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 Taiyuan University of Technology

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) straight parallel cutters, b) straight inclined cutters, c) rotary cutters.

a) straight parallel cutters  F = τ A  F M =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Z=Dd -dp DdDd  the space between the punch and the die opening  c=(D d – d p ) /2

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

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  Z >> Zr, the slug burnish zone contracts  Z = Zr, the contraction=the expansion  Z << Zr, 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 2.The effects of clearance on the process of blanking

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

§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 calculation  deformation law: punching→punch d p blanking →die opening D d  accuracy of dimension  proper clearance  wearing law: D min, d max  machining method dpdp DdDd

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

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 ) 1. Principles of calculation

2.1 processed separately  Suitable condition: simple contour especially circle or rectangular  Premise: δ d +δ p ≤ Z max – Z min or δ d =0.6( Z max – Z min ) δ p =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 →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

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

2. Methods of calculation Type of dimension (wearing law) : → increscent→ as blanking → decrescent→as punching → invariable 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 →D e ; (3) D e → D d Punching: (1) D p →D e ; (2) D e → D d

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

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 carbon steel t=3mm Z=0.46~0.64

Homework 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 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

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

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

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

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

elastic stripper, upwards return-blank die

elastic stripper, downwards drop-blank die

stationary stripper,downwards drop-blank die

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)

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 1. Layout 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 Homework: Calculate the materials efficiency for the workpiece on Page45.No3 and No 4.