In a punching operation in which a part of the material with a width near the plate thickness remains, since it is difficult in terms of the strength of the die to punch both the left and right sides at the same time, the left and right portions are punched alternately. When this is done, frequently the shape shown in Fig. 1 is obtained.

During a punching operation, the punching force acting in the plate thickness direction, the sideward force trying to push out laterally, and a bending moment are acting on the material.
If the punching width is large, the sideward force is suppressed and there is no effect on the product after forming. If the width becomes narrow, the material bends because it cannot withstand these forces, and the tip opens up in the case of a cutout shape shown in the figure.
In addition, the cross-section gets twisted and the shape gets deformed as the effect of the bending moment during the punching operation as shown in the cross-section a−a.
As a countermeasure for these problems, the punching width is made wide to more than twice the plate thickness.

When narrow shapes are necessary, as shown in Fig. 2(a), as large a rounding R as possible is provided at the base of the narrow portions. In addition, another method is punching by making the blank holder strong at the narrow punching part. Caution should be exercised because making the blank holder strong can cause reduction in the plate thickness.
As shown in (b), there is also a method in which punching is done of a shape in which the tips of the thin parts are connected, and then the tip part is cut off.

When punching such shapes, the effect is increased if some of the measures are combined together rather than taking only one measure.

Although it was said above that it is not possible to punch the parts to the right and left of a narrow part at the same time, there is a method of punching simultaneously, but in this case the die structure will be special. Another measure is to change the clearance in different parts.

Increase the limits to punching of such shapes by combining various methods.