May 2010 Archives

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.

Reverse redrawing is explained using Fig. 1 "Details of formation". The condition before formation is prepared by ordinary drawing. This shape is drawn so that the outside becomes the inside as is shown by the figure of "In the middle of formation". The product after forming is such that the inside of the shape before forming has become the outside. By doing this, the material of the product is made to move by a substantial amount, the stress of drawing changes, and it is possible to obtain larger reduction ratio than normal drawing.

The structure of the drawing die will be as shown in Figure 2 to Fig. 4. The shape of the die is common to all the structures. This looks very weak. However, the inner diameter of the product to be drawn gets added to the external shape of the die, and in the process of drawing, the product moves so as to tighten the die and becomes a resistance to the force during drawing that tries to open the die outwards, and hence since a balance is achieved, breakage of the die is not common. It is better that the straight part of the die interior is as short as possible.

Whether or not a stripper is necessary is determined by the relationship between the drawing diameter and the material plate thickness. If the material plate thickness is small compared to the drawing diameter, the structure becomes one like that shown in Fig. 4. This is because generation of wrinkles is suppressed at the part where the material makes U turn.

is the ease of operation of press formation work. This is because it is difficult to use for transfer forming of progressive forming depending on the details of the drawing work (work of stopping in the middle without inverting completely is a different case). Since single process formation is done frequently because of this, the ease of operation is judged to be poor.

Even redrawing work when carried out several times, cause the diameter reduction ratio to become small. Because of this, although the inner diameter guide becomes weak and easily breakable and hence can no longer be used, there is no problem because there is no fear of buckling of the drawn side wall part because of the small diameter reduction ratio. These types of redrawing die structures are shown in Fig. 1 and Fig. 2.

The structure of Fig. 1 is suitable for redrawing of products with a flange. The flange part of the product goes on top of the stripper, and the inner diameter of the product is guided by the tip of the punch that is projecting beyond the stripper.

The structure shown in Fig. 2 is structure that is suitable for redrawing of products without a flange. In the case of a product without a flange, the edge of drawing does not become flat but becomes wavy. Because of this, if the drawn edge part of the product is placed downwards, the product becomes slant. Becoming slant is a problem in drawing work. Therefore, drawing the material so that the bottom part of the drawn product is at the bottom causes the drawing process to become stable.

The feature of this redrawing die structure is in the killer pin. The killer pin presses down the stripper according to the timing of drawing, and makes sure that the product is not strongly gripped between the die and the stripper. If there is no killer pin, to begin with, the product is held between the stripper and the die, and the drawing is started thereafter. If this form is used, the plate thickness will not become uniform in the side wall part, and becomes the cause of generation of cracks in the bottom of the drawn product.

The killer pin is held by a spring. At the end of drawing, the spring of the killer pin contracts so that the stripper comes into contact with the die or the positioning plate.

In drawing work, there is a limit to the extent to which drawing can be done in one operation. Therefore, the desired diameter is obtained by repeating drawing several times. Such repeated drawing is called "redrawing". A die that is used for redrawing is called a "redrawing die". Since the drawing diameter difference is large in the step next to first drawing, a die with a structure shown in Fig. 1 or in Fig. 2 is used.

The product to be formed from now on is formed after placing a guide (inner diameter guide) in the gap between the punch and the inner diameter difference part of the drawn product by the blank holder (wrinkle suppressor in the case of a downward drawing die). Although wrinkles appear in the flange part during first drawing, in redrawing in which the difference between the diameters before and after drawing is large, wrinkles appear as vertical wrinkles in the side wall part. The inner diameter guide supports so that the side wall part does not buckle.

Next the relationship between the inner diameter guide and the rounded part of the die is explained here. The material of the product is gripped between the inner diameter guide and the rounded part of the die, slides over the inner diameter guide, moves to the rounded part of the die, and the drawing is done as the product moves to the interior of the die. At this time, the pressing force on the material held between the inner diameter guide and the rounded part of the die is determined either by the pressure of the die cushion or by the pressure of the spring. If this force is too strong, the deformation resistance becomes large causing breakage defects of the product or breakage of the inner diameter guide. If this force is too weak, it will lead to wrinkles. It is desirable to maintain a fixed gap between the inner diameter guide and the rounded part of the die. To do this, techniques are used such as placing a pin in the die, etc., and also a stopper is provided so that the gap between the inner diameter guide and the rounded part of the die is fixed.

This can be said to be the basic structure of a redrawing die.