Design for Manufacturability: - Introduction by kp0vUawa

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									Direct Extrusion or Forward Extrusion

• Similar to polymer extrusion, except it is not a
  continuous process - a small portion of billet, called
  the butt, remains that cannot be forced through die,
  and is separated from the extrudate by cutting.
• Starting billet cross section usually round
Indirect Extrusion
• Also called backward or reverse extrusion
• Limitations of indirect extrusion are imposed by
  – Lower rigidity of hollow ram
  – Difficulty in supporting extruded product as it exits




  (a) Solid extrudate, and (b) hollow cross-section extrudate
Hot vs. Cold Extrusion
• Hot extrusion - prior heating of billet to above
  its                    temperature
  – Reduces strength and increases ductility of the
    metal, permitting more size reductions and more
    complex shapes
• Cold extrusion - generally used to produce
  discrete parts
  – The term impact extrusion is used to indicate
    high speed cold extrusion
  – Material possess some degree of
    Complex Cross Section




Figure 19.36 A complex extruded cross
 section for a heat sink (photo courtesy of
     Aluminum Company of America)
Wire and Bar Drawing
• Cross-section of a bar, rod, or wire is reduced
  by pulling it through a die opening
• Similar to extrusion except work is
  through the die in drawing.
• Both tensile and compressive stress deform the
  metal as it passes through die opening.
 Features of a Draw Die
• Entry region - funnels lubricant into the die to prevent
  scoring of work and die
• Approach - cone-shaped region where drawing occurs
• Bearing surface - determines final stock size
• Back relief - exit zone - provided with a back relief angle
  (half-angle) of about 30°
 Continuous Wire Drawing
• Continuous drawing machines consisting of multiple draw
  dies (typically 4 to 12) separated by accumulating drums
   – Each drum (or capstan) provides proper force to draw wire
     stock through its upstream die
   – Each die provides only a small portion of the overall reduction
   – Annealing is sometimes required between dies to relieve work
     hardening
Sheet Metal working

 Cutting Operations

 Bending Operations

 Drawing or Forming

 Other Sheet Metal Forming Operations
Advantages of Sheet Metal Parts

• High strength

• Good dimensional accuracy

• Good surface finish

• Relatively low cost

• Economical mass production for large
  quantities
Basic Types of Sheet Metal Processes
1. Cutting
   – Shearing to separate large sheets
   – Blanking to cut part perimeters out of
     sheet metal
   – Punching to make holes in sheet metal
2. Bending
   – Straining sheet around a straight axis
3. Drawing or Forming
   – Forming of sheet into convex or concave
     shapes
Sheet Forming Examples




               Cutting




   Bending
                    Drawing
Soft Tooled v. Hard Tooled Processes
1. Soft Tooled (Programmable - Expendable
   tooling – medium to low volume applications)
  – Laser, Plasma, and Oxy-fuel Cutting
  – Bend Brake
  – Turret Press
2. Hard Tooled (Stamping Dies – high capital
   investment)
  –   Standard Press Brake (manual batch)
  –   Stage Tooling (manual line transfer)
  –   Progressive Die
  –   Transfer Presses
                      Sheet metal cutting examples:
Sheet Metal Cutting   Shearing, Blanking, & Punching

                            (1) punch before contact,
                            clearance ‘c’ between
                            punch and die

                            (2) punch causes material
                            to plastically deform

                            (3) smooth cut surface is
                            formed

                            (4) fracture initiated at the
                            opposing cutting edges
                            which separates the sheet.
  1. Shearing
Sheet metal cutting operation along a
  between two cutting edges
• Typically used to cut large sheets




  (a) side view of the shearing operation; (b) front view of
  power shears equipped with inclined upper cutting blade.
2. Blanking and Punching
Blanking - sheet metal cutting to separate piece
  (called a        ) from surrounding stock
Punching - similar to blanking except cut piece is
  scrap, called a




            (a) Blanking and (b) punching.
Clearance in Sheet Metal Cutting
• Distance between punch cutting edge and die
  cutting edge (typically 4 - 8% of thickness)
  – If too     (a), fracture lines pass each other,
    causing double burnishing and larger force
  – If too     (b), metal is pinched between cutting
    edges and excessive burr results
  Clearance in Sheet Metal Cutting
• Recommended clearance is calculated by:
             c = a∙t
      where           c = clearance;
                      a = allowance;
                      t = stock thickness

• Allowance is determined according to type of metal:

  Metal group                                                a _
  aluminum alloys (1100, 5052)                             0.045
  aluminum alloys (2024 and 6061); brass,                  0.060
     soft cold rolled steel, soft stainless steel
  cold rolled steel, stainless steel, (hard & half-hard)   0.075
   Punch and Die Sizes
• For a round blank of dia.
  Db and clearance c:
   – Blanking punch diameter =



   – Blanking die diameter =


• For a round hole of dia.
  Dh and clearance c :
   – Hole punch diameter =


   – Hole die diameter =
  Angular Clearance
Purpose: allows slug or blank to drop through die
• Typical values: 0.25O to 1.5O on each side



Cutting Force:
      F=StL
  where S = shear strength;
         t = stock thickness,
        L = length of cut edge
              (disregard ‘c’)
              1 ton ≈ 8896 N
Progressive Die
  Types of Sheet Metal Bending
• V-bending - performed
  with a V-shaped die
  – Performed on a
      press brake
  – V-dies are simple and
    inexpensive
• Edge bending - performed
  with a wiping die
  – Pressure pad
     required
  – Dies are more complicated
    and costly
  Stretching during Bending
• If bend radius is
  relative to stock thickness,
  metal tends to stretch during
  bending
• Important to estimate
  amount of stretching, so
  final part length = specified
  dimension
• Problem: to determine the
  length of neutral axis of the
  part before bending
Bend Allowance Formula
                                        L1


         α
Ab = 2π     ( R + K bat )
        360

L = L 1 + Ab + L 2
                                   L2


where Ab = bend allowance; α = bend angle;
   R= bend radius; t = stock thickness;
   Kba is factor to estimate stretching
           If R < 2t, Kba = 0.33
           If R ≥ 2t, Kba = 0.50
   Springback
Increase in included angle of bent part relative to included
   angle of forming tool after tool is removed




   (1) during bending, the work is forced to take radius Rb and
   included angle αb‘ of the bending tool, (2) after punch is removed,
   the work springs back to radius R and angle α‘
  Springback




• Y: yield strength of the
  material                       1 1      Y        2 Y 
                                                             3

• E: modulus of elasticity of      3        4 Ri     
  the material                    Ri R f 
                                          TE         TE 
• T: thickness of the material
                                 Aluminum Alloy v.s. low C steel ?
Bending Force
Maximum bending force estimated as follows:
                                where F = bending force;
        K bf  TS  w  t   2

  F                                  TS = tensile strength;
                D                     w = part width;
                                      t = stock thickness.
For V- bending, Kbf = 1.33;
                                      D = die opening
For edge bending, Kbf = 0.33
   Drawing
• Sheet metal forming to
  make cup-shaped,
  box-shaped, or other
  hollow-shaped parts
• clearance c =     t
• where t = stock
  thickness
• In other words,
  clearance is about
  10% greater than stock
  thickness
 Tests of Drawing Feasibility
                                                                  Db
• Drawing ratio                                              DR 
                                                                  Dp
   – where Db = blank diameter; and Dp = punch diameter

   – Indicates severity of a given drawing operation

   – Upper limit:
                                                               Db  Dp
• Reduction                                               r
                                                                 Db
   – Value of r should be less than 0.50 for a cylinder

• Thickness-to-diameter ratio
   – Thickness of starting blank divided by blank diameter

   – Desirable for t/Db ratio to be          than 1%

   – As t/Db            , tendency for wrinkling increases
Blank Size Determination

• For final dimensions of drawn shape to be

  correct, starting blank diameter Db must be right

• Solve for Db by setting starting sheet metal blank

  volume = final product volume

• To facilitate calculation, assume negligible

  thinning of part wall
Other Sheet Metal Forming

• Ironing
• Embossing
• The Guerin process
• Stretch forming
• Roll bending
• Roll forming
• Spinning
 Ironing

• Makes wall thickness of cylindrical cup more
  uniform




Ironing to achieve more uniform wall thickness in a drawn cup:
(1) start of process; (2) during process. Note thinning and
elongation of walls.
Embossing

 Creates indentations in sheet, such as raised
  (or indented) lettering, or strengthening ribs




Embossing: (a) cross-section of punch and die configuration
during pressing; (b) finished part with embossed ribs.
Guerin Process

                 Advantages:

                 • Low tooling cost

                 • Rubber pad can
                   be used with
                   different form
                   blocks

                 • Suited for
                   prototype
                   production
   Stretch Forming
  Sheet metal is stretched and simultaneously bent:
                                                         φ


                                                               F  L  t Yf


• F = stretching force; L = length in direction perpendicular to
  stretching (e.g.        ); t = thickness; Yf = flow stress
• Initially assume ε = 0.002, calculate Yf =
• As ‘length’ is increased ‘t’ is
   – Find tf by conservation of volume: wotoLo= wf ∙ tf ∙ Lf
• Die force Fdie can be determined by balancing vertical force
  components: Fdie = 2F sin φ
Roll Bending

  Large metal sheets and plates are formed
  into curved sections using rolls
Roll Forming

Continuous bending process in which opposing rolls
produce long sections of formed shapes from coil or
strip stock
Force and Springback estimates of a V-Bend
 •   A square Ti Alloy work piece being bent:
 •   E=300GPa, Y=344MPa, TS=415MPa,
 •   t=2mm, w=150mm, D=75mm
 •   Ri=6mm
 •   Kbf = 1.33, or 0.33

       K bf  TS  w  t   2   1   1    Y        2 Y 
                                                           3

F                               3        4 Ri     
               D               
                                Ri R f 
                                         TE         TE 
Springback Estimation:
              1   1    Y        2 Y 
                                          3
Using           3        4 Ri     
Formula        Ri R f 
                       TE         TE 
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• Non-traditional & advanced Manufacturing

								
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