May 2012 Archives

(1) Sideward Force (F)

As shown in Fig. 1, the sideward force is the force that is generated in a direction at right angles to the forming force. When the remaining width of the material is small, the material is pressed by the sideward force and can get deformed.

The effect on the punch is that it is pushed to one side, and the clearance changes thereby changing the blanking state. The die can break if its cutting edge part is weak. The sideward force increases in proportional to the blanking process. It also changes depending on the size of the blanking clearance. The sideward force is expressed as follows.

Fig. 2 shows the coefficient (Kf) of the sideward force when the clearance is 3%. In the case of a soft steel plate, care should be taken because this can become even larger than 30% of the blanking force.

(2) Scrap Removing Force (Ps)

This is also called the stripping force. This is the force required to detach the material that has got stuck to the punch. (See Fig. 3.)

This force is indicated as follows with respect to the blanking force (P).

The scrap removing force varies between 0.03 and 0.08. The scrap removing force changes greatly with the clearance. This force becomes large as the clearance becomes small and becomes a minimum at a clearance of about 20%.
In the case of a movable stripper type die, this scrap removing force is required to determine the strength of the spring. When flatness is obtained by pressing the material using a movable stripper, the above value of Ks is insufficient, and it will have to be made much larger. The value of Ks in this case will be about 0.1 to 1.0. The more frequently used value of Ks is in the range of 0.1 to 0.3.

Knowing the forming force necessary for carrying out a press blanking operation (shearing operation) is indispensable for selecting the press machine and for carrying out the die and punch design.

The blanking force (P) is obtained using the following equation.

However, when it is difficult to know the shear resistance (S), it is substituted by a value equal to 80% of the tensile strength (Ts) of the material. The equation in this case will be as follows.

Taking the example of Fig. 1, the blanking force will be as follows. In this case the tensile strength of SPCC is taken as 30 kgf/mm2.

As a method of reducing the blanking force, there is a method of providing a shearing angle as shown in Fig. 2. The shearing angle is provided in the die in the case of blanking operations and in the punch in the case of hole punching operations. Most often the shearing angle is provided so that the dimension H is roughly equal to or more than the plate thickness. By the way, the blanking force can be reduced by about 30% when the dimension H of the shearing angle is made equal to the plate thickness.

Sometimes it is difficult to refer by names to detailed parts when investigating a product of drawing. The names of some parts that are convenient if remembered are given below.

[Fig. 1] shows the names related to the shape of a product formed by cylindrical drawing.

[Fig. 2] shows the names related to the shape of a product formed by square container drawing.