apparatus, die or machine, and it is essential that these metal parts be within the required dimensions and tolerance limits. Obtaining the required dimensions and tolerance limits depends on the amount of spring-back of the material bent.

Shaping materials without removing any chips around a definite axis through or without heat is called bending. Bending is the process of placing a sheet of metal over the matrix on the press bed where the sheet is bent around the tip of the punch as it enters the die. Bending dies are the setup, proper to the required piece shape, consisting of a female die and punch, and making permanent changes on steel sheet material [1]. In bending processes, stress–strain distribution is very important for achieving the perfect bending profile required. In bending process, elasticity limits of materials can be exceeded while that of flow stress cannot. Therefore, the material still keeps a portion of its original flexibility [1], [2], [3]. When the load is removed, the material tries to retrieve its original form and the material bent expands backwards with some amount of stretching. This behavior of the material is called “spring-back”, and the methods such as increasing the bending angle with respect to the “spring-back”, crushing the bending zone to prevent spring-back, bending the piece through stretching, and increasing the time to hold the load on the material are the ways commonly used to avoid spring-back in products [1].

Many research conducted recently have shown the importance of spring-back in sheet metal industry, and studied how this permanent physical variation can be avoided. One common point of all these research is that they have dwelled on estimating or determining the amount of spring-back in advance, and accordingly, designing and later manufacturing dies based on this spring-back amount. That is why some mathematical models are developed to predict the spring-back. While some researchers have dealt, in order to minimize spring-back values, with bipartite functions of both die and material resulting in the ideal die or matrix measurements, others have brought forward all functions to act on spring-back and determined how to minimize them.

One of the most important problems in “V” bending dies is the control of the spring-back. It is known that different spring-backs are possible regarding die design and convenience of the material. A combination of various materials and processes make it difficult to obtain the predicted spring-back. Material parameters such as elasticity, yield stress, hardening property, and process parameters such as the load applied, thickness of sheet metal, die angle, punch radius and die gap affect spring-back in a complex way. These parameters are presented in Fig. 1. In this study, parameters related to bending are expressed as: punching radius R, die upper corner radius Rd, die gap W, material thickness T, punch depth h (total of H_p and H_e) and bending angle α_b, the angle between two feet of a bend [2].

In the research conducted by Tan et al. [2], a simple method is proposed to prevent the spring-back. In this method, sheet metal is exposed to a set of loading and unloading process. Each bending angle and the corresponding spring-back values are measured, and the output signal is computerized. Thus, the appropriate press depth of the punch, which corresponds to excessive bending, is determined.

The first objective of the research conducted by Shu and Hung [4] was to employ finite elements method to analyze the relation between coupled bending technique, spring-back and variables. Later, in order to reduce spring-back and find optimum shaping parameters, they combined the finite elements analysis and optimization techniques. The results obtained from the study were compared with the experimental data, and it was concluded that increasing die gap results in reduction of spring-back values. Gan and Vogener [5] have developed a new method to design general sheet in forming dies to produce a desired final part shape by taking spring-back into account. This application method is general in that it is not limited to operations having particular symmetry, die shapes, or magnitude of spring-back shape change.

Tseng et al. [6] worked on the issue designing a V-die to examine the spring-back behaviors of copper–beryllium (CuBe) sheets regarding all the principles. Tseng et al. [7] researched the importance of spring-back values in the shaping of CuNiBe alloy. They have suggested many analytic formulas on estimated spring-back, and made the designed of the elements obtained by means of computer. Pourboghrat and Chu [8], Papeleux and Ponthot [9], Micari et al. [10], Forcellese et al. [11], also conducted studies employing the finite elements method. They eventually showed and supported experimentally that finite elements method could be employed in calculating die parts in addition to spring-back calculations. Ling et al. [12], studied spring-back values in L-bending using the finite elements method.

Yuan [13] tried to lessen spring-back, in the material subjected to plastic deformation, through redistribution of elastic stresses inside the material after the release of the affecting load. Up to now, spring-back has been limited to homogenous materials. Upon the increase in the use of composite materials in engineering, Yuan looked for and presented a mathematical model solution to determine spring-back for such materials. A new mathematical method is prepared by Zhang et al. [14] for the analysis of spring-back in metal sheet forming. The model is used to determine the spring-back and bending moment force. Lo et al. [15] have studied in their research the spring-back of metal parts and conformity of surface stress deformation with power transfer theories.