Drawing Process

DRAWING PROCESS

Drawing is the operation of producing cylindrical or shell shaped components by applying the required force using a die set. The cylinder or shell is having either straight or taper sides. The metal flows over a radius to make curved corners. The pressure pad helps to apply uniform pressure on the workpiece. At the same time, it holds the workpiece tightly between die and punch.

First, the required blank is cut to make the shell. Next, the blank is kept on the die. Then the punch descends to push the blank into the die. After finishing drawing, the punch moves with stripper and the pressure pad holds the work. The simple arrangement of drawing die is shown in Figure 4.29 and a specially designed die with air vent for cylindrical cup is shown in Figure 4.30.

Based on the blank holder pressure, drawing operations are classified as follows:

1. Shallow drawing

2. Deep drawing..

Shallow drawing methods are preferred for the cup that is not deeper than half its diameter. But, deep drawing is mainly preferred for deeper cups. Deep drawing is the manufacturing process of forming the sheet metal stock into geometrical or irregular shapes which are more than half of their diameter in depth.

1. Factors affecting Drawing

1. Type of material

2. Ductility

3. Yield strength

4. Force of friction

5. Blank holder pressure

6. Lubrication

7. Radius on punch

8. Radius on die

9. Drawing speed

10. Die clearance.

2. Blank Holding Pressure

Blank holding pressure prevents the wrinkling of metal. Too high value of blank holder pressure causes higher frictional forces and finally, it results the tear of cup wall. It varies from 1/3 or more of drawing force.

⸫ Blank holding force, B = π/4 × (D − d)2 × σ

where

σ ⇒ Stress

D ⇒ Blank diameter

d ⇒ Shell diameter.