4 Deep Drawing

  • Uploaded by: mck_medo
  • 0
  • 0
  • March 2021
  • PDF

This document was uploaded by user and they confirmed that they have the permission to share it. If you are author or own the copyright of this book, please report to us by using this DMCA report form. Report DMCA


Overview

Download & View 4 Deep Drawing as PDF for free.

More details

  • Words: 1,488
  • Pages: 26
Loading documents preview...
4. Deep Drawing: 4.1Definition: Deep drawing is the forming of smooth (sheet) blanks into hollow parts. It is a process which involves forming by tensile and compressive forces.

D0

D dm

The flange is the deformation zone.

The deformation takes place using: a deep drawing ring, drawing punch and blank holder. In the process, the punch draws the material through the gap formed by the punch and the drawing ring, forming it into a cup.

Tooling and workpiece layout during the deep drawing process: 1 drawing punch, 2 blank holder, 3 drawing ring (drawing die), 4 container, 5 base plate, 6 ejector

4.2 Determination of the drawing force: Main assumptions made for the calculation of the ideal drawing force: 1. All of the energy expended is used to deform the material in the flange. The work against friction and the work to bend and unbend the material as it flows over the die lip are neglected in the initial treatment, but will be accounted for later by an efficiency factor. 2. The material does not strain harden. 3. Flow in the flange is characterized by plane strain, εz = 0, so the thickness of the cup wall is the same as the thickness of the blank.

The the maximum drawing force ( FD ) can be expressed as:

D FD   d S  f m ln  /  F d h

where S is the sheet thickness fm is the mean flow stress of the material during the drawing d is the punch diameter D is the outer diameter of the plank at the present state F is the forming efficiency,  F  60% ~ 80%

at fiber (1):

D0

l  D0    D0  1  ln   ln ln     l D     D  0

D1 (1)

(2)

at fiber (2):

 D1   2  ln  d   

4

 4

 4

D02 

D  2 0

D  2 1

h

d

To determine “D0”, “D” and “D1” and based on the constant surface area:



D



d2  d H



D0  d 2  4 d H

D2   d h



D  D02  4 d h

d2  d h



D1  d 2  4 d h

4

 4

 4

H

After the calculation of the strains 1 and 2 the flow stresses f1 and f2 can be determined from the flow curve of the drawn material.

f  m

 f  f 1

2

2

In general, the total drawing force (FD) is comprised of the following:

FD  Fi  FB  F f1  F f 2 where FB is the stress due to bending Ff1 is the force to overcome friction under blank holder Ff2 is the force to overcome friction at die edge Condition of successful drawing:

rmax

FD < force required for tearing the cup wall

  D  f m ln  /  F    ut of the cup wall  dm   

+ r rmax

rmax

4.3 Determining the blank size for axisymmetric (rotationally symmetric) parts with large radii (r > 10 mm): Bottom radii of > 10 mm must be given particular consideration when determining the blank size. This takes place using Guldin’s (Pappus’s) theorem: “The surface of a body of revolution generated by rotating a curve around its axis is equal to the product of the generating curve and the distance travelled by its geometric centroid at distance rs from the axis of rotation.”

4.4 Blank holder calculations: Pressure dm   fm  2 . p   actual  1   200.S  400  dm 0

fm 0

Area  ABH  D02  d e2 . 4 d e  d  2.w  2.rM

Force FBH  p . ABH

4.5 Redrawing:

D0

The parts which cannot be produced in a single drawing are manufactured by following redrawings:

dmf

D0

Drawing 1st Redrawing

d1

Total drawing ratio T 

d2

Redrawing ratio  2nd

Redrawing

Dm d mf

diam. before redrawing diam. after redrawing

 D  d  d 

d3

T   0  1  2    d1  d 2  d 3  0 1 2 Intermediate annealing is taking place between the redrawing operations

Direct Redrawing

Reverse Redrawing before

d1

d1

Blank holder

Die

d2

after

d2

4.6 Drawing tooling: Drawing clearance, w The drawing clearance, w, is half the

difference between the diameter of the drawing ring and the diameter of the punch.

Punch radius, rp ,for cylindrical parts This depends upon the part being drawn.

Die edge curvature, rM For cylindrical parts: Too small radii subject the sheet to additional strain. Too large radii lead to the formation of wrinkles at the end of the draw, as then the blank holder is no longer effective.

With low drawing depths, a smaller rM must be selected as otherwise, the area where the blank holder presses is too small. The parts of the drawing rings which come into contact with the sheet must be kept smoothly ground and polished to reduce frictional forces.

Structural design of drawing tooling The structural design of drawing tooling is determined by the following factors: 1. The type of deep drawing

Deep drawing tooling for the second draw:

2. The press which is available (single or double-action presses) If a deep drawing operation is carried out on a single-action press, then the drawing ring must be fixed onto the ram and the blank holder must be operated from the die cushion.

The principle of the deep drawing process with a single-action press, a)drawing punch, b)Blank holder, c)Drawing ring

Drawing tooling for the second darw for a single-action press

3. Drawing tooling for irregular flat forms e.g. for automobile manufacturing Drawing tooling for irregular, flat forms, such as those which occur often in automobile parts, is characterised by difficult forming conditions. Instead of a conventional drawing ring, a shaping drawing die is used which has the negative form of the workpiece. This tooling is produced by casting, often full mould (lost foam) casting followed by a chip-producing (cutting) finishing process.

To form the shape, the tensile-compressive stresses must be accompanied by uniaxial or biaxial tensile and bending stresses created by a suitable tooling design. The (draw) bead is generally put on the die and the draw beads (lock beads) are arranged on the blank holder, with gaps in the die to fit them.

The lock beads are of more importance than the draw beads and are positioned in places where problems occur with varying stress ratios and an associated material flow. The draw beads and lock beads mean that the material flow can be controlled, fine-tuned by FEM simulation and ultimately the toolmaker’s experience, so that the defects of cracking, over-reduction of sheet thickness and wrinkling can be avoided.

4.7 Defects during deep drawing:

4.8 Hydromechanical deep drawing (HDD) Definitions In hydromechanical deep drawing the circular blank to be formed (first draw) is pressed directly onto the downward-moving drawing die by a pressurised water pad giving it the exact shape of the drawing punch.

Advantages of hydromechanical deep drawing: When a processing medium is used in place of the rigid drawing ring, as the punch is moved down, this processing medium produces pressure on all sides, pressing the workpiece being formed onto the punch.

Frictional forces occur between the punch and the workpiece which transmit part of the drawing force onto the cup wall. This means that the force passed across the bottom of the drawn part is lower and the area of the workpiece under the most stress changes from the bottom of the workpiece towards the drawing radius. This provides the following advantages: – The possible draw ratio is far better than with the conventional drawing process. Draw ratios of up to  = 2.4 are possible, e.g. for St 13 and d/s= 100. – Conical and parabolic drawn parts are produced in one draw. In the conventional drawing process, 4-5 drawing operations with 1-2 intermediate anneals are often necessary for this kind of part. – The reduction in sheet thickness at the bottom radii is very low, enabling thinner sheets to be deployed in many cases. Very small bottom radii can still be drawn efficiently for this reason. – Blanks of varying thickness and different kinds of material can be processed with the same tooling. – Production costs are lower than with conventional deep drawing. – Low tooling costs – Fewer steps in drawing – Lower annealing costs. – Any down-acting double-action press can be fitted with a hydromechanical unit, even as a later addition. So expensive special-purpose machines are not necessary.

Related Documents

4 Deep Drawing
March 2021 0
Deep Drawing
March 2021 0
Deep Drawing
March 2021 0
Deep Drawing
March 2021 0
5 Deep Drawing
March 2021 0