SHEET METAL PROCESSES

1. RAMCO INSTITUTE OF TECHNOLOGY Mr.M.LAKSHMANAN Assistant Professor (Senior Grade) Department of Mechanical Engineering RIT - M.Lakshmanan, AP (S.G)/MECH 1

2. UNIT IV SHEET METAL PROCESSES RIT - M.Lakshmanan, AP (S.G)/MECH 2

3. Syllabus Sheet metal characteristics – shearing, bending and drawing operations – Stretch forming operations – Formability of sheet metal – Test methods –special forming processes-Working principle and applications – Hydro forming – Rubber pad forming – Metal spinning– Introduction of Explosive forming, magnetic pulse forming, peen forming, Super plastic forming – Micro forming. RIT - M.Lakshmanan, AP (S.G)/MECH 3

4. Introduction Sheet Metal Introduction ◼ Sheet metal is a metal formed into thin and flat pieces. It is one of the fundamental forms used in metalworking, and can be cut and bent into a variety of different shapes. ◼ Countless everyday objects are constructed by this material. Thicknesses can vary significantly, although extremely thin sheets are considered as foil or leaf, and sheets thicker than 6 mm (0.25 in) are considered as plate. RIT - M.Lakshmanan, AP (S.G)/MECH 4

5. Sheet Metal Processing ▪ The raw material for sheet metal manufacturing processes is the output of the rolling process. ▪ Typically, sheets of metal are sold as flat, rectangular sheets of standard size. ▪ If the sheets are thin and very long, they may be in the form of rolls. Therefore the first step in any sheet metal process is to cut the correct shape and sized blank from larger sheet. RIT - M.Lakshmanan, AP (S.G)/MECH 5

6. Sheet Metal Working ▪ Performing Cutting and forming operations on relatively thin sheets of metal ▪ Thickness of sheet metal = 0.4 mm to 6 mm ▪ Thickness of plate stock > 6 mm ▪ Operations usually performed as cold working RIT - M.Lakshmanan, AP (S.G)/MECH 6

7. Sheet Metal Forming Sheet metal forming is a process that materials undergo permanent deformation by cold forming to produce a variety of complex three dimensional shapes. The process is carried out in the plane of sheet by tensile forces with high ratio of surface area to thickness. RIT - M.Lakshmanan, AP (S.G)/MECH 7

8. Sheet and plate metal parts for consumer and industrial products such as ➢ Automobiles and trucks ➢ Airplanes ➢ Railway cars and locomotives ➢ Farm and construction equipment ➢ Small and large appliances ➢ Office furniture ➢ Computers and office equipment RIT - M.Lakshmanan, AP (S.G)/MECH 8

9. Advantages of sheet Metal • High strength • Good dimensional accuracy • Good surface finish • Relatively low cost • Economical mass production for large quantities RIT - M.Lakshmanan, AP (S.G)/MECH 9

10. Applications of sheet metals • Aircraft Bodies • Automobiles bodies • Domestic purposes • Beverage cans RIT - M.Lakshmanan, AP (S.G)/MECH 10

11. Sheet Metal operations Introduction ◼ Sheet metal formingis a grouping of many complementary processes that are used to form sheet metal parts. ◼ One or more of these processes is used to take a flat sheet of ductile metal, and mechanically apply deformation forces that alter the shape of the material. Before deciding on the processes, one should determine whether a particular sheet metal can be formed into the desired shape without failure. ◼ The sheet metal operations done on a press may be grouped into two categories, cutting (shearing) operations and forming operations. RIT - M.Lakshmanan, AP (S.G)/MECH 11

12. Sheet Metal operations RIT - M.Lakshmanan, AP (S.G)/MECH 12

13. Sheet Metal operations ◼ The art of sheet metal lies in the making of different shapes by adopting different operations. The major types of operations are given below ❑ Shearing (Cutting) ❑ Bending ❑ Drawing ❑ Squeezing RIT - M.Lakshmanan, AP (S.G)/MECH 13

14. Sheet Metal operations ◼ Shearing ❑ Cutting to separate large sheets; or cut part perimeters or make holes in sheets ◼ Bending ❑ Straining sheet around a straight axis ◼ Drawing ❑ Forming of sheet into convex or concave shapes ◼ Squeezing ❑ Forming of sheet by gripping and pressing firmly – Coining & Embossing RIT - M.Lakshmanan, AP (S.G)/MECH 14

15. Sheet Metal Characteristics • Sheet metal is characterized by high ratio of surface area to thickness. • Forming is generally carried out in tensile forces • Decrease thickness should be avoided as far as possible as they can lead to necking and failure. • The major factors that contribute significantly include elongation, anisotropy, grain size, residual stresses, spring back, and wrinkling. RIT - M.Lakshmanan, AP (S.G)/MECH 15

16. Metal characteristics affecting sheet metal processing • Strength • Elongation • Anisotropy • Grain size • Spring back • wrinkling • Residual stresses • Surface condition of the sheets RIT - M.Lakshmanan, AP (S.G)/MECH 16

17. 1. Strength: The strength of a sheet metal is the property of resistance to external loads or stresses while not causing the structural damage. ❖ Tensile Strength Its ability of a metal to resist being pulled apart by opposing forces acting in a straight line. ❖ Shear Strength Its ability of a material to resist being fractured by opposing forces acting in a straight line but not in the same plane. ❖ Compressive Strength Its ability of a material to withstand to pressures acting on a given plane. RIT - M.Lakshmanan, AP (S.G)/MECH 17

18. 2. Elongation: It’s the capability of the sheet metal to stretch without necking and failure. A specimen subjected to tension undergoes uniform elongation. When the load is exceeded, the ultimate tensile strength the specimen begins to neck. A test to measure the ductility of a material. When a material is tested for tensile strength it elongates a certain amount before fracture takes place. Materials like low carbon steels exhibit a behavior called yield point elongation, exhibiting upper yield and lower yield points. Higher elongation leads to Lueder’s band (strain marks) To avoid this reduce yield point elongation by reducing the thickness of sheet by 0.5% to 1.5% by cold rolling, known as Temper rolling. RIT - M.Lakshmanan, AP (S.G)/MECH 18

19. LUEDER’S BAND or STRAIN MARKS RIT - M.Lakshmanan, AP (S.G)/MECH 19

20. 3. Elasticity: It’s the ability of material to return to its original size, shape and dimensions after being deformed. Also the property of regaining the original dimensions upon removal of the external load is known as elasticity. 4. Modulus of Elasticity: It’s the ratio of the internal stress to the strain produced. It expresses the stiffness of a material. RIT - M.Lakshmanan, AP (S.G)/MECH 20

21. 5. Ductility It’s the capacity of a material to be drawn or stretched under tension loading and permanently deformed without rupture or fracture. 6. Malleability: It’s the property of a metal to be deformed or compressed permanently without rupture or fracture. 7. Plasticity: It’s the ability of a metal to be deformed extensively without rupture. RIT - M.Lakshmanan, AP (S.G)/MECH 21

22. 8. Toughness: It’s a combination of high strength and medium ductility. It’s a ability of material or metal to resist the fracture after the damage has begun. 9. Hardness: Its ability of a material to resist penetration and wear by another material. 10. Brittleness: It’s the property of breaking the material without visible permanent deformation. It’s the reverse of toughness. RIT - M.Lakshmanan, AP (S.G)/MECH 22

23. 11. Corrosion Resistance: It’s the resistance to eating away or wearing by the atmosphere, moisture or other agents such as acid. 12. Anisotropy: It is the property of being directionally dependent, which implies different properties in different directions, as opposed to isotropy. Its acquired during the thermo mechanical processing. ❖ Crystallographic anisotropy (Grain orientation) ❖ Mechanical fibering (Alignment of impurities, inclusions and voids) RIT - M.Lakshmanan, AP (S.G)/MECH 23

24. 13. Springback: The tendency of the metal that tries to resume its original position causing a decrease in bend angle is known as springback. It varies from 0.50 to 50 for steel. Greater springback is caused by a larger bend radius. It depends on the following factors: • Material type • Thickness • Hardness • Bend Radius RIT - M.Lakshmanan, AP (S.G)/MECH 24

25. 14. Wrinkling: One of the primary defects that occurs in deep drawing operations is the wrinkling of sheet metal material, generally in the wall or flange of the part. The flange of the blank undergoes radial drawing stress and tangential compressive stress during the stamping process. RIT - M.Lakshmanan, AP (S.G)/MECH 25

26. 15. Grain size: It determines the surface roughness on a stretched sheet metal. It affects both mechanical properties and surface appearance. Smaller grain size will be stronger metal. 16. Residual Stress: It can develop in sheet metal forming due to non uniform deformation that take place. When disturbed such as removing a portion of it, the part may distort. Tensile residual stresses can lead to stress corrosion cracking of the part unless it is properly relieved. RIT - M.Lakshmanan, AP (S.G)/MECH 26

27. Sheet Metal Processes Processes involving shear stress • Blanking • Piercing • Shaving • Notching • Punching RIT - M.Lakshmanan, AP (S.G)/MECH 27

28. • Processes involving compressive stresses – Coining – Sizing – Ironing – Stamping • Processes involving tensile stresses – Stretch forming • Processes involving both tensile and compressive stresses – Spinning – Drawing – Bending, forming RIT - M.Lakshmanan, AP (S.G)/MECH 28

29. Shearing Process Shearing is the process of cutting sheet metal strip. The shearing action takes place in a sheet metal. Cutting processes are those in which a piece of sheet metal is separated by applying a great enough force to caused the material to fail. The most common cutting processes are performed by applying a shearing force, and are therefore sometimes referred to as shearing processes. RIT - M.Lakshmanan, AP (S.G)/MECH 29

30. Shearing Process RIT - M.Lakshmanan, AP (S.G)/MECH 30

31. Cutting Processes: • Shearing - Separating material into two parts • Blanking - Removing material to use for parts ▪ Conventional blanking ▪ Fine blanking • Punching - Removing material as scrap ▪ Piercing ▪ Slotting ▪ Perforating ▪ Notching ▪ Nibbling ▪ Lancing ▪ Slitting ▪ Parting ▪ Cutoff ▪ Trimming ▪ Shaving ▪ Dinking RIT - M.Lakshmanan, AP (S.G)/MECH 31

32. Shearing • Sheet thickness: 0.005-0.25 inches • Tolerance: ±0.1 inches (±0.005 inches feasible) • Surface finish: 250-1000 μin (125-2000 μin feasible) RIT - M.Lakshmanan, AP (S.G)/MECH 32

33. Blanking During which a metal work piece is removed from the primary metal strip or sheet when it is punched. Blanking is a cutting process in which a piece of sheet metal is removed from a larger piece of stock by applying a great enough shearing force. In this process, the piece removed, called the blank, is not scrap but rather the desired part. RIT - M.Lakshmanan, AP (S.G)/MECH 33

34. RIT - M.Lakshmanan, AP (S.G)/MECH 34

35. Fine blanking Fine blanking is a specialized type of blanking in which the blank is sheared from the sheet stock by applying 3 separate forces. This technique produces a part with better flatness, a smoother edge with minimal burrs, and tolerances as tight as ±0.0003. As a result, high quality parts can be blanked that do not require any secondary operations. RIT - M.Lakshmanan, AP (S.G)/MECH 35

37. Punching Punching is a cutting process in which material is removed from a piece of sheet metal by applying a great enough shearing force. Punching is very similar to blanking except that the removed material, called the slug, is scrap and leaves behind the desired internal feature in the sheet, such as a hole or slot. Punching can be used to produce holes and cutouts of various shapes and sizes. The most common punched holes are simple geometric shapes (circle, square, rectangle, etc.) RIT - M.Lakshmanan, AP (S.G)/MECH 37

38. Punching RIT - M.Lakshmanan, AP (S.G)/MECH 38

39. Piercing The typical punching operation, in which a cylindrical punch pierces a hole into the sheet. RIT - M.Lakshmanan, AP (S.G)/MECH 39

40. Slotting A punching operation that forms rectangular holes in the sheet. Sometimes described as piercing despite the different shape. RIT - M.Lakshmanan, AP (S.G)/MECH 40

41. Perforating Punching a close arrangement of a large number of holes in a single operation. RIT - M.Lakshmanan, AP (S.G)/MECH 41

42. Notching Punching the edge of a sheet, forming a notch in the shape of a portion of the punch. RIT - M.Lakshmanan, AP (S.G)/MECH 42

43. Nibbling Punching a series of small overlapping slits or holes along a path to cutout a larger contoured shape. This eliminates the need for a custom punch and die but will require secondary operations to improve the accuracy and finish of the feature. RIT - M.Lakshmanan, AP (S.G)/MECH 43

44. Lancing Creating a partial cut in the sheet, so that no material is removed. The material is left attached to be bent and form a shape, such as a tab, vent, or louver. RIT - M.Lakshmanan, AP (S.G)/MECH 44

45. Slitting Cutting straight lines in the sheet. No scrap material is produced. RIT - M.Lakshmanan, AP (S.G)/MECH 45

46. Parting Separating a part from the remaining sheet, by punching away the material between parts. RIT - M.Lakshmanan, AP (S.G)/MECH 46

47. Cutoff Separating a part from the remaining sheet, without producing any scrap. The punch will produce a cut line that may be straight, angled, or curved. RIT - M.Lakshmanan, AP (S.G)/MECH 47

48. Trimming Punching away excess material from the perimeter of a part, such as trimming the flange from a drawn cup. RIT - M.Lakshmanan, AP (S.G)/MECH 48

49. Shaving Shearing away minimal material from the edges of a feature or part, using a small die clearance. Used to improve accuracy or finish. Tolerances of ±0.001 inches are possible. RIT - M.Lakshmanan, AP (S.G)/MECH 49

50. Dinking A specialized form of piercing used for punching soft metals. A hollow punch, called a dinking die, with beveled, sharpened edges presses the sheet into a block of wood or soft metal. RIT - M.Lakshmanan, AP (S.G)/MECH 50

51. BENDING RIT - M.Lakshmanan, AP (S.G)/MECH 51

52. BENDING Bending of sheet metal is a common and vital process in manufacturing industry. Sheet metal bending is the plastic deformation of the work over an axis, creating a change in the part's geometry. Similar to other metal forming processes, bending changes the shape of the work piece, while the volume of material will remain the same. In addition to creating a desired geometric form, bending is also used to impart strength and stiffness to sheet metal. RIT - M.Lakshmanan, AP (S.G)/MECH 52

54. Bending Processes 1. ‘V’ Bending: One of the most common types of sheet metal manufacturing processes is V bending. The V shaped punch forces the work into the V shaped die and hence bends it. This type of process can bend both very acute and very obtuse angles, also anything in between, including 90 degrees. RIT - M.Lakshmanan, AP (S.G)/MECH 54

55. 2. Edge bending: Edge bending is another very common sheet metal process and is performed with a wiping die. Edge bending gives a good mechanical advantage when forming a bend. However, angles greater than 90 degrees will require more complex equipment, capable of some horizontal force delivery. The punch then applies force to the cantilever beam section, causing the work to bend over the edge of the die. RIT - M.Lakshmanan, AP (S.G)/MECH 55

56. 3. Rotary bending Rotary bending forms the work by a similar mechanism as edge bending. However, rotary bending uses a different design than the wiping die. A cylinder, with the desired angle cut out, serves as the punch. The cylinder can rotate about one axis and is securely constrained in all other degrees of motion by its attachment to the saddle. The sheet metal is placed cantilevered over the edge of the lower die, similar to the setup in edge bending. Unlike in edge bending, with rotary bending, there is no pressure pad. Force is transmitted to the punch causing it to close with the work. RIT - M.Lakshmanan, AP (S.G)/MECH 56

57. The groove on the cylinder is dimensioned to create the correctly angled bend. The groove can be less than or greater than 90 degrees allowing for a range of acute and obtuse bends. RIT - M.Lakshmanan, AP (S.G)/MECH 57

58. 4. Air bending Air bending is a simple method of creating a bend without the need for lower die geometry. The sheet metal is supported by two surfaces a certain distance apart. A punch exerts force at the correct spot, bending the sheet metal between the two surfaces. RIT - M.Lakshmanan, AP (S.G)/MECH 58

59. 5. ‘U’ and Channel Bending Punch and die are manufactured with certain geometries, in order to perform specific bends. Channel bending uses a shaped punch and die to form a sheet metal channel. U bend is made with a U shaped punch of the correct curvature. RIT - M.Lakshmanan, AP (S.G)/MECH 59

60. 6. Offset Bending: Many bending operations have been developed to produce offsets and form the sheet metal for a variety of different functions. RIT - M.Lakshmanan, AP (S.G)/MECH 60

61. 7. Roll Bending: This process uses a three rollers set to bend a sheet by adjusting the distance between rolls. This process can utilize to form various curvatures. RIT - M.Lakshmanan, AP (S.G)/MECH 61

62. Roll bending provides a technique that is useful for relatively thick work. Although sheets of various sizes and thicknesses may be used, this is a major manufacturing process for the metal bending of large pieces of plate. Roll bending uses three rolls to feed and bend the plate to the desired curvature. The arrangement of the rolls determines the exact bend of the work. Different curves are obtained by controlling the distance and angle between the rolls. A moveable roll provides the ability to control the curve. RIT - M.Lakshmanan, AP (S.G)/MECH 62

64. 8. Four Slide machine Bending: This process uses three movable and one stationary slide to bent a work sheets as shown in figure. This is used to bent small work pieces. RIT - M.Lakshmanan, AP (S.G)/MECH 64

65. 9. Beading: It is a process in which the periphery of the sheet metal is bent into the cavity of a die. It increases moment of inertia of the section and stiffness. It also eliminates exposed sharp edges. RIT - M.Lakshmanan, AP (S.G)/MECH 65

66. Sheet metal beading processes produce a bead with a single die. In a process called wiring, the metal's edge is bent over a wire. RIT - M.Lakshmanan, AP (S.G)/MECH 66

67. 10. Flanging: It is a process of bending the edges of the metal sheet at perpendicular to the length. It can be further divided according the shape like straight flange, stretch flange, joggled flange, shrink flange etc. as shown in figure. RIT - M.Lakshmanan, AP (S.G)/MECH 67

68. Flanging RIT - M.Lakshmanan, AP (S.G)/MECH 68

69. 11. Dimpling: It is a process in which first a hole in made into the sheet metal and then it is expanded into a flange using punch die system. RIT - M.Lakshmanan, AP (S.G)/MECH 69

70. 12.Hemming: It is an operation in which the edge of the sheet is folded over itself. This process increases stiffness of the part and eliminate sharp edges. RIT - M.Lakshmanan, AP (S.G)/MECH 70

71. Bending Terminology RIT - M.Lakshmanan, AP (S.G)/MECH 71

72. • Neutral Axis: It is an imaginary axis which does not undergo any stress during bending. • Bend Allowance: The length of the neutral axis in the bend zone is known as bend allowance. • Bend Angle: The angle form by the bend area at the center of bend is known as bend angle. • Bend Radius: Distance between bend center and neutral axis is known as bend radius. It is denoted by r. RIT - M.Lakshmanan, AP (S.G)/MECH 72

73. • Springback: When load is removed, the sheet metal shows some elastic recovery and tends to achieve its original position. This phenomenon is called springback. It will increase the final bend radius and decrease the bend angle after spingback. RIT - M.Lakshmanan, AP (S.G)/MECH 73

74. Bend Allowance (BA) RIT - M.Lakshmanan, AP (S.G)/MECH 74

75. Where, • Lf = flat length of the sheet • BA = bend allowance • BD = bend deduction • R = inside bend radius • K = K-Factor, which is t / T • T = material thickness • t = distance from inside face to the neutral line • A = bend angle in degrees (the angle through which the material is bent) RIT - M.Lakshmanan, AP (S.G)/MECH 75

76. Bend Deduction (BD) The bend deduction BD is defined as the difference between the sum of the flange lengths (from the edge to the apex) and the initial flat length.(OSSB-Outside Set Back) RIT - M.Lakshmanan, AP (S.G)/MECH 76

77. The bend deduction (BD) is twice the outside setback minus the bend allowance. BD is calculated using the following formula, where A is the angle in radians (=degrees*π/180) K -Factor to the Bend Allowance; RIT - M.Lakshmanan, AP (S.G)/MECH 77

78. Drawing/ Deep Drawing Operations Deep drawing is a manufacturing process that is used extensively in the forming of sheet metal into cup or box like structures. Pots and pans for cooking, containers, sinks, automobile parts, such as panels and gas tanks, are among a few of the items manufactured by sheet metal deep drawing. This process is sometimes called drawing. For the primary sheet metal deep drawing process the part will have a flat base and straight sides. RIT - M.Lakshmanan, AP (S.G)/MECH 78

79. Deep drawing of sheet metal is performed with a punch and die. The punch is the desired shape of the base of the part, once drawn. The die cavity matches the punch and is a little wider to allow for its passage, as well as clearance. This setup is similar to sheet metal cutting operations. RIT - M.Lakshmanan, AP (S.G)/MECH 79

80. The punch travels towards the blank. After contacting the work, the punch forces the sheet metal into the die cavity, forming its shape. RIT - M.Lakshmanan, AP (S.G)/MECH 80

81. Equipment for sheet metal deep drawing processes would involve a double action, one for the blank holder and one for the punch. Both mechanical and hydraulic presses are used in manufacturing industry. Typically the hydraulic press can control the blankholder and punch actions separately, but the mechanical press is faster. Punch and die materials, for the deep drawing of sheet metal, are usually tool steels and iron. Parts are usually drawn at speeds of 4 to 12 inches per second. RIT - M.Lakshmanan, AP (S.G)/MECH 81

82. Drawing Ratio Measurement of the amount of drawing performed on a sheet metal blank can be quantified. This can be done with the drawing ratio. The higher the drawing ratio, the more extreme the amount of deep drawing. Due to the geometry, forces, metal flow and material properties of the work, there is a limit to the amount of deep drawing that can be performed on a sheet metal blank in a single operation. Db is the diameter of the blank and Dp is the diameter of the punch. DR = Db/Dp RIT - M.Lakshmanan, AP (S.G)/MECH 82

83. Reduction Another way to express drawing ratio is the reduction (r). Reduction is measured using the same variables as drawing ratio. Reduction can be calculated by r = (Db - Dp)/(Db) Db and Dp being blank and punch diameters respectively. Percentage of reduction: r = (Db - Dp)/(Db) X 100% In this case the reduction should be 50% or under. RIT - M.Lakshmanan, AP (S.G)/MECH 83

84. Redrawing Sheet Metal If required percent reduction of sheet metal is over 50%, the part must be formed in multiple operations. Redrawing is the subsequent deep drawing of a work that has already undergone a deep drawing process. By using more than one operation, a greater magnitude of deep drawing can be accomplished. Initial reduction is usually 35% to 45%. First redraw is commonly performed at a 20% to 30% reduction. Second redraw can typically range from 13% to 16% reduction. RIT - M.Lakshmanan, AP (S.G)/MECH 84

85. Redrawing RIT - M.Lakshmanan, AP (S.G)/MECH 85

86. Reverse Redrawing Containers or shells that are too difficult to draw in one operation are generally redrawn. In reverse redrawing, the metal is subjected to bending in the direction opposite to its original bending configuration. This reversal in bending results in strain softening. This operation requires lower forces than direct redrawing and the material behaves in a more ductile manner. RIT - M.Lakshmanan, AP (S.G)/MECH 86

87. Reverse Redrawing RIT - M.Lakshmanan, AP (S.G)/MECH 87

89. Stretch Forming Stretch forming is a metal forming process in which a piece of sheet metal is stretched and bent simultaneously over a die in order to form large contoured parts. Stretch forming is performed on a stretch press, in which a piece of sheet metal is securely gripped along its edges by gripping jaws. The gripping jaws are each attached to a carriage that is pulled by pneumatic or hydraulic force to stretch the sheet. RIT - M.Lakshmanan, AP (S.G)/MECH 89

91. Types of Stretch Forming Process ❖Simple stretch forming ❖Tangential stretch forming ❖Stretch forming according to Cyril-Bath ❖Multi-sided stretch forming RIT - M.Lakshmanan, AP (S.G)/MECH 91

92. Simple stretch forming RIT - M.Lakshmanan, AP (S.G)/MECH 92

93. Tangential stretch forming RIT - M.Lakshmanan, AP (S.G)/MECH 93

94. Cyril-Bath Process RIT - M.Lakshmanan, AP (S.G)/MECH 94

95. Multi-sided stretch forming RIT - M.Lakshmanan, AP (S.G)/MECH 95

96. Formability of Sheet Metal Formability is the ability of a given metal workpiece to undergo plastic deformation without being damaged. The plastic deformation capacity of metallic materials, however, is limited to a certain extent, at which point, the material could experience tearing or fracture (breakage). Processes affected by the formability of a material include: deep drawing, cup drawing, bending etc. involve extensive tensile deformation. RIT - M.Lakshmanan, AP (S.G)/MECH 96

97. Formability = f(f1,f2) Where, f1= Material Variables f2= Process variables Formability Test Methods: Simple uniaxial tensile test is not much useful for the formability of sheet metals. Its due to the biaxial or triaxial nature of stresses acting on the sheet metal during forming operations. 1. Formability test for bulk deformation 2. Formability test for elastic –Plastic deformation 3. Simulative test for forming operation 4. Full scale forming tests RIT - M.Lakshmanan, AP (S.G)/MECH 97

98. 1. Formability test for bulk deformation • Stress –strain characteristics under actual working conditions • Process Economic Analysis • Full scale experiments RIT - M.Lakshmanan, AP (S.G)/MECH 98

99. 2. Formability test for elastic –Plastic deformation ❖ Test methods based on tensile test: – Tensile test for stretch forming operations: Fracture of sheet metal is predicted by local thinning. At the same time, failure is avoided. The important property of work hardening is predicted in terms of stress- strain. RIT - M.Lakshmanan, AP (S.G)/MECH 99

100. f = A*ɛn Where, f = Stress ɛ = Strain A and n are Constants (Values from 0.22 to 0.24) –Tensile Test for Drawing Operations: The sheet metal is deformed on the lower punch by thinning under bi-axial stresses. Then the average value of sheet metal radius is determined by orienting the axis of the metal flow at 00 to 450 and 900. Mean value of Radius, rm = ¼(r0+2r45+r90)1/2 Good drawability rm varies from 1.0 to 1.7.RIT - M.Lakshmanan, AP (S.G)/MECH 100

101. ❖ Simulative Drawing tests This test conducted in various cup forming operations such as 1. Erichsen Test 2. Olsen Test 3. Swift Test 4. Fukui Test RIT - M.Lakshmanan, AP (S.G)/MECH 101

102. Erichsen Test • The standard specimen of 90mm wide is rigidly clamped against a die having 27mm diameter opening. A spherical punch of 20mm diameter is moved against the sheet metal. • The cup height at the fracture point is a measure of the stretching ability. • The maximum load point is measured during the tests. It is hard to obtain repeatable data, because the friction affects the results. RIT - M.Lakshmanan, AP (S.G)/MECH 102

104. Olsen Test The size of the standard specimen and rest are taken same as mentioned in Erichsen test. But the die opening size of 50mm diameter is used. This test is also carried out for assessing the stretchability. RIT - M.Lakshmanan, AP (S.G)/MECH 104

105. Swift Test In this test, flat bottomed cups of uniform diameter are formed from a series of metallic sheet blanks. These metallic blanks are of different diameters. This process is continued until the fracture occurs in all cups. Limiting Draw ratio, LDR = Blank Diameter Punch Diameter RIT - M.Lakshmanan, AP (S.G)/MECH 105

108. Fukui Test The sheet is both drawn and stretched over a cup of conical shape. So, the strechability and drawability can be assessed. Both die and punch are in the form of conical shape. The cup depth is measured at maximum load which is referred as formability index. RIT - M.Lakshmanan, AP (S.G)/MECH 108

109. • The Fukui conical cup value is determined by the ratio of the diameter of the base of the conical cup formed and the diameter of the original specimen. RIT - M.Lakshmanan, AP (S.G)/MECH 109

110. 4. Full scale Forming Test Forming Limit Diagram is obtained to describe the different strain conditions and their combinations with load to failure of sheet metal. The strain distribution is assessed from the surface. RIT - M.Lakshmanan, AP (S.G)/MECH 110

111. Forming Limit Diagram (FLD) A forming limit diagram, also known as a forming limit curve, is used in sheet metal forming for predicting forming behaviour of sheet metal. The diagram attempts to provide a graphical description of material failure tests, such as a punched dome test. The mechanical test is performed by placing a circular mark on the workpiece prior to deformation, and then measuring the post-deformation ellipse that is generated from the action on this circle. Actual strain on the sheet metal ɛ = (l-d)/d RIT - M.Lakshmanan, AP (S.G)/MECH 111

112. Where, l= Length of major or minor axes d= Corresponding concentric circle Max surface strain, ɛ1 = Length of major axis Min Surface strain, ɛ2 = Length of Minor axis RIT - M.Lakshmanan, AP (S.G)/MECH 112

114. Keeler – Goodwin Forming Limit Diagram RIT - M.Lakshmanan, AP (S.G)/MECH 114

117. Special Forming Processes ❖Hydroforming ❖Rubber Pad Forming ❖Metal Spinning ❖Explosive Forming ❖Magnetic Pulse Forming ❖Peen Forming ❖Superplastic Forming RIT - M.Lakshmanan, AP (S.G)/MECH 117

118. Hydroforming Hydroforming is a drawing process. This process is carried out by two ways. ➢Hydro-Mechanical Forming ➢Electro-Hydraulic Forming RIT - M.Lakshmanan, AP (S.G)/MECH 118

119. Hydro-Mechanical Forming RIT - M.Lakshmanan, AP (S.G)/MECH 119

120. Hydro-Mechanical Forming The punch is connected to the lower die. The required shape of inner configuration is made on the punch. A Diaphragm or seal is used for making perfect sealing between top and bottom die. The pressure forming chamber is filled with a hydraulic fluid. Then the blank is correctly positioned over the top or lower die. The required shape of the blank is obtained only by drawing rather than by bending. RIT - M.Lakshmanan, AP (S.G)/MECH 120

121. Hydro-Mechanical Forming RIT - M.Lakshmanan, AP (S.G)/MECH 121

122. Electro-Hydraulic Forming RIT - M.Lakshmanan, AP (S.G)/MECH 122

123. Electro-Hydraulic Forming (EHF) Electrohydraulic forming is a type of metal forming in which an electric arc discharge in liquid is used to convert electrical energy to mechanical energy and change the shape of the workpiece. A capacitor bank delivers a pulse of high current across two electrodes, which are positioned a short distance apart while submerged in a fluid (water or oil). The electric arc discharge rapidly vaporizes the surrounding fluid creating a shock wave. The workpiece, which is kept in contact with the fluid, is deformed into an evacuated die. RIT - M.Lakshmanan, AP (S.G)/MECH 123

125. Advantages of EHF: • A single-step process • Fine details and sharp lines can be easily formed • Forming of negative and positive shapes • Only a single one-sided die is required • Enables extremely deep forming (much more than is possible with conventional stamping) • Even distribution and higher strength of thin material • Extremely fast • Equipment has small footprint • No need for a press – the forming chamber is a self-balanced system • Allows forming of parts up to a few square meters in size RIT - M.Lakshmanan, AP (S.G)/MECH 125

126. Rubber Pad Forming (RPF) RPF is a metalworking process where sheet metal is pressed between a die and a rubber block, made of polyurethane. Under pressure, the rubber and sheet metal are driven into the die and conform to its shape, forming the part. Rubber pad forming is a deep drawing technique that is ideally suited for the production of small and medium-sized series. Deep drawing makes it possible to deform sheet metal in two directions, which offers great benefits in terms of function integration, weight reduction, cleanability. RIT - M.Lakshmanan, AP (S.G)/MECH 126

128. Advantages: • Short time to market through simple tools • Low tooling costs • Excellent for low and medium numbers • With the use of rubber, a polished or sharpened surface remains undamaged • Suitable for steel, stainless steel, aluminum etc. up to a thickness of about 4mm Disadvantages: • For very large numbers too laborable so too expensive. • Somewhat less freedom in form compared to the regular deep drawing process • In most cases not suitable for sheet thicknesses greater than 4mm. RIT - M.Lakshmanan, AP (S.G)/MECH 128

129. Metal Spinning Metal spinning, also known as spin forming or spinning or metal turning most commonly, is a metalworking process by which a disc or tube of metal is rotated at high speed and formed into an axially symmetric part. Spinning can be performed by hand or by a CNC lathe. RIT - M.Lakshmanan, AP (S.G)/MECH 129

132. 1. Shear Spinning Shear spinning is a process related to conventional spinning and is also known as flow turning or spin forging. In a conventional spinning operation the work is essentially formed by bending. There is usually not much change in the thickness of the sheet metal. The diameter of the work in conventional spinning must be large enough to account for the size of the final part. Shear spinning involves forming the work over the mandrel, causing metal flow within the work. This metal flow will act to reduce the thickness of the work as it is formed. The initial diameter of the work can be smaller in shear spinning. RIT - M.Lakshmanan, AP (S.G)/MECH 132

134. 2. Tube Spinning Tube spinning is performed on cylindrical parts. Tube spinning is similar to shear spinning in that metal flow occurs within the work. This metal flow acts to reduce the thickness of the metal. While using tube spinning to reduce the thickness of the tube, the tube's length will be increased. This manufacturing process can be performed externally with the tube over a mandrel or internally with the tube enclosed by a die. RIT - M.Lakshmanan, AP (S.G)/MECH 134

136. Explosive Forming Explosive forming is a metalworking technique in which an explosive charge is used instead of a punch or press. It can be used on materials for which a press setup would be prohibitively large or require an unreasonably high pressure, and is generally much cheaper than building a large enough and sufficiently high-pressure press; on the other hand, it is unavoidably an individual job production process, producing one product at a time and with a long setup time. RIT - M.Lakshmanan, AP (S.G)/MECH 136

137. Explosive Forming It is used for blanking, cutting, expanding, coining, embossing, flanging, drawing operations etc,. Explosives used can be high energy chemicals such as TNT, RDX and Dynamite or Gaseous mixture or propellants. These chemicals are used in various forms such as rod, sheet, liquid, stick etc,. According to the placement of explosives the operations can be divided into the following categories: ❖Unconfined or Stand-off Technique ❖Confined System or Contact TechniqueRIT - M.Lakshmanan, AP (S.G)/MECH 137

138. Unconfined or Stand-off Technique RIT - M.Lakshmanan, AP (S.G)/MECH 138

139. An explosive forming process commonly used for the production of large parts is called a standoff system. Typically the mold and work piece are submerged in water. The sheet metal is secured over the mold by a ring clamp. Air is drawn out, creating a vacuum in the die cavity. An explosive is placed between the die cavity and the work, a certain distance from the work. This distance is called the standoff distance. Standoff distance depends on the size of the work, for larger parts it is usually about half the diameter of the blank. The explosive itself is also deeply submersed in water. Upon detonation, the shock wave travels through the water and delivers great energy to the work, forming it to the die cavity near instantaneously. This high energy rate forming process can be used to form big thick plates. RIT - M.Lakshmanan, AP (S.G)/MECH 139

141. Confined System or Contact Technique This is usually used for relatively smaller parts than the standoff system. All of the energy is directed into a closed container, the walls of which contain the die cavity. The energy from the canned explosive forces the sheet metal into the walls of the mold, forming the part. Safety is always a consideration when manufacturing by explosive forming, particularly with the confined system, where die failure is a significant concern. RIT - M.Lakshmanan, AP (S.G)/MECH 141

143. Explosive Forming: Unconfined and Confined RIT - M.Lakshmanan, AP (S.G)/MECH 143

144. Magnetic Pulse Forming/ Electromagnetic Forming The magnetic pulse forming process which uses opposing magnetic fields to force a sheet of metal onto a mandrel or other form. First, an extremely large current discharge is directed through a coil which creates a magnetic field. Capacitor banks are used to store charge for larger discharges. In the nearby sheet of metal, an opposing magnetic field is induced which causes the metal sheet to be pushed into a form of some shape. The method generates pressures up to 50 Kpsi creating velocities up to 900 fps. The process production rate can climb to 3 parts a second. RIT - M.Lakshmanan, AP (S.G)/MECH 144

147. A capacitor bank is charged up and a large electrical surge is sent through the coil. The current creates a magnetic field. When a conductive material disrupts a magnetic field it produces a current in that material, this is called an eddy current. Due to the close proximity of the conductive sheet metal to the coil, the coil's magnetic field is disrupted and eddy currents are generated in the work piece. These currents in the sheet metal produce their own magnetic field that opposes the original magnetic field of the coil. The opposing forces push these fields apart and form the work. RIT - M.Lakshmanan, AP (S.G)/MECH 147

148. Advantages: • Improved formability (the amount of stretch available without tearing) • Wrinkling can be greatly suppressed • Forming can be combined with joining and assembling with dissimilar components including glass, plastic, composites and other metals. • Close tolerances are possible as springback can be significantly reduced. • Single sided dies are sufficient which can reduce tooling costs. Disadvantages: • Non conductive materials cannot be formed directly, but can be formed using a conductive drive plate • The high voltages and currents involved require careful safety considerations.RIT - M.Lakshmanan, AP (S.G)/MECH 148

149. Peen Forming It is also called as shot peening. The free forming obtain where the stream of small steel balls are forced together again the metal surface, when the metal forming process restricted to fulfillment of fairly specialized function. The peen forming is used to form of various irregular shapes on the aluminium sheet plate. That requires large plate and small control in the process. The peen forming process not requires any die and forming press. The part to be made by the sheet metal placed on block or it suspended from support and it has blasting together with shot, small steel balls. During the operation blanks are clamped over simple form blocks. RIT - M.Lakshmanan, AP (S.G)/MECH 149

151. The ball forced by compressed air or rotating blade. The ball is having high velocity directly imping the sheet metal to the form of block. There is repeated force by sheet metal get the require form block shape. The numerous small balls is having diameter of 2.5 mm size cast- steel ball blast again the metal surface. The ball discharged from the rotating wheel or by air blast from nozzle. The balls travelling speed of 60 m/s. The residual stress are induced to the compressive surface which improve the fatigue strength of sheet metal. RIT - M.Lakshmanan, AP (S.G)/MECH 151

153. Advantages of peen forming: • Tooling cost will be low • Require no maintenance cost of tooling • The compound curvatures are easily produced • It is a die less forming, So that require minimum lead time • That process permit rework and design changes for improve the fitness of sheet metal • Formed product improve the fatigue strength and stress corrosion resistance RIT - M.Lakshmanan, AP (S.G)/MECH 153

154. Disadvantage of peen forming process: • Exhibited increase the resistance to flexural bending fatigue • The surface of metal compressed together, that prevent stress corrosion cracking. Application of peen forming: • Provide smoothing and complex curvature of aircraft wings • From of large tubular shapes • Military air craft RIT - M.Lakshmanan, AP (S.G)/MECH 154

155. Superplastic Forming Superplastic forming (SPF) is an industrial process used for creating precise and complex components out of certain types of materials called superplastic materials. To begin with, the material is heated up to promote superplasticity. For titanium alloys: E.g. Ti 6Al 4V and some stainless steels this is around 900 °C (1,650 °F) and For aluminium alloys: E.g. AA5083 it is between 450–520 °C. In this state the material becomes soft so processes that are usually used on plastics can be applied, such as: thermoforming, blow forming, and vacuum forming. Inert gas pressure is applied on the superplastic sheet forcing it into a female die. RIT - M.Lakshmanan, AP (S.G)/MECH 155

157. Superplastic Forming During the super plastic forming process the elevated temperature will occur the sheet metal stress is very low. When the forming process the sheet metal and tooling are heated together and then high pressure of gas is applied. That case, the sheet metals are plastically deformed into desire shape of die cavity. RIT - M.Lakshmanan, AP (S.G)/MECH 157

158. Superplastic Forming RIT - M.Lakshmanan, AP (S.G)/MECH 158

159. Advantage of superplastic forming: • Lower strength require the tooling • The most complex can be formed • No residual stress induced during the parts of formed • Tooling cost low • High rate of forming RIT - M.Lakshmanan, AP (S.G)/MECH 159

160. Materials: The superplastic forming process suitable for some certain material only. That having very fine grain structure of less than 10 to 15 µm size. The material of SFP: • Iron based high carbon alloy • Incol 100 • 7475- T6 Aluminium alloy • Incoloy 718 • T1- 6A1- 4v Titanium alloy RIT - M.Lakshmanan, AP (S.G)/MECH 160

161. Application: • Fresh coater panels • Water closet • Cooler outlet ducts • Heat- exchanger ducts • Electric vehicle aluminum battery tray RIT - M.Lakshmanan, AP (S.G)/MECH 161

162. Micro forming It involves forming of parts and features with dimensions below 1mm. It’s a recent area of research in the wide field of metal forming technologies which is expanding the limits for applying metal forming towards micro technology. The major sheet metal processes for micro forming are shearing, blanking, bending, stamping, deep drawing, hydro forming, stretch forming, super plastic forming, spinning, explosive forming , etc. RIT - M.Lakshmanan, AP (S.G)/MECH 162

163. Similar to conventional sheet metal, the mechanical properties of the materials such as elasticity, Plasticity, stress strain relations, work hardening, temperature effect, anisotropy, grain size and residual stress involve in analysing the deformation of micro forming products. Micro Forming Products: • Micro gears and micro shafts • Connector pin • IC Chip lead frame and IC Sockets • Miniature Fasteners • Micro springs for micro switches • Micro knifes for micro surgery • Hard disc drives • Sensors RIT - M.Lakshmanan, AP (S.G)/MECH 163

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