MECHANICS OF DEEP DRAWING

80 Deep Drawing T I Fig. 6.8 Developing corner contour: Rb = 0. In all three examples, the center of the radii R and is the same. Considering that the flat shell sides are bent, for calculating the flat blank dimensions of the following formula may be used: 6.15 However, calculating the flat blank size for rectangular drawn shells Fig. 6.6, Fig. 6.7, and Fig. 6.8 in this way is not satisfactory because the sharp transition between the corner arcs and the flat sides will result in cracks. The shape of the blank needs to be modified as shown in Fig. 6.9, according to the following steps: a Draw a rectangle with dimensions a and b. b At each side of the rectangle, add a value c From the center of radius point 0 draw an arc with radius = where: 0.0185 - + 0.982 d Reduce the height of each side by the following values: 2 = - , and 6.17 2 = - b e Round the comers by radii and whose value is defined graphically. Note that the subtracted surfaces should be equal to the added surfaces. Deep Drawing 81 When laying out the blank it is usually advisable to plan for a form that will produce corners a little higher than the sides. The wear on the die is at the corners, and when it occurs, the metal will thicken and the drawn part will be low at the corners if no allowance for wear has been made on the blank. Blank Fig. 6.9 Modified blank of rectangular shell.

6.5.2 Square Shells

A flat blank for square shells without flanges, Fig. 6.10, has a circular shape whose diameter may be cal- culated by the formula: D = + - + 2.28 - -

5.3 R,

6.18 The height of the workpiece is = the value of Ac may be found from Table 6.4, or calcu- lated by the formula: Ac = 0.7 Fig. 6.10 Blank for square shell. 82 Deep Drawing 6.6 DRAWING PRACTICE 6.6.1 Defects During Deep Drawing Deep drawing operations are governed by many complex factors that may result in either successful or defective products. Clearance. If the shell fractures during the deep drawing operation, the problem may be that the clearance between the punch and the die is incorrect. This problem can be a direct result of the punch and die hav- ing been designed or made with incorrect clearance. Chapter 11 may help in making a correct choice for the clearance. Fractures can also result if the thickness of the work piece is out of tolerance or not uniform, or if the punch and die are not properly aligned. Blank holderpressure. If too much force is applied to the blank, the punch load will be increased because of the increase in friction, and this increase will lead to fracture of the shell wall. To calculate the blank holder force, see Chapter 1 1. The corner radius of the punch and die radius. These radii are important for a successful deep drawing operation. If the radii are too small, the corner may fracture because of the increased force required to draw the cup. Scratches, dirt, or any surface defect of the punch or die increase the required drawing force and may cause a shell to tear Fig. and Fig. . For correct calculation of the corner punch and die radii, see Chapter 1 1. If the blank holder exerts too little pressure, or if the die radius is too large, wrinkles will appear at the top flange of the part, as shown in Fig 6.1 1. Fig. 6.1 Wrinkles on the workpiece. Drawing beads are useful in controlling the flow of the blank into the die cavity. They are necessary for drawing nonsymmetrical shells. For the proper design and location of drawing beads, see Chapter

6.6.2 Lubrication in Drawing

During deep drawing, different lubrication conditions exist, from hydrodynamic lubrication in the blank holder to boundary lubrication at the drawing radius, where breakdown of the film very often occurs. Lubrication in deep drawing is important in lowering forces, increasing drawability, reducing wear of the tool, and reducing defects in the workpiece. Lubricant selection is based on the difficulty of the operation, Deep Drawing 83 the type of drawing operation; and the material; recommendations are given in Table 6.5. In this table, a mild operation typically is a shallow draw on low-carbon steel, a medium operation is a deep draw on carbon steel, and a severe operation is a cartridge-case draw or a seamless tube draw. Table 6.6 Lubricants commonly used in deep drawing processes. DRAWING OF MATERIAL Steel - carbon and low alloy Stainless Steel Aluminum and Aluminum-alloys Titanium Copper LUBRICANT Mild Operation: Mineral oil of medium heavy to heavy viscosity, Soap solutions 0.03 to 2.0 percent high-titer soap, Fatty oil + mineral oil, emulsions, and lanolin. Medium Operation: Fat of oil in soap-base emulsions, fatty oil + mineral oil, soap +wax, dried soap film. Severe Operation: Dried soap or wax film, sulphide or phosphate coatings + emulsions with finely divided fillers and sometimes sulphurized oils. Mild Operation: Corn oil or castor oil, castor oil + emulsified soap, waxed or oiled paper. Medium Operation: Powdered graphite suspension dried on work piece before operation to be removed before annealing, solid wax film. Severe Operation: Lithopone and boiled linseed oil, white lead and linseed oil in a heavy consistency. Mild Operation: Mineral oil, fatty oil blends in mineral oil 10 to 20 fatty oil. Tallow and paraffin, sulphurized fatty oil blends to 15 preferably enriched with 10 fatty oil. Severe Operation: Dried soap film or wax film, mineral oil or fatty oil, fat emulsions in soap water + finely divided fillers. Chlorinated paraffin, soap, polymer, and wax. Fatty oil + soap emulsions, fatty oil + mineral oil, lard oil blends 25 to 50 in mineral oil, dried soap properly applied.

7.1 Stretch Forming

7.2 Nosing

7.3 Expanding 7.4 Dimpling 7.5 Spinning 7.6 Flexible Die Forming VARIOUS FORMING PROCESSES

7.1 STRETCH FORMING

The process of stretch drawing was developed as a method of putting metals under combined bending and tension stresses at the same time. Sometimes, a part that has been previously bent may be used as an ini- tial material in stretch draw forming. In stretch forming, the sheet is clamped around its edges and stretched over a die or form block. This process strains the metal beyond the elastic limit to set the work- piece shape permanently. Workpieces may have single or double curvatures, as in aircraft skin panels and structure frames, or automobile body parts. To assess the formability of sheet metals while forming a workpiece, circle grid analy- sis is used to construct a forming limit diagram of the material to be used. In such an analysis, a circular pat- tern is etched on the sheet blank The blank is then formed in a die. Each circle on the blank will deform in a different manner due to local forming patterns. After a series of such tests on a particular metal sheet, the deformed circles are analyzed to produce a forming limit diagram FLD that shows the overall forming pat- tern of the blank during plastic deformation. In the forming limit diagram, the major strain is always positive. However, minor strains can be positive and negative at the same time. Fig. 7.1 shows the FLD that bounds the deformation of the sheet metal. Above the curves is the failure zone, and below the curves is the safe zone, and the actual strains used in stretch draw forming must be below the curve for any given material. Two methods are used in stretch forming: the form block method and the mating die method. a The form block method is shown in Fig. 7.2. Each end of theblank is securely held in tension by an adjustable gripper, which is moved to stretch the blank over a form block. The desired shape of the workpiece is formed by the action of the form block as the material is moved hydraulically against the block. 85 86 Various Forming Processes After Before stretching Minor axis I I -40 40 60 Minor strain Fig. Forming limit diagram FLD. Fig. 7.2 Stretch draw forming with a form block. b The mating die method is shown in Fig. 7.3. The blank is held in tension by grippers which, as they move, perform two actions: they stretch the workpiece by a predetermined amount to approximately 2 elongation over the form block. The punch then descends onto the blank to thus form the workpiece into the desired shape by pressing the metal against the dies. The process is used primarily for aerospace and automobile applications with a variety of materials. The workpieces may have single or double curvatures, such as in aircraft wings or fuselage skin panels, automobile body parts, etc. Typical shapes of workpieces formed by these methods are shown in Fig. 7.4. Various Forming Processes 87 Fig. 7.3 Stretch draw forming with mating dies: a work material is held in tension; b punch moves to form workpiece. Fig. 7.4 Typical shapes of part formed with mating dies.

7.2 NOSING

Nosing is a die reduction method whereby the top of a cup or a tubular shape may be made closer or small- er in diameter than its body. There are three types of end profiles after nosing: Frustum of cone Neck Segment of sphere. It is possible to reduce the top of the cup if the material is not too thin, to about 20 of its diameter in one operation. Nosing compresses the work metal, resulting in an increase in length and wall thickness. Fig. 7.5 shows schematic illustrations of three types of die reduction methods. The calculation of the height of a drawn shell or tubing by nosing its end is different for each type.