Sheet Metal Forming

forming method, very widely used one.

1. SHEET METAL FORMING
VINAY

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. Sheet Metal Forming
•AGENDAAGENDA
Introduction
Sheet Metal characteristics
Shearing
Piercing and Blanking
Die types
Machines

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. 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. Applications of sheet metals
•Aircraft Bodies
•Automobiles bodies
•Utensils used for domestic purposes
•Beverage cans

7. Sheet Metal Operations
•Shearing
•Blanking
•Bending
•Stretch forming
•Deep drawing
•Redrawing etc

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. 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. LUEDER’S BAND or STRAIN MARKS

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 = ε
w
/ ε
t
normal anisotropy

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. 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. 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. Spring back (wiping die)

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. SHEARING
•Shearing is a process for cutting sheet
metal to size out of a larger stock such as
roll stock.

18. Shearing Machine

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. 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. 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. 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. 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. 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. 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. 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. •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. •Die cutting
•Slitting
•Nibbling
•Steel rules

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. SLITTING
•Shearing operation –using circular blades
•These blades follow straight line or circular path or
curved path – depending upon requirement.

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. 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. BLANKING:
•It is also a shearing operation.
•It enlarges earlier pierced hole.
•Removed metal is desired one.
PARTPART
SLUG

34. •Very smooth and square edges can be obtained
by fine blanking.

35. DIE TYPES
•Open type
•Compound types
•Progressive types
•Transfer type

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. Open type

38. Compound type

39. Progressive type

40. Transfer type

41. Dies and Presses Dies 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. 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. CRANK PRESS

44. FRICTION PRESS

45. HYDRAULIC PRESS

47. SHEET METAL FORMING
CONTENTS
Bending
Stretch forming
Deep drawing
ANGAD S NAIK

48. 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

49. Bending Processes
ME 4210: Manufacturing Processes and Engineering 4
Prof. J.S. Colton

50. 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

51. 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).

52. 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.

53. Bendability
ME 4210: Manufacturing Processes and Engineering 25
Prof. J.S. Colton

54. 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.

55. 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

56. 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.

57. 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.

58. Spring back (wiping die)
ME 4210: Manufacturing Processes and Engineering 24
Prof. J.S. Colton

59. Bending Operations
ME 4210: Manufacturing Processes and Engineering 5
Prof. J.S. Colton

60. 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 .

61. Press Brake
ME 4210: Manufacturing Processes and Engineering 6
Prof. J.S. Colton

62. 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.

63. ROLL-bending
ME 4210: Manufacturing Processes and Engineering 8
Prof. J.S. Colton

64. Air Bending
ME 4210: Manufacturing Processes and Engineering 9
Prof. J.S. Colton

65. ME 4210: Manufacturing Processes and Engineering 10
Prof. J.S. Colton
ROLL BENDING

66. 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

67. 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.

68. Stretch Forming
ME 4210: Manufacturing Processes and Engineering 13
Prof. J.S. Colton

69. Stretch Forming
ME 4210: Manufacturing Processes and Engineering 14
Prof. J.S. Colton

70. 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.

71. 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.

72. Deep Drawing
ME 4210: Manufacturing Processes and Engineering 19
Prof. J.S. Colton

75. Cracks
•It is regarded as the ultimate defect.
•Development of cracks destroys its
structural integrity.

76. 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.

77. 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.

78. 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.

79. 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.

80. LUEDER’S BAND or STRAIN MARKS

81. 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 =R
0 + R
90 -2 * R
45
•Subscripts represents degree of orientation
of fibers.

82. 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.