DRAWING PRACTICE .1 Defects During Deep Drawing

Various Forming Processes 93 Fig. 7.10 Schematic illustration of segmented punch. A method of enlarging portions of a drawn shell in a press is shown in Fig. 7.1 1. The value of the expanding ratio = Dd is dependent on the kind and heat-treatment condition of the material and the rel- ative thickness of the material Some values of are given in Table 7.2. The thickness of the material at the deformation zone of the workpiece is reduced by the expansion operation and is given by the formula: = e where: T = thickness of the material before expanding T = thickness of the material in the deformation zone D = main diameter of workpiece before and after expanding. Punch \ F Die 7.12 Fig. Expanding a drawn shell with a punch. 94 Various Forming Processes Low-carbon steel Aluminum and copper Table 7.2 Values of expanding ratio K for a drawn shell. 0.45 to 0.35 0.32 to 0.28 Annealed Hard Annealed Hard 1.20 1.10 1.15 1.25 1.20 1.20 1.15 MATERIAL Material thickness Relative thickness of material Die corner radius mm 4 to 2 to 3T The punch force can be calculated by the formula: where: = for E = 0 and = main deformation impedance. k = = 0.5 + k , I k , = for E = d 7.4 DIMPLING Dimpling is the process of bending and stretching flanging the inner edges of sheet metal components Fig. 7.12. A hole is drilled or punched and the surrounding metal is expanded into a flange. Sometimes a shaped punch pierces the sheet metal and is expanded into the hole. Stretching of the metal around the hole subjects the edges to high tensile strains, which could lead to cracking and tearing. As the ratio of flange diameter to hole diameter increases, the strain increases proportionately. The diameter of the hole may be calculated by the formula: d = D - - - where: d hole diameter before dimpling, = height of flange, = die corner radius Table 7.3. T = material thickness. Table 7.3 Values of die corner radius Rm, for dimpling 7.13 Various Forming Processes 95 Final T Phase Fig. 7.12 Phases in the dimpling process. The height of the flange is given by the formula: D - d 2 + + 7.14 The punch force is given by: F = D - d 7.15 The thickness of the material at the end of the flange is reduced by the stretching of the material. The min- imum thickness of cylindrical flanges is given by the formula: 7.16 In the dimpling process, the ratio of the hole diameter to the flange diameter is very important and is given by the formula: d D The value of m is given in Table 7.4. 7.17 96 Process used in making hole: Drilling Punching Various Forming Processes Relative thickness of material x 100 2 3 5 8 10 15 20 30 70 0.75 0.57 0.48 0.41 0.40 0.34

0.32 0.26

0.22 0.70 0.60 0.52 0.50 0.50 0.48 0.46 0.45 7.5 SPINNING Spinning is the process of forming a metal part from a circular blank of sheet metal or from a length of tubing over a mandrel with tools or rollers. There are three types of spinning processes: Type I Spinning In type I spinning, also called manual spinning, a circular blank of sheet metal is held against a form block and rotated while a rigid tool is used to deform and shape the workpiece over the form block. Fig. 7.13 schematically illustrates the manual spinning process. The tools may be operated manually or by a hydraulic mechanism. Forming stages Workpiece Form block Fig. 7.13 Manual spinning process. Fig. 7.14 illustrates representative shapes that can be produced by manual spinning. The advantages of manual spinning, as compared to other drawing processes, are the speed and economy of producing prototype samples or small lots, normally less than 1,000 pieces. Tooling costs are less and the invest- ment in equipment is relatively small. However, spinning requires more skilled labor. In Fig. 7.15 is shown a relative cost comparison for manufacturing a round sheet metal shell by deep drawing and by manual spinning.