1. -Sheet-Metal Forming Processes
Fundamental of Materials Forming
-Sheet-Metal Forming Processes
板金成形工艺

2. INTRODUCTION
Products made by sheet-metal forming processes are all around us; they include metal desks, file cabinets, appliances, car bodies, aircraft fuselages(机身）, and beverage（饮料） cans. Sheet forming dates back to 5000 B.C., when household utensils(器具） and jewelry were made by hammering and stamping gold, silver, and copper.

3. 冲压生产场景

4. Compared to those made by casting and by forging, sheet-metal parts offer the advantages of light weight and versatile（万能的） 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.

5. Outline of Sheet-Metal Forming Processes
Figure 16.1

6. Characteristics of Sheet-Metal Forming Processes

7. SHEARING剪切
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. 16.2a). Typical features of the sheared edges of the sheet and of the slug are shown in Figs. 16.2b and c, respectively. Note that the edges are not smooth, nor are they perpendicular to the plane of the sheet.

9. The major processing parameters
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 determin-ing the shape and the quality of the sheared edge.

10. Clearance
Figure (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.

11. Punch force
The force required to punch is, basically, the product of the shear strength of the sheet metal
by the area being sheared. Friction between the punch and the workpiece can, however, increase this force substantially. The maximum punch force, F, can be estimated from the equation
F = 0.7TL(UTS),
where T is 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 is also reduced.

12. Shearing Operations
Several operations based on the shearing process are performed. We first define two terms. In punching, the sheared slug is discarded (Fig. 16.4a). In blanking, the slug is the part and the rest is scrap. Many of the operations described below, as well as those described throughout the rest of this chapter, can now be carried out on computer- numerical-controlled machines (see Chapter 39) with quick-change toolholders.

13. Shearing Operations notching开槽 lancing切缝 Slitting切口 parting切断
perforating成孔

14. Shearing dies
Fig 16.3 (a) Effect of the clearance,c, between punch and die in the deformation zone in shearing .As the clearance increases the material trends to be pulled into the die rather than be sheared .

15. Fine Blanking.
Very smooth and square edges can be produced by fine blanking(Fig. 16.5a).
The fine-blanking operation is usually carried out on triple-action hydraulic presses, where the movements of the punch, of the pressure pad, and of the die are separately controlled. The process usually involves a part having holes that are punched simultaneously with its blanking. Suitable sheet hardness is typically between 50 and 90 HRB.

16. Fine Blanking精冲 (a) (b)
Figure (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.

17. Slitting(切口）
Slitting. Shearing operations can be carried out by means of a pair of circular blades(刀片） similar to those in a can opener (Fig. 16.6); this process is called slitting. The blades follow either a straight line or a circular or curved path. A slit edge normally has a burr毛刺, which may be plastically folded over the sheet surface by rolling the sheet between two rolls.

18. Figure 16. 6 Slitting with rotary knives
Figure Slitting with rotary knives. This process is similar to opening cans.

19. steel rule钢尺
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.

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

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

22. Shearing Dies剪切模具
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（韧性）, and its thickness and of the size of the blank and its proximity to the edges of the original sheet. 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. Holes which are small (as compared to sheet thickness) require greater clearances than ones which are larger.
Clearances generally range between 2% and 8% of the sheet thickness, but they may be as small as 1% or as large as 30%.

23. 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 ). The geometry is similar to that of a paper punch; you can see that by looking closely at the tip of thepunch. Beveling is particularly suitable for the shearing of thick blanks, because it reducesthe force at the beginning of the stroke; it also reduces the operation's noise level.

24. Shaving and Shear Angles
Bevel shear斜面剪切
Double-Bevel shear双斜面剪切
convex shear凸面剪切
Figure Examples of the use of shear angles on punches and dies.

25. Compound Die复合模
Several operations on the same strip may be performed in one stroke at one station with a compound die (Figs. 16. l I a and b). Such combined operations are usually limited to relatively simple shapes, because they are somewhat slow and because the dies (with increasing complexity) rapidly become much more expensive than those for individual shearing operations.

26. (a)
(b)
Figure 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).

28. Progressive Die级进模
Parts requiring multiple operations, such as punching, blanking, and notching（开槽）, can be made at high production rates in progressive dies. The sheet metal is fed through as a coil strip, and a different operation is performed at the same station with each stroke of a scries of punches (Fig I c). An example of a part made in progressive dies is shown in Fig ld.

29. (c)
Figure Schematic illustrations: (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.
(d)

30. Transfer dies
Sheet metal undergoes different operations at different stations in a straight line or circular path.
Tool and Die Material :
Carbides are used for high production rates.

31. Other sheet metal cutting methods
Band saw
Flame cutting
Laser beam cutting
Friction sawing
Water-jet cutting

32. Characteristics of Metals Important in Sheet Forming

33. Yield-Point Elongation
(b)
(c)
Figure (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.

34. Cupping test
The sheet metal specimen is clamped between two circular flat dies and a steel or round punch is pushed hydraulically into the sheet metal until a crack begins to appear on the stretched specimen
Fig (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 farthest right is subjected to equal biaxial stretching

35. Erichsen and Bulge-Tests杯突试验
Figure (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 farthest right is subjected to equal biaxial stretching. Source: Inland Steel Company.
(b)

36. Major and Minor Strain
Figure (a) Strains in deformed circular grid patterns. (b) Forming-limit diagrams (FLD) for various sheet metals. Although the major strain is always positive (stretching), the minor strain may be either positive or negative. In the lower left of the diagram, R is the normal anisotropy of the sheet, as described in Section Source: S. S. Hecker and A. K. Ghosh.

37. Bending
Figure Bending terminology. Note that the bend radius is measured to the inner surface of the bent part.
In Bending ,after plastic deformation there is an elastic recovery this recovery is called spring back.Spring back can be calculated approximately in terms if radii Ri and Rf
Ri/Rf = 4 ( Ri Y / ET )3 – 3 (Ri Y /ET) + 1 Spring back Increases as (R/T ratio & yield stress of material ) increases as elastic modulus E decreases

38. Bending sheet and plate
In bending outer fibers are in tension and inner fibers are in compression
Fig (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 degree.Note the narrowing of the top surface due to the Poisson effect

39. The engineering strain on a sheet during bending is
e=1/(2R/T)+1)
As R/T decreases (the ratio of the bend radius to the thickness becomes smaller), the tensile strain at the outer fiber increases, and the material eventually cracks.

40. Bending force Maximum bending force, P = KYLT2 W For a V-die
K – constant ranges from 0.3(wiping die) – 0.7(u-die)-1.3(V-die)
Y – yield stress
L- length of the bend
T- thickness of sheet
For a V-die
Max bending force, P = (UTS)LT 2
UTS – Ultimate tensile strength

41. Common bending operations
Press brake forming
Used for sheets 7M(20ft) or longer and other narrow pieces
Long dies in a mechanical or hydraulic press for small production runs
Die material range from hardwood to carbides.

42. Bending
(a)
(b)
(c)
Figure (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 the narrowing of the tope surface due to the Poisson effect.

43. Minimum Bend Radius for Various Materials at Room Temperature

44. Springback
Because all materials have a finite modulus of elasticity, plastic deformation is followed,when the load is removed, by some elastic recovery (see Fig. 2.3). In bending, this recovery is called springback.
  与所有塑性变形一样，塑性弯曲时伴随有弹性变形，当外载荷去除后，塑性变形保留下来，而弹性变形会完全消失，使弯曲件的形状和尺寸发生变化而与模具尺寸不一致，这种现象叫回弹。

45. Figure 16. 19 Springback in bending
Figure 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).

46. 合理设计弯曲模
In forming operations, springback is usually compensated for by overbending the part (Figs a and b); Another method is to coin the bend area by subjecting it to high localized compressive stresses between the technique tip of the punch and the die surface (Figs c and d);

47. Bending Operations air bending 气动弯曲机
Figure Common die-bending operations, showing the die-opening dimension, W, used in calculating bending forces.
air bending 气动弯曲机
Figure Examples of various bending operations.

48. Bending in a Press Brake 折弯机上弯曲
Figure (a) through (e) Schematic illustrations of various bending operations in a press brake. (f) Schematic illustration of a press brake. Source: Verson Allsteel Company.

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

50. Flanging
 Flanging is a process of bending the edges of sheet metals, usually to90.
 翻边是在模具的作用下，将坯料的孔边缘或外边缘冲制成竖立边的成形方法，根据坯料的边缘状态和应力、应变状态的不同，翻边可以分为内孔翻边和外缘翻边，也可分为伸长类翻边和压缩类翻边。

51. In shrink flanging (Fig. 16
In shrink flanging (Fig a), the flange is subjected to compressive hoop stresses
which, if excessive, can cause the flange edges to wrinkle. The wrinkling tendency increases
with a decrease in the radius of curvature of the flange. In stretch flanging, the flange edges
arc subjected to tensile stresses that, if excessive, can lead to cracking along the periphery(外围）.

52. Figure 16. 25 Various flanging operations. (a) Flanges on a flat sheet
Figure 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.

53. Dimpling :
First hole is punched and expanded into a flange
Flanges can be produced by piercing with shaped punch
When bend angle < 90 degrees as in fitting conical ends its called flanging

54. Hemming : Seaming : The edge of the sheet is folded over itself
This increases stiffness of the part
The metal strip is bent in stages by passing it through a series of rolls
Seaming :
Joining two edges of sheet metal by hemming specifically shaped rollers used for watertight and airtight joints

55. Roll Forming成形轧制
Figure Schematic illustration of the roll-forming process.
This process is used for forming continuous lengths of sheet metal and for large production runs; it is also called contour rollforming(滚型)or cold roll forming. Init, the metal strip is bent in stages by passing it through a series of rolls (Fig ). The parts are then usually sheared and stacked continuously.

56. Tube Bending
The bending and forming of tubes and of other hollow sections requires special tooling in order to avoid buckling and folding. The oldest and simplest method of bending a tube or pipe is first, to pack the inside with loose particles, commonly sand, and then to bend it in a suitable fixture. The filling prevents the tube from buckling. After the tube has been bent, the sand is shaken out. Tubes can also be plugged with various flexible internal mandrels (Fig ). A relatively thick tube having a large bend radius can be bent without filling it with particulates and without using plugs.

57. Figure 16. 27 Methods of bending tubes
Figure 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.

58. Bulging胀形
Figure (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 bottomof the piece is then punched out to produce a "T." Source: J. A. Schey, Introduction to Manufacturing Processes (2d ed.) New York: McGraw-Hill Publishing Company, 1987.

59. Manufacturing of Bellows
Figure Steps in manufacturing a bellows.

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

61. Deep drawing
Punch forces a flat sheet metal into a deep die cavity
Round sheet metal block is placed over a circular die opening and held in a place with blank holder & punch forces down into the die cavity

62. Deep drawing process Wrinkling occurs at the edges
Fig (a) Schematic illustrations 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.

63. Deep drawability Deep drawability is expressed in LDR
Limiting drawing ratio (LDR)
LDR – Max blank dia/punch dia =Do/Dp
Drawability of metal is determined by normal anisotropy( R ) or plastic anisotropy.
R = width strain / thickness strain =Ew /Et

64. Steps in Manufacturing an Aluminum Can
Figure The metal- forming processes involved in manufacturing a two-piece aluminum beverage can

65. Deep Drawing(深冲压）
Figure (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.

66. Anisotropy各向异性
Figure 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 The relationship between average normal anisotropy and the limiting drawing ratio for various sheet metals. Source: M. Atkinson.

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

68. Earing
Edges of cups may be wavy this phenomenon is called Earing The above condition is called planar anisotropy
Del R = R0 – 2 R45 + R 90 / 2
Where Del R = 0 => no ears formed Height of the ears increases Del R increases.
Figure Earing in a drawn steel cup, caused by the planar anisotropy of the sheet metal.

69. Deep drawing Practice
Blank holder pressure – 0.7% -1.0 % of Yield strength + UTS
Clearance usually – 7% -14 % > sheet thickness
Draw beads are used to control flow of blank into die cavity.
Ironing is a process in which the thickness of a drawn cup is made constant by pushing of the cup through ironing rings.
Redrawing – Containers or shells which are too difficult to draw in one operation undergo redrawing

70. Lubricants Lowers forces and increases drawability
commonly used lubricants are mineral oils ,soap solutions,heavy duty emulsions.
Tooling & equipment for drawing :
Tool & die materials are tool steels cast irons carbides
Equipment is hydraulic press or mechanical press

71. Rubber forming
In bending and embossing of sheet metal , the female die is replaced with rubber pad
Hydro-form (or) fluid forming process :
The pressure over rubber membrane is controlled through out the forming cycle ,with max pressure up to 100 Mpi
As a result the friction at the punch-cup interface increases ,this increase reduces the longitudinal tensile stresses in the cup and delays fracture

72. Spinning
Shaping thin sheets by pressing them against a form with a blunt tool to force the material into a desired form
Conventional spinning :
A circular blank if flat or performed sheet metal hold against a mandrel and rotated ,while a rigid metal is held against a mandrel and rotated ,wile a rigid tool deforms and shapes the material over the mandrel.

73. Shear Spinning
Fig (a) Schematic illustration of the conventional spinning process (b) Types of parts conventionally spun.All parts are antisymmetric

74. Shear spinning
Known as power spinning, flow turning, hydro-spinning, and spin forging
Produces axisymmetric conical or curvilinear shape
Single rollers and two rollers can be used
It has less wastage of material
Typical products are rocket-motor casing and missile nose cones.

75. Tube spinning
Thickness of cylindrical parts are reduced by spinning them on a cylindrical mandrel rollers
Parts can be spun in either direction
Large tensile elongation up to 2000 % are obtained within certain temperature ranges and at low strain rates.

76. Drawbeads拉延筋
Drawbeads (Fig a) are often necessary to control the flow of the blank into the die cavity. Beads restrict the flow of the sheet metal by bending and unbending it during drawing; they thereby increase the force required to pull the sheet into the die cavity. Drawbeads also help to reduce the required blankholder压边 forces, because the beaded sheet has a higher stiffness and hence a lower tendency to wrinkle. Drawbead diameters may range from 13 mm to 20 mm (0.50 in. to 0.75 in.), the latter being for large stampings such as automotive panels.

77. Drawbeads拉延筋
Figure (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.

78. Embossing轧花
Figure An embossing operation with two dies. Letters, numbers, and designs on sheet-metal parts and thin ash trays can be produced by this process.
Figure Examples of the bending and the embossing of sheet metal with a metal punch and with a flexible pad serving as the female die. Source: Polyurethane Products Corporation.

79. Figure 16. 39 The hydroform (or fluid forming) process
Figure 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.

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

81. Figure (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.

82. Diffusion Bonding and Superplastic Forming 扩散粘结和超塑性成形
Figure Types of structures made by diffusion bonding and superplastic forming of sheet metal. Such structures have a high stiffness-to-weight ratio. Source: Rockwell International Corp.

83. Super Plastic forming Advantages :
Lower strength is required and less tooling costs
Complex shapes with close tolerances can be made
Weight and material savings
Little or no residual stress occurs in the formed parts
Disadvantages :
Materials must not be super elastic at service temperatures
Longer cycle times

84. Explosive forming Explosive energy used s metal forming
Sheet-metal blank is clamped over a die
Assembly is immersed in a tank with water
Rapid conversion of explosive charge into gas generates a shock wave .the pressure of this wave is sufficient to form sheet metals

85. caused due to explosion , generated in water
Peak pressure (due to explosion):
caused due to explosion , generated in water
P = k( 3sqrt(w) /R)9
P- in psi
K- constant
TNT- trinitrotoluene
W-weight of explosive in pounds
R- the distance of explosive from the work piece

86. Magnetic-Pulse Forming磁脉冲成形
(b)
(a)
Figure (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.

87. Cost Comparison for Spinning and Deep Drawing
Figure 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.