Chapter 12 Sheet Metal Forming PDF

CHAPTER 12 SHEET METAL FORMING BMFS 2613 MANUFACTURING PROCESS 1  Introduction Shearing Sheet Metal Characteristics and Chapter Formability Outline Formability Tests for Sheet Metals Bending Sheets, Plates and...

CHAPTER 12 SHEET METAL FORMING BMFS 2613 MANUFACTURING PROCESS 1  Introduction Shearing Sheet Metal Characteristics and Chapter Formability Outline Formability Tests for Sheet Metals Bending Sheets, Plates and Tubes Deep Drawing 2  Sheet metal parts offer the advantages of lightweight and versatile shapes. Introduction A sheet metal part produced in presses is called stamping. Low carbon steel is the most used sheet metal, because of its low cost and generally good strength and formability characteristics. For automotive application, a higher strength steel is used; for providing good crash protection in a lightweight design. Aluminum and titanium are also used for corrosion resistance application in household items, aircraft and aerospace applications. Examples of sheet metal parts (Fig. 16.1). Sheet metal forming commonly performed at room temperature. Hot stamping is occasionally performed in order to increase formability and decrease forming loads on 3 FIGURE 16.1 Examples of sheet-metal parts. (a) Stamped parts. (b) Parts produced by spinning. Source: Courtesy of Williamsburg Metal Spinning & Stamping Corp.  Shearing All sheet metal forming operations begin with a blank of suitable dimensions and removed from a large sheet by shearing. Shearing subjects the sheet to shear stresses, generally using a punch and a die (Fig. 16.2a). The typical features of the sheared edges of the sheet metal and of the slug are shown in Fig. 16.2b and c, respectively. Shearing generally starts with a formation of cracks on both the top and bottom edges of the workpiece, at points A and B, and C and D (Fig. 16.2a) These cracks eventually meet each other, and complete separation occurs. The rough fracture surfaces are due to the cracks; the smooth and shiny burnished surfaces on the hole and the slug are from the contact and rubbing of the sheared edge against the walls of the 5 FIGURE 16.2 (a) Schematic illustration of shearing with a punch and die, indicating some of the process variables. Characteristic features of (b) a punched hole and (c) the slug; note that the scales of (b) and (c) are different. A burr is a thin edge or ridge. Burr height increases with increasing clearance and ductility of the sheet metal. Dull tool edges contribute greatly to large burr formation. The height, shape and size of the burr can significantly affect subsequent forming operations Deburring processes remove the burr  The major processing parameters in shearing are: The shape of the punch and die The clearance, c, between the punch and the die The speed of punching Lubrication The clearance is a major determining the shape and the quality of the sheared edge. Shearing As clearance increases, the deformation zone (Fig. 16.3a) becomes larger and the sheared edge surface becomes rougher. With excessive clearances, the sheet tends to be pulled into the die cavity, and the perimeter or edges of the sheared zone become rougher. Secondary operations may be necessary to make them smoother, which will 7 FIGURE 16.3 (a) Effect of the clearance, c, between punch and die on the deformation zone in shearing; as the clearance increases, the material tends to be pulled into the die rather than be sheared. In practice, clearances usually range between 2 and 10% of sheet thickness. (b) Microhardness (HV) contours for a 6.4-mm (0.25-in.) thick AISI 1020 hot-rolled steel in the sheared region. Source: After H.P. Weaver and K.J. Weinmann.  Punch Force The force required to punch out a blank is basically the product of the shear strength of the sheet metal and the total area being sheared. The maximum punch force, F, can be estimated from the equation Where T is the sheet thickness, L is the total length sheared (such as the perimeter of a hole), and UTS is the ultimate tensile strength of the material. As the clearance increases, the punch force decreases, and the wear on dies and punches also is reduced. Friction between the punch and workpiece, increases the punch force. 9  Example: Calculation of Punch Force A 25 mm diameter hole is to be punched through a 3.2 mm thick annealed titanium alloy Ti-6Al-4V sheet at room temperature. UTS is given at 1000MPa. Estimate the force required. Solution: 10 Shearing Operations The most common shearing operations are punching (where the sheared slug is scrap or may be used for some other purpose) and blanking (where the slug is the part to be used and the rest is scrap). Die cutting is the shearing operation that consists of the following basic processes: Fine blanking is square edges with very smooth sheared surfaces (Fig. 16.5). Slitting is shearing operation by means of a pair of circular blades, similar those in a can opener (Fig. 16.6). Steel rules consists of a thin strip of hardened steel (die). The die is pressed against the (soft) sheet, and it shears the sheet along the shape defined by the steel rule. 11 FIGURE 16.4 (a) Punching (piercing) and blanking. (b) Examples of various die-cutting operations on sheet metal; lancing involves slitting the sheet to form a tab. FIGURE 16.5 (a) Comparison of sheared edges produced by conventional (left) and by fine-blanking (right) techniques. (b) Schematic illustration of one setup for fine blanking. Source: Reprinted by permission of Feintool U.S. Operations. FIGURE 16.6 Slitting with rotary knives, a process similar to opening cans.  Characteristics and Types of Shearing Dies Clearance control is important because the formability of the sheared part can be influenced by the quality of its sheared edges. The clearance depends on: Type of material and its temper Thickness and size of the blank Proximity to the edges of other sheared edges or the edges of the original blank Clearance generally ranges between 2% and 8% of the sheet thickness, although they may be as small as 1% (as in fine blanking) or as large as 30%. General guideline: Clearance for soft materials are less than those for hard grades The thicker the sheet, the larger the clearance As the ratio of hole diameter to sheet thickness decreases, clearance 15  Punch and Die Shape The location of the regions being sheared at any particular instant can be controlled by beveling the punch ad die surfaces. Several operations may be performed on the same sheet in one stroke, and at one station, with a compound die. Parts requiring multiple forming operations can be made at high production rates, using progressive dies. Tool and die materials for shearing generally are tool steel and carbides. Lubrication is important for reducing tool and die wear, thus maintaining edge quality. Examples of shear angles on punches and dies. 16 FIGURE 16.11 Schematic illustrations (a) before and (b) after blanking a common washer in a compound die; note the separate movements of the die (for blanking) and the punch (for punching the hole in the washer). (c) Schematic illustration of making a washer in a progressive die. (d) Forming of the top piece of an aerosol spray can in a progressive die; the part is attached to the strip until the last operation is completed.  Formability Tests for Sheet Metals Sheet metal formability is defined as the ability of the sheet metal to undergo the required shape change without failure, such as by cracking, wrinkling, necking, or tearing. Sheet metal may undergo 2 basic modes of deformation: 1. Stretching 2. drawing There are important distinctions between these 2 modes and different parameters are involved in determining formability under different conditions. 20  Cupping Tests. The sheet metal is clamped between 2 circular flat dies, and a steel ball (or a round punch) is forced into the sheet until a crack begins to appear on the stretched specimen. The punch depth, d, at which a crack appears is a measure of the formability of the sheet. FIGURE 16.13 (a) A cupping test (the Erichsen test) to determine the formability of sheet metals. (b) Bulge-test results on steel sheets of various widths; the specimen farthest left is subjected to, basically, simple tension. The specimen that is farthest right is subjected to equal biaxial stretching. Source: Courtesy of (a) Arcelor Mittal and (b) Inland Steel Company.  Bending Sheets, Plates and Tubes Bending is one of the most common forming operations, as evidenced by observing automobiles bodies, exhaust pipes, appliances, paper clips, or file cabinets. Bending also imparts stiffness to the part, by increasing its moment of inertia. Fig. shows the terminology used in bending sheet or plate. Outer fibers of the material are in tension, while the inner fibers are in compression. Because of the Poisson effect, the width of the part (bend length, L) 24 has become smaller in the outer  Bending Sheets, Plates and Tubes The bend allowance, Lb, is the length of the neutral axis in the bend; it is used to determine the length of the blank for a part to the bent. The position of the neutral axis, however, depends on the bend radius and bend angle, as described in texts on mechanics of materials. Formula for the bend allowance is Where  is the bend angle (in radius), T is the sheet thickness, R is the bend radius, and k is a constant, in the range of 0.33 (for R  2T) to 0.5 (for R  2T). For ideal case, the neutral axis is at the center of the sheet thickness, k = 0.5, 25 FIGURE 16.17 (a) and (b) The effect of elongated inclusions (stringers) on cracking as a function of the direction of bending with respect to the original rolling direction of the sheet. (c) Cracks on the outer surface of an aluminum strip bent to an angle of 90°. Note also the narrowing of the top surface in the bend area (due to the Poisson effect) Bending Sheets, Plates and Tubes Minimum Bend Radius The radius at which a crack first appears at the outer fibers of a sheet being bent is referred to as the minimum bend radius. It can be shown that the engineering strain on the outer and inner fibers of a sheet during bending is given by the expression; Thus, R/T decreases, the tensile strain at the outer fiber increases, and the material eventually develops cracks (Fig. 16.17) 27  Springback Springback - the recovery/tendency of a material to return to its original shape (1) after being bent or formed, when the forces are removed. As noted in Fig. 16.19, the final bend angle of a sheet metal after springback is smaller than the angle to which the sheet was bent, and the final bend radius is larger than before springback. Springback can be calculated approximately in terms of radii Ri and Rf as; Springback increases as the R/T ratio and the yield stress, Y, of the material increase, and as the elastic modulus, E, decreases. 28 FIGURE 16.19 Springback in bending; the part tends to recover elastically after bending, and its bend radius becomes larger. Under certain conditions, it is possible for the final bend angle to be smaller than the original angle (negative springback). FIGURE 16.22 Examples of various bending operations. Negative springback does not occur in air bending, shown in Fig. 16.22a (also called free bending), because of the absence of constraint that a V-die imposes on the bend area.  by overbending the part, although several trials may be necessary to obtain the desired results. Another method is to coin the bend area by subjecting it to highly localized compressive stresses Compensati between the tip of the punch at the die surface on for (Fig. 16.20c and d), a technique called bottoming Springback the punch. Another method, the part is subjected to stretch bending, in which it is under external tension while being bent. In Fig. 16.20e, the upper die rotates clockwise as the  In V-bending FIGURE 16.20 Methods of reducing or eliminating springback in bending operations. (Fig. 16.20), it is also possible for the material to also exhibit negative springback. This is a condition caused by the nature of the deformation occurring within the sheet metal just when the punch completes the bending operation at the  Bending Force The bending for sheets and plates can be estimated by assuming that the process is simply bending of a rectangular beam. Thus, the bending force is function of the strength of the material, the length, L, of the bend, the thickness, T, of the sheet, and the die opening, W. Excluding friction, the maximum bending force, P is; Where the factor k ranges from about 0.3 for a wiping die, to about 0.7 for a U-die, to about 1.3 for a V-die, and Y is the yield stress of the material. 33  Miscellaneous Bending and Related Forming Operations 34 FIGURE 16.23 (a) through (e) Schematic illustrations of various bending operations in a press brake. (f) Schematic illustration of a press brake. Source: Enprotech Industrial Technologies Inc. FIGURE 16.24 (a) Bead forming with a single die. (b) through (d) Bead forming with two dies in a press brake. FIGURE 16.25 Various flanging operations. (a) Flanges on flat sheet. (b) Dimpling. (c) The piercing of sheet metal to form a flange. In this operation, a hole does not have to be prepunched before the punch descends; note the rough edges along the circumference of the flange. (d) The flanging of a tube; note the thinning of the edges of the flange. FIGURE 16.26 (a) Schematic illustration of the roll-forming process. (b) Examples of roll-formed cross-sections. Source: (b) Courtesy of Sharon Custom Metal Forming, Inc. FIGURE 16.27 Methods of bending tubes. Internal mandrels or filling of tubes with particulate materials such as sand are often necessary to prevent collapse of the tubes during bending. Tubes also can be bent by a technique in which a stiff, helical tension spring is slipped over the tube. The clearance between the outer diameter of the tube and the inner diameter of the spring is small; thus, the tube cannot kink and the bend is uniform. FIGURE 16.28 A method of forming a tube with sharp angles, applying an axial compressive force; compressive stresses are beneficial in forming operations because they delay fracture. Note that the tube is supported internally with rubber or fluid to avoid collapsing during forming. Source: After J.L. Remmerswaal and A. Verkaik. FIGURE 16.29 (a) The bulging of a tubular part with a flexible plug; water pitchers can be made by this method. (b) Production of fittings for plumbing by expanding tubular blanks under internal pressure; the bottom of the piece is then punched out to produce a “T.” (c) Steps in manufacturing bellows. Source: (b) After J.A. Schey, Introduction to Manufacturing Processes, 3rd ed., 2000, McGraw-Hill, p. 425. ISBN No. 0-07-031136-6. FIGURE 16.30 Schematic illustration of a stretch-forming process; aluminum skins for aircraft can be made by this method. Source: (a) Courtesy of Cyril Bath Co. Deep Drawing  Deep Drawing Numerous sheet metal parts are cylindrical or box shaped, such as pots and pans, all types of containers for food and beverages, stainless steel kitchen sinks, canisters, and automotive fuel tanks. Such parts usually are made by a process in which a punch forces a flat sheet metal blank into a die cavity (Fig. 16.32a) – Deep Drawing 44 FIGURE 16.32 (a) Schematic illustration of the deep-drawing process on a circular sheet-metal blank; the stripper ring facilitates the removal of the formed cup from the punch. (b) Process variables in deep drawing. Except for the punch force, F, all the parameters indicated in the figure are independent variables.  Deep Drawing A round sheet metal blank placed over a circular die opening, and held in place with a blankholder, hold down ring (Fig. 16.13b). The punch travels downward, forcing the blank into the die cavity, thus forming a cup. The major variables in this process are: a. Properties of the sheet metal b. Ratio of blank diameter, Do c. Punch diameter, Dp d. Clearance, c, between punch and die e. Punch radius, Rp f. Die corner radius, Rd g. Blankholder force h. Friction and lubrication between all contacting interfaces  Deep Drawing During the drawing operation, the movement of the blank into the die cavity induces compressive circumferential (hoop) stresses in the flange, which tend to cause the flange to wrinkle during drawing. This phenomenon can be demonstrated simply by trying to force a circular piece of paper into a round cavity. Wrinkling can be reduced or eliminated if a blankholder is pressed downward with a certain force. 47  Deep Drawing – Punch Force Because of several variables involved, the punch force, F, is difficult to calculate directly. The maximum punch force, Fmax, can be estimated from the formula; Force increases with increasing blank diameter, sheet thickness, strength, and the ratio (D0 /Dp). The wall of the cup being drawn is subjected principally to a longitudinal (vertical) tensile stress, due to the punch force. Elongation under this stress causes the cup wall to become thinner, and, if excessive, can cause tearing of the cup. 48 The metal-forming processes employed in manufacturing two-piece aluminum beverage cans The process starts with 140 mm diameter blanks produced from rolled sheet stock. The blanks are deep drawn to a diameter of about 90 mm. Redrawn to the final diameter of around 65 mm. Ironed through two or three ironing rings in one pass. Domed for shaping the can bottom. Necking of the can body is performed either through spinning or by die necking and then; THANK YOU 50