1

CHAPTER 16

Sheet-Metal Forming Processes

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Formed and shaped parts in a typical automobile

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4

Outline

16.1 Introduction

16.2 Shearing

16.3 Sheet-Metal Characteristics

16.4 Test Methods for Formability of Sheet Metals

16.5 Bending Sheet and Plate

16.6 Common Bending Operations

16.7 Tube Bending and Forming

16.8 Stretch Forming

16.9 Deep Drawing

16.10 Rubber Forming

16.11 Spinning

16.12 Superplastic Forming

16.13 Explosive, Magnetic-Pulse, Peen, and Other Forming Processes

16.14 The Manufacturing of Honeycomb Structures

16.15 The Dent Resistance of Sheet-Metal Parts

16.16 Equipment for Sheet-Metal Forming

16.17 The Economics of Sheet-Metal Forming Processes

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16.1 INTRODUCTION • Products made by sheet-metal forming processes are all around us;

they include metal desks, file cabinets, appliances, car bodies, aircraft fuselages, and beverage cans.

• Sheet

• forming dates back to 5000 B.C., when household utensils and jewelry were made by hammering and stamping gold, silver, and copper.

• Compared to those made by casting and by forging, sheet-metal parts offer the advantages of light weight and versatile shape. Because of its low cost and generally good strength and formability characteristics.

• low-carbon steel is the most commonly used sheet metal. For aircraft and aerospace applications, the common sheet materials are aluminum and titanium.

• This chapter first describes the methods by which blanks are cut from large rolled sheets, then further processed into desired shapes by a wide variety of traditional methods as well as by such techniques as superplastic forming and diffusion bonding.

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Outline of Sheet-Metal Forming Processes

Figure 16.1

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Characteristics of Sheet-Metal Forming Processes

TABLE 16.1

Process Characteristics

Roll forming Long parts with constant complex cross-sections; good surface finish; high production rates; high

tooling costs.

Stretch forming Large parts with shallow contours; suitable for low-quantity production; high labor costs; tooling

and equipment costs depend on part size.

Drawing Shallow or deep parts with relatively simple shapes; high production rates; high tooling and

equipment costs.

Stamping Includes a variety of operations, such as punching, blanking, embossing, bending, flanging, and

coining; simple or complex shapes formed at high production rates; tooling and equipment costs

can be high, but labor cost is low.

Rubber forming Drawing and embossing of simple or complex shapes; sheet surface protected by rubber

membranes; flexibility of operation; low tooling costs.

Spinning Small or large axisymmetric parts; good surface finish; low tooling costs, but labor costs can be

high unless operations are automated.

Superplastic

forming

Complex shapes, fine detail and close tolerances; forming times are long, hence production rates are

low; parts not suitable for high-temperature use.

Peen forming Shallow contours on large sheets; flexibility of operation; equipment costs can be high; process is

also used for straightening parts.

Explosive

forming

Very large sheets with relatively complex shapes, although usually axisymmetric; low tooling costs,

but high labor cost; suitable for low-quantity production; long cycle times.

Magnetic-pulse

forming

Shallow forming, bulging, and embossing operations on relatively low-strength sheets; most

suitable for tubular shapes; high production rates; requires special tooling.

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16.2 shearing

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Shearing

•Schematic illustration of the shearing process with a punch and die. This process with a punch and die. This process is a common method of producing various openings in sheet metals.

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Characteristics of Hole and Slug

•Characteristic features

of (a) a punched hole

and (b) the punched

slug. Note that the slug

has been sealed down

as compared with the

hole.

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Shearing

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 the two figures are different.

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16.2 SHEARING • Before a sheet-metal part is made, a blank of suitable dimensions is

first removed from a large sheet (usually from a coil) by shearing; that is, the sheet is cut by subjecting it to shear stresses, typically ones developed between a punch and a die (Fig. l6.2a).

• Typical features of the sheared edges of the sheet and of the slug are shown in Figs. l6.2b and c, respectively.

• Note that the edges are not smooth, nor are they perpendicular to the plane of the sheet.

• Shearing usually starts with the formation of cracks on both the top and bottom edges of the workpiece (A and B, and C and D in Fig. l6.2a).

• These cracks eventually meet each other and separation occurs.

• The rough fracture surfaces are due to these 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 punch and die.

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Clearance

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 the thickness of the sheet. (b) Microhardness (HV) contours for a 6.4-mm (0.25-in) thick AISI 1020 hot-rolled steel in the sheared region. Source: H. P. Weaver and K. J. Weinmann.

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16.2 SHEARING • The major processing parameters in shearing are the

shape of and the materials for the punch and die. the speed of the punching, the lubrication, and the clearance, c, between the punch and the die.

• The clearance is a major factor in determining the shape and the quality of the sheared edge. As the clearance increases, the sheared edge becomes rougher, and the zone of deformation (Fig. l6.3a) becomes larger. The sheet tends to be pulled into the clearance zone, and the edges of the sheared zone become rougher. Unless such edges are acceptable as produced, secondary operations (which will add to the production cost) may be required, to make them smoother.

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16.2 SHEARING

• The width of the deformation zone in Fig. 16.3

depends on punch speed.

• With increasing speed, the heat generated by

plastic deformation is confined to a smaller and

smaller zone; consequently, the sheared zone is

narrower, and the surface is smoother and

exhibits less burr formation.

• A burr is a thin edge or ridge, as shown in Figs.

l6.2b and c. Burr height increases with increases

in the clearance and in the ductility of the sheet

metal.

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16.2 SHEARING • Dull tool edges contribute greatly to burr

formation. The height, shape, and size of the burr can significantly affect subsequent forming operations.

• Edge quality has been found to improve with increasing punch speed; speeds may be as high as 10-12 m/s (33-39 ft/s).

• As shown in Fig. l6.3b, sheared edges can undergo severe cold working, which is due to the high shear strains involved.

• The resultant work hardening can adversely affect the formability of the sheet during subsequent operations.

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16.2.1 punch force

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16.2.2 Shearing Operations Several operations based on the shearing process are

performed.

• In punching, the sheared slug is discarded (Fig. l6.4a).

• In blanking, the slug is the part and the rest is scrap.

• Die Cutting. Die cutting is a shearing process that consists of the following operations ;",

• a. perforating-punching a number of holes in a sheet;

• b. parting-shearing the sheet into two or more pieces;

• c. notching-removing pieces (or various shapes) from the edges; and

• d. lancing-leaving a tab without removing any material.

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Shearing Operations

Figure 16.4 (a) Punching (piercing) and blanking. (b) Examples of various shearing operations on sheet metal.

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16.2.2 Shearing Operations

• Fine Blanking. Very smooth and square edges

can be produced by fine blanking (Fig. 16.5a).

One basic die design is shown in Fig. 16.5b. A V-

shaped stinger , or impingement, locks the sheet

tightly in place and prevents the type of

distortion of the material shown in Figs. 16.2b

and 16.3.

• Slitting. Shearing operations can be carried out

by means of a pair of circular blades similar to

those in a can opener (Fig. 16.6); this process is

called slitting.

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Fine Blanking

(a) (b)

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: Feintool U.S. Operations.

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Slitting

Figure 16.6 Slitting with rotary knives. This process is similar to opening cans.

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16.2.2 Shearing Operations • Steel Rules. Soft metals (as well as paper, leather, and

rubber) can be blanked with steel-rule dies. Such a die consists of a thin strip of hardened steel, bent into the shape to be produced (a concept similar to that of a cookie cutter) and held on its edge on a flat wooden base. The die is pressed against the sheet, which rests on a flat surface, and it shears the sheet along the shape of the steel rule.

• Nibbling. In nibbling, a machine called a nibbler moves a small straight punch up and down rapidly into a die. A sheet is fed through the gap, and many overlapping holes are made. This operation is similar to the making of a large elongated slot in a sheet of paper by the successive punching of holes with a paper punch.

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16.2.2 Shearing Operations • Scrap in Shearing. The amount of scrap

(the trim loss) produced in shearing operations can be significant; it can be as high as 30% on large stampings. A significant factor in manufacturing cost, scrap can be reduced substantially by proper arrangement of the shapes on the sheet to be cut (nesting). Computer-aided design techniques have been developed to minimize the scrap from shearing operations.

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Laser Welding

Figure 16.7 Production of an outer side panel of a car body, by laser butt-welding and stamping. Source: After M. Geiger and T. Nakagawa.

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Examples of Laser Welded Parts

Figure 16.8 Examples of laser butt-welded and stamped automotive body components. Source: After M. Geiger and T. Nakagawa.

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16.2.3 Shearing Dies The features and types of various shearing dies are described in this

section.

• Clearances. Because the formability of the sheared part can be influenced by the quality of its sheared edges, clearance control is important. The appropriate clearance is a function of the type of material, its temper, and its thickness and of the size of the blank

• As a general guideline, clearances for soft materials are less than those for harder grades. Also, the thicker the sheet is, the larger the clearance must be.

• Clearances generally range between 2% and 8% of the sheet thickness, but they may be as small as 1 % or as large as 30%.

• In the use of larger clearances, attention must be paid to the rigidity and the alignment of the presses and to the dies and their setups. The smaller the clearance, the better the quality of the edge. In a process called shaving (Fig. 16.9), the extra material from a rough sheared edge is trimmed by cutting.

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16.2.3 Shearing Dies • Punch and Die Shapes. Note in Fig. 16.2a that the

surfaces of the punch and of the die are both flat. The punch force, therefore, builds up rapidly during shearing, because the entire thickness is sheared at the same time.

• The location of the regions being sheared at any moment can be controlled by beveling the punch and die surfaces (Fig. 16.10). The geometry is similar to that of a paper punch; you can see that by looking closely at the tip of the punch. Beveling is particularly suitable for the shearing of thick blanks, because it reduces the force at the beginning of the stroke; it also reduces the operation's noise level.

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Shaving and Shear Angles

Figure 16.9 Schematic illustrations of the shaving of a sheared edge. (a) Shaving a sheared edge. (b) Shearing and shaving, combined in one stroke.

Figure 16.10 Examples of the use of shear angles on punches and dies.

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Compound and Progressive Die

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. Note that the part is attached to the strip until the last operation is completed.

(a) (b) (c)

(d)

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Progressive Dies

(a) Schematic illustration of the making of a washer in a progressive die. (b) Forming of the top piece of an aerosol spray can in a progressive die. Note that the part is attached to the strip until the last operation is completed.

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16.3 Sheet metal Characteristics

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16.3 Sheet metal Characteristics

TABLE 16.2

Characteristic Importance

Elongation Determines the capability of the sheet metal to stretch without necking and

failure; high strain-hardening exponent (n)and strain-rate sensitivity exponent

(m)desirable.

Yield-point elongation Observed with mild-steel sheets; also called Lueder’s bands and stretcher strains;

causes flamelike depressions on the sheet surfaces; can be eliminated by temper

rolling, but sheet must be formed within a certain time after rolling.

Anisotropy (planar) Exhibits different behavior in different planar directions; present in cold-rolled

sheets because of preferred orientation or mechanical fibering; causes earing in

drawing; can be reduced or eliminated by annealing but at lowered strength.

Anisotropy (normal) Determines thinning behavior of sheet metals during stretching; important in

deep-drawing operations.

Grain size Determines surface roughness on stretched sheet metal; the coarser the grain, the

rougher the appearance (orange peel); also affects material strength.

Residual stresses Caused by nonuniform deformation during forming; causes part distortion when

sectioned and can lead to stress-corrosion cracking; reduced or eliminated by

stress relieving.

Springback Caused by elastic recovery of the plastically deformed sheet after unloading;

causes distortion of part and loss of dimensional accuracy; can be controlled by

techniques such as overbending and bottoming of the punch.

Wrinkling Caused by compressive stresses in the plane of the sheet; can be objectionable or

can be useful in imparting stiffness to parts; can be controlled by proper tool and

die design.

Quality of sheared edges Depends on process used; edges can be rough, not square, and contain cracks,

residual stresses, and a work-hardened layer, which are all detrimental to the

formability of the sheet; quality can be improved by control of clearance, tool and

die design, fine blanking, shaving, and lubrication.

Surface condition of

sheet

Depends on rolling practice; important in sheet forming as it can cause tearing

and poor surface quality; see also Section 13.3.

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Stress-Corrosion Cracking •Stress-corrosion cracking. The cracks developed over a period of time. Brass and austenitic (300 series) stainless steels are among metals that are susceptible to stress-corrosion cracking.

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Yield-Point Elongation

Figure 16.12 (a) Yield-point elongation in a sheet-metal specimen. (b) Lueder's bands in a low-carbon steel sheet. Source: Courtesy of Caterpillar Inc. (c) Stretcher strains at the bottom of a steel can for household products.

(a) (b) (c)

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16.4 TEST METHODS FOR FORMABILITY OF SHEET METALS

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16.4 TEST METHODS FOR FORMABILITY OF SHEET METALS

• Sheet-metal formability is of great technological and economic interest. It is normally defined as the ability of the sheet metal to undergo the desired shape change without such failure as necking or tearing.

• sheet metals may (depending on part geometry) undergo two basic modes of deformation: (a) stretching and (b) drawing

• This section describes the methods that are generally used in manufacturing industries to predict formability.

• 16.4.1 Cupping Tests: Because sheet-forming is basically a process of stretching the material, the earliest tests developed to predict formability were cupping tests (Fig. 16.13a).

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Erichsen and Bulge-Tests

Figure 16.13 (a) A cupping test (the Erichsen test) to determine the formability of sheet metals.

(a)

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16.5 BENDING SHEET AND PLATE

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16.5 BENDING SHEET AND PLATE • Bending is one of the most common forming operations.

• look at the components in an automobile or an appliance-or at a paper clip or a file cabinet-to appreciate how many parts are shaped by bending.

• The terminology used in bending is shown in Fig. 16.16. Note that, in bending, the outer fibers of the material are in tension, while the inner fibers are in compression.

• Because of the Poisson's ratio, the width of the part (bend length, L) in the outer region is smaller, and in the inner region it is larger, than the original width (see Fig. 16.17c).

• This phenomenon may easily be observed by bending a rectangular rubber eraser.

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Tearing and Bending

Figure 16.15 The deformation of the grid pattern and the tearing of sheet metal during forming. The major and minor axes of the circles are used to determine the coordinates on the forming-limit diagram in Fig. 16.14b. Source: S. P. Keeler.

Figure 16.16 Bending terminology. Note that the bend radius is measured to the inner surface of the bent part.

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Bending 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 90o. Note the narrowing of the tope surface due to the Poisson effect.

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16.5 BENDING SHEET AND PLATE

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16.5.1 Minimum Bend Radius for Various Materials at Room Temperature

TABLE 16.3

Condition

Material Soft Hard

Aluminum alloys

Beryllium copper

Brass, low-leaded

Magnesium

Steels

Austenitic stainless

Low-carbon, low-alloy, and HSLA

Titanium

Titanium alloys

0

0

0

5T

0.5T

0.5T

0.7T

2.6T

6T

4T

2T

13T

6T

4T

3T

4T

soft=smooth edges Hard=rough edges------more stress concentration

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R/T Ratio versus % Area Reduction

Figure 16.18 Relationship between R/T ratio and tensile reduction of area for sheet metals. Note that sheet metal with a 50% tensile reduction of area can be bent over itself, in a process like the folding of a piece of paper, without cracking.

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16.5.2 Springback

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16.5.2 Springback

Figure 16.19 Springback in bending. The part tends to recover elastically after ending, 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 16.20 Methods of reducing or eliminating springback in bending operations..

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16.5.3 bending force

• P=kYLT2/W

• K=0.3 for wipping die

• K=0.7 for U die

• K=1.3 for V die

For V-die the above equation can be written as

P=(UTS)LT2/W

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Bending Operations

Figure 16.21 Common die-bending operations, showing the die-opening dimension, W, used in calculating bending forces.

Figure 16.22 Examples of various bending operations.

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16.6 common bending operations

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Bending in a Press Brake

آلة لتشكيل الصفائح المعدنية

Figure 16.23 (a) through (e) Schematic illustrations of various bending operations in a press brake. (f) Schematic illustration of a press brake. Source: Verson Allsteel Company.

The edge is folded over itself

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Bending Operations In a Press Brake

Schematic illustration of various bending operations in a press brake.

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Bending Operations

Figure 16.22 Examples of various bending operations.

Free bending

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Bead Forming يتخذ شكل شبه كروي

Figure 16.24 (a) Bead forming with a single die. (b) Bead forming with two dies, in a press brake.

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Flanging operations

Figure 16.25 Various flanging operations. (a) Flanges on a 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 bunch descends. Note, however, the rough edges along the circumference of the flange. (d) The flanging of a tube; note the thinning of the edges of the flange.

First hole is punched and then expanded in a flang

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Roll-Forming Process

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16.7 Tube Bending

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16.7 Tube Bending

Figure 16.27 Methods of bending tubes. Internal mandrels, or the filling of tubes with particulate materials such as sand, are often necessary to prevent collapse of the tubes during bending. Solid rods and structural shapes can also be bent by these techniques.

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Tube Forming

•A method of forming a tube with sharp angles, using axial compressive forces. 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.

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Bulging Figure 16.28 (a) The bulging of a tubular part with a flexible plug. Water

pitchers بريقإا can be made by this method. (b) Production of fittings for plumbing,

by expanding tubular blanks under internal pressure. The bottomof the piece is then punched out to produce a "T."

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16.8 Stretch Forming

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16.8 Stretch Forming Figure 16.30 Schematic illustration of a stretch-forming process. Aluminum skins for aircraft can be made by this method. Source: Cyril Bath Co.

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16.8 Stretch Forming

• In stretch forming the sheet metal is clamped along its edge and then stretched over a die or form block.

• Die are generally made of zinc alloys, steel plastic or wood.

• Little or no lubrication

• Various accessories

• Low volume production, economical and versatile.

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16.9 Deep Drawing

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Top of Aluminum Can

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Steps in Manufacturing an Aluminum Can

Figure 16.31 The metal- forming processes involved in manufacturing a two-piece aluminum beverage can

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Deep Drawing

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.

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Anisotropy

Figure 16.33 Strains on a tensile-test specimen removed from a piece of sheet metal. These strains are used in determining the normal and planar anisotropy of the sheet metal.

Figure 16.34 The relationship between average normal anisotropy and the limiting drawing ratio for various sheet metals. Source: M. Atkinson.

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Typical Range of Average Normal Anisotropy, R, for Various Sheet Metals

TABLE 16.4

Zinc alloys

Hot-rolled steel

Cold-rolled rimmed steel

Cold-rolled aluminum-killed steel

Aluminum alloys

Copper and brass

Titanium alloys (a)

Stainless steels

High-strength low-alloy steels

0.4–0.6

0.8–1.0

1.0–1.4

1.4–1.8

0.6–0.8

0.6–0.9

3.0–5.0

0.9–1.2

0.9–1.2

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Earing

Figure 16.45 Earing in a drawn steel cup, caused by the planar anisotropy of the sheet metal.

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Drawbeads

Figure 16.36 (a) Schematic illustration of a draw bead. (b) Metal flow during the drawing of a box- shaped part, while using beads to control the movement of the material. (c) Deformation of circular grids in the flange in deep drawing.

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Embossing

Figure 16.37 An embossing operation with two dies. Letters, numbers, and designs on sheet-metal parts and thin ash trays can be produced by this process.

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16.10 Rubber forming

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16.10 Rubber forming • In the processes described in the preceding sections, it

has been noted that the dies are generally made of solid materials. However, in rubber forming, one of the dies in a set can be

• made of a flexible material, such as a polyurethane membrane.

• Polyurethanes is used widely, because of their resistance to abrasion, their resistance to cutting by burrs or by sharp edges on the sheet metal, and their long fatigue life.

• In the bending and embossing of sheet metal, the female die is replaced with a rub ber pad (Fig. 16.38). Note that the outer surface of the sheet is protected from damage or scratches because it is not in contact with a hard metal surface during forming.

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• When selected properly, rubber-forming

processes have the advantages of (a) low

tooling cost, (b) flexibility and ease of

operation, (c) low die wear, (d) avoidance

of damage to the surface of the sheet, and

(e) capability to form complex shapes.

• Parts can also be formed with laminated

sheets of various nonmetallic materials or

coatings.

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Forming with a Flexible Pad

Figure 16.38 Examples of bending and

embossing sheet metal with a metal punch and

a flexible pad serving as the female die. Source:

Polyurethane Products Corporation.

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Hydroform Process

Figure 16.39 The hydroform (or fluid forming) process. Note that, in contrast to the ordinary deep-drawing process, the pressure in the dome forces the cup walls against the punch. The cup travels with the punch; in this way, deep drawability is improved.

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16.11 Spinning

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16.11 spinning • Spinning is an old process which involves the forming of

axisymmetric parts over a mandrel, by the use of various

tools and rollers.

• This process is somewhat similar to that of forming clay

on a potter's wheel.

• As this section describes, there are three basic type

spinning techniques: conventional (or manual), shear,

and tube spinning.

• The equipment used in these processes is similar to a

lathe, but it has special features.

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16.11.1 Conventional Spinning • In conventional spinning, a circular blank of flat or

preformed sheet metal is held against a mandrel and rotated, while a rigid tool deforms and shapes the material over the manderel.

• The tool may be activated either manually or by a computer-controlled hydraulic mechanism.

• The process involves a sequence of passes, and it requires considerable skill

• Part diameters may range up to 6 m .

• Although most spinning is performed at room temperature, thick parts, or metals with high strength or low ductility, require spinning at elevated temperatures.

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16.11.1 Conventional Spinning Figure 16.40 (a) Schematic illustration of the conventional spinning process. (b) Types of parts conventionally spun. All parts are axisymmetric.

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Shapes in Spinning Processes

Typical shapes produced by the conventional-spinning process. Circular marks on the external surfaces of components usually indicate that the parts have been made by spinning. Examples include aluminum kitchen utensils and light reflectors.

84

16.11.2 Shear Spinning • Also known as power spinning, flow turning,

hydrospinning, and spin forging, shear spinning produces an axisymmetric conical or curvilinear shape, while maintaining the part's maximum diameter and reducing its thickness (Fig. 16.41a).

• Although a single roller can be used, two rollers are preferable in order to balance the forces acting on the mandrel. Typical parts made are rocket-motor casings and missile nose cones.

85

16.11.2 Shear and Tube Spinning

Figure 16.41 (a) Schematic illustration of the shear spinning process for making conical parts. The mandrel can be shaped so that curvilinear parts can be spun. (b) Schematic illustration of the tube spinning process.

Spinnability ????

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Spinning Processes

•Schematic illustration of spinning processes: (a) conventional spinning and (b) shear spinning. Note that in shear spinning, the diameter of the spun part, unlike in conventional spinning, is the same as that of the blank. The quantity f is the feed (in mm/rev or in./rev).

87

16.11.3 Tube Spinning •Examples of external and internal tube spinning and the

variables involved.

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Spinning of a Compressor Shaft

Figure 16.42 Steps in tube and shear spinning of a compressor shaft for the Olympus jet engine of the supersonic Concorde aircraft.

89

16.12 Super plastic forming

90

16.12 Super plastic forming

• Superplastic alloys (particularly Zn-22Al

and Ti-6Al-4V) can be formed by such bulk

deformation processes as compression

molding, closed-die forging, coining,

hubbing, and extrusion.

• Sheet forming of these materials can also

be carried out using such operations as

thermoforming, vacuum forming, and blow

molding (Chapter 18)

91

16.12 Super plastic forming • An important development is the ability to

fabricate sheet-metal structures by combining diffusion bonding with superplastic forming (SPF/DB). Typical structures in which flat sheets are diffusion bonded (as described in Section 28.7) and formed are shown in Fig. 16.43.

• After diffusion bonding the selected locations of the sheets, the unbonded regions (stop-off) are expanded into a mold by pressurized argon gas.

• These structures are thin, and

• they have high stiffness-to-weight ratios. As a result, they are particularly important in aircraft and aerospace applications.

92

Diffusion Bonding and Superplastic Forming

Figure 16.43 Types of structures made by diffusion bonding and superplastic forming of sheet metal. Such structures have a high stiffness-to-weight ratio..

93

16.13 EXPLOSIVE, MAGNETIC-PULSE, PEEN,

AND OTHER FORMING PROCESSES

94

• By controlling their quantity and shape,

however, one can use explosives as a

source of energy for metal forming

16.13.1 Explosive Forming

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16.13.1 Explosive Forming

Figure 16.44 (a) Schematic illustration of the explosive forming process. (b) Illustration of the confined method of explosive bulging of tubes.

96

16.13.2 Magnetic-Pulse Forming

Figure 16.45 (a) Schematic illustration of the magnetic-pulse forming process used to form a tube over a plug. (b) Aluminum tube collapsed over a hexagonal plug by the magnetic-pulse forming process.

97

16.13.3 Peen forming

•?

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16.13.4 other sheet forming process

• Laser forming

• Laser assisted forming

• Electrohydraulic forming (underwater

spark, or electric discharge forming :

source of the energy is spark between

electrodes

• Gas mixtures: (the source of energy): as

internal combustion engines.

• Liquefied gases: nitrogen ( liquid and gas

phase)

99

16.14 manufacturing honeycomb structures

Figure 16.46 Methods of manufacturing honeycomb structures: (a) Expansion process; (b) Corrugation process; (c) Assembling a honeycomb structure into a laminate.

100

Questions

101

Stamping Press and

Press Frames

Figure 16.47 (a) and (b) Schematic illustration of types of press frames for sheet- forming operations. Each type has its own characteristics of stiffness, capacity, and accessibility. Source: Engineer's Handbook, VEB Fachbuchverlag, 1965. (c) A large stamping press. Source: Verson Allsteel Company.

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Cost Comparison for Spinning and Deep Drawing

Figure 16.48 Cost comparison for manufacturing a round sheet-metal container either by conventional spinning or by deep drawing. Note that for small quantities, spinning is more economical.