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Chapter 20 SHEET METALWORKING

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ISE 316 - Manufacturing Processes Engineering
Chapter 20SHEET METALWORKING
Cutting OperationsBending OperationsDrawingOther Sheet Metal Forming OperationsDies and Presses for Sheet Metal ProcessesSheet Metal Operations Not Performed on PressesBending of Tube Stock
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Sheet Metalworking Defined
Cutting and forming operations performed on relatively thin sheets of metalThickness of sheet metal = 0.4 mm (1/64 in) to 6 mm (1/4 in)Thickness of plate stock > 6 mmOperations usually performed as cold working
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Sheet and Plate Metal Products
Sheet and plate metal parts for consumer and industrial products such asAutomobiles and trucksAirplanesRailway cars and locomotivesFarm and construction equipmentSmall and large appliancesOffice furnitureComputers and office equipment
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Advantages of Sheet Metal Parts
High strengthGood dimensional accuracyGood surface finishRelatively low costFor large quantities, economical mass production operations are available
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Sheet Metalworking Terminology
“Punch‑and‑die”Tooling to perform cutting, bending, and drawing“Stamping press”Machine tool that performs most sheet metal operations“Stampings”Sheet metal products
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Three Major Categories ofSheet Metal Processes
CuttingShearing to separate large sheets; or cut part perimeters or make holes in sheetsBendingStraining sheet around a straight axisDrawingForming of sheet into convex or concave shapes
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Figure 20.1 ‑ Shearing of sheet metal between two cutting edges:(1) just before the punch contacts work
CuttingShearing between two sharp cutting edges
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Figure 20.1 ‑ Shearing of sheet metal between two cutting edges:(2) punch begins to push into work, causing plastic deformation
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Figure 20.1 ‑ Shearing of sheet metal between two cutting edges:(3) punch compresses and penetrates into work causing a smooth cut surface
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Figure 20.1 ‑ Shearing of sheet metal between two cutting edges:(4) fracture is initiated at the opposing cutting edges which separates the sheet
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Shearing, Blanking, and Punching
Three principal operations in pressworking that cut sheet metal:ShearingBlankingPunching
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Shearing
Sheet metal cutting operation along a straight line between two cutting edgesTypically used to cut large sheets into smaller sections for subsequent operations
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Figure 20.3 ‑ Shearing operation:side view of the shearing operation(b) front view of power shears equipped with inclined upper cutting blade Symbolvindicates motion
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Blanking and Punching
Blanking- sheet metal cutting to separate piece from surrounding stockCut piece is the desired part, called ablankPunching- sheet metal cutting similar to blanking except cut piece is scrap, called aslugRemaining stock is the desired part
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Figure 20.4 ‑ (a) Blanking and (b) punching
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Clearance in Sheet Metal Cutting
Distance between the punch and dieTypical values range between 4% and 8% of stock thicknessIf too small, fracture lines pass each other, causing double burnishing and larger forceIf too large, metal is pinched between cutting edges and excessive burr results
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Clearance in Sheet Metal Cutting
Recommended clearance can be calculated by:c =atwhere c = clearance;a= allowance; andt= stock thicknessAllowanceais determined according to type of metal
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AllowanceaforThree Sheet Metal Groups
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Punch and Die Sizes forBlanking and Punching
For a roundblankof diameterDb:Blanking punch diameter =Db‑ 2cBlanking die diameter =Dbwherec= clearanceFor a roundholeof diameterDh:Hole punch diameter = DhHole die diameter = Dh+ 2cwherec= clearance
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Figure 20.6 ‑ Die size determines blank sizeDb; punch size determines hole sizeDh.;c= clearance
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Angular ClearancePurpose: allows slug or blank to drop through dieTypical values: 0.25to 1.5on each side
Figure 20.7 ‑ Angular clearance
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Cutting Forces
Important for determining press size (tonnage)F = S t Lwhere S = shear strength of metal;t= stock thickness, and L = length of cut edge
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BendingStraining sheetmetal around a straight axis to take a permanent bend
Figure 20.11 ‑ (a) Bending of sheet metal
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Metal on inside of neutral plane is compressed, while metal on outside of neutral plane is stretched
Figure 20.11 ‑ (b) both compression and tensile elongation of the metal occur in bending
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Types of Sheetmetal Bending
V‑bending- performed with a V‑shaped dieEdge bending- performed with a wiping die
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V-BendingFor low productionPerformed on apress brakeV-dies are simple and inexpensive
Figure 20.12 ‑(a) V‑bending
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Edge BendingFor high productionPressure pad requiredDies are more complicated and costly
Figure 20.12 ‑ (b) edge bending
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Stretching during Bending
If bend radius is small relative to stock thickness, metal tends to stretch during bendingImportant to estimate amount of stretching, so that final part length = specified dimensionProblem: to determine the length of neutral axis of the part before bending
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Bend Allowance Formula
whereBA= bend allowance;A= bend angle;R= bend radius;t= stock thickness; andKbais factor to estimate stretchingIf R < 2t,Kba= 0.33If R2t,Kba= 0.50
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Springback in Bending
Springback= increase in included angle of bent part relative to included angle of forming tool after tool is removedReason for springback:When bending pressure is removed, elastic energy remains in bent part, causing it to recover partially toward its original shape
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Figure 20.13 ‑ Springback in bending shows itself as a decrease in bend angle and an increase in bend radius: (1) during bending, the work is forced to take the radiusRband included angleAb'of the bending tool (punch in V‑bending), (2) after punch is removed, the work springs back to radiusRand angleA'
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Bending Force
Maximum bending force estimated as follows:
whereF= bending force;TS= tensile strength of sheet metal;w= part width in direction of bend axis; andt= stock thickness. For V- bending,Kbf= 1.33; for edge bending,Kbf= 0.33
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Figure 20.14 ‑ Die opening dimension D: (a) V‑die, (b) wiping die
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Drawing
Sheet metal forming to make cup‑shaped, box‑shaped, or other complex‑curved, hollow‑shaped partsSheet metal blank is positioned over die cavity and then punch pushes metal into openingProducts: beverage cans, ammunition shells, automobile body panels
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Figure 20.19 ‑Drawing of a cup‑shaped part:start of operation before punch contacts worknear end of stroke(b) Corresponding workpart:(1) starting blank(2) drawn part
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Clearance in Drawing
Sides of punch and die separated by a clearancecgiven by:c= 1.1twheret= stock thicknessIn other words, clearance = about 10% greater than stock thickness
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Drawing RatioDR
where Db= blank diameter; and Dp= punch diameterIndicates severity of a given drawing operationUpper limit = 2.0
Most easily defined for cylindrical shape:
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Reductionr
Again, defined for cylindrical shape:
Value ofrshould be less than 0.50
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Thickness‑to‑Diameter Ratio
Thickness of starting blank divided by blank diameterThickness-to-diameter ratio =t/DbDesirable fort/Dbratio to be greater than 1%Ast/Dbdecreases, tendency for wrinkling increases
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Blank Size Determination
For final dimensions of drawn shape to be correct, starting blank diameterDbmust be rightSolve forDbby setting starting sheet metal blank volume = final product volumeTo facilitate calculation, assume negligible thinning of part wall
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Shapes other than Cylindrical Cups
Square or rectangular boxes (as in sinks),Stepped cups,Cones,Cups with spherical rather than flat bases,Irregular curved forms (as in automobile body panels)Each of these shapes presents its own unique technical problems in drawing
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Other Sheet Metal Forming on Presses
Other sheet metal forming operations performed on conventional pressesOperations performed with metal toolingOperations performed with flexible rubber tooling
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IroningMakes wall thickness of cylindrical cup more uniformExamples: beverage cans and artillery shells
Figure 20.25 ‑ Ironing to achieve a more uniform wall thickness in a drawn cup: (1) start of process; (2) during processNote thinning and elongation of walls
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EmbossingUsed to create indentations in sheet, such as raised (or indented) lettering or strengthening ribs
Figure 20.26 ‑ Embossing: (a) cross‑section of punch and die configuration during pressing; (b) finished part with embossed ribs
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Figure 20.28 ‑ Guerin process: (1) before and (2) afterSymbolsvandFindicate motion and applied force respectively
Guerin Process
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Advantages of Guerin Process
Low tooling costForm block can be made of wood, plastic, or other materials that are easy to shapeRubber pad can be used with different form blocksProcess attractive in small quantity production
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Dies for Sheet Metal Processes
Most pressworking operations performed with conventionalpunch‑and‑dietoolingCustom‑designed for particular partThe termstamping diesometimes used for high production dies
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Figure 20.30 ‑ Components of a punch and die for a blanking operation
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Figure 20.31 ‑Progressive die;associated strip development
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Figure 20.32 ‑ Components of a typical mechanical drive stamping press
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Types of Stamping Press Frame
Gap frame– configuration of the letter C and often referred to as aC‑frameStraight‑sided frame– box-like construction for higher tonnage
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Figure 20.33 ‑ Gap frame press for sheet metalworking(photo courtesy of E. W. Bliss Company)Capacity = 1350 kN (150 tons)
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Figure 20.34 ‑Press brake with bed width of 9.15 m (30 ft) and capacity of 11,200 kN (1250 tons); two workers are positioning plate stock for bending(photo courtesy of Niagara Machine & Tool Works)
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Figure 20.35 ‑ Several sheet metal parts produced on a turret press, showing variety of hole shapes possible(photo courtesy of Strippet, Inc.)
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Figure 20.36 ‑ Computer numerical control turret press(photo courtesy of Strippet, Inc.)
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Figure 20.37 ‑Straight‑sided frame press(photo courtesy Greenerd Press & Machine Company, Inc.)
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Power and Drive Systems
Hydraulic presses - use a large piston and cylinder to drive the ramLonger ram stroke than mechanical typesSuited to deep drawingSlower than mechanical drivesMechanical presses – convert rotation of motor to linear motion of ramHigh forces at bottom of strokeSuited to blanking and punching
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Sheet Metal OperationsNot Performed on Presses
Stretch formingRoll bending and formingSpinningHigh‑energy‑rate forming processes.
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Stretch FormingSheet metal is stretched and simultaneously bent to achieve shape change
Figure 20.39 ‑ Stretch forming: (1) start of process; (2) form die is pressed into the work with forceFdie, causing it to be stretched and bent over the form.F= stretching force
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Force Required in Stretch Forming
whereF= stretching force;L= length of sheet in direction perpendicular to stretching;t= instantaneous stock thickness; andYf= flow stress of work metalDie forceFdiecan be determined by balancing vertical force components
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Roll BendingLarge metal sheets and plates are formed into curved sections using rolls
Figure 20.40 ‑ Roll bending
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Roll FormingContinuous bending process in which opposing rolls produce long sections of formed shapes from coil or strip stock
Figure 20.41 ‑ Roll forming of a continuous channel section:straight rollspartial formfinal form
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Spinning
Metal forming process in which an axially symmetric part is gradually shaped over a rotating mandrel using a rounded tool or rollerThree types:Conventional spinningShear spinningTube spinning
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Figure 20.42 ‑ Conventional spinning: (1) setup at start of process; (2) during spinning; and (3) completion of process
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High‑Energy‑Rate Forming (HERF)
Processes to form metals using large amounts of energy over a very short timeHERF processes include:Explosive formingElectrohydraulic formingElectromagnetic forming
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Explosive Forming
Use of explosive charge to form sheet (or plate) metal into a die cavityExplosive charge causes a shock wave whose energy is transmitted to force part into cavityApplications: large parts, typical of aerospace industry
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Figure 20.45 ‑ Explosive forming:(1) setup, (2) explosive is detonated, and(3) shock wave forms part and plume escapes water surface
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Electromagnetic Forming
Sheet metal is deformed by mechanical force of an electromagnetic field induced in workpart by an energized coilPresently the most widely used HERF processApplications: tubular parts
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Figure 20.47 ‑ Electromagnetic forming: (1) setup in which coil is inserted into tubular workpart surrounded by die; (2) formed part

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Chapter 20 SHEET METALWORKING