In drawing work the material gets deformed due to the anisotropy of the material. For example, when a circular drawing is made from a circular blank, the blank does not become round but becomes a shape that is closer to a rectangle. An ear is generated when forming a drawn shape without a flange, and the edge becomes wavy. "Trimming" is the operation of cutting off the parts that have deviated from the shape as described above thereby making the shape correct.
A typical die structure for trimming is shown in Fig. 1. An inverted placement structure is being used. The positioning of the product is done using the internal diameter.
The same clearance between the punch and the die as well as the shape of the cutting edge as in normal blanking work are used as they are in this case also.
The trimming is done by the top die, and the product enters inside the top die. The product inside the die rises above the top die and is knocked out near the top dead point, and is thus ejected from the die. The knock out in Fig. 1 touches the bottom of the drawn product. In the case of a product with a large flange, there is the possibility that the flange is warped at the time of ejecting. In the case of a product with a large flange, the knock out is made to escape according to the diameter of drawing, and the ejecting is done at the flange part.
One of the methods of trimming products without flanges is shown in Fig. 2. This method is called "pinch trimming". In pinch trimming, a rounding close to the rounding radius of the flange is provided. The tip of the punch is an edge. The clearance is made zero or close to zero. The trimming is done by pushing the product into the die. Basically, no step is formed in the outer periphery (if a clearance is provided, a step corresponding to the size of the clearance will be formed). Rounding remains inside the product. When the cut edge has to be made neat, although cutting is made horizontally using a cam, etc., the structure of the die becomes complex.
The scrap of trimming has the shape of a ring, and this remains on the outer periphery of the punch. If it is left as it is, it will not be possible to take it out of the die. This problem is solved by a scrap cutter. A part with the shape of the letter V hits against the scrap. While the scrap is pushed down during the subsequent trimming operations, at that time, the scrap is cut by the V-shape and gets detached from the punch. At least two scrap cutters should be provided, and their number is increased as the shape of the product becomes large so that the scrap is cut into smaller pieces thereby making it easy to handle it.