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Sheet metal Forming Process
Sheet metal stamping was developed in the 1890s for mass production of bicycles, playing an important role in making interchangeable parts economical. Basic sheet forming processes include shearing, bending, drawing, and involve tools like shear presses, brake presses, and finger presses. Material selection is critical, balancing formability with strength, weight, cost, and corrosion resistance. Stretch forming allows tighter tolerances than stamping but is difficult for complex shapes. New developments include tailored blanks, binder force control, and quick die exchange. Alternative auto body materials offer cost and environmental benefits compared to steel.
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Sheet metal Forming Process
- 1. Sheet Metal Forming 2.810Fall 2002 Professor Tim Gutowski Minoan gold pendant of bees encircling the Sun, showing the use of granulation, from a tomb at Mallia, 17th century BC. In the Archaeological Museum, Iráklion, Crete.
- 2. Historical Note; Sheet metalstamping was developed as a mass production technology for the production of bicycles around the 1890’s. This technology played an important role in making the system of interchangeable parts economical (perhaps for the first time).
- 3. Steps in makingHub Steps in Sprocket making
- 4. Stress Strain diagram– materials selection
- 5. Basic Sheet FormingProcesses (from http://www.menet.umn.edu/~klamecki/Forming/mainforming.html) Shearing Bending Drawing
- 6. Shear and cornerpress
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- 9. Shearing Operation Force Requirement Die Sheet PunchT D Part or slug F = 0.7 T L (UTS) T = Sheet Thickness L = Total length Sheared UTS = Ultimate Tensile Strength of material
- 10. Yield Criteria σ τ Y Y/2 Tresca Mises τmax = (2/3)1/2 Yτ max = (1/2) Y
- 11. Schematic of aBlanked Edge
- 12. Bending Force Requirement Punch WorkpieceT Die L Force T = Sheet Thickness W = Total Width Sheared (into the page) L =Span length UTS = Ultimate Tensile Strength of material Engineering Strain during Bending: e = 1/((2R/T) + 1) R = Bend radius Minimum Bend radius: R = T ((50/r) – 1) r = tensile area reduction in percent )( 2 UTS L WT F =
- 13. Stress distribution throughthe thickness of the part σ σ yY Y -Y σ h -Y Y Elastic Elastic-plastic Fully plastic
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- 16. Stretch Forming Loading Pre-stretching WrappingRelease * source: http://www.cyrilbath.com/sheet_process.html
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- 19. Stretch Forming Force Requirement F= (YS + UTS)/2 * A F = stretch forming force (lbs) YS = material yield strength (psi) UTS = ultimate tensile strength of the material (psi) A = Cross-sectional area of the workpiece (in2 ) • Example of Force Calculation Calculate the force required to stretch form a wing span having a cross- sectional area of .50X120” made from 2219 aluminum alloy having a yield strength of 36,000 psi and a UTS of 52,000 psi: F = 88000/2 * 60 = 2,640,000 lbs = 1320 tons Calculate the force required to shear a 10” diameter, 1/8” thick blank from mild steel with a UTS of 45,000 psi: F = 0.7 (.125)(π)(10) 45,000 = 62 tons
- 20. Auto body panels 10- 11 panels •3 to 5 dies each • ~$0.5M each • ~$20M investment
- 21. Tooling for AutomotiveStamping
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- 23. Material Selection Material selectionis critical in both product and process design. Formability is the central material property. This property must be balanced with other product and process considerations such as strength, weight, cost, and corrosion resistance. Auto vs. Aerospace Example Auto Body Panel Airplane Body Panel Progressive stamping stretch forming 1010 Steel, cold-rolled 2024 Aluminum, T3 temper .04” sheet, custom order .08” sheet, oversize Double-sided Zinc clad mechanically polished Cost ~ $.35-.45/lb Cost ~ $4.0/lb UTS ~ 300 MPa UTS ~ 470 MPa YS ~ 185 MPa YS ~ 325 MPa Elongation ~ 42% Elongation ~ 20% n = .26 n = .16
- 24. Comparison of representative Parts:Aero and Auto Auto Aero Part Description Body Panel Body Panel 54"X54" 54"X54" Forming Process Progressive Stamping Stretch Forming MATERIAL Material 1010 Steel, cold-rolled, .04" sheet, custom order double-sided Zinc clad 2024 Aluminum, T3 temper, .08" sheet, oversize mechanically polished Scrap 40% 20% Material Cost $0.45/lb $4.00/lb Per part $15.75 $105.00 LABOR Set-up Time 1.5hr 1.0hr Parts/Run 2,000 30 Cycle Time 0.25 min 2.5 min Total Labor 0.30 min 4.5 min Labor Rate** $20.00/hr $20.00/hr Stretch-Form Labor Cost $0.10 $1.50 FIXED Equipment $5,000,000 $1,000,000 Tools/Dies $900,000 $45,000 (200 manhours labor) TOTAL TRANSFER COST $25 $265
- 25. Parts Received Mylar Protection Applied ‘Burr’ Edges intension Stretch Forming Index to Block ‘Burr’ Edges and Inspect Hand Trim Chemica l Milling Aerospace Stretch Forming Body Panel Process Clad and Prime Surfaces Process Flow for Automobile Door Stamping Operation Raw material Blank material starting dimensions Drawing Pierce FlangeRestrike
- 26. Design: Stretch Formingvs. Stamping Stretch Forming Advantages over Stamping Tighter tolerances are possible: as tight as .0005 inches on large aircraft parts Little problem with either wrinkling or spring back Large, gently contoured parts from thin sheets Stretch forming Disadvantages over Stamping Complex or sharply cornered shapes are difficult or impossible to form Material removal – blanking, punching, or trimming – requires secondary operations Requires special preparation of the free edges prior to forming
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- 28. Elastic Springback Analysis L x y h b 1.Assume plane sections remain plane: εy = - y/ρ (1) 2. Assume elastic-plastic behavior for material M ρ = 1/K M y σ ε E εy σY σ= E ε ε <ε σ= σY ε >ε
- 29. M 1/ρ EI 1/ρY MY Loading EI Unloading 1/R01/R1 3. Wewant to construct the following Bending Moment “M” vs. curvature “1/ρ” curve Springback is measured as 1/R0 – 1/R1 (2) Permanent set is 1/R1
- 30. 4. Stress distributionthrough the thickness of the beam σ σ yY Y -Y σ h -Y Y Elastic Elastic-plastic Fully plastic
- 31. 5. M =∫A σ y dA Elastic region At the onset of plastic behavior σ = - y/ρ E = - h/2ρ E = -Y (4) σ Y This occurs at 1/ρ = 2Y / hE = 1/ρY (5) dσ y dA b h dy Substitution into eqn (3) gives us the moment at on-set of yield, MY MY = - EI/ρY = EI 2Y / hE = 2IY/h (6) After this point, the M vs 1/r curve starts to “bend over.” Note from M=0 to M=MY the curve is linear. ρρ σ EI dA y EydAM −=−== ∫ ∫ 2 (3)
- 32. In the elastic– plastic region σ yY Y Ybyy h Yb y b y Yy Yb Ybydy y y YbydyybdyM YY y Y h y h y y Y Y Y Y Y 22 2 0 32/2 2/ 0 3 2 ) 4 ( 3 2 2 2 22 +−= += +== ∫ ∫ ∫σ −= 22 2/3 1 1 4 h y Y bh M Y Note at yY=h/2, you get on-set at yield, M = MY And at yY=0, you get fully plastic moment, M = 3/2 MY (7)
- 33. To write thisin terms of M vs 1/ρ rather than M vs yY, note that the yield curvature (1/ρ)Y can be written as (see eqn (1)) 2/ 1 h Y Y ε ρ = (8) Where εY is the strain at yield. Also since the strain at yY is -εY, we can write Y Y y ε ρ = 1 (9) Combining (8) and (9) gives ρ ρ 1 )1( 2/ YY h y = (10)
- 34. Substitution into (7)gives the result we seek: −= 2 1 )1( 3 1 1 2 3 ρ ρ Y YMM (11) M 1/ρ EI 1/ρY MY Loading EI Unloading 1/R01/R1 Eqn(11) Elastic unloading curve −= 1 11 )1( R M M Y Y ρρ (12)
- 35. Now, eqn’s (12)and (13) intersect at 1/ρ = 1/R0 Hence, −= − 2 010 1 )1( 3 1 1 2 311 )1( R M RR M Y Y Y Y ρ ρ Rewriting and using 1/ρ = 2Y / hE, we get 3 2 0 10 43 11 −= − hE Y R hE Y RR (13)
- 37. New developments Tailored blanks Binderforce control Segmented dies Quick exchange of dies Alternative materials; cost issues
- 38. - SHAPE MEASUREMENT SHAPE CONTROLLER WORKPIECE desired shape + shape error finished part DISCRETEDIE SURFACE DISCRETE DIE FORMINGPRESS CONTROLLER TRACING CMM Part Error Die Shape Change New Part Shape The Shape Control Concept
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- 40. 60 Ton MatchedDiscrete Die Press(Robinson et al, 1987) Tool Setup Actuators Programmable Tool Passive Tool Press Motion
- 41. Cylindrical Part Error Reduction 0 10 20 30 40 50 60 P1P2 P3 P4 PART CYCLE 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 RMSError[x0.001in.] MAX RMS SSYYSSTTEEMM EERRRROORR TTHHRREESSHHOOLLDD MAXIMALSHAPEERROR [x0.001in.]
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- 43. Stretch Forming withReconfigurable Tool @ Northrop Grumman
- 44. Stamping and TPS: QuickExchange of Dies Ref. Shigeo Shingo, “A Revolution in Manufacturing: The SMED System” Productivity Press. 1985 •Simplify, Organize, Standardize, •Eliminate Adjustments, •Convert Internal to External Set-Ups
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- 46. Alternative materials forauto body panels
- 47. Comparison Steel Vs SMC $0.35/lb 0.03thick 7.6 lb 40% scrap $4.25 mat’l cost 400/hr 5 workers $18.90/hr (Union) $0.24 labor cost $5,000,000 equipment $900,000 tools $7.71 unit cost at 100,000 units $0.65/lb .0.12 thick 7.0 lb 6% scrap $4.84 mat’l cost 40/hr $12.50/hr (non-Union) $0.63 labor cost $1,200,000 eqipment $250,000 tools $7.75 unit cost at 100,000 units Ref John Busch
- 48. Cost comparison betweensheet steel and plastics and composites for automotive panels ref John Busch
- 49. Environment punching Vs machining hydraulicfluids and lubricants scrap energy painting, cleaning
- 50. Steel can productionat Toyo Seikan See Appendix D; http://itri.loyola.edu/ebm/
- 51. Summary Note on HistoricalDevelopment Materials and Basic Mechanics Aerospace and Automotive Forming New Developments Environmental Issues Solidworks and Metal Forming your Chassis
- 52. Readings 1. “Sheet MetalForming” Ch. 16 Kalpakjian (3rd ed.) 2. “Economic Criteria for Sensible Selection of Body Panel Materials” John Busch and Jeff Dieffenbach 3. Handout from Shigeo Shingo, The SMED System 4. “Steps to Building a Sheet Metal Chassis for your 2.810 Car Using Solidworks”, by Eddy Reif 5. “Design for Sheetmetal Working”, Ch. 9 Boothroyd, Dewhurst and Knight
Editor's Notes
- #42 Max and RMS error results Nearly identical results were found for spherical and saddle shaped parts