stretch forming, explosive forming

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Sheet metal basics- Stretch
forming- Explosive forming

2. • UNIT IV SHEET METAL PROCESSES
Sheet metal characteristics –
shearing, bending and drawing
operations – Stretch forming
operations– Formability of sheet
metal – Test methods –special
forming processes-Working principle
and applications – Hydro forming –
Rubber pad forming – Metal
spinning– Introduction of Explosive
forming, magnetic pulse forming,YoucaN

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6. • Sheet metal parts offers the advantages of light weight
and versatile shape as compared to those made by
casting/ forging.
• Low carbon steel is the most commonly used sheet
metal.
• Aluminum and titanium are most common sheet
materials for aircraft and aerospace applications.
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8. • The process of cutting a straight line across a strip, sheet
or bar is known a shearing process.
• Stage 1- plastic deformation
• Stage 2- shear
• Stage 3- fracture
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9. • Cutting operations
• Blanking(punched out piece is workpiece)
• Punching(used to produce a hole in workpiece)
• Shearing(cuts along a single line)
• Parting(shearing the sheet into two or more
pieces)
• Notching (metal pieces are cut from the edge of
a sheet)
• Lancing- cutting the sheet metal through a small
length and bending this small cut portion
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11. • Bending (Forming the metal between a suitably shaped punch
and forming block. )
• Drawing (consist of a punch forcing a sheet metal blank to flow
plastically into the clearance available between the punch and
die surfaces so as to acquire top shape, a cylindrical shape or
a box shape )
• Sequeezing (metal is caused to flow to all portions of a die
cavity under the action of compressive force).
• Embossing (producing required shapes on sheet metal blanks
means of punches and dies- name plate details)
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12. • For shearing the blank holder pressure is important.
• If the blank holder pressure(depends upon part it vary) is low in
drawing operation.
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22. It denotes the property of sheet
metal to partially fall back from its
bent position when the punch
retards after completing the
operation
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23. • Angle bending (sheet is bent at an angle to each edge.)
• Roll bending (sheet is bent in the form of rolled edges)
• Roll forming (desired shapes or any impression in the
form of bend is made on the sheet metal)
• Seaming (process of providing lock between two edges
of different work sheet)
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26. • Used to produce large accurate contoured sheets.
• Stretching the work beyond its elastic limit by moving a
form block towards the blank or sheet metal.
• In stretch forming, the sheet metal is clamped along its edges and
then stretched over a male die (form block or form punch).
• The die moves upward, downward, or sideways, depending on the
particular design of the machine
• Stretch forming is used primarily to make aircraft wing-
skin panels, fuselages, and boat hulls.
• Aluminum skins for the Boeing 767 and 757 aircraft, for example,
are made by stretch forming-with a tensile force of 9 MN.
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28. • The rectangular sheets are 12 m >< 2.5 m >< 6.4 mm. Although this
process generally is used for low volume production, it is
versatile and economical, particularly for the aerospace industry.
• In most operations, the blank is a rectangular sheet clamped
along its narrower edges and stretched lengthwise, thus allowing
the material to shrink in width.
• Controlling the amount of stretching is important in order to
prevent tearing.
• Stretch forming cannot produce parts with sharp contours
or with reentrant corners (depressions on the surface of the die).
• Various accessory equipment can be used in conjunction with stretch
forming, including further forming with both male and female dies
while the part is under tension.
• Dies for stretch forming generally are made of zinc alloys, steel,
plastics, or wood. Most applications require little or no lubrication.YoucaN

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33. • Mating die method
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34. • Blank can be stretched in a single operation
• Spring back is reduced or eliminated when compared
to other forming methods
• Plastic deformation is due to pure tension only
• Larger parts can be formed
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Materials used
Aluminium Alloy
Nickel and Cobalt base alloys – Stainless
Steel

35. • Blank thickness should be uniform throughout the length.
• Sudden changes in contour surfaces cannot be
stretched.
• Maintenance cost of hydraulic cylinders is high
• The process requires high quality form blocks
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36. •limitations
• Uneven thickness of blank cannot be stretched
• Stretching of blank to the required shape of contour is
limited
• Applications
• Production of aircraft wing parts
• Production of contoured panels for truck trailer and bus
bodies in automobile industry.
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Some of the applications of sheet meta

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Some of the applications of sheet me

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Chemical energy in explosives- shock waves-
Energy transmitting medium(air or water) -
work holding die – vaccum – plastic work-milli
seconds Explosives:
Trinitrotoluene (TNT)
Research and development explosives
(RDX)
Dynamite

42. • In explosive forming the chemical energy stored in
explosives is used to process the materials.
• The shock waves developed depends on the compressibility
of the energy transmitting medium.(water or air)
• Water is being less compressible than air is a better
medium.
• Terms: distance between workpiece and explosive,
amount of explosive, energy transmitting medium
• Two systems
• 1- confined system OR Contact system
• 2- unconfined system OR stand off distance methodYoucaN

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44. • Two or more die piece, completely encloses the workpiece.
• Between the die and the workpiece vacuum should be
produced.
• When the explosive is denonated a shock wave is
produced in the energy transmitting medium.
• This forces the workpiece into the die.
• The already created vacuum between the die and workpiece
helps to transfer the energy completely.
• That vacuum helps to form completely.
• Used to small and tubular parts for flaring and bulging
operations.
• Being a closed system there is a greater hazard of die
failure due to high pressure generated.YoucaN

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46. • Used to form very thin sheet to large ship plates about
25mm thick.
• Workpiece is clamped In die assembly. And the entire die
assembly is lowered in to a swimming pool. Vacuum should
be created between the plate and die.
• An explosive is denoted above or with in the water.
• The produced shock waves in the water body forces the
workpiece into the die.
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47. • Low pressure explosives
• Black powder and smokeless powder
• Pressure is only 700MPa
• High pressure explosives
• Trinitrotoluene (TNT)
• Research and development explosives (RDX)
• Dynamite
• Pressure is 14000MPa.
• Deformation velocity is 9
• 230m/s
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48. • Applications
• Radar dishes
• Rocket motor cases
• Parts which are not manufactured in conventional
methods can be prepared.
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