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#139 Design of Blanking Dies (12) Key Points of the Design of a Blanking Die with the Fixed Stripper Construction

Category : Shear Forming
December21, 2012

This is a summary of the key points of the design of a blanking die with the fixed stripper construction.

(1) Reasons for using a fixed stripper

In the case of a blanking die, the item obtained after passing through the die is the product. Therefore, the material that remains on the die plate is scrap. Since there is no effect on the product even if there is some deformation, it is sufficient to use a construction in which there is no need to keep the material pressed.

(2) Fixed stripper and stock guide

The role of a stripper is to pull away the material that has got stuck to the punch. It is useful if the stripper is firmly fixed. The space below the fixed stripper is the space through which the material passes. If a large space is created, the material moves inside the space and it becomes difficult to carry out the blanking operation. If the width and height of this space is restricted to match with the material, it becomes easy to carry out the blanking operation. By doing this, the space below the fixed stripper becomes a space in which the material is guided (= stock guide). It is good to take measures in the stock guide so as to make the press operations easy.

(3) The interior of the construction of the fixed stripper cannot be seen

When carrying out press operations, it is very uneasy if the working point cannot be seen. This is the drawback of the fixed stripper construction. Let us cut out the uneasiness and try to make the working point visible as far as possible.

(4) Mounting the die (die setup)

Since a die is used after installing it in the press machine, the punch, die, and the die holder are designed considering the method of mounting. Although a shank is a convenient means for mounting the top die, it can also be dangerous depending on the usage. It is good to consider a small die used in a press machine of about 30 tons or less.

(5) Dimensions of the punch and die

The blanking clearance has a very important role in blanking operations. This clearance is the gap between the punch and the die. The dimensions of the punch and the die vary depending on the amount of clearance. In blanking operations the die dimensions will be equal to the product dimensions. The punch dimensions will be slightly smaller than the die dimensions.

(6) By how much to insert the punch in the die

The material is blanked by placing it on the die and pushing in by the punch. Logically speaking, if the bottom surface of the punch reaches the top surface of the punch, the blank should separate from the material and should enter into the die. However, in practice, if the thickness of the material is about 1 mm, the punch is inserted in the die by about 1 mm.
The reason for doing this is that, even the press machine extends or shrinks slightly. The frame of the machine extends when the punch comes into contact with the material and starts applying pressure. While extending, it pushes the punch inside the die. As the forming progresses, the necessary blanking force decreases. When a certain limit is exceeded, the extended frame of the press machine suddenly returns to its original position. At this time, it moves beyond the original position. This reduction is called breakthrough. Apart from this, even the various parts of the moving section have their effect. The bottom dead center varies due to such reasons. Therefore, at the time of blanking operations, the punch is made to enter slightly inside the die.

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