Taiyuan University of Technology
Sheet Metal Forming
Processes and Die Design
冲压工艺及模具设计
College of Materials Science
and Engineering
Lecturer：CAO Xiao-qing（曹晓卿）
May,2009
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;
Key points:
 Blanking and punching clearance ;
 Punch force, punch power and methods
to decrease punch force;
 Calculate the cut edges of punch and die
New words:
clearance（间隙）
punch and die edge（凸凹模刃口）
lay-out（排样）scrap bridge（搭边）
punch force（冲裁力）
straight parallel cutter（平刃）
straight inclined cutter/bevel-cut edge(斜刃)
obtuseness(钝)
punch penetration（凸模行程）
§2.1 Shearing
1. Definition
2. Shearing process
3. Shearing force
1. Definition
 the cutting of flat material forms
 done by different types of blades or
cutters
 machines driven by mechanical,
hydraulic, or pneumatic power
2. Shearing process
3 phases:elastic deformation, plastic deformation,
and fracture
3. Shearing force
May be calculated according to the edge types
of the cutters :
a)
b)
c)
straight parallel cutters,
straight inclined cutters,
rotary cutters.
a) straight parallel cutters
 F=τ•A
 FM =1.3 F (reasons: p24)
b) straight inclined cutters
c) rotary cutters
§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)
1. Deformation process
Three phases:
 elastic deformation
plastic deformation
 fracture

1. Deformation process

Deformation zone: spindly area between the cutting edges of the punch and
die.
Grammar
 Note the articles on page30:
•During phase III, …in …,…turn into…,
followed by….
• …in …start at…of …on…of …,
at…on…of ;…propagate along…from…
2. Stress analysis
Forces applied to sheet
a) section analysis; b) force analysis
2. Stress analysis
Forces caused by moment
Left: without press pad ; Right: with press pad
2. Stress analysis
 five characteristic points in deformation zone
3. Section quality (Features of edges)
3.1 Four parts: rollover, burnish zone, fracture, burr
3. Section quality (Features of edges)
3.2 Affecting factors
material’s property (plasticity)
▲clearance:large, proper, small
 cutting edge condition:wearing and obtuse
 Lubrication: good or bad

§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
1.The definition and significance of clearance
 Z=Dd -dp
 c=(Dd –dp ) /2
 the space
between the
punch and the die
opening
Dd
2.The effects of clearance on the process
of blanking
a)
b)
c)
d)
section quality
dimension precision
power consumption
die life
2.The effects of clearance on the process
of blanking
a) section quality
 Z >> Zr , burnish zone rollover burr and fracture
 Z = Zr
 Z << Zr, burnish zone rollover fracture burr (thin and
long)
2.The effects of clearance on the process
of blanking
b) dimension precision
 d  D  Dd  p  d  d p
 Z >> Zr ,  d  0  p  0 the slug burnish zone contracts
 Z = Zr,
d  0  p  0
 Z << Zr,
the contraction=the expansion
 d  0  p  0 the slug burnish zone expands
2.The effects of clearance on the process
of blanking
c) power consumption( punch force)
 Z >> Zr, F
excessive
 Z = Zr , F
(See Ch-t p11,Fig.2-14)proper
 Z << Zr, F not sufficient especially the power
d) die life
 Z >> Zr, life
 Z = Zr
 Z << Zr, life
3.How to determine and choose reasonable
value of clearance
 Theoretical way
Z  2t (1  h0 t )tg



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
Z=k·t
§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
1. Principles of calculationdp
 deformation law:
punching→punch dp
blanking →die opening Dd
 accuracy of dimension
 proper clearance
 wearing law: Dmin, dmax
 machining method
Dd
1. Principles of calculation
1.1 benchmarks
taped edge of
parts
punching→punch,
blanking→die
1. Principles of calculation
1.2 limit dimension
die wearing law:
punch→upper
blanking→lower
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)
2. Methods of calculation
2.1 processed separately
 Suitable condition: simple contour especially
circle or rectangular
 Premise: δd +δp ≤ Z max – Z min
or δd=0.6( Z max – Z min )
δp=0.4( Z max – Z min )
 Features:
advantages----interchangeability, short machine time
disadvantages----high cost (small tolerance)
2. Methods of calculation
blanking
punching
2. Methods of calculation
Dd  ( Dmax  x  ) 0 d
→blanking, – Z min →D p
D p  ( Dmax  x    Z min ) 0d p
d p  (d min  x  )
0
 p
d d  (d min  x    Z min )
 d
0
→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 0.5~1
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
 Method:benchmark
→calculate
→wearing line →dimension change
2. Methods of calculation
Type of dimension(wearing law):
→ increscent→ as blanking

A  ( Amax  x  )0
→ decrescent→as punching
B  ( Bmin  x  ) 
0
→ invariable
C  (Cmin  0.5x)   '
δ=Δ/4, δ’ =Δ/8
2. Methods of calculation
 blanking
2. Methods of calculation
 punching
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
2. Methods of calculation
Conversion: (only for blanking)
(A→B)

0
B p  ( Ad   Z min ) 

4
4

A p  ( Bd   Z min ) 0
4

(B→A)

4
3. Examples
 Separately: Q235,t=1.5mm
tractor part: gasket
3. Examples
 Coordinately: H62,t=0.5mm
Homework
 Page45.No3 and No 4
 2 supplements
1. 20 carbon steel
t=3mm
Z=0.46~0.64
Homework
2. 10 carbon steel, t=1.5mm, Z=0.132~0.24
§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
1. The calculation of punch force
1.1 calculation formula
F  k  L  t 
k =1.1~1.3
 unequal thickness of the material;
 Inhomogeneous mechanical property of the
material;
 friction between the punch and the work part;
 poorly sharpened edges.
1. The calculation of punch force
1.2 affecting factors
work part : L、 t
material :τ
clearance : c/Z
2. Methods to reduce the force
L↓: stepping punch(multiple punches)
2. Methods to reduce the force
τ↓:heated blanking and punching
Lt↓:bevel-cut edges
3. Press choosing
3.1 other forces needed
 stripping force F x :
 knockout force:
ejecting force F d -against
knocking force Ft -along
Fx  k x  Fb ,
Fd  kd  Fb ,
Ft  kt  Fb  n
k d  kt  k x
3. Press choosing
3.2 total force and press choosing
Ftotal  Fb  Fx  Fd  Ft
Fp  Ftotal
According to different die structure:
•elastic stripper, upwards
•elastic stripper, downwards
•stationary stripper,downwards
elastic stripper, upwards
Ftotal  Fb  Fx  Fd
return-blank die
elastic stripper, downwards
Ftotal  Fb  Fx  Ft
drop-blank die
stationary stripper,downwards
drop-blank die
Ftotal  Fb  Ft
3. Press choosing
3.2 examples
 Simple/plain die
 compound die: positive assembly
inverted assembly
 progressive die: multi-station(p103)
compound die: positive assembly
compound die: inverted assembly
§2.6 Material economy
1. Layout
 meaning
 form
2. Scrap
 function and determination
 width of sheet scrap
3. Calculation
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:
1. Layout
1.2 form of layout
A) type of scrap:
(1)m>0, n>0; with scrap layout
(2)m=0,n>0; less scrap layout
(3) m=0,n=0. no scrap layout
1. Layout
m: distance from the edge of the blank to the side of the strip
n: the distance from blank to blank.
E=L - (N.t + n)
1. Layout
B) position of blanks
(1) straightforward: square, rectangular
(2)opposite layout: L/T-shaped, triangular,
trapeziform(梯形)，half circular
(3) in an angle layout: L/T-shaped, cross-shaped,
ellipse(椭圆)
(4)single-pass, multi-line layout (Used for smaller,
simple-shaped workpiece)
1. Layout
62.5%→76.5%→ 81.8%The→
greatest material economy
Alternate multi-line layout
B = D+ 0.87(D+ n)·( i +1) + 2m
where: D- -width of blank,
i --number of lines.
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).
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
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= D+2m
3．Calculation
  A A0  100%
Homework:
Calculate the materials efficiency for
the workpiece on Page45.No3 and
No 4.