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#159 Design of Compound Punching Dies (8) Design of a Knockout - 3

Category : Die Design
May31, 2013

The method of preventing a knockout from falling down.

In the case of a large product, as shown in Fig. 1, several tapped holes made inside the knockout, and the knockout is held by screwing suspension bolts (stripper bolts) into these tapped holes. If this method is used, the design of the die becomes considerably easy. Even the preparation of the die becomes easy because there is no turning upside down of the die. For the purpose of preventing breakage of the suspension bolts, it is necessary to use as thick screws as possible, and to retighten the bolts frequently. Fig. 1

In the case of a round shape or a simple shape, a flange is provided at a part of or all around the knockout and the knockout is prevented from falling down by this flange (see Fig. 2).

Fig. 2, Fig. 3

Since a knockout is usually hardened, even the flange part becomes hardened. Since this makes it brittle, if the flange portion is thin, it can easily break due to repeated movements, and hence care should be taken to design the flange to be somewhat thick, and to suppress the hardening to about 56HRC.

If the shape of the knockout becomes complex, the body of the knockout and the flange portion are prepared separately and are used by combining them together. The combining is done using screws if there is enough space for tapping threads. In the case of small shapes, the combining is done by passing a pin through them, and then crimping both ends of the pin.

In this method, the body portion is hardened, and the flange portion is used without hardening thereby taking measures against breaking. If the flange portion is thin, care should be taken because the plate of the flange gets warped with use.

In the method of using a flange, since the strength of the die is affected if the flange is made too big, the size should be designed considering the balance with the size of the product so that there is no excessiveness.

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