DEEP DRAWING

DRAWING:-

One of the most common metalworking methods is drawing, which involves forming flat sheet metal into “cup-shaped” parts. If the depth of the formed cup is equal to or greater than the radius of the cup, the process is called deep drawing.
Deep drawing is a process of cold forming a flat pre cut blank in to a hollow vessel .
In an idealized forming operation in which drawing is the only deformation process that occurs, the clamping force of the hold-down dies is just sufficient to permit the material to flow radically into the die cavity without wrinkling. Deformation of the sheet takes place in the flange and over the lip of the die; no deformation occurs over the nose of the punch.

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Analysis indicates that the flange is compressed circumferentially and pulled radically in the plane of the sheet into the side wall of the part. This is analogous to wire drawing in that a large cross section is drawn into a smaller cross section of greater length; and for this reason, this kind of forming process is called drawing to distinguish it from stretching.

The capability of the metal to withstand drawing depends on two factors.

1. The ability of the material in the flange region to flow easily in the plane of the sheet under a condition of pure shear. This means it is desirable to have low flow strength in all directions of the plane of the sheet.
2. Draw ability factor is the ability of the material in the side wall to resist deformation in the thickness direction.
3 .When the punch of the drawing tool  forces the metal blank through the  bore of the drawing die, different  forces come into the action to cause  rather complicated plastic flow of    the  material.
4.The volume and the thickness of the material of the metal  remain essentially constant and final shape of  the component will be similar to the  contour of the punch.
5. The relationship between the  diameters and depth of the drawn sheets vary widely and relationship is  an important factor in the design of  the drawing dies.
6. If the drawing ratio exceeds a certain limit the material will fail due to excessive stresses.
7. Then it is necessary to draw the component in two or more stages. This  increases the tool cost.

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SUMMARY OF METAL FLOW

Little or no metal deformation takes place in the blank area which forms the bottom of the cup.

The metal flow  taking place during the forming of the cup wall uniformly increases with the cupheight. 

The metal flow of the volume elements at the.periphery of the blank is extensive and involves a increase in metal thickness caused by severecircumferential compression. This increase in thewall thickness caused by severe circumferential compression. This increase in the wall thickness isat the open end of the cup wall. The increase isusually slight because it is restricted by the clearance between the punch and the bore wall of the die

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Wrinkling and Puckering:-
Deep drawing necessitates severe cold working and involves plastic flow of the metal.
The metal may buckle rather than shrink,
‘wrinkles’ occur at the edge of the blank  and ‘puckers’ appear on only one part of the blank.

Cause	Remedy
Insufficient the blank holding force (mostly in flanges).	Increase spring pressure Redesign punch and die
Too large area of unsupported metal between punch and draw ring	Use draw beads
Broken spring behind blank holder	Use homogeneous material

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Puckering:

Cause	Remedy
Wrinkling is called as puckers if they appear in any other part of the cup like sidewall. It may be due the improper clearance between punch and die.	Reset correct clearance Use optimum die radius
In addition, it will occur when the die radius is too large.	Proper blank holding force

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Sequence of drawing operation on double action press
Deep drawing involves placing a sheet metal blank over a shaped die and pressing the metal into the die with a punch. The piece produced may be cylindrical or box-shaped with straight or tapered sides or with a combination of straight, tapered, or curved sides.
The punch must provide enough force so that the metal is drawn over the edge of the die opening and allowed to flow into the die.

Sequence of drawing operation on single action press

Single-action drawing with a draw cushion works the other way round:
The forming force is exerted by the slide above through the die and the blank holder onto the draw cushion in the press bed (Fig.)
The draw punch and the blank holder of the drawing tool are both located in a base plate on the press bed.  Pressure pins, which come up through the press bed and the base plate transfer the blank holder force from the draw cushion onto the blank holder.
The female die and the ejector are mounted on the press slide. At the start of the forming process the blank is held under pressure between the draw die and the blank holder. The slide of the press pushes the blank holder downwards over the draw die against the upward acting force of the draw cushion.
The part is formed via the downward movement of the die over the stationary draw punch. The press slide must apply both the pressing and the blank holder forces.

Thus, using a single-action tool, the part does not have to be rotated after the drawing process. Furthermore, today the use of hydraulically controlled draw cushions, even with deep drawing processes up to depths of 250 mm, produces a work piece quality comparable to that of double-action presses.

Sequence of operations in reverse drawing

An energy saving and cost effective alternative in stamping is counter drawing or reverse drawing (Fig.)
In this operation, once again, a single-action press with a draw cushion is used normally a hydraulic one.  The top die is attached to the slide. The lower die is mounted on the press bed with the blank holder.

The punch is located in an opening in the center of the press bed on the draw cushion. During deformation, the blank holding force is transferred via the slide from above and the draw punch force acts from below through the active draw cushion.
The punch forms the part by means of its upward movement while the blank holder rests on the die. The active counter drawing combines the advantages of static blank holding and the low power usage of double-action dies with the, advantage of a single action die, in which it is not necessary to rotate the part.
Admittedly, it is necessary to have a specialmodification of the dies for performing this operation,since the forces operate in the opposite direction tothose of a normal single-action deep drawing presswith draw cushion. Thus, these dies cannot be installed in a normal single-action press.

Lubrication for deep drawing.
The punch must provide enough force so that the metal is drawn over the edge of the die opening and allowed to flow into the die. The sheet metal blank must be strong and ductile enough to avoid breaking in areas where the metal flows from the punch face to the sides of the punch.
Characteristic of deep drawing is the high pressure on the order of 100,000 pounds per square inch (PSI) involved in the operation. To deal with such force, the choice of lubricant is critical to the success of the operation.
Under such pressure, the drawing lubricant should:

Cool the die and the work piece.
lubricate between the die and the work piece.
Prevent metal-to-metal adhesion or welding.
Cushion the die during the drawing operation.

Drawing compounds used in deep drawing are known as boundary lubricants. The tooling and sheet metal surfaces are pressed so tightly together that the liquid is squeezed out and only a very thin adsorbed film remains.
Different types of drawing lubricants are used, depending on the depth of a particular draw. Generally, the effectiveness of a deep drawing lubricant depends on its ability to form an adsorbed film of sufficient strength and oiliness on the metal surface being drawn.

Three types of drawing lubricants are used:
Drawing oils. Drawing oils form an adsorbed film, and they take the form of light or soluble oils such as straight mineral oil or emulsions of soluble oil and soap, or of heavy oils, fats, and greases such as tallow or lard oil.
Emulsions. Aqueous solutions of nonoil lubricants containing some suspended solids are called emulsions. These lubricants are not widely used in deep drawing because they contain little or no oil.
Lubricants containing both oil and solid substances. Used in applications involving severe drawing, these lubricants contain oily components that reduce friction and heat. The combination of the oil and the solids together produces enough lubrication for severe drawing applications such as deep drawing.
A lubricant compound can be used as a paste or as a liquid after being diluted with water, depending on the required concentration and the severity of the drawing operation. Methods for applying lubricant to sheet metal include dips, swabs, brushes, wipers, rollers, or recirculation.

Of these, the three most common are:
1 Manually wiping lubricant onto a surface with a rag.
2. Roll coating, during which metal blanks pass through rollers that apply the compound.
3 Flooding, during which tooling and metal sheets are drenched with lubricant, and the excess liquid is recovered via a filtration and recirculation system.

BLANK DEVELOPMENT
(Cylindrical shell)

The first calculation to be made to determining the blank diameter. How much material will it take to produce the final draw? 
This is done by calculating the area of the final draw and then adding enough material for trimming and carrying the part.

This can be done by;
Algebraic method
Graphical method
Area of element method
Layout method
Center of gravity
Centre of stock

The next step is Determining how many draws or redraws are needed to make the final product, this is
calculated as a percentage of reduction from blank diameter to final part geometry. As a general rule, the
maximum reduction for most materials is about 40%.After we have the calculations, we then can develop a
tooling approach.

Algebraic method

The following equations may be used to calculate the blank size for cylindrical shells of relatively thin
metal. The ratio of shell diameter to corner radius can effect the diameter and should be taken into consideration. The cylindrical shells can be considered as consisting of circular pipes or disc.

D = Blank Dia.
d = Shell dia
h = Shell dia.
R= Corner radius.

Solved Examples: (Algebraic method)

Determine the blank size required to produce a cup of Ø65mm , height 75mm,  corner radius 3.5 drawn from a 1mm of DD Quality steel

Solution:
Given: Cup dia = 65
Cup height = 75
Conner radius = 3.5

D / r = 65  / 3.5
=  18.6

As d / r is in between 15 and 20 following equation can be used

D = √( d2 + 4 d h ) -0.5R
D = √( 652 + 4 x 65 x 75 ) - 0.5 x 3.5
D = 152.2

Graphical method

Knowing the finished shell dimensions it is possible to  graph the diameter using the reference plane.
Step 1.
From O reference plane Raise the perpendicular  to height h 
Step 2.
From the top[ of the perpendicular Draw hypotenuse of length h+(d/2) to intersect reference plane.
The horizontal component x between the  intersections on the reference plane, equal s the  radius of the necessary circular blank diameter D

Solved Examples: (Graphical method)

Determine the force required to produce a cup of Ø65mm , height 75mm,  corner radius 3.5 drawn from a 1mm of DD Quality steel

Solution:
Given: Cup dia.   = 65
Cup height = 75
Conner radius = 3.5

Acceding to PYTHAGORUS Theorem
(h+ d/2)2 = h2 + x2
           x2  = (h+ d/2)2 - h2
= ( 75 + 65 /2 ) 2 - 75 2
  = 77.1
So D = X x 2
    = 77.1 x 2
    =154.1

Area of element method

To calculate the blank diameter for complex Circular shells, it can be divided into simple elements
Of shapes. The elements are numbered  1,2,3,..etc.,. Then each element is calculated for it’s development
by using the equations given bellow. Then all divided elements are added and root of the answer will be the
required diameter of the  blank.

Solved Examples: ( Blank development for cup)

Area of element method
To calculate the blank diameter for the circular shells, of  diameter 50, height 40, and corner radius 10
Solution:

In the example shown the elements are numbered 1, 2,3 etc
1 is a cylinder
2 is a portion of cylinder.
3 is a disc.
The area of these elements can be found by using equations given in the standard chart. From the total area the diameter of the blank can be calculated
Element 01 (cylinder)
= π x d x h
= π x 50 x 40
=  6283.2
Element 02 (portion of cylinder)
= π2 x r ( D -  0.7 r)
                                     4
= π2 x 10 ( 50 -  0.7 x 10 )
                                     2
=  2122

Element 02 (disc)
=  d2
=  30 2
=  900

So Total area of cup D=

D = √ (Element 01+ Element 02+ Element 03)
D = √ (6283.2 + 2122 + 900 )
D = 96.5 mm

Layout method

The graphical or layout method for determining the blank diameter for the same shell or cup is as follows:
Make an accurate layout of the part including a line through the centre of the stock.
Number each dissimilar section starting from the extreme edge of the part.
Draw a vertical line x y and mark off the length of each section accurately starting with section 1 at the top of the line (i.e.. the length of sections).
Number each section to correspond with the same section of the shell.
Through the centre of gravity of each section draw a line downward parallel to x y.

From point x and y draw a line A and D at 45°. Mark the meeting point at these two lines as P draw line to end at each section. 1 and 2 mark lines as B and C is about midway between x and y.
Draw a line A' parallel to A intersecting the lines drawn through the centers of gravity.
Draw parallel lines B'. C' and D'. B' starts where A‘ intersects the first centre of gravity line and so on until where D' starts where C' intersects the third centre of gravity line and continues to intersect A'.
Through the intersection of A' and D' draw a horizontal line Z to the centre line of the shell.
Construct a circle using y as centre and z as diameter.
Using x as centre draw an arc tangent to the circle.
Draw a horizontal line tangent to the top of the circle until it intersects the large arc.
The distance from this intersection to the line x y is the radius of the blank.

Centre of gravity  method.
The blank size for a symmetrical drawn cup can be determined by Guldinuss theorem. Guldinuss rule states that the area is equal to the length of the profile lines the length of the path of its centre of gravity.
The centre of gravity point in this method can be found out graphically or can be calculated arithmetically.

The centre of gravity distance can be calculated arithmetically from the formula.

Centre of stock method:-