#104 Fundamentals of Press Die Structure Design (13) Relationship Between a Round Punch and Hole
The stud part of the punch (shaft = D) is held and supported when inserted into the hole in the punch plate. The methods of supporting are divided into two types of the fixed stripper structure shown in Fig. 1 (including even a movable stripper structure in which the tip of the punch is not guided by the stripper) and the structure shown in Fig. 2 in which the tip of the punch is guided by a movable stripper. These are explained below.
In a mold having the fixed stripper structure of Fig. 1, since it is necessary to maintain the position and perpendicularity of the punch by the punch plate, the hole for inserting a round punch has to be a press-fit hole. The "tightening margin" is very often set at 0.005 mm to 0.01 mm and it is very rare to use a mating tolerance. When a small number of punches are to be inserted in a punch plate, there is no problem even if a mating tolerance is used. However, when a large number of punches are to be inserted, the press-fit becomes too strong if a mating tolerance is used and the punch plate can become warped. Because of this problem, the conditions of the holes are determined by keeping the tightening margin fixed (the machining of the hole becomes more difficult compared to when a mating tolerance is used), thereby making the effect due to press-fitting small, and also, making the supporting of the punch very firm.
Although the seating surface dimension (H) has been taken as 5.3 mm, this has been determined because the tolerance of the flange dimension is +0.3/0 for a standard round punch. If possible it is better to finish the punch to have a flange dimension of 5.00 mm 0/-0.02 and an H dimension of about 5.00 mm. It is not good if the play between the H dimension and the punch flange dimension is too large and the punch moves in the axial direction.
Fig. 2 shows the different hole diameters for the structure when the stripper guides the tip of the punch (stripper reference). With respect to the dimension P of the punch, the stripper hole is made about equal to the dimension P1. The size of the number and the removing clearance (the plate thickness of the work to be formed) are proportional to each other. This is made loose with respect to the punch plate hole (D1). This method of fixing the punch is also called the punch free fixing method. Punch supporting with the stripper as the reference is considered better than when the relationship between the punch and the die is a fixing method based on the punch plate as the reference.
The relationship with the dimension H does not differ from that of Fig. 1. There is also a structure that is used in which the tip of the punch is guided by the stripper, and also the shaft of the punch is press-fitted into the punch plate, but this makes the preparation of dies considerably difficult.
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