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SHEET METAL FORMING
VINAY
Other Cutting OperationsOther Cutting Operations
• Cutoff
– Each cut is a new part
• Parting
– 2 cutting edges
– Less effi...
Sheet Metal Forming
• AGENDAAGENDA
Introduction
Sheet Metal characteristics
Shearing
Piercing and Blanking
Die types
...
1.0 Introduction
 Sheet metal is simply metal formed into thin and flat pieces.
 Sheet metal is essentially metal presse...
Sheet Metal Forming
How to differentiate a sheet and a plate ?
• If thickness is less than 6 mm (1/ 4 inches) then
it is r...
Applications of sheet metals
• Aircraft Bodies
• Automobiles bodies
• Utensils used for domestic purposes
• Beverage cans
Sheet Metal Operations
• Shearing
• Blanking
• Bending
• Stretch forming
• Deep drawing
• Redrawing etc
2.0 Sheet metal characteristics
• Sheet metal is characterized by high ratio of
surface area to thickness.
• Forming is ge...
Elongation
A test to measure the ductility of a material. When a
material is tested for tensile strength it elongates a
ce...
LUEDER’S BAND or STRAIN MARKS
ANISOTROPY
• They may be two types i.e. crystallographic
anisotropy (grain orientation) and mechanical
fibering (alignment...
Anisotropy
• Planar anisotropy R’ is given by
R’ = (R0 + 2R45 + R90) / 4
where, subscripts 0, 45, 90 refer to angular
orie...
Residual stresses :
• Residual stresses can develop in sheet
metal forming due to non uniform
deformation that take place....
Spring back
• Because sheet metal generally are thin and are subjected to
relatively small strains during forming, sheet m...
Spring back (wiping die)
Wrinkling
• Compressive stresses are formed in plane of the
sheet results in wrinkling (buckling).
The tendency of wrinkli...
SHEARING
• Shearing is a process for cutting sheet
metal to size out of a larger stock such as
roll stock.
Shearing Machine
Shearing
• Material thickness ranges from 0.125 mm to
6.35 mm (0.005 to 0.250 in). The dimensional
tolerance ranges from ±...
Variables affecting shearing
Shear force
• Shear force is basically the product of shear
strength of sheet metal and the c...
Engineering Analysis (cont.)Engineering Analysis (cont.)
• Cutting Forces (F)
– F = S * t * L
• S - shear strength
• t - t...
HARDNESS, BURR, RADIUS ON
CUTTING SURFACES
• There should be optimum hardness, more than
required hardness would make it b...
CLEARANCE = THE MEASURED SPACE
BETWEEN THE MATING MEMBERS OF A
DIE SET ( C )
P
d
d1
C / 2C / 2
D D
Clearance between Punch...
Engineering AnalysisEngineering Analysis
• Clearance (C)
– Distance between punch and die
– 4%-8% of sheet thickness
– Sma...
CLEARANCE = C = d – d1
therefore CLEARANCE PER SIDE = C= C / 2
ALSO CLEARANCE PER SIDE C / 2 IS GIVEN
BY
c /2 = 0.01* s* s...
Clearance – its effects
• Greater clearance – rough edges, formation of
burr, material is pulled rather being cut
• Smalle...
• Tool and die materials : generally steels are
used, ex:- Die steel, HSS for most common
operations.
• For higher product...
• Die cutting
• Slitting
• Nibbling
• Steel rules
DIE CUTTING
• It is combination of following operations
• Perforating.
• Parting or shearing a sheet into two.
• Notching ...
SLITTING
• Shearing operation –using circular blades
• These blades follow straight line or circular path or
curved path –...
NIBBLING
• Nibbler (tool) move straight up and moves down
rapidly.
• Sheet metal is fed in gap between two cutting
tools a...
PIERCING
• It is a shearing operation.
• Creates open hole in sheet metal by
separating the interior section.
• Removed me...
BLANKING:
• It is also a shearing operation.
• It enlarges earlier pierced hole.
• Removed metal is desired one.
PARTPART
...
• Very smooth and square edges can be obtained
by fine blanking.
DIE TYPES
• Open type
• Compound types
• Progressive types
• Transfer type
Dies and PressesDies and Presses
for Sheet Metal Processesfor Sheet Metal Processes
• Dies
– Components of Dies (picture)
...
Open type
Compound type
Progressive type
Transfer type
Dies and PressesDies and Presses
for Sheet Metal Processesfor Sheet Metal Processes
• Presses
– Definition- In sheet metal...
Dies and PressesDies and Presses
for Sheet Metal Processesfor Sheet Metal Processes
• Solid- one piece construction,up to ...
CRANK PRESS
FRICTION PRESS
HYDRAULIC PRESS
SHEET METAL FORMING
CONTENTS
Bending
Stretch forming
Deep drawing
ANGAD S NAIK
BENDING
• Most commonly used metal forming process.
• Used – form parts such as flanges curls
seams and corrugation
• To i...
Bending Processes
ME 4210: Manufacturing Processes and Engineering 4
Prof. J.S. Colton
Terminology used in bending
• Bend allowance – length of neutral axis in bend
area.
• Used to determine blank length and b...
Terminology used in bending
MINIMUM BEND RADIUS
As load is applied compression – inner fiber,
tension –outer fiber.
Theore...
MINIMUM BEND RADIUS
• As R/ t increases tensile strain decreases
increases in outer fiber and material may
crack a certain...
Bendability
ME 4210: Manufacturing Processes and Engineering 25
Prof. J.S. Colton
Factors affecting bend ability
• Bend ability can be increased by increasing
tensile stresses
• It can be increased by inc...
Bend length L
• As the length increases outer fiber change
from uniaxial stresses to biaxial stresses.
• Reason for this: ...
Edge condition
• Edges - being rough bend ability
decreases.
• Removal of cold work regions such as
shaving, machining, or...
Spring back
• Plastic deformation is followed by elastic
recovery upon removal of load this
recovery is SPRING BACK.
• Thi...
Spring back (wiping die)
ME 4210: Manufacturing Processes and Engineering 24
Prof. J.S. Colton
Bending Operations
ME 4210: Manufacturing Processes and Engineering 5
Prof. J.S. Colton
Common bending operations
PRESS BRAKE FORMING
Sheet metal can be bent using simple fixtures,
and presses.
Press brake is...
Press Brake
ME 4210: Manufacturing Processes and Engineering 6
Prof. J.S. Colton
AIR BENDING
• Here only one die is used
• Bending is carried out with a pair of rolls, the
larger one is made of polyureth...
ROLL-bending
ME 4210: Manufacturing Processes and Engineering 8
Prof. J.S. Colton
Air Bending
ME 4210: Manufacturing Processes and Engineering 9
Prof. J.S. Colton
ME 4210: Manufacturing Processes and Engineering 10
Prof. J.S. Colton
ROLL BENDING
Common bending operations
• BEADING – here sheet is bent into cavity
of the die, improve stiffness, imparts
moment of iner...
STRETCH FORMING
• SHEETS ARE CLAMPED AROUND ITS
EDGES AND STRECHTED OVER A DIE.
• Can be moved upwards downwards
sideways ...
Stretch Forming
ME 4210: Manufacturing Processes and Engineering 13
Prof. J.S. Colton
Stretch Forming
ME 4210: Manufacturing Processes and Engineering 14
Prof. J.S. Colton
STRECTH FORMING
• In most operation blank is rectangular
sheet, clamped along narrow edges and
stretched length wise.
• Co...
DEEP DRAWING
• Used to shaping flat sheets into cup
shaped articles.
• This is done by placing blank of
appropriate shaped...
Deep Drawing
ME 4210: Manufacturing Processes and Engineering 19
Prof. J.S. Colton
Cracks
• It is regarded as the ultimate defect.
• Development of cracks destroys its
structural integrity.
Buckling or wrinkling
• Wrinkling of the edges results in buckling of
the sheet due to high circumferential
compressive st...
Wrinkling
• Compressive stresses are formed in plane of the
sheet results in wrinkling (buckling).
The tendency of wrinkli...
SURFACE DEFECTS
• SINCE SHEET METAL IS CHARACTERISED
BY HIGH SURFACE AREA – SURFACE IS
PRONE TO DEFECTS.
• Susceptible to ...
Stretcher marks or worms
• This is regarded another serious defect.
• This defect is characterized by flame like patterns ...
LUEDER’S BAND or STRAIN MARKS
EARING
• Here directional properties are essential.
• This usually occurs in deep drawing
processes.
• It is formation of ...
BURR
• This defects is usually seen in shearing and
blanking operations
• These are real productivity killers.
• They not ...
Sheet Metal Forming
Sheet Metal Forming
Sheet Metal Forming
Sheet Metal Forming
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Sheet Metal Forming Slide 1 Sheet Metal Forming Slide 2 Sheet Metal Forming Slide 3 Sheet Metal Forming Slide 4 Sheet Metal Forming Slide 5 Sheet Metal Forming Slide 6 Sheet Metal Forming Slide 7 Sheet Metal Forming Slide 8 Sheet Metal Forming Slide 9 Sheet Metal Forming Slide 10 Sheet Metal Forming Slide 11 Sheet Metal Forming Slide 12 Sheet Metal Forming Slide 13 Sheet Metal Forming Slide 14 Sheet Metal Forming Slide 15 Sheet Metal Forming Slide 16 Sheet Metal Forming Slide 17 Sheet Metal Forming Slide 18 Sheet Metal Forming Slide 19 Sheet Metal Forming Slide 20 Sheet Metal Forming Slide 21 Sheet Metal Forming Slide 22 Sheet Metal Forming Slide 23 Sheet Metal Forming Slide 24 Sheet Metal Forming Slide 25 Sheet Metal Forming Slide 26 Sheet Metal Forming Slide 27 Sheet Metal Forming Slide 28 Sheet Metal Forming Slide 29 Sheet Metal Forming Slide 30 Sheet Metal Forming Slide 31 Sheet Metal Forming Slide 32 Sheet Metal Forming Slide 33 Sheet Metal Forming Slide 34 Sheet Metal Forming Slide 35 Sheet Metal Forming Slide 36 Sheet Metal Forming Slide 37 Sheet Metal Forming Slide 38 Sheet Metal Forming Slide 39 Sheet Metal Forming Slide 40 Sheet Metal Forming Slide 41 Sheet Metal Forming Slide 42 Sheet Metal Forming Slide 43 Sheet Metal Forming Slide 44 Sheet Metal Forming Slide 45 Sheet Metal Forming Slide 46 Sheet Metal Forming Slide 47 Sheet Metal Forming Slide 48 Sheet Metal Forming Slide 49 Sheet Metal Forming Slide 50 Sheet Metal Forming Slide 51 Sheet Metal Forming Slide 52 Sheet Metal Forming Slide 53 Sheet Metal Forming Slide 54 Sheet Metal Forming Slide 55 Sheet Metal Forming Slide 56 Sheet Metal Forming Slide 57 Sheet Metal Forming Slide 58 Sheet Metal Forming Slide 59 Sheet Metal Forming Slide 60 Sheet Metal Forming Slide 61 Sheet Metal Forming Slide 62 Sheet Metal Forming Slide 63 Sheet Metal Forming Slide 64 Sheet Metal Forming Slide 65 Sheet Metal Forming Slide 66 Sheet Metal Forming Slide 67 Sheet Metal Forming Slide 68 Sheet Metal Forming Slide 69 Sheet Metal Forming Slide 70 Sheet Metal Forming Slide 71 Sheet Metal Forming Slide 72 Sheet Metal Forming Slide 73 Sheet Metal Forming Slide 74 Sheet Metal Forming Slide 75 Sheet Metal Forming Slide 76 Sheet Metal Forming Slide 77 Sheet Metal Forming Slide 78 Sheet Metal Forming Slide 79 Sheet Metal Forming Slide 80 Sheet Metal Forming Slide 81 Sheet Metal Forming Slide 82 Sheet Metal Forming Slide 83
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Sheet Metal Forming

  1. 1. SHEET METAL FORMING VINAY
  2. 2. Other Cutting OperationsOther Cutting Operations • Cutoff – Each cut is a new part • Parting – 2 cutting edges – Less efficient • scrap • Slotting – punching a whole • Perforating – punching of a pattern • Notching – edge portion removed • Trimming,Shaving and Fine Blanking – operations to clean up and smooth out edges
  3. 3. Sheet Metal Forming • AGENDAAGENDA Introduction Sheet Metal characteristics Shearing Piercing and Blanking Die types Machines
  4. 4. 1.0 Introduction  Sheet metal is simply metal formed into thin and flat pieces.  Sheet metal is essentially metal pressed into sheets. These sheets are used at various places. These sheets can be bent, cut and molded into any shape for use anywhere.  Sheet metal is generally produced in sheets by reducing the thickness of work piece by compressive forces applied through a set of rolls. Sheets
  5. 5. Sheet Metal Forming How to differentiate a sheet and a plate ? • If thickness is less than 6 mm (1/ 4 inches) then it is regarded as sheet. • If thickness is greater than 6 mm (1/ 4 inches) then it is regarded as plate.
  6. 6. Applications of sheet metals • Aircraft Bodies • Automobiles bodies • Utensils used for domestic purposes • Beverage cans
  7. 7. Sheet Metal Operations • Shearing • Blanking • Bending • Stretch forming • Deep drawing • Redrawing etc
  8. 8. 2.0 Sheet metal characteristics • Sheet metal is characterized by high ratio of surface area to thickness. • Forming is generally carried out in tensile forces • Decrease thickness should be avoided as far as possible as they can lead to necking and failure. • The major factors that contribute significantly include elongation, anisotropy, grain size, residual stresses, spring back, and wrinkling.
  9. 9. Elongation A test to measure the ductility of a material. When a material is tested for tensile strength it elongates a certain amount before fracture takes place. The two pieces are placed together and the amount of extension is measured against marks made before starting the test and is expressed as a percentage of the original gauge length. Materials like low carbon steels exhibit a behavior called yield point elongation, exhibiting upper yield and lower yield points. Higher elongation leads to Lueder’s band or strain marks To avoid this reduce yield point elongation by reducing the thickness of sheet by 0.5% to 1.5% by cold rolling, known as temper rolling.
  10. 10. LUEDER’S BAND or STRAIN MARKS
  11. 11. ANISOTROPY • They may be two types i.e. crystallographic anisotropy (grain orientation) and mechanical fibering (alignment of impurities, voids, inclusions). The anisotropy present in plane of the sheet is called planar anisotropy, and the anisotropy present in thickness direction is called normal anisotropy. R = εw / εt normal anisotropy
  12. 12. Anisotropy • Planar anisotropy R’ is given by R’ = (R0 + 2R45 + R90) / 4 where, subscripts 0, 45, 90 refer to angular orientation of the specimen with respect to rolling direction of sheet. • Grain size : coarser the grain rougher the surface finish Generally ASTM grain size of No. 7 is preferred for general sheet metal forming.
  13. 13. Residual stresses : • Residual stresses can develop in sheet metal forming due to non uniform deformation that take place. • When disturbed such as by removing a portion of it, the part may distort. • Tensile residual stresses can lead to stress corrosion cracking of the part unless it is properly relieved.
  14. 14. Spring back • Because sheet metal generally are thin and are subjected to relatively small strains during forming, sheet metal parts are likely to experience considerable springback. • This effect is particularly significantly in bending and other forming operations where the bend radius to thickness ratio is high, such as in automotive body parts.
  15. 15. Spring back (wiping die)
  16. 16. Wrinkling • Compressive stresses are formed in plane of the sheet results in wrinkling (buckling). The tendency of wrinkling increases with Unsupported length of sheet metal, Decreasing thickness, Non uniformity in thickness, Lubricants trapped can also contribute to wrinkling.
  17. 17. SHEARING • Shearing is a process for cutting sheet metal to size out of a larger stock such as roll stock.
  18. 18. Shearing Machine
  19. 19. Shearing • Material thickness ranges from 0.125 mm to 6.35 mm (0.005 to 0.250 in). The dimensional tolerance ranges from ±0.125 mm to ±1.5 mm (±0.005 to ±0.060 in). • The shearing process produces a shear edge burr, which can be minimized to less than 10% of the material thickness. • The major variables that affect the shearing are punch force, speed of the punch, lubrication, The hardness of punch and die materials, the corner radii of the punch and the clearance between die and punch.
  20. 20. Variables affecting shearing Shear force • Shear force is basically the product of shear strength of sheet metal and the cross sectional area being sheared • maximum force can be obtained only when maximum speed is derived or vice versa. Fmax = 0.7 * UTS * t * L Where, UTS – ultimate tensile strength t – Thickness L – Total length of sheared edge (For round hole of diameter D, L = π * D)
  21. 21. Engineering Analysis (cont.)Engineering Analysis (cont.) • Cutting Forces (F) – F = S * t * L • S - shear strength • t - thickness • L - length of cutting edge – F = 0.7TS * t *L • TS - Ultimate tensile strength – Max F is used to determine press for operation
  22. 22. HARDNESS, BURR, RADIUS ON CUTTING SURFACES • There should be optimum hardness, more than required hardness would make it brittle, and lesser would not withstand higher force . • Burr are small projection that are formed on outer surface of a formed component. These are undesirable as they increase NVA operations, cost, time. • Small radius should be given to cutting surfaces to impart strength, it should be optimum
  23. 23. CLEARANCE = THE MEASURED SPACE BETWEEN THE MATING MEMBERS OF A DIE SET ( C ) P d d1 C / 2C / 2 D D Clearance between Punch and Die or Clearance between two cutting blades
  24. 24. Engineering AnalysisEngineering Analysis • Clearance (C) – Distance between punch and die – 4%-8% of sheet thickness – Small Clearance • double burnishing • large cutting force – Large Clearance • Sheet becomes pinched • excessive burr
  25. 25. CLEARANCE = C = d – d1 therefore CLEARANCE PER SIDE = C= C / 2 ALSO CLEARANCE PER SIDE C / 2 IS GIVEN BY c /2 = 0.01* s* sqr (Tb) S=material thickness Tb= shear stress Generally clearance is of 5 to 10% of material thickness. CLEARANCE
  26. 26. Clearance – its effects • Greater clearance – rough edges, formation of burr, material is pulled rather being cut • Smaller clearance – though has good cut edges – forces required is more – also tool withdrawal is difficult .
  27. 27. • Tool and die materials : generally steels are used, ex:- Die steel, HSS for most common operations. • For higher production carbides are employed. Scraps : amount of scrap can be a high as upto 30% of original sheet. This can be reduced by proper nesting.
  28. 28. • Die cutting • Slitting • Nibbling • Steel rules
  29. 29. DIE CUTTING • It is combination of following operations • Perforating. • Parting or shearing a sheet into two. • Notching or removing pieces in edges. • Lancing or leaving a tab with removing any material
  30. 30. SLITTING • Shearing operation –using circular blades • These blades follow straight line or circular path or curved path – depending upon requirement.
  31. 31. NIBBLING • Nibbler (tool) move straight up and moves down rapidly. • Sheet metal is fed in gap between two cutting tools and produces overlapping holes • Suitable small runs, several intrcate shapes can be produced.
  32. 32. PIERCING • It is a shearing operation. • Creates open hole in sheet metal by separating the interior section. • Removed metal is discarded as scrap PART slug
  33. 33. BLANKING: • It is also a shearing operation. • It enlarges earlier pierced hole. • Removed metal is desired one. PARTPART SLUG
  34. 34. • Very smooth and square edges can be obtained by fine blanking.
  35. 35. DIE TYPES • Open type • Compound types • Progressive types • Transfer type
  36. 36. Dies and PressesDies and Presses for Sheet Metal Processesfor Sheet Metal Processes • Dies – Components of Dies (picture) – Types • Simple- single operation with a single stroke • Compound- two operations with a single stroke • Combination- two operations at two stations • Progressive- two or more operations at two or more stations with each press stroke, creates what is called a strip development
  37. 37. Open type
  38. 38. Compound type
  39. 39. Progressive type
  40. 40. Transfer type
  41. 41. Dies and PressesDies and Presses for Sheet Metal Processesfor Sheet Metal Processes • Presses – Definition- In sheet metal working it is a machine tool with a bed and powered ram that can be driven toward and away from the frame to perform various cutting and forming operations
  42. 42. Dies and PressesDies and Presses for Sheet Metal Processesfor Sheet Metal Processes • Solid- one piece construction,up to 1000 tons • Adjustable bed-accommodates different die sizes • Open Back-tilted for easy removal of stampings • Press Brake-wide bed for use of various dies • Turret-suited for sequence of punching and notching Straight Sided Frame-high tonnage presses (4000), greater structural stability Power Systems • Hydraulic-driven by a piston/cylinder system • Mechanical-eccentric,crankshaft,knuckle joint
  43. 43. CRANK PRESS
  44. 44. FRICTION PRESS
  45. 45. HYDRAULIC PRESS
  46. 46. SHEET METAL FORMING CONTENTS Bending Stretch forming Deep drawing ANGAD S NAIK
  47. 47. BENDING • Most commonly used metal forming process. • Used – form parts such as flanges curls seams and corrugation • To impart stiffness by increasing moment of inertia • Here straight length is transformed into curved length
  48. 48. Bending Processes ME 4210: Manufacturing Processes and Engineering 4 Prof. J.S. Colton
  49. 49. Terminology used in bending • Bend allowance – length of neutral axis in bend area. • Used to determine blank length and bend part. Lb = α (R = kt) Where = bend angle in radians R = bend radius k = constant t= thickness of sheet For ideal case k = 0.5 αα
  50. 50. Terminology used in bending MINIMUM BEND RADIUS As load is applied compression – inner fiber, tension –outer fiber. Theoretically strains are equal eo = et But due to shifting of neutral axis length of bend is smaller in outer surface hence there is difference in strains. eo = e t = 1 / ((2R /t)+1).
  51. 51. MINIMUM BEND RADIUS • As R/ t increases tensile strain decreases increases in outer fiber and material may crack a certain strain • The radius R @ which crack appears on outer surface is called minimum bend radius • It usually expressed in 2t, 3t, 4t………. • If R / t approaches 0 then it can completely bendable ex: paper. This characteristic is called bend ability.
  52. 52. Bendability ME 4210: Manufacturing Processes and Engineering 25 Prof. J.S. Colton
  53. 53. Factors affecting bend ability • Bend ability can be increased by increasing tensile stresses • It can be increased by increasing temperature • It can be increased by increasing pressure. • It can be increased by increasing compressive stresses in the plane of the sheet and minimizing tensile stresses in outer stresses.
  54. 54. Bend length L • As the length increases outer fiber change from uniaxial stresses to biaxial stresses. • Reason for this: L tends to become smaller due to stretching of outer fiber. • Biaxial stretching tends to reduce ductility, increases chances of failure. • As L increases minimum bend radius also increases
  55. 55. Edge condition • Edges - being rough bend ability decreases. • Removal of cold work regions such as shaving, machining, or heat treating greatly improves the resistance to edge cracking.
  56. 56. Spring back • Plastic deformation is followed by elastic recovery upon removal of load this recovery is SPRING BACK. • This is shown in figure NEXT SLIDE • Can be observed in short strip metal. • It can occur in any cross section.
  57. 57. Spring back (wiping die) ME 4210: Manufacturing Processes and Engineering 24 Prof. J.S. Colton
  58. 58. Bending Operations ME 4210: Manufacturing Processes and Engineering 5 Prof. J.S. Colton
  59. 59. Common bending operations PRESS BRAKE FORMING Sheet metal can be bent using simple fixtures, and presses. Press brake is usually machine that is used. This M/C uses mechanical or hydraulic press, usually suitable for short runs and can be automated. Die materials can of wood, steels ,carbides .
  60. 60. Press Brake ME 4210: Manufacturing Processes and Engineering 6 Prof. J.S. Colton
  61. 61. AIR BENDING • Here only one die is used • Bending is carried out with a pair of rolls, the larger one is made of polyurethane. • Upper roll pushes into flexible lower roll. ROLL BENDING •Here three rolls is used, adjusting distance between three rolls produces various curvatures. •Here short pieces can also be bent.
  62. 62. ROLL-bending ME 4210: Manufacturing Processes and Engineering 8 Prof. J.S. Colton
  63. 63. Air Bending ME 4210: Manufacturing Processes and Engineering 9 Prof. J.S. Colton
  64. 64. ME 4210: Manufacturing Processes and Engineering 10 Prof. J.S. Colton ROLL BENDING
  65. 65. Common bending operations • BEADING – here sheet is bent into cavity of the die, improve stiffness, imparts moment of inertia of edges. • FLANGING – here sheet edges are bent @ 90 deg. This causes hoops stress, if excessive bent then wrinkling chances are more – leads to cracking
  66. 66. STRETCH FORMING • SHEETS ARE CLAMPED AROUND ITS EDGES AND STRECHTED OVER A DIE. • Can be moved upwards downwards sideways depending upon requirement. • Primarily used to make aircraft wing skin, automobile door panels and window frames. • Although used for low volume prod. It is versatile and economical.
  67. 67. Stretch Forming ME 4210: Manufacturing Processes and Engineering 13 Prof. J.S. Colton
  68. 68. Stretch Forming ME 4210: Manufacturing Processes and Engineering 14 Prof. J.S. Colton
  69. 69. STRECTH FORMING • In most operation blank is rectangular sheet, clamped along narrow edges and stretched length wise. • Controlling amount of stretch is important to avoid tearing. • This process cannot produce parts with sharp edges • Dies- zinc alloys, hard plastics, wood.
  70. 70. DEEP DRAWING • Used to shaping flat sheets into cup shaped articles. • This is done by placing blank of appropriate shaped die and pressed into with punch.
  71. 71. Deep Drawing ME 4210: Manufacturing Processes and Engineering 19 Prof. J.S. Colton
  72. 72. Cracks • It is regarded as the ultimate defect. • Development of cracks destroys its structural integrity.
  73. 73. Buckling or wrinkling • Wrinkling of the edges results in buckling of the sheet due to high circumferential compressive stresses. • If a blank diameter is too high punch load will also rise which may exceed critical buckling value. this may lead to failure of sheet . • To prevent this it is necessary to have sufficient hold pressure to suppress the buckling.
  74. 74. Wrinkling • Compressive stresses are formed in plane of the sheet results in wrinkling (buckling). The tendency of wrinkling increases with Unsupported length of sheet metal, Decreasing thickness, Non uniformity in thickness, Lubricants trapped can also contribute to wrinkling.
  75. 75. SURFACE DEFECTS • SINCE SHEET METAL IS CHARACTERISED BY HIGH SURFACE AREA – SURFACE IS PRONE TO DEFECTS. • Susceptible to surface blemishes, peeling of surface also known as orange peeling – it is mostly occurs in sheets having large grain size. • This can be corrects by using sheets having smaller grain size.
  76. 76. Stretcher marks or worms • This is regarded another serious defect. • This defect is characterized by flame like patterns or depressions on the surface. • These depressions first appear along planes of shear stresses, they continue growing as they join – they give depression like surface or rough surface. • This directly related to yield point elongation. • Main difficulty – it appears in regions where strain is less than yield point • The remedy is to give sheet metal a small cold reduction, temper rolling, cold works can reduce the defect. To avoid this, reduce yield point elongation by reducing the thickness of sheet by 0.5% to 1.5% by cold rolling, known as temper rolling.
  77. 77. LUEDER’S BAND or STRAIN MARKS
  78. 78. EARING • Here directional properties are essential. • This usually occurs in deep drawing processes. • It is formation of wavy edges on a top of a drawn cup – necessitates extensive trimming. • It is related to planar isotropy, can be co- related as R =R0 + R90 -2 * R45 • Subscripts represents degree of orientation of fibers.
  79. 79. BURR • This defects is usually seen in shearing and blanking operations • These are real productivity killers. • They not only increase time but cost to deburr. • Deburring operations add no value to process. • This is related to clearance between cutting edges • This can be reduced by decreasing the cutting clearances but note that clearance can be decreased without increasing cutting forces.
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forming method, very widely used one.

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