Chapter 17 Sheet Forming Processes EIN 3390 Manufacturing Processes Summer A, 2011

1. Chapter 17Sheet Forming ProcessesEIN 3390 Manufacturing ProcessesSummer A, 2011

2. Sheet metal processes involve plane stress loadings and lower forces than bulk forming • Almost all sheet metal forming is considered to be secondary processing • The main categories of sheet metal forming are: • Shearing • Bending • Drawing 17.1 Introduction

3. Shearing- mechanical cutting of material without the formation of chips or the use of burning or melting • Both cutting blades are straight • Curved blades may be used to produce different shapes • Blanking • Piercing • Notching • Trimming 17.2 Shearing Operations

4. Fracture and tearing begin at the weakest point and proceed progressively or intermittently to the next-weakest location • Results in a rough and ragged edge • Punch and die must have proper alignment and clearance • Sheared edges can be produced that require no further finishing Shearing Operations

5. Shearing Operations Figure 17-1 Simple blanking with a punch and die.

6. Shearing Operations

7. Classification of Metal forming Operations

8. Types of Shearing • Simple shearing- sheets of metal are sheared along a straight line • Slitting- lengthwise shearing process that is used to cut coils of sheet metal into several rolls of narrower width Figure 17-5 Method of smooth shearing a rod by putting it into compression during shearing. Figure 17-6 A 3-m (10ft) power shear for 6.5 mm (1/4-in.) steel. (Courtesy of Cincinnati Incorporated, Cincinnati, OH.)

9. Shearing Operations Figure 17-2 (Left) (Top) Conventionally sheared surface showing the distinct regions of deformation and fracture and (bottom) magnified view of the sheared edge. (Courtesy of Feintool Equipment Corp., Cincinnati, OH.)

10. Piercing and Blanking • Piercing and blanking are shearing operations where a part is removed from sheet material by forcing a shaped punch through the sheet and into a shaped die • Blanking- the piece being punched out becomes the workpiece • Piercing- the punchout is the scrap and the remaining strip is the workpiece Figure 17-8 (Above) (Left to Right) Piercing, lancing, and blanking precede the forming of the final ashtray. The small round holes assist positioning and alignment. Figure 17-7 Schematic showing the difference between piercing and blanking.

11. Fine Blanking - the piece being punched out becomes the workpiece and pressure pads are used to smooth edges in shearing Fine Blanking Operations Figure 17-3 (Top) Method of obtaining a smooth edge in shearing by using a shaped pressure plate to put the metal into localized compression and a punch and opposing punch descending in unison.

12. Shearing Operations Figure 17-4 Fineblanked surface of the same component as shown in Figure 17-2. (Courtesy of Feintool Equipment Corp., Cincinnati, OH.)

13. Lancing- piercing operation that forms either a line cut or hole • Perforating- piercing a large number of closely spaced holes • Notching- removes segments from along the edge of an existing product • Nibbling- a contour is progressively cut by producing a series of overlapping slits or notches Types of Piercing and Blanking

14. Sheet-metal Cutting Operations

15. Types of Piercing and Blanking • Shaving- finishing operation in which a small amount of metal is sheared away from the edge of an already blanked part • Cutoff- a punch and a die are used to separate a stamping or other product from a strip of stock • Dinking- used to blank shapes from low-strength materials such as rubber, fiber, or cloth

16. Figure 17-10 The dinking process. Sheet-metal Shaving Operations Figure The shaving process.

17. Tools and Dies for Piercing and Blanking • Basic components of a piercing and blanking die set are: punch, die, and stripper plate • Punches and dies should be properly aligned so that a uniform clearance is maintained around the entire border • Punches are normally made from low-distortion or air-hardenable tool steel Figure 17-11 The basic components of piercing and blanking dies.

18. Figure 17-12 Blanking with a square-faced punch (left) and one containing angular shear (right). Note the difference in maximum force and contact stroke. The total work (the are under the curve) is the same for both processes. Blanking Operations

19. Design rules • Diameters of pierced holes should not be less than the thickness of the metal with a minimum 0f 0.3 mm (0.025”) • Minimum distance between holes or the edge of the stock should be at least equal to the metal thickness • The width of any projection or slot should be at least 1 times the metal thickness and never less than 2.5 mm (3/32”) • Keep tolerances as large as possible • Arrange the pattern of parts on the strip to minimize scrap Design for Piercing and Blanking

20. Design Clearance

21. The recommended clearance is: C = at Where c – clearance, in (mm); a – allowance; and t = stock thickness, in (mm). Allowance a is determined according to type of metal. From Mikell P. Groover “Fundamentals of Modern Manufacturing”. Clearance Calculation

22. For a round blank of diameter Db is determined as: Blank punch diameter = Db - 2c Blank die diameter = Db For a round hole (piercing) of diameter Dh is determined as: Hole punch diameter = Dh Hole die diameter = Db + 2c Design Die and Punch Sizes

23. Cutting forces are used to determine size of the press needed. F = StL Where S – shear strength of the sheet metal, lb/in2 (Mpa); t – sheet thickness in. (mm); and L – length of the cut edge, in. (mm). In blanking, punching, slotting, and similar operations, L is the perimeter length of blank or hole being cut. Note: the equation assumes that the entire cut along sheared edge length is made at the same time. In this case, the cutting force is a maximum. Cutting Forces

24. for slug or blank to drop through the die, the die opening must have an angular clearance of 0.25 to 1.50on each side. Angular Clearance

25. Round disk of 3.0” dia. is to be blanked from a half-hard cold-rolled sheet of 1/8” with shear strength = 45,000 lb/in2. Determine (a) punch and die diameters, and (b) blanking force. (a). From table , a = 0.075, so clearance c = 0.075(0.125) = 0.0094”. Die opening diameter = 3.0” Punch diameter = 3 – 2(0.0094) = 2.9812 in (b). Assume the entire perimeter of the part is blanked at one time. L = p Db = 3.14(3) = 9.426” F = 45,000(9.426)(0.125) = 53,021 = 24.07 tons What is the difference if the operation is a piercing? Example for Calculating Clearance and Force

26. 17.3 Bending • Bending is the plastic deformation of metals about a linear axis with little or no change in the surface area • Forming- multiple bends are made with a single die • Drawing and stretching- axes of deformation are not linear or are not independent • Springback is the “unbending” that occurs after a metal has been deformed Figure 17-19 (Top) Nature of a bend in sheet metal showing tension on the outside and compression on the inside. (Bottom) The upper portion of the bend region, viewed from the side, shows how the center portion will thin more than the edges.

27. Example of Bending

28. Design for Bending • Several factors are important in specifying a bending operation • Determine the smallest bend radius that can be formed without cracking the metal • Metal ductility • Thickness of material Figure 17-24 Relationship between the minimum bend radius (relative to thickness) and the ductility of the metal being bent (as measured by the reduction in area in a uniaxial tensile test).

29. Considerationsfor Bending • If the punch radius is large and the bend angle is shallow, large amounts of springback are often encountered • The sharper the bend, the more likely the surfaces will be stressed beyond the yield point Figure 17-25 Bends should be made with the bend axis perpendicular to the rolling direction. When intersecting bends are made, both should be at an angle to the rolling direction, as shown.

30. Design Considerations • Determine the dimensions of a flat blank that will produce a bent part of the desired precision • Metal tends to thin when it is bent Figure 17-26 One method of determining the starting blank size (L) for several bending operations. Due to thinning, the product will lengthen during forming. l1, l2, and l3 are the desired product dimensions. See table to determine D based on size of radius R where t is the stock thickness.

31. Engineering Analysis of Bending • Bending radius R is normally specified on the inside of the part, rather than at the neutral axis. The bending radius is determined by the radius on the tooling used for bending. • Bending Allowance: If the bend radius is small relative to sheet thickness, the metal tends to stretch during bending. • BA = 2pA(R + Kbat)/360 • Where BA – bend allowance, in. (mm); A - bend angle, degrees; R – bend radius, in. (mm); t – sheet thickness; and Kba - factor to estimate stretching. According to [1], if R < 2t, Kba = 0.33; and if R>=2t, Kba =0.5. [1]: Hoffman, E.G., Fundamentals of Tool Design, 2nd ed.

32. Engineering Analysis of Bending Spring back: When the bending pressure is removed at the end of deformation, elastic energy remains in the bend part, causing it to recover partially toward its original shape. SB = (A’ – Ab’)/Ab’ Where SB – springback; A’ – included angle of sheet-metal part; and Ab’ – included angle of bending tool, degrees. From Mikell P. Groover “Fundamentals of Modern Manufacturing”.

33. Engineering Analysis of Bending Bending Force: The force required to perform bending depends on the geometry of the punch and die and the strength, thickness, and width of the sheet metal. The maximum bending force can be estimated by means of the following equation based on bending of a simple beam: F = (KbfTSwt2)/D Where F – bending force, lb (N),; TS – tensile strength of the sheet metal, lb/in2. (Mpa); t – sheet thickness, in. (mm); and D – die opening dimension. Kbf – a constant that counts for differences in an actual bending processes. For V-bending Kbf =1.33, and for edge bending Kbf =0.33

34. Example for Sheet-metal Bending Metal to be bent with a modulus of elasticity E = 30x106 lb/in2., yield strength Y = 40,000lb/in2 , and tensile strength TS = 65,000 lb/in2. Determine (a) starting blank size, and (b) bending force if V-die will be used with a die opening dimension D = 1.0in. (a) W = 1.75” and the length of the part is: 1.5 +1.00 + BA. R/t = 0.187/0.125 = 1.5 < 2.0, so Kba =0.33 For an included angle A’ = 1200, then A = 600 BA = 2pA(R + Kbat)/360 =2p60(0.187 + 0.33 x 0.125)/360 = 0.239” Length of the bank is 1.5+1+0.239 = 2.739” (b) Force: F = (KbfTSwt2)/D = 1.33 (65,000)(1.75)(0.125)2/1.0 = 2,364 lb

35. Drawing refers to the family of operations where plastic flow occurs over a curved axis and the flat sheet is formed into a three-dimensional part with a depthmore than several times the thickness of the metal • Application: a wide range of shapes, from cups to large automobile and aerospace panels. 17.4 Drawing and Stretching Processes

36. Types of Drawing and Stretching • Spinning • Shear forming or flow turning • Stretch forming • Deep drawing and shallow drawing • Rubber-tool forming • Sheet hydroforming • Tube hydroforming • Hot drawing • High-energy-rate forming • Ironing • Embossing • Superplastic sheet forming 17.4 Drawing and Stretching Processes

37. Spinning is a cold forming operation • Sheet metal is rotated and progressively shaped over a male form, or mandrel • Produces rotationally symmetrical shapes • Cones, spheres, hemispheres, cylinders, bells, and parabolas 17.4 Spinning

38. Figure 17-34 (Above) Progressive stages in the spinning of a sheet metal product. Spinning

39. Spinning

40. Figure 17-35 (Below) Two stages in the spinning of a metal reflector. (Courtesy of Spincraft, Inc. New Berlin, WI.) Spinning

41. Tooling cost can be extremely low. The form block can often be made of hardwood or even plastic because of localized compression from metal. With automation, spinning can also be used to mass-produce high-volume items such as lamp reflectors, cooking utensils, bowls, and bells. Spinning is usually considered for simple shapes that can be directly withdrawn from a one-piece form. More complex shapes, such as those with reentrant angles, can be spun over multipiece or offset forms. Spinning

42. Shear forming is a version of spinning • A modification of the spinning process in which each element of the blank maintains its distance from the axis of rotation. • No circumferential shrinkage • Wall thickness of product, tc will vary with the angle of the particular region: tc = tb(sin a) where tb is the thickness of the starting blank. • Reductions in wall thickness as high as 8:1 are possible, but the limit is usually set at about 5:1, or 80% Shear Forming

43. Shearing Forming

44. Material being formed moves in the same direction as the roller Figure 17-36 Schematic representation of the basic shear-forming process. Direct Shear Forming

45. Material being formed moves in the opposite direction as the roller • By controlling the position and feed of the forming roller, the reverse process can be used to shape con- cave, convex, or conical parts without a matching form block. Reverse Shear Forming

46. Deep Drawing and Shallow Drawing • Drawingis typically used to form solid-bottom cylindrical or rectangular containers from sheet metal. • When depth of the product is greater than its diameter, it is known “Deep drawing”. • When depth of the product is less than its diameter, it is known “shallow drawing”. Figure 17-40 Schematic of the deep-drawing process.

47. Deep Drawing and Shallow Drawing • Key variables: • Blank and punch diameter • Punch and die radius • Clearance • Thickness of the blank • Lubrication • Hold-down pressure Figure 17-4 Flow of material during deep drawing. Note the circumferential compression as the radius is pulled inward

48. During drawing, the material is pulled inward, so its circumference decrease. Since the volume of material must be the same, V0 = Vf the decrease in circumferential dimension must be compensated by a increase in another dimension, such as thickness or radial length. Since the material is thin, an alternative is to relieve the circumferential compression by bulking or wrinkling. The wrinkling formation can be suppressed by compressing the sheet between die and blankholder service. Deep Drawing and Shallow Drawing

49. Once a drawing process has been designed and the tooling manufactured, the primary variable for process adjustment is hold-down pressure or blankhoder force. If the force is too low, wrinkling may occur at the start of the stroke. If it is too high, there is too much restrain, and the descending punch will tear the disk or some portion of the already-formed cup wall. Deep Drawing and Shallow Drawing

50. As cup depth increases or material is thin, there is an increased tendency for forming the defects. Thin Thick Good blankholder force Deep Drawing