اثر پارامترهای شکل دهی بر روی کشش ورق در فرآیند شکل دهی تدریجی ورق (ISF) بر روی ماشین تراش CNC The Effect of Forming Parameters on the Sheet Stretch in Incremental Sheet Forming (ISF) process on CNC Lathe Machine

Introduction Forming of sheet metal can be carried out using different methods which are usually based on the use of punches and dies. Other methods like stamping and drawing are always in use in mass production. As the cost of dies is relatively high and it doesn’t justify the batch production environment; it seems necessary to use a new sheet forming procedure, namely, Incremental Sheet Forming (ISF). Nowadays, low production and small batch series are more appropriate in many industries (automotive, aircraft industries, rapid prototyping and medical implants) and ISF appears to be significant to fill this gap between the batch and mass production with less lead-time and investment. There are some parameters which are rather important in ISF than in the other sheet forming process such as spindle speed, tool material, feed rate, temperature and so on. Standardized charts and diagrams can help make this method more applicable. However, none of the previous works has produced a valid table or figure in order to successfully use the method in the industrial workshops. In addition, one can change the output from ISF just by changing one single parameter, for example, the tool coordinate. Nevertheless, ISF has some limitations such as the product volume, the shape of the sheet metal part and the surface quality. There are two methods of finding the optimized parameters in ISF process. The first method is through the use of Computer Numerical Control (CNC) milling machine and the second is by using the CNC lathe machine. In this paper a CNC lathe machine was employed due to its simplicity in programming and low working time which is more suitable to be used as a rapid prototyping machine to receive the program and make a circular shape on a stainless steel 316 sheet metal. Most of the parts used in automotive, ship, aircraft and recently medical implants are made by sheet forming method. These parts have been produced using the conventional sheet forming process either by stamping or drawing which requires the use of dies or punches. If a part needs a replacement, sometimes the delivery of the part is time consuming and it causes long idle time. ISF is an effort to reduce the part lead-time as it is a die-less or punch-less technique. The method is particularly advantageous for customized replacement parts especially in medical implants. Research Background ISF is a technique developed to manufacture sheet metal parts quickly. It is treated as a method to produce prototypes from sheet metals. In 1960s engineers found that if the sheet metal was formed incrementally, forming force would de-crease and the process could run on smaller machines so that more complicated shapes could be formed incrementally [1]. This process is called AISF, Asymmetric Incremental Sheet Forming, which includes sheet metal forming process, small sized solid tool, connection between the forming tool and sheet metal; but there is no need to dedicate a die as in conventional forming process [2]. ISF is divided into two important types called TPIF, Two Point Incremental Forming; and SPIF, Single Point Incremental Forming. In both SPIF and TPIF, forming is the combination of stretching and drawing [3]. There are some parameters which are the same as machining parameters but used in ISF such as spindle speed, feed rate, chip depth and so on; but there are some other parameters like sheet thickness, tool radius, step size and wall angle which depend on the forming process [4]. Some other crucial factors are focused on feedback control for part geometry, male and female die tool and path design [5]. Some of these parameters have a close relation to the tools so a high performance tool is needed [6]. All of these parameters take into account the increasing ability of force prediction in the forming procedure [7]. The material of the tools was also considered, as Fiorotto [8] mentioned, it can be made of stainless steel, high speed steel or high carbon steel. It can also be made of brass or bronze depending on the material of the sheet metal. Among all parameters that affect this process, friction between the tool and the sheet metal is an important parameter that needs to be taken care of because it directly affects on the sheet metal surface quality. Shear failure can also occur when there is a lack of lubrication or when there is an over load on the sheet metal that causes heating by friction [2]. If the relative motion between the sheet metal and tool surface is small the heating is minimized during the forming. In addition, if the tool rotates at a high speed, the quality of surface decreases and the temperature at the forming area would rise. Moreover, hot condition causes tool wear which decreases tool life during the process. The amount of friction also depends on some parameters such as lubrication, spindle speed, sheet material, and tool material and so on. If the tool and sheet metal are of the same material, then there is a definite rise in the forming temperature. Previous researchers have found some ways to proportion the amount of friction stress to normal contact stress [10].