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

17.1 Introduction 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

17.2 Shearing Operations 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

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

Shearing Operations

Classification of Metal forming Operations

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.)

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.)

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.

Fine Blanking Operations Fine Blanking - the piece being punched out becomes the workpiece and pressure pads are used to smooth edges in shearing 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.

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

Types of Piercing and Blanking 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

Sheet-metal Cutting Operations

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

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

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.

Blanking Operations 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.

Design for Piercing and Blanking 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 Clearance

Clearance Calculation 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”.

Design Die and Punch Sizes 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

Cutting Forces 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.

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

Example for Calculating Clearance and Force 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?

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.

Example of Bending

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).

Considerations for 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.

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.

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.

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”.

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

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

17.4 Drawing and Stretching Processes 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 depth more 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 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 Spinning 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

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

Spinning

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

Spinning 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.

Shear Forming 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%

Shearing Forming

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

Reverse Shear Forming 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.

Deep Drawing and Shallow Drawing Drawing is 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.

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

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

Limitations of Deep Drawing Wrinkling and tearing are typical limits to drawing operations Trimming may be used to reach final dimensions Figure 17-45 Pierced blanked, and drawn part before and after trimming

Defects in Drawing Parts

Deep Drawing Two alternatives for converting drawn parts into deeper cups: Forward redraw – the material undergoes reverse bending as it flows into the die. Reverse redraw – the stating cup is placed over a tubular die, and the punch acts to turn it inside out. Cup redrawing to further reduce diameter and increase wall height. (Left) forward redraw; (right) reverse redraw.

Properties of Sheet Material Tensile strength of the material is important in determining which forming operations are appropriate. Sheet metal is often anisotropic- properties vary with direction or orientation. A metal with low-yield, high-tensile, and high-uniform elongation has a good mechanical property for sheet-forming operations. Majority of failures during forming occur due to thinning or fracture Strain analysis can be used to determine the best orientation for forming

Engineering Analysis of Drawing

Engineering Analysis of Drawing

Engineering Analysis of Drawing It is important to assess the limitation of the amount of drawing that can be accomplished. Measures of Drawing: 1) Drawing ratio (cylinder) DR = Db/Dp Where Db – blank diameter, Dp – punch diameter The greater the ratio, the more severe is the drawing. An approximate upper limit on the drawing ratio is a value of 2.0. The actual limiting value for a given drawing depends on punch and die corner radii (Dp and Dd), friction conditions, depth of draw, and characteristics of the sheet metal (ductility, degree of directinality of strength in the metal).

Engineering Analysis of Drawing 2) Reduction r (another way to characterize a given drawing) r = (Db - Dp )/Db It is very closely related to drawing ratio. Consistent with Dr <= 2.0, the value of r should be less than 0.5. 3) Thickness-to-diameter ratio: t/Db Where t – thickness of the starting blank, Db – blank diameter. The ratio t/Db is greater than 1%. As t/Db decreases, tendency for wrinkling increases. If DR , r, t/Db are exceeded by the design, blank must be draw in two or more steps, sometimes with annealing between steps.

Engineering Analysis of Drawing Example: Cup Drawing For a cylindrical cup with inside diameter = 3.0” and height = 2.0”, its starting blank size Db = 5.5”, and its thickness t = 3/32”, please indicate its manufacturing feasibility. Solution: DR = Db/Dp = 5.5/3.0 = 1.833 <2.0 r = (Db - Dp )/Db = (5.5 – 3.0)/5.5 = 45.45% < 50% t/Db = (3/32)/5.5 = 0.017 > 1% So the drawing operation is feasible.

Engineering Analysis of Drawing Drawing Force F = pDpt(TS)(Db/Dp – 0.7) Where F – drawing force, lb(N); t – thickness of blank, in. (mm); TS - tensile strength, ib/in2 (Mpa); Db and D p – starting blank diameter and punch diameter, in. (mm). 0.7 – a correction factor for friction. The equation is the estimation of the maximum force in the drawing. The drawing force varies throughout the downward movement of the punch, usually reaching its maximum value at about one-third the length of the punch stroke. Clearance c: about 10% than the stock thickness (t) c = 1.1 t

Engineering Analysis of Drawing Holding Force Fh = 0.015Yp[Db2 – (Dp + 2.2t + 2Rd)2] Where Fh – holding force in drawing, ib (N); Y – yield strength of the sheet metal, lb/in2 (Mpa); t – starting stock thickness, in. (mm); Rd – die conner radius, in. (mm). The holding force is usually about one-third the drawing force [1]. [1]: Wick, C., et al., “Tool and Manufacturing Engineers, 4th ed. Vol. II.

Engineering Analysis of Drawing Example Forces in Drawing Determine the (a) drawing force, and (2) holding force for the case in previous example for feasibility, where tensile strength of the metal = 70,000 lb/in 2 and yield strength = 40,000 lb/in 2 , the die corner radius = 0.25”. Solution: (a) F = pDpt(TS)(Db/Dp – 0.7) =p(3.0)(3/32)(70,000)(5.5/3.0 – 0.7) =70,097 lb (b) Fh = 0.015Yp[Db2 – (Dp + 2.2t + 2Rd)2] = 0.015(40,000)p{5.52 – [3.0 + 2.2(3/32) + 2(0.25)]2} = 1,885 (30.25 – 13.74) = 31,121 lb

Engineering Analysis of Drawing Blank Size Determination Assume that the volume of the final product is the same as the that of the starting sheet-metal blank and the thinning of the part wall is negligible. For a cup with its height H and the same diameters Dp in the bottom and top: pDb2/4 = pDp2/4 + pDp H, and Db = SQRT(Dp2 + 4Dp H)

Summary Sheet forming processes can be grouped in several broad categories Shearing Bending Drawing Basic sheet forming operations involve a press, punch, or ram and a set of dies Material properties, geometry of the starting material, and the geometry of the desired final product play important roles in determining the best process

HW (Not required to turn in) Review questions (page 457 - 458): 1, 2, 10, 20, 22, 26