January 2010 Archives

A blanking process is that of forming contour shapes as is shown in Fig. 1. The contour shape may become a product by itself, or some times may become a blank for bending or drawing operations. Normally, in the blanking process, the product passes through the die and falls down. A die of this type of structure is called a blanking die.

In a blanking die, the punch and the die are prepared to match with the shape of the product. The punch is made smaller than the die by the amount of the blanking clearance. When the material is processed, blanking chaff gets adhered to the punch. The stripper is the part used for the purpose of removing this blanking chaff from the punch. Since it is sufficient for the stripper to merely remove the blanking chaff from the punch, there is no problem even if the stripper is fixed on the side of the die plate. A stripper of this form is called a "fixed stripper".

Since this is a blanking die using a fixed stripper, it is called a blanking die of the fixed stripper structure. Since the material passes in between the fixed stripper and the die plate, a suitable gap is provided between them. Although there is no rule about this, but a guideline for the size of this gap is about 5 to 10 times the material plate thickness. Further, in this gap part, the material is guided along the width direction of the material.

Although a form is shown in Fig. 2 in which the punch is studded in the punch plate, if the cross section of the punch is large, the punch may also be affixed directly to the punch holder. On the other hand, if the punch is small and a thick material is being blanked, a punch packing plate may be placed between the punch plate and the punch holder. There can be may variations even in the basic structure.

Blanking operations always result in generation of burrs. Because of that, burrs are very frequently removed by flattening the burrs. However, there are problems about Product Liability laws, and recently there is an increasing trend of removing burrs. One of the methods of eliminating burrs is given here. This method is generally called the flash pressing method.

In this method, to begin with partial blanking is done as shown in Fig. 1. Usually the punch is slightly larger than the die. This condition is called "negative clearance". By pushing the punch inside the material, the material is pushed inside the die and single sided shear droop is formed. The amount of pushing in by the punch at this time is very important, and should be 75% or more of the material plate thickness.

The next process is the flash pressing process. In the flash pressing processing although the operation is very simple, it content is unique. This is explained in Fig. 2. The material is in the partially blanked condition. This condition is considered dividing it into three parts, namely, the virtual punch part, the virtual die part, and the part that is gripped between the virtual punch and the virtual die. The material in this part is blanked using the virtual punch and the die. In this process the material blanks itself the forms the die roll (shear droop) part.

The condition after flash pressing is shown in Fig. 3. The amount of pushing in the partial blanking process is the one that makes the flash pressing process successful. If this is shallow, the virtual punch and die are not effective. If this is too deep, the material gets cut before flash pressing. The reason for having this negative clearance is to make sure that the material does not get fused during the flash pressing process.

The shape after separation is shown in Fig. 4. Compare this shape carefully with that in Fig. 1. The finished dimensions, in terms of the external dimensions of the blanked product, will be almost the same as the punch dimensions in the partial blanking process. This is slightly different from the ordinary blanking process.

This method has not been tried for large products. Try out this process for small products.