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#129 Design of Blanking dies (2) Outline of Blanking Punch Design

Category : Shearing work
July 6, 2012

In blanking work, the shape of the blanking punch is the same as the shape of the blanked product. The dimensions of the blanking punch at that time are made smaller than the blank dimensions by the amount of clearance.

* See also
Lesson 5:Clearance in the Blanking Operation
Lesson 11:Basics of Blanking Work (1) Blanking Work
Lesson 26:Basics of Die Structure (1) : Structure of Blanking Dies - 1

During the design, it is important to make the shape simple and easy to prepare. The changes depending on the size of the blanking punch are indicated below.

(1) Design of a large sized punch (See Fig. 1)

[Fig. 1] Design of a large sized punch

Since the punch area is large in the case of a large sized punch, it is possible to drill threaded holes for fixing the punch and holes for dowel pins (knock pins) inside that area.
This is shown schematically in Fig. 1 (a). This type of shape can be machined easily using a wire cut discharge machine.
Fig. 1 (b) is a standard shape of an assembly of the top die. The punch plate is omitted and the punch is directly installed to the punch holder thereby simplifying the structure. In the case of simple shapes such as a circle, etc., it is possible to prepare in further simplified manner by integrating the shank and the punch as shown in Fig. 1 (c). However, as explained in Lesson No. 168, the problem of adjusting the clearances of the punch and the die remains.

(2) Design of a medium sized punch (See Fig. 2)

[Fig. 2] Design of a medium sized punch

This is a type of punch whose size makes it difficult to drill threaded holes or knock holes inside it.
In the case of a simple shape such as a circle, etc., it is possible to provide a flange and to drill threaded holes or knock holes in the flange part, and to design again a structure in which the punch plate is omitted.
When the shape is complex, a flange is provided at a simple part, and the design is made in a similar fashion.
When this has been determined to be difficult to do, the punch will have to be fixed using a punch plate. Understand that the punch plate is not an indispensable part.

(3) Design of a small sized punch (See Fig. 3)

[Fig. 3] Design of punch and punch plate

When the product becomes small, even the punch becomes small. In that case, it becomes difficult to fix only the punch to the top die, and the method is used of fixing the punch after embedding it in a plate. This plate for fixing the punch is called a punch plate.
As shown in figure (f), the punch is made as straight as possible. When there is some problem in the strength due to the relationship between the size of the punch and the punch length, the design is made to provide a step in the punch. The relationship between the punch and the punch plate is designed so that the punch is lightly press fitted inside the hole in the punch plate (so that it can enter the hole with light hit with a plastic hammer).
This ensures the verticality and position of the punch.
Since it is difficult to ensure the verticality of the punch when it is fixed by swaging, this method should be avoided as far as possible.
It is standard practice to use S50C or SS400 as the material for the punch plate. The thickness should be about 30% to 40% of the punch length.

(4) Punch material

The standard punch material is SKD11. The heat treated hardness should be about 60 HRC.
The major method of preparing dies is using wire cut electric discharge machining. This is because the wire cut electric discharge machining is suitable for the material SKD11 and it is also good for wear resistance.
The punch material is changed taking SKD11 as the reference depending on the number of work steps, the material of the product to be worked, etc.

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