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# Sheet metal Forming Process

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Sheet metal Forming Process & Methods of Forming

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• Max and RMS error results
Nearly identical results were found for spherical and saddle shaped parts
• ### Sheet metal Forming Process

1. 1. Sheet Metal Forming 2.810 Fall 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. 2. Historical Note; Sheet metal stamping 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. 3. Steps in making Hub Steps in Sprocket making
4. 4. Stress Strain diagram – materials selection
5. 5. Basic Sheet Forming Processes (from http://www.menet.umn.edu/~klamecki/Forming/mainforming.html) Shearing Bending Drawing
6. 6. Shear and corner press
7. 7. Brake press
8. 8. Finger press
9. 9. Shearing Operation Force Requirement Die Sheet Punch T D Part or slug F = 0.7 T L (UTS) T = Sheet Thickness L = Total length Sheared UTS = Ultimate Tensile Strength of material
10. 10. Yield Criteria σ τ Y Y/2 Tresca Mises τ max = (2/3)1/2 Yτ max = (1/2) Y
11. 11. Schematic of a Blanked Edge
12. 12. Bending Force Requirement Punch Workpiece T 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. 13. Stress distribution through the thickness of the part σ σ yY Y -Y σ h -Y Y Elastic Elastic-plastic Fully plastic
14. 14. Springback •Over-bend •Bottom •Stretch
15. 15. tension compression Pure Bending Bending & Stretching
17. 17. Stretch Forming
18. 18. Stretch forming
19. 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. 20. Auto body panels 10 - 11 panels •3 to 5 dies each • ~\$0.5M each • ~\$20M investment
21. 21. Tooling for Automotive Stamping
22. 22. Machines
23. 23. Material Selection Material selection is 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. 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. 25. Parts Received Mylar Protection Applied ‘Burr’ Edges in tension 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. 26. Design: Stretch Forming vs. 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
27. 27. Springback
28. 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. 29. M 1/ρ EI 1/ρY MY Loading EI Unloading 1/R01/R1 3. We want 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. 30. 4. Stress distribution through the thickness of the beam σ σ yY Y -Y σ h -Y Y Elastic Elastic-plastic Fully plastic
31. 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. 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. 33. To write this in 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. 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. 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)
36. 36. New developments Tailored blanks Binder force control Segmented dies Quick exchange of dies Alternative materials; cost issues
37. 37. - SHAPE MEASUREMENT SHAPE CONTROLLER WORKPIECE desired shape + shape error finished part DISCRETEDIE SURFACE DISCRETE DIE FORMING PRESS CONTROLLER TRACING CMM Part Error Die Shape Change New Part Shape The Shape Control Concept
38. 38. Conventional Tooling Tool Pallet Parking Lot
39. 39. 60 Ton Matched Discrete Die Press(Robinson et al, 1987) Tool Setup Actuators Programmable Tool Passive Tool Press Motion
40. 40. Cylindrical Part Error Reduction 0 10 20 30 40 50 60 P1 P2 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.]
41. 41. Large Scale Tool 6 feet
42. 42. Stretch Forming with Reconfigurable Tool @ Northrop Grumman
43. 43. Stamping and TPS: Quick Exchange 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
44. 44. Standard fixtures
45. 45. Alternative materials for auto body panels
46. 46. Comparison Steel Vs SMC \$0.35/lb 0.03 thick 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
47. 47. Cost comparison between sheet steel and plastics and composites for automotive panels ref John Busch
48. 48. Environment punching Vs machining hydraulic fluids and lubricants scrap energy painting, cleaning
49. 49. Steel can production at Toyo Seikan See Appendix D; http://itri.loyola.edu/ebm/
50. 50. Summary Note on Historical Development Materials and Basic Mechanics Aerospace and Automotive Forming New Developments Environmental Issues Solidworks and Metal Forming your Chassis
51. 51. Readings 1. “Sheet Metal Forming” 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