Sheet Metal Forming

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Taiyuan University of Technology
Sheet Metal Forming Processes and Die Design
?????????
College of Materials Science and Engineering
LecturerCAO Xiao-qing(???) May,2009
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CHAPTER 2 SHEARING AND BLANKING

• Main contents
• Shearing
• Deformation mechanism of blanking and punching
• Clearance of blanking and punching
• Punch force, power and methods to decrease the
  force
• The principle and methods to calculate the cut
  edges of punch and die
• Materials economy

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Key points

• Blanking and punching clearance
• Punch force, punch power and methods to decrease
  punch force
• Calculate the cut edges of punch and die

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New words

• clearance(??)
• punch and die edge(?????)
• lay-out(??)scrap bridge(??)
• punch force(???)
• straight parallel cutter(??)
• straight inclined cutter/bevel-cut edge(??)
• obtuseness(?)
• punch penetration(????)

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2.1 Shearing

1. Definition
2. Shearing process
3. Shearing force

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1. Definition

• the cutting of flat material forms
• done by different types of blades or cutters
• machines driven by mechanical, hydraulic, or
  pneumatic power

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2. Shearing process

• 3 phaseselastic deformation, plastic
  deformation,
• and fracture

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3. Shearing force

• May be calculated according to the edge types
  of the cutters
• a)      straight parallel cutters,
• b)      straight inclined cutters,
• c)       rotary cutters.

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 a) straight parallel cutters

• F t A
• FM 1.3 F (reasons p24)

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b) straight inclined cutters
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c) rotary cutters
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2.2 Deformation mechanism of blanking and
punching

• 1. Deformation process three phases, deformation
  zone
• 2. Stress analysis five characteristic points
  in deformation zone
• 3.    ?Section quality(Features of edges)

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1. Deformation process

• Three phases
• elastic deformation
• plastic deformation
• fracture

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1. Deformation process

• Deformation zone spindly area between the
  cutting edges of the punch and die.

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Grammar

• Note the articles on page30
• During phase III, in ,turn into, followed
  by.
• in start atof onof , atonof propagate
  alongfrom

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2. Stress analysis
Forces applied to sheet a) section analysis b)
force analysis
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2. Stress analysis
Forces caused by moment Left without press pad
Right with press pad
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2. Stress analysis

• five characteristic points in deformation zone

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3.  Section quality (Features of edges)

• 3.1 Four parts rollover, burnish zone, fracture,
  burr

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3.  Section quality (Features of edges)

• 3.2 Affecting factors
• materials property (plasticity)
• ?clearancelarge, proper, small
• cutting edge conditionwearing and obtuse
• Lubrication good or bad

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2.3 Clearance of blanking and punching

• 1.The definition and significance of clearance
• 2.The effects of clearance on the process of
  blanking
• 3.How to determine and choose reasonable value of
  clearance

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1.The definition and significance of clearance

• ZDd -dp

• c(Dd dp ) /2

• the space between the punch and the die opening

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2.The effects of clearance on the process of
blanking

• a)   section quality
• b)   dimension precision
• c)   power consumption
• d) die life

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2.The effects of clearance on the process of
blanking

• a)   section quality
• Z gtgt Zr , burnish zone rollover burr and
  fracture
• Z Zr
• Z ltlt Zr, burnish zone rollover fracture
  burr (thin and long)

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2.The effects of clearance on the process of
blanking

• b) dimension precision
• Z gtgt Zr , the slug
  burnish zone contracts
• Z Zr, the
  contractionthe expansion
• Z ltlt Zr, the slug
  burnish zone expands

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2.The effects of clearance on the process of
blanking

• c) power consumption( punch force)
• Z gtgt Zr, F excessive
• Z Zr , F (See Ch-t p11,Fig.2-14)proper
• Z ltlt Zr, F not sufficient especially the power
• d) die life
• Z gtgt Zr, life
• Z Zr
• Z ltlt Zr, life

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3.How to determine and choose reasonable value of
clearance

• Theoretical way
• Enable the fractures to start ideally at the
  cutting edge of the punch and also at the die.
• The fracture will proceed toward each other until
  they meet .
• Function of the kind, thickness, and temper of
  the material
• Experimental way
• Zkt

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2.4 The calculation of punch and die cutting edge

• 1.   Principles of calculation
• benchmarks
• limit dimension
• accuracy of dimension
• 2.   Methods of calculation
• separately
• coordinately
• electric spark machining
• 3. Examples

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1.   Principles of calculation

• deformation law
• punching?punch dp
• blanking ?die opening Dd
• accuracy of dimension
• proper clearance
• wearing law Dmin, dmax
• machining method

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1.   Principles of calculation

• 1.1 benchmarks
• taped edge of parts
• punching?punch,
• blanking?die

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1.   Principles of calculation

• 1.2 limit dimension
• die wearing law
• punch?upper
• blanking?lower

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1.   Principles of calculation

• 1.3 accuracy of die dimension
• maintenance and cost
• determined by the accuracy of parts
• (see Ch-t p11,table 2-1)

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2. Methods of calculation

• 2.1 processed separately
• Suitable condition simple contour especially
  circle or rectangular
• Premise dd dp Z max Z min
• or dd0.6( Z max Z min )
• dp0.4( Z max Z min )
• Features
• advantages----interchangeability, short machine
  time
• disadvantages----high cost (small tolerance)
  

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2. Methods of calculation
blanking punching
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2. Methods of calculation

• ?blanking,
  Z min ?D p
• 
  ?punching, Z min ?d d
• x-a coefficient to make the dimension of punching
  and blanking part be close to the middle in
  tolerance band. In the range of 0.51

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2. Methods of calculation

• 2.2 processed coordinately
• Suitable condition complex contour or thin sheet
• Aim ensure the clearance
• Benefit small clearance, expand the tolerance of
  benchmark, easy made
• Methodbenchmark ?wearing line ?dimension change
  ?calculate

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2. Methods of calculation

• Type of dimension(wearing law)
• ? increscent? as
  blanking
• ? decrescent?as
  punching
• ? invariable
• d?/4, d ?/8

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2. Methods of calculation

• blanking

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2. Methods of calculation

• punching

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2. Methods of calculation

• 2.3 electric spark machining
• Both tolerance and dimension are marked on
• punch.
• Blanking (1)D d ? - Z min ?D p
• (2) D p ?De
• (3) De ? D d
• Punching (1) D p ?De
• (2) De ? D d

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2. Methods of calculation

• Conversion (only for blanking)
• (A?B)
• (B?A)

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3. Examples

• Separately Q235,t1.5mm
• tractor part gasket

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3. Examples

• Coordinately H62,t0.5mm

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Homework

• Page45.No3 and No 4
• 2 supplements

1. 20 carbon steel t3mm Z0.460.64
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Homework
2. 10 carbon steel, t1.5mm, Z0.1320.24
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2.5 Punch force, power and decreasing methods

• 1. The calculation of punch force
• calculation formula
• affecting factors
• 2.  Methods to reduce the force
• stepping punch
• heated blanking and punching
• bevel-cut edges
• 3. Press choosing
• other forces needed
• total force and press choosing
• examples

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1. The calculation of punch force

• 1.1 calculation formula
• k
  1.11.3
• unequal thickness of the material
• Inhomogeneous mechanical property of the
  material
• friction between the punch and the work part
• poorly sharpened edges.

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1. The calculation of punch force

• 1.2 affecting factors
• work part L? t
• material t
• clearance c/Z

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2.  Methods to reduce the force

• L? stepping punch(multiple punches)

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2.  Methods to reduce the force

• t?heated blanking and punching
• Lt?bevel-cut edges

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3. Press choosing

• 3.1 other forces needed
• stripping force F x
• knockout force
• ejecting force F d -against
• knocking force Ft -along

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3. Press choosing

3.2 total force and press choosing

• According to different die structure
• elastic stripper, upwards
• elastic stripper, downwards
• stationary stripper,downwards

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elastic stripper, upwards
return-blank die
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elastic stripper, downwards
drop-blank die
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stationary stripper,downwards
drop-blank die
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3. Press choosing

• 3.2 examples
• Simple/plain die
• compound die positive assembly
• inverted assembly
• progressive die multi-station(p103)

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compound die positive assembly
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compound die inverted assembly
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2.6 Material economy

• 1. Layout
• meaning
• form
• 2. Scrap
• function and determination
• width of sheet scrap
• 3. Calculation

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1.  Layout

• 1.1 meaning
• layout relative position of the blanks on the
  work material
• scrap
• technical scrap edge of the blank to side of
  strip
• distance from blank to
  blank
• structural scrap

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1.  Layout

• 1.2 form of layout
• A) type of scrap
• (1)mgt0, ngt0 with scrap layout
• (2)m0,ngt0 less scrap layout
• (3) m0,n0. no scrap layout

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1.  Layout

m distance from the edge of the blank to the
side of the strip n the distance from blank to
blank. EL - (N.t n)
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1.  Layout

• B) position of blanks
• (1) straightforward square, rectangular
• (2)opposite layout L/T-shaped, triangular,
  trapeziform(??),half circular
• (3) in an angle layout L/T-shaped, cross-shaped,
  ellipse(??)
• (4)single-pass, multi-line layout (Used for
  smaller, simple-shaped workpiece)

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1.  Layout
Alternate multi-line layout B D 0.87(D n)(
i 1) 2m where D- -width of blank, i --number
of lines.

• 62.5?76.5? 81.8The?
• greatest material economy

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1.  Layout

Utilize the scrap from one piece as a material
for another piece (scrap from piece Ias the
material for piece 2 scrap from piece 2 as
the material for piece 3).
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2. Scrap

• 2.1 function and determination
• function
• (1) compensate orientation error
• (2) keep the rigidity of the strip to ensure the
  quality and stock movement
• (3) protect dies
• determination
• (1)  materials property hard, m, n? soft and
  brittle, m, n?
• (2)  thickness of the sheet t??m, n?
• (3)  shape of part complex and large, small
  radii m, n?
• (4) die structure guiding and stopping mode

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2. Scrap

• 2.2 width of sheet scrap
• The minimum value should ensure the rational
  scrap around work part, and the maximum value
  should ensure the strip move well between guiding
  rails and is of certain distance between the edge
  of strip and guiding rails.
• B D2m

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3.Calculation
Homework Calculate the materials efficiency for
the workpiece on Page45.No3 and No 4.