Deep drawing process: What is it, Formula, Defects, Calculation, Applications, Advantages [with Pdf]

Hello Friends, Let’s Discuss about the Deep drawing. Deep drawing is combination of sheet metal operation and metal forming process. Lets see more about it.

What is Deep drawing ?

1] Deep drawing is the process of converting blank of sheet metal into a cylindrical containers with a flat or hemispherical base.

2] It is combination of metal forming process & sheet metal process.

3] If the ratio of height to diameter of the product is greater than 0.5 then this drawing operation is known as deep drawing.

`frac{h}{d_{p}}geq 0.5`

4] It basically consists of die, punch, blank holders.

5] After the application of drawing force, the blank of material forms & takes shape of the punch or shape made onto the punch. To avoid frictional resistance between blank & die, the lubricant is used.

6] The applications of deep drawing process are kitchen utensils, cylindrical containers, fire extinguishers etc.

Process Diagram

Construction

The components of deep drawing process are as follows :-

1)  Blank :-

It is work-piece on which drawing operation is done. It is a blank / Piece of sheet metal which is placed onto the die hole.

The diameter of blank is denoted by `D_{b}` and thickness is denoted by ‘t’. After application of drawing force, the material of blank form and take shape of punch.

2) Punch :-

Punch is used to apply drawing force on blank material. The shape of product depends upon shape of punch.

The punch pushes the blank material into the die cavity. The diameter of punch is denoted by Dp.

3) Die :-

It is generally made of cast iron or tool steel. The blank bend over the circular shape of die.

The clearance between the die and punch should be enough to allow drawing and to avoid shearing failure in material. The corners of the die should have fillets to avoid shearing.

4) Blank holders :-

Blank holders are used to hold the blank surface onto the die surface. The blanking force should be optimum.

If it is greater than optimum limit then it will results in shearing at die corners. If it is lower thank optimum limit then it will results in wrinkling defect at flange surface of product.

The optimum holding force is one third of drawing force.

`F_{h}= frac{F_{d}}{3}`

Working

1) The blank of sheet metal is placed onto surface of die.

2) The blank holders hold the blank onto the die.

3) The punch moves downward into the die operation to complete drawing operation because of which blank material will form & take the shape of punch or shape on punch.

4) In some cases, single stroke drawing will not give required height to the product. Hence in this case drawing operation is done in number of stages.

Deep drawing calculation

A] Diameter of blank (Db) :-

For cylindrical shape :-

Here, 

dp = Inner diameter of cylinder.

h = height of cylinder

r = Corner radius

Considering surface area of blank and inner surface area of cup are equal.

One side surface area of blank = Inner area of cup

 `frac{pi }{4}D_{b}^{2}= frac{pi }{4}D_{p}^{2}+pi d_{p}h`

`D_{b}= sqrt{d_{p}^{2}+4d_{p}h}`

For considering corner radius (r) the blank size is given by,

i) Case 1:- When `frac{D_{p}}{r}geq 20` , The effect of corner radius is neglected.

`D_{b}= sqrt{d_{p}^{2}+4d_{p}h}`

ii) Case 2 :- When `15< frac{D_{p}}{r}< 20`

`D_{b}= sqrt{d_{p}^{2}+4d_{p}h}-0.5r`

iii) Case 3 :- When `10< frac{d_{p}}{r}< 15` 

`D_{b}= sqrt{d_{p}^{2}+4d_{p}h}-2r`

For hemispherical shape :-

In this case, equating surface area of blank & hemisphere.

`frac{pi }{4}d^{2}= 2pi r^{2}`

`d_{b}= sqrt{8}r`

B] Maximum drawing force (Fd) :-

The maximum drawing force applied onto the punch for drawing operation is given by,

 `F_{d}= pi d_{p}t+sigma _{ut} [ frac{D_{b}}{d_{p}}-0.7 ]`

Where, 

dp = Punch diameter

t = Thickness of sheet

`sigma_{ut} ` = Ultimate tensile stress of blank material

Db = Diameter of blank

0.7 = Correction factor for friction.

C] Blank holding force :-

The blank holding force should be sufficient to avoid wrinkling defect and to avoid shear failure. Generally it equal to one third of drawing force.

`F_{h}= frac{F_{d}}{3}` 

D] Clearance between die & punch :-

The clearance between the die and punch should be optimum to avoid shear failure in blank.

The clearance is 10% more than thickness of sheet metal.

Clearance, ( C ) = (1.1) x t

Deep drawing defects

The defects occurs in material of blank are as follows

A] Shearing :- It is the shearing failure of blank material at corners of die and at corners of punch. It occurs when the stresses in blank material overcomes ultimate stresses.

B] Wrinkling / Earing :- When the blank holding force is less than optimum holding force then it will results in wrinkling of collar of product.

Advantages

The advantages of deep drawing process are as follows

1. It is quick operation.
2. Skilled operators are not required.
3. It is simple process.
4. It requires less setup cost.

Application

The applications of deep drawing process are as follows

1. Kitchen utensils
2. Cylindrical & hemispherical shaped containers.
3. Fire extinguishers cylinders.

FAQ on Deep drawing

Read also :-

• Non Traditional Machining or Non Conventional Machining Process 
• Traditional machining process or Conventional machining process