Evaluation of Metal Strip-Layout Selectionusing AHP and TOPSIS Method

Abstract

Stamping dies are used to produce very large numbers of identical parts from sheet metal. Due to the high volumes of parts produced, even small inefficiencies in material utilization per part can lead to very large amounts of wasted material over a die's life . Strip-layout design is an important step in the planning stage of sheet metal work on metal stamping. It is an experience-driven activity and the quality of strip-layout is highly dependent on the knowledge and skill of die designers. However, due to the complexity in strip-layout, it is impossible to judge the efficient layout manually by the designer. This paper presents a strip-layout selection procedure pertaining to metal die stamping work in complex layout situations. The procedure is based on a combined TOPSIS and AHP method. The proposed Strip layout index helps to evaluate and rank of any given set of strip-layout alternatives for a given engineering design. The procedure is illustrated by means of an example. During the design process for stamping dies, decisions must be made about the orientation of the stamped part on the strip. The orientation determines how efficiently raw material is utilized, and in an operation such as stamping where large amounts of material are processed, small inefficiencies per piece can accumulate into huge wastes of material in the long term. In stamping, sheet metal parts of various levels of complexity are produced rapidly, often in very large volumes, using hard tooling. Maximizing material utilization in stamping is of paramount importance. Raw materials typically represent 75% or more of total costs in stamping facilities, [1] so a poorly designed die can significantly increase a company's operating costs over its life. Due to the high volume of parts produced, even small inefficiencies in material utilization per part, can lead to very large amounts of wasted material over a die’s life. Hence, the choice of an efficient strip-layout is an important step during die design, because as only the optimum layout can reduce wastage of the strip material and reduce the overall cost of production. Traditional strip-layout design is manual, highly experiencebased activity and therefore tedious, time consuming and errorprone [2-3]. The blanks cutting from cardboard were manipulated to obtain a good strip-layout. This trial and error procedure of obtaining suitable strip-layout with maximum material utilization is still being used in most of the small scale and even in some medium scale sheet metal industries worldwide. The quality of strip-layout achieved by using traditional methods depends on the experience and knowledge of designers. On the advent of computer aided design (CAD) systems, the process of strip-layout design was somewhat made easier and the design lead-time was reduced. However, welltrained and experienced die designers were still required to operate these CAD systems. Most of the applications of CAD in strip-layout design are aimed mainly at achieving better material utilization by rotating and placing the blanks as close as possible in the strip. However, the strip-layout with maximum material saving may not be the best strip-layout, indeed the die construction may become more complex, which could offset the savings due to material economy unless a large number of parts are to be produced. The system developed by Schaffer [4] reported to calculate the stresses due to bending moment on cantilevered die projections and if the system finds that the stress level is above the yield stress of die steel material, then the system distributes the cutting operations over several stages in order to keep the stresses within the reasonable limit. One of the