JMEPEG (1993) 2:863-866 9 International Drawbeads in Sheet Metal Forming M. Y. Demeri Drawbeads are used to control the flow of sheet metal into the die cavity during stretch-draw forming of large panels. They prevent wrinkling in formed panels, reduce the blankholder force, and minimize the blank size needed to make a part. Drawbead restraining forces (pulling forces) and failure locations in the formed sheets are usually evaluated by using drawbead simulation tooling. In this article, a drawbead simulation apparatus is used to assess the influence of variation in material, bead penetration, and fric- tion conditions on the drawbead restraining force. Results from the test can be used as input for sheet metal forming simulation programs. Keywords computer simulation, drawbeads, forming 1. Introduction The stamping of large sheet metal panels is generally a two- step process. In the first step, the binder clamps the sheet metal to hold it in place, and in the second step, the punch contacts the sheet and forms the part. The binder surface may be fiat, or it may contain drawbeads. A fiat binder surface provides little braking action (restraining force) as the sheet metal flows into the die cavity. Friction in a fiat binder may not provide enough restraining force to control metal flow and prevent wrinkling in the formed panels. The presence of drawbeads in the binder area increases the braking action substantially and provides ex- tra restraining force because the sheet metal would have to bend around the bead as it is being pulled through the binder. The pulling force (drawbead restraining force) is the force re- quired to pull the sheet metal through the drawbeads. The mag- nitude of this force and, consequently, the amount of draw-in allowed is determined mainly by the geometry of the drawbead and the clamping force in the binder area. The clamping force is the force needed to maintain a certain depth of the bead. It is important to note that too much metal flow into the die cavity may produce wrinkling, whereas too little metal flow may cause splitting in the formed panel. Drawbead restraining force is caused, to a large extent, by deformation resulting from bending and unbending of the sheet metal in the drawbead area and, to a lesser extent, by the fric- tional forces resulting from the sliding contact between the binder and the sheet metal. The bending force was found to be inversely proportional with the radius of curvature, whereas the friction force was found to be directly proportional with the co- efficient of friction (It) and the binder hold-down force (BH). The bending force was also found to be larger than the friction force by a factor of about 3:1 for good lubrication condition and about 1:1 for poor lubrication.Ill Drawbead action occurs in two stages. In the first stage, the binder closes, and a protrusion on one binder face pushes the sheet metal into a matching groove on the other binder face to M.Y. Demeri, Scientific Research Labs, Ford Motor Company, Dear- born, M148121-2053. form a bead. The depth of movement of the protrusion into the sheet surface determines the amount of bead penetration. In the second stage, the sheet metal is pulled through the bead and into the die cavity. The restraining force of a drawbead increases with increase in penetration because the metal would be sub- jected to more bending. The amount of bead penetration is con- trolled by the magnitude of the normal force (hold-down pressure). During prototype development and tryout of sheet metal panels, drawbeads are physically adjusted or altered to obtain the proper restraining force. Adjustments are usually done by an elaborate method of welding and hand grinding until the proper amount of metal flow in the die cavity is achieved. In some situations, a single drawbead with more restraining force may replace multiple drawbeads, and this may result in reduc- ing the blank size needed for the finished product. 2. Computer Simulation of Panels Computer modeling and simulation is now replacing proto- typing to help die designers, during tool development, to ex- plore alternative die designs and to evaluate tradeoffs on the computer. Using this methodology, proposed product designs can be tested without the need for expensive tooling and time- consuming tryouts. A successful computer simulation, how- ever, depends greatly on the relevance of the simulation model and the accuracy of the input parameters. Such parameters usu- ally are derived from experience or from experimental evalu- ation. An important input for sheet metal forming simulation pro- grams is the drawbead restraining force. This input is essential to determine the proper amount of metal flow needed for suc- cessful stamping. Because the magnitude of the drawbead re- straining force, for a specific bead geometry, is determined largely by the amount of bead penetration, a relationship be- tween the two parameters must be established for various mate- rials. The importance of drawbeads in sheet metal forming simu- lation has been demonstrated by two numerical simulation ex- amples for autobody panels.[21 In the first example, the introduction of a full set of drawbeads in the binder area pro- duced the proper amount of stretching in the plane of the sheet and resulted in the elimination of the loose metal problem. In Journal of Materials Engineering and Performance Volume 2(6) December 1993---863