1. Sheet Metal Forming
Processes and Die Design

2. 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;

3. Key points:
Blanking and punching clearance ;
Punch force, punch power and methods to decrease punch force;
Calculate the cut edges of punch and die

4. 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）

5. §2.1 Shearing
Definition
Shearing process
Shearing force

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

7. 2. Shearing process 3 phases:elastic deformation, plastic deformation,
and fracture

8. 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.

9. a) straight parallel cutters
F = τ • A
FM =1.3 F (reasons: p24)

10. b) straight inclined cutters

11. c) rotary cutters

12. §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)

13. 1. Deformation process Three phases: elastic deformation
plastic deformation
fracture

14. 1. Deformation process
Deformation zone: spindly area between the cutting edges of the punch and die.

15. 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…

16. 2. Stress analysis Forces applied to sheet
a) section analysis; b) force analysis

17. 2. Stress analysis Forces caused by moment
Left: without press pad ; Right: with press pad

18. 2. Stress analysis
five characteristic points in deformation zone

19. 3. Section quality (Features of edges)
3.1 Four parts: rollover, burnish zone, fracture, burr

20. 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

21. §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

22. 1.The definition and significance of clearance
Z=Dd -dp Dd
c=(Dd –dp ) /2
the space between the punch and the die opening

23. 2.The effects of clearance on the process of blanking
a)   section quality
b)   dimension precision
c)   power consumption
d) die life

24. 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)

25. 2.The effects of clearance on the process of blanking
b) dimension precision
Z >> Zr , the slug burnish zone contracts
Z = Zr, the contraction=the expansion
Z << Zr, the slug burnish zone expands

26. 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

27. 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
Z=k·t

28. §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

29. 1. Principles of calculationdp Dd
deformation law:
punching→punch dp
blanking →die opening Dd
accuracy of dimension
proper clearance
wearing law: Dmin, dmax
machining method

30. 1. Principles of calculation
1.1 benchmarks
taped edge of parts
punching→punch,
blanking→die

31. 1. Principles of calculation
1.2 limit dimension
die wearing law:
punch→upper
blanking→lower

32. 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)

33. 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)

34. 2. Methods of calculation
blanking punching

35. 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 0.5~1

36. 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 →wearing line →dimension change →calculate

37. 2. Methods of calculation
Type of dimension(wearing law):
→ increscent→ as blanking
→ decrescent→as punching
→ invariable
δ=Δ/4, δ’ =Δ/8

38. 2. Methods of calculation
blanking

39. 2. Methods of calculation
punching

40. 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

41. 2. Methods of calculation
Conversion: (only for blanking)
(A→B)
(B→A)

42. 3. Examples
Separately: Q235,t=1.5mm
tractor part: gasket

43. 3. Examples
Coordinately: H62,t=0.5mm

44. Homework Page45.No3 and No 4 2 supplements 1. 20 carbon steel t=3mm
Z=0.46~0.64

45. Homework
2. 10 carbon steel, t=1.5mm, Z=0.132~0.24

46. §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

47. 1. The calculation of punch force
1.1 calculation formula
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.

48. 1. The calculation of punch force
1.2 affecting factors
work part : L、 t
material :τ
clearance : c/Z

49. 2. Methods to reduce the force
L↓: stepping punch(multiple punches)

50. 2. Methods to reduce the force
τ↓:heated blanking and punching
Lt↓:bevel-cut edges

51. 3. Press choosing 3.1 other forces needed stripping force F x :
knockout force:
ejecting force F d -against
knocking force Ft -along

52. 3. Press choosing 3.2 total force and press choosing
According to different die structure:
elastic stripper, upwards
elastic stripper, downwards
stationary stripper,downwards

53. elastic stripper, upwards
return-blank die

54. elastic stripper, downwards
drop-blank die

55. stationary stripper,downwards
drop-blank die

56. 3. Press choosing 3.2 examples Simple/plain die
compound die: positive assembly
inverted assembly
progressive die: multi-station(p103)

57. compound die: positive assembly

58. compound die: inverted assembly

59. 1. Layout 2. Scrap 3. Calculation §2.6 Material economy meaning form
function and determination
width of sheet scrap
3. Calculation

60. 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:

61. 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

62. 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)

63. 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)

64. 1. Layout Alternate multi-line layout 62.5%→76.5%→ 81.8%The→
B = D+ 0.87(D+ n)·( i +1) + 2m
where: D- -width of blank,
i --number of lines.
62.5%→76.5%→ 81.8%The→
greatest material economy

65. 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).

66. 2. Scrap 2.1 function and determination function： determination:
(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

67. 2. Scrap 2.2 width of sheet scrap B= D+2m
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

68. 3．Calculation Homework:
Calculate the materials efficiency for the workpiece on Page45.No3 and No 4.