Chapter 17Chapter 17

Sheet Forming ProcessesSheet Forming Processes(Review)(Review)

EIN 3390 Manufacturing ProcessesEIN 3390 Manufacturing ProcessesSummer A, 2011Summer A, 2011

17.1 Introduction17.1 IntroductionSheet 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 Operations17.2 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 OperationsShearing OperationsFracture 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

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

Shearing Operations

Shearing OperationsShearing Operations

Classification of Metal forming Classification of Metal forming OperationsOperations

Types of ShearingTypes of ShearingSimple 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.)

Piercing and BlankingPiercing and BlankingPiercing 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 OperationsFine Blanking OperationsFine 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.

Types of Piercing and BlankingTypes of Piercing and BlankingLancing- piercing operation that forms

either a line cut or holePerforating- piercing a large number

of closely spaced holesNotching- removes segments from

along the edge of an existing productNibbling- a contour is progressively cut by

producing a series of overlapping slits or notches

Sheet-metal Cutting OperationsSheet-metal Cutting Operations

Types of Piercing and BlankingTypes of Piercing and BlankingShaving- 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

Tools and Dies for Piercing and Tools and Dies for Piercing and BlankingBlanking

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 OperationsBlanking 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 BlankingDesign for Piercing and BlankingDesign 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 ClearanceDesign Clearance

Clearance CalculationClearance CalculationThe recommended clearance is:

C = atWhere 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 SizesDesign Die and Punch Sizes

For a round blank of diameter Db is determined as:

Blank punch diameter = Db - 2cBlank die diameter = Db

For a round hole (piercing) of diameter Dh is determined as:

Hole punch diameter = Dh

Hole die diameter = Db + 2c

Cutting ForcesCutting ForcesCutting forces are used to determine size of the

press needed.

F = StLWhere 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 ClearanceAngular 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 Example for Calculating Clearance and Forceand ForceRound 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 = Db = 3.14(3) = 9.426”

F = 45,000(9.426)(0.125) = 53,021 = 24.07 tonsWhat is the difference if the operation is a piercing?

17.3 Bending17.3 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 BendingExample of Bending

Design for BendingDesign for BendingSeveral 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).

ConsiderationsConsiderations for Bendingfor 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 ConsiderationsDesign ConsiderationsDetermine the dimensions of a flat blank that will

produce a bent part of the desired precisionMetal 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 BendingEngineering 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 = 2A(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 BendingEngineering 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”.

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

Engineering Analysis of 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 = 2A(R + Kbat)/360 =260(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

Example for Sheet-metal Bending

17.4 Drawing and Stretching 17.4 Drawing and Stretching ProcessesProcesses

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 17.4 Drawing and Stretching ProcessesProcessesTypes 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 Spinning17.4 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

SpinningSpinning

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

SpinningSpinning

SpinningSpinning

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

SpinningSpinning

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 FormingShear FormingShear forming is a version of spinningA modification of the spinning process in which

each element of the blank maintains its distance from the axis of rotation.

No circumferential shrinkageWall thickness of product, tc will vary with the

angle of the particular region:tc = tb(sin

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 FormingShearing Forming

Direct Shear FormingDirect Shear Forming

Figure 17-36 Schematic representation of the basic shear-forming process.

Material being formed moves in the same direction as the roller

Reverse Shear FormingReverse 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 Deep Drawing and Shallow DrawingDrawing 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 Deep Drawing and Shallow DrawingDrawing Key variables:

◦ Blank and punch diameter

◦ Punch and die radius

◦ Clearance◦ Thickness of the

blank◦ Lubrication◦ Hold-down

pressureFigure 17-4 Flow of material during deep drawing. Note the circumferential compression as the radius is pulled inward

Deep Drawing and Shallow DrawingDeep 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 DrawingDeep 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 DrawingDeep Drawing

As cup depth increases or material is thin, there is an increased tendency for forming the defects.

Th

inT

hic

k

Good blankholder force

Deep DrawingDeep Drawing

Cup redrawing to further reduce diameter and increase wall height. (Left) forward redraw; (right) reverse redraw.

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.

Properties of Sheet MaterialProperties of Sheet MaterialTensile 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 DrawingEngineering Analysis of Drawing

Engineering Analysis of DrawingEngineering Analysis of Drawing

Engineering Analysis of DrawingEngineering Analysis of DrawingIt 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 diameterThe 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 DrawingEngineering Analysis of Drawing2) 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 DrawingEngineering Analysis of DrawingExample: Cup DrawingFor 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 <2.0r = (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 DrawingEngineering Analysis of DrawingDrawing Force

F = Dpt(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 DrawingEngineering Analysis of DrawingHolding Force

Fh = 0.015Y[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 DrawingEngineering Analysis of DrawingExample 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 = Dpt(TS)(Db/Dp – 0.7)

=(3.0)(3/32)(70,000)(5.5/3.0 – 0.7) =70,097 lb

(b) Fh = 0.015Y[Db2 – (Dp + 2.2t + 2Rd)2]

= 0.015(40,000){5.52 – [3.0 + 2.2(3/32) + 2(0.25)]2}= 1,885 (30.25 – 13.74) = 31,121 lb

Engineering Analysis of DrawingEngineering Analysis of DrawingBlank 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:

Db2/4 = Dp

2/4 + Dp H, and

Db = SQRT(Dp2 + Dp

H)

SummarySummarySheet 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 HW ((Not required to turn inNot required to turn in))

Review questions (page 457 - 458): 1, 2, 10, 20, 22, 26